A Resource Document

Volume 2 Pam Clunie & John Koehn

Silver Perch:

A Resource Document

Pam Clunie and John Koehn

Freshwater Ecology Arthur Rylah Institute for Environmental Research Department of Natural Resources and Environment 123 Brown Street, Heidelberg VIC 3084 tel: (03) 9450 8600 fax: (03) 9450 8730

July 2001

ISBN 978-1-74208-128-1 (Print) ISBN 978-1-74208-129-8 (Online)

Final Report for Natural Resource Management Strategy Project R7002 to the Murray Darling Basin Commission

Silver perch – A Resource Document

The preparation of this publication was funded by the Murray Darling Basin Commission, Natural Resource Management Strategy and Department of Natural Resources and Environment, . The views expressed are those of the authors and do not necessarily reflect those of the Murray Darling Basin Commission.

Copyright © 2001

Apart from fair dealing for the purposes of private study, research, criticism or review as permitted under the Copyright Act, no part of this publication may be reproduced by any means without the permission of Murray Darling Basin Commission.

Acknowledgments The authors which to thank all staff of the Freshwater Ecology Group, in particular John McKenzie, John Mahoney, Damien O'Mahony, Louise Grgat, Tom Ryan, Jason Lieschke, Andrew Bearlin, Tim O'Brien, Simon Nicol, Steve Saddlier and Justin O'Connor. The authors also wish to thank Bill O'Connor and Julia Reed, Parks Flora and Fauna, Alan Baxter, Craig Balinger Fisheries, Department of Natural Resources and Environment, Brett Ingram, Geoff Gooley and Gus Strongman, Department of Natural Resources and Environment, Snobs Creek, Donna Tippett, Marine and Freshwater Resources Institute, Ray Mephan 'Little Valley', Alistair Dove University of , Jane Roberts CSIRO, Alex Hamlyn Department of Primary Industries, Andrew Sanger, Alan Lugg, Cameron Lay, Paul O'Connor, Craig Schiller, Ian Lyall NSW Fisheries, Deborah Love Department of Land and Water Conservation, Robyn Watts , Anthony Moore, Southern Cross University, Phil Cadwallader Queensland Fisheries Management Authority and Lance Lloyd Murray Darling Basin Commission, John Humphrey, Department of Natural Resources and Environment, John Winwood, formerly SA Recreational Fishing Council, Ross Hyne, Environment Protection Agency and John Whittington, New South Wales Agriculture.

Recovery Team Membership: • John Koehn, Manager, Freshwater Ecology, Arthur Rylah Institute for Environmental Research, Parks, Flora and Fauna, Department of Natural Resources and Environment • Pam Clunie, Freshwater Ecology, Parks, Flora and Fauna, Department of Natural Resources and Environment • Tarmo Raadik, Freshwater Ecology, Parks, Flora and Fauna, Department of Natural Resources and Environment • Dr Stuart Rowland, NSW Fisheries, Grafton, New South Wales • Dr John Harris, formerly NSW Fisheries, New South Wales • David Moffatt, Department of Natural Resources, , Queensland • Dr Michael Hutchison, Department of Primary Industries, Deception Bay, Queensland • Dr Clive Keenan, Department of Primary Industries, Bribie Island, Queensland • Dr Alistair Brown, Aquatic Ecosse, , Victoria

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Silver perch – A Resource Document • Linda Selg, Environment • Bryan Pierce, South Australian Research and Development Institute

Steering Committee Membership: • John Koehn, Manager, Freshwater Ecology, Parks, Flora and Fauna, Department of Natural Resources and Environment • Pam Clunie, Freshwater Ecology, Parks, Flora and Fauna, Department of Natural Resources and Environment • Brian Lawrence, Murray Darling Basin Commission, Canberra • Dr Alistair Brown, Aquatic Ecosse, Melbourne, Victoria. • Dr Martin Mallen-Cooper, Fishways Consulting Services, , New South Wales

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Silver perch – A Resource Document

Summary

This document outlines the existing biological information for silver perch bidyanus and reviews existing and potential threats to the species, including an assessment of their possible roles in the species' decline. It was prepared as the first step in developing a recovery plan for silver perch (Clunie and Koehn, in press). It is essential to understand the available knowledge and knowledge gaps when determining recovery actions for a species. This ensures that decisions can be made using the best possible information and that research efforts are not repeated unnecessarily.

The collation of information for this project has indicated that silver perch has significantly declined within the majority of the Murray Darling Basin. Silver perch clearly warrants the formal recognition it currently has as a threatened species within Australia. There was a relatively consistent pattern of decline in commercial catches of silver perch in New South Wales since the early 1960s and in South Australia since the early 1980s. Many recent surveys within the Murray Darling Basin, including the New South Wales Survey, have recorded extremely low numbers of silver perch. Some surveys have indicated there has been a good population in the Murray below Torrumbarry. The species' high fecundity means that in years were conditions are suitable, good recruitment and strong year classes are likely to be recorded.

While this species has a wide distribution and was once abundant within the Murray Darling Basin, our knowledge of its habitat requirements and habitat preferences is poor. There have been some observations that the species prefers faster flowing, open waters, however the significance of habitat components such as woody debris, aquatic vegetation and riparian vegetation is unknown. Some research has been undertaken on movement patterns, indicating that both adults and juveniles undertake upstream movements. Upstream movements appeared to be stimulated by rises in water temperature and water level. Adults may move upstream prior to spawning. The reasons for upstream movement are not well understood. Whether there is variation in movement and spawning patterns across the species' range requires investigation. The behaviour, habitat preferences and physico chemical tolerances of eggs and larvae are also not well known. Silver perch has an omnivorous diet in which the significance of algal components appears to increase in adult fish.

With the exception of high altitude habitats, silver perch naturally occurs throughout the entire Murray Darling Basin, an area which encompasses a wide range of climates, habitats and environmental conditions. There are numerous threats which exist across its range and various combinations of threats operate in different ways, over different areas and different timeframes. This resource document reviews threats in the following categories; river regulation, introduced species, water quality, instream and surrounding habitats, diseases, fishing and breeding and genetic issues. Many threats are closely interlinked and complex, making it difficult to distinguish between causes of decline and the effects of threats. Our knowledge of the distribution and severity of particular threats,

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Silver perch – A Resource Document as well as their impact on the aquatic environment and individual species is incomplete. The reviews of each threat outline background, management and an assessment of the significance of each threat. Some research has been undertaken on silver perch, however there is much we do not know of its ecological requirements or its response to many threats. There is no doubt the species has suffered a significant decline throughout the majority of its range, this decline occurring over many decades, but there is a lack of detailed information to provide an accurate pattern of decline for particular areas. How each threat specifically affects silver perch is also an important factor. For example, whether a threat causes sublethal or lethal effects and whether it affects each life history stage is crucial to understand. There is also often a time lag between the occurrence of a particular threat and when the ecological consequences are recognised. Since recent aging has indicated silver perch can live up to 27 years, the decline may be a result of changes and threats which occurred in the last 20-30 years.

In this context, we cannot definitively state exact reasons for the decline of silver perch. It is likely that numerous threats have contributed to this decline, although some are clearly more significant and broad ranging than others. Threats such as river regulation have affected the majority of rivers within the Murray Darling Basin in a range of different ways. The changes to flow regimes may well have significantly affected the movement patterns and spawning success of silver perch. Barriers such as and weirs have restricted fish movements. Cold water pollution may have restricted successful reproduction as well as growth and survival for large stretches downstream of several major rivers. Barriers and thermal pollution are clear threats to silver perch. Whether carp has played any role in the decline of silver perch is unknown. It is not known how broad ranging threats such as sedimentation affect silver perch either in relation to adults, juveniles, larvae or eggs. Threats such as salinity may not have played a significant role in the decline of silver perch since the species demonstrates a reasonable tolerance to salinity. However this may become more significant in the future given the predicted increases in salinity over many areas. The toxicity of algal blooms is unknown.

There are a number of other threats, such as predation by redfin or pesticide contamination which operate in specific parts of the Murray Darling Basin. These threats may have contributed to the decline of the species but are less broad ranging that threats such as river regulation. The susceptibility of silver perch to previous threats such as DDT is entirely unknown. Some research has been undertaken on the species' tolerance to endosulphan, a pesticide now commonly used. In comparison to a number of native and introduced species, silver perch was one of the least sensitive species to endosulphan, however sublethal effects of this pesticide have not been investigated. There are a number of diseases such as Epizootic Haematopoietic Necrosis virus and Viral Encephalopathy and Retinopathy which silver perch is susceptible to, however the prevalence of these viruses and other diseases in the wild, and whether silver perch may experience sublethal effects is unknown.

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Silver perch – A Resource Document Management issues such as stocking programs are likely to become more significant in the future, given the significant decline of silver perch. There are now large scale aquaculture operations, as well as releases of large numbers of silver perch fingerlings for put-and-take fisheries. The responsible management of genetic stocks is essential to ensure that if hatchery-reared stocks are released into the wild, they have appropriate genetic variation.

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Silver perch – A Resource Document

CONTENTS

ACKNOWLEDGMENTS...... I

SUMMARY...... III

SECTION 1 EXISTING ECOLOGICAL INFORMATION ...... 1

1 INTRODUCTION...... 1

2 DESCRIPTION ...... 2

2.1 ...... 2 2.2 GENETICS ...... 3 3 DISTRIBUTION ...... 3

3.1 PAST DISTRIBUTION ...... 3 3.2 PRESENT DISTRIBUTION ...... 5 3.2.1 South Australia...... 5 3.2.2 Victoria...... 5 3.2.3 New South Wales ...... 6 3.2.4 Australian Capital Territory...... 7 3.2.5 Queensland...... 7 3.3 TRANSLOCATIONS ...... 8 3.3.1 Victoria...... 8 3.3.2 New South Wales ...... 8 3.3.3 Australian Capital Territory...... 10 3.3.4 Queensland...... 10 3.3.5 Other States ...... 13 4 CONSERVATION STATUS AND LEGAL PROTECTION...... 14

5 BIOLOGY AND ECOLOGY...... 16

5.1 GROWTH, LONGEVITY AND MATURITY ...... 16 5.2 DIET ...... 17 5.3 HABITAT PREFERENCE ...... 19 5.4 MOVEMENT ...... 19 5.5 REPRODUCTION ...... 21 5.5.1 Spawning cues ...... 21 5.5.2 Spawning behaviour...... 21 5.5.3 Fecundity and spawning...... 21 5.5.4 Larval growth...... 22 5.5.5.Phototactic Responses...... 22 SECTION 2 - REVIEW OF THREATS...... 25

6 RIVER REGULATION...... 25

6.1 CHANGES TO FLOW REGIMES ...... 25

6.1.1 BACKGROUND ...... 26 6.1.2 MANAGEMENT...... 28 6.1.3 ASSESSMENT OF THE SIGNIFICANCE OF THE THREAT...... 30

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Silver perch – A Resource Document 6.2 CHANGES TO TEMPERATURE REGIMES...... 36

6.2.1 BACKGROUND ...... 36 6.2.2 MANAGEMENT...... 39 6.2.3 ASSESSMENT OF THE SIGNIFICANCE OF THE THREAT...... 40 6.2.4 INCREASES IN WATER TEMPERATURE ...... 42 6.3 BARRIERS ...... 44

6.3.1 BACKGROUND ...... 45 6.3.2 MANAGEMENT...... 46 6.3.3 ASSESSMENT OF SIGNIFICANCE OF THE THREAT...... 50 7 INTRODUCED SPECIES ...... 55

7.1 CARP...... 55

7.1.1 BACKGROUND ...... 56 7.1.2 MANAGEMENT...... 58 7.1.3 ASSESSMENT OF SIGNIFICANCE OF THE THREAT...... 58 7.2 REDFIN...... 65

7.2.1 BACKGROUND ...... 65 7.2.2 MANAGEMENT...... 66 7.2.2 ASSESSMENT OF SIGNIFICANCE OF THE THREAT...... 66 7.3 OTHER INTRODUCED FISH SPECIES ...... 69

8 WATER QUALITY ...... 71

8.1 SEDIMENTATION...... 71

8.1.1 BACKGROUND ...... 71 8.1.2 MANAGEMENT...... 73 8.1.3 ASSESSMENT OF SIGNIFICANCE OF THE THREAT...... 73 8.2 SALINITY...... 77

8.2.1 BACKGROUND ...... 77 8.2.2 MANAGEMENT...... 78 8.2.3 ASSESSMENT OF SIGNIFICANCE OF THE THREAT...... 79 8.3 ALGAL BLOOMS ...... 84

8.3.1 BACKGROUND ...... 84 8.3.2 MANAGEMENT...... 86 8.3.3 ASSESSMENT OF SIGNIFICANCE OF THE THREAT...... 86 8.4 AGRICULTURAL CHEMICALS...... 88

8.4.1 BACKGROUND ...... 89 8.4.2 MANAGEMENT...... 89 8.4.3 ASSESSMENT OF THE SIGNIFICANCE OF THE THREAT...... 91 9 RIPARIAN VEGETATION...... 95

9.1 BACKGROUND ...... 95 9.2 MANAGEMENT...... 97 9.3 ASSESSMENT OF THE SIGNIFICANCE OF THE THREAT...... 98 10 REMOVAL OF WOODY DEBRIS...... 99

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Silver perch – A Resource Document

10.1 BACKGROUND ...... 99 10.2 MANAGEMENT...... 100 10.3 ASSESSMENT OF SIGNIFICANCE OF THE THREAT...... 101 11 AQUATIC VEGETATION ...... 103

11.1 BACKGROUND ...... 103 11.2 MANAGEMENT...... 107 11.3 ASSESSMENT OF SIGNIFICANCE OF THE THREAT...... 107 12 DISEASES...... 108

12.1 BACKGROUND ...... 109 12.2 MANAGEMENT...... 113 12.3 ASSESSMENT OF THE SIGNIFICANCE OF THE THREAT...... 113 13 AQUACULTURE INDUSTRY, TRANSLOCATIONS AND GENETIC IMPLICATIONS ...... 115

13.1 BACKGROUND ...... 115 13.2 MANAGEMENT...... 123 13.3 ASSESSMENT OF THE SIGNIFICANCE OF THE THREAT...... 125 14 COMMERCIAL FISHING ...... 126

14.1 BACKGROUND ...... 126 14.2 MANAGEMENT...... 132 14.3 ASSESSMENT OF SIGNIFICANCE OF THE THREAT...... 132 15 RECREATIONAL FISHERY...... 134

15.1 BACKGROUND ...... 134 15.2 MANAGEMENT...... 135 15.3 ASSESSMENT OF SIGNIFICANCE OF THE THREAT...... 138 16 THREAT ASSESSMENT BY RECOVERY TEAM...... 140

17 REFERENCES ...... 142

18 PERSONAL COMMUNICATIONS ...... 172

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Silver perch – A Resource Document

Section 1 Existing Ecological Information

1 Introduction

This resource document is presented in two sections. Section 1 includes a description of silver perch, outlines its past and present distribution, its conservation status and legal protection throughout the Murray Darling Basin, as well as our current state of knowledge on aspects of its biology and ecology. Section 2 provides a review of existing potential threats to the species and its habitat. Each review includes a background to the threat, a discussion of current management as well as an assessment of the significance of each threat to silver perch. Each review provides a summary of each threat. The resource document also includes an initial assessment of the significance of threats undertaken by the recovery team at the commencement of this project.

Silver perch was once one of the most common fish species within the Murray Darling Basin. As is the case for many native fish species, detailed information concerning the past and present distribution of silver perch is limited, patchy and often anecdotal. However, it is apparent that the species has experienced a significant decline in distribution and abundance throughout most of its range. This observation comes from a collation of past and present survey data, the views of scientists, fisheries managers and anglers as well as commercial catch information.

The collation of existing information of silver perch has indicated that while there has been some targeted research on the species, further research is required to fully understand the species' habitat preferences and requirements, the significance of threats to its aquatic environment, and the reasons for its recent decline in abundance and distribution. Some research has been carried out on

Outline of research Much of the research carried out on silver perch relates to aquaculture. Little work has been undertaken concerning the species’ ecology, habitat preferences and requirements, the significance of threats to its aquatic environment, or reasons for its decline in abundance and distribution.

Initial research was undertaken in the 1960s on reproductive biology, including inducement to spawn in rearing ponds (Lake 1967a) and morphogenesis and ontogeny (Lake 1967b). Following the recognition that silver perch has a high aquaculture potential (Lake 1967c), techniques for the effective hatchery production of silver perch have been developed; these are summarised by Thurstan and Rowland (1995). Detailed research has been carried out on artificial diet (Allan and Rowland 1992), diets in farm dams (Barlow et al. 1986), larval growth, development, behaviour and diet (Thurstan 1991), stocking success of different aged fish and marking of individuals (Willett 1993, 1994, 1996, Ingram 1993) disease (Rowland and Ingram 1991, Callinan and Rowland 1995) and water quality in intensive pond culture (Rowland 1995a). Silver perch has Freshwater Ecology, NRE & Murray Darling Basin Commission 1

Silver perch – A Resource Document been found to be highly susceptible to the epizootic haematopoietic necrosis virus (EHNV) (Langdon 1989) and the Viral Encephalopathy and Retinopathy (VER) (Glazebrook 1995). Silver perch is also sensitive to low concentrations of the insecticide endosulphan (Sunderam et al. 1992). The tolerance of eggs and larvae (Guo et al. 1993) and juvenile fish (Guo et al. 1995) to varying levels of salinity have been tested, while Ryan et al. (1999) investigated the in situ tolerance of juvenile fish to high levels of salinity and low dissolved oxygen in stratified saline pools. The ability of larvae to detect and respond to gradients of light, depth, flow and leachates has been investigated (Gehrke 1990, Gehrke 1994). Reynolds (1983) carried out some work on movement patterns of adult silver perch, which recorded some undergoing extensive upstream migrations. Recent detailed research undertaken by (Mallen-Cooper et al. 1995) has found that both immature and adult silver perch undergo upstream movements. Mallen-Cooper (1994) investigated the ability of fish to negotiate an experimental fishway, although results were inconclusive. The population genetics of silver perch within the Murray Darling Basin has also been studied (Keenan et al. 1996).

2 Description

2.1 Taxonomy D.XII,12-13. P.14-17. A.III, 7-9. V.1,5. L.Lat. 70-90.

Silver perch (Mitchell 1838) is a member of the family , which is a family restricted to the Indo-Pacific region (Merrick and Schmida 1984). There are fifteen genera in this large family of small to moderately sized perch-like fish. Similar species to the silver perch include welch’s grunter Bidyanus welchi and the barcoo grunter Scortum barcoo (Merrick 1996).

The silver perch is also known as the grunter, black bream, silver bream, bidyan and Murray perch. It has an oval-elongate body which is highly compressed. The head is quite small and the snout pointed, appearing increasingly beak-like as size increases (Merrick and Schmida 1984). The eyes are small and located on the side of the head near the dorsal profile. The mouth is small, protractile, terminal and located ventrally; its gape reaches to approximately level with the posterior nostril. Narrow bands of small, pointed teeth occur along each jaw. The preorbital bone and preoperculum are strongly denticulate. The operculum has two flat spines, with the lower spine larger. Small, thin, ctenoid scales cover the body to the back of the head and preoperculum. A continuous lateral line follows the dorsal profile although it is not prominent. There is a single, long- based ; the anterior section is spiny while the posterior section is soft. The dorsal fin is deeply notched between the spines, with only a small notch between the spiny and soft sections. The spines are longer than the rays, and depress into a groove along the dorsal surface. The anal fin has a short base with three strong spines and a soft ray section. A scaley sheath covers the bases of both the anal and dorsal fins. The pectoral fins are located low on the sides below the largest operculum spine. They are small and rounded. The pelvic spines are small and pointed, and positioned forward on the abdomen. The caudal fin is slightly formed, and the upper and lower lobes are pointed Freshwater Ecology, NRE & Murray Darling Basin Commission 2

Silver perch – A Resource Document to slightly rounded (Cadwallader and Backhouse 1983). Shape can vary with age, with adults having a deeper and more compressed body than juveniles. While there is no sexual dimorphism, Lake (1967d) observed near ripe specimens to have a different body contour.

Fish are dark grey to grey brown on the dorsal surface, silvery laterally, and lighter and close to white ventrally; the dorsal and caudal fins are grey while the pelvic fins are white (Merrick and Schmida 1984). Colour intensity can vary with water turbidity. The dark margins on the scales give a reticulated appearance. Fish smaller than 100 mm may be mottled, with vertical dark bars (Cadwallader and Backhouse 1983).

Plate 2.1 Silver perch Bidyanus bidyanus

2.2 Genetics It is only recently that any research has been carried out on the population genetics of silver perch. Keenan et al. (1996) investigated the extent of genetic subdivision and inbreeding in populations of silver perch in rivers and impoundments within the Murray Darling Basin, using electrophoretic and morphometric techniques. Despite extensive sampling, only three polymorphic loci were identified. Another four loci showed unresolvable variability (C. Keenan, DPI, pers. comm. 1998). Three of the five stocked populations showed a reduced genetic variability compared to two wild populations and two other stocked populations. This limited genetic variation within some stocked populations was probably the result of hatcheries using small numbers of broodstock.

3 Distribution

3.1 Past Distribution The natural range of silver perch includes most of the Murray Darling drainage, excluding the cool, higher altitude, upper reaches of streams on the western side of (Merrick 1996). The species has been recorded in southern Queensland, western New South Wales, northern Victoria and South Australia (Cadwallader and Backhouse 1983). Previous records of the species from Western Australia and the Philippines are questionable and probably a result of misidentification (Vari 1978).

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Silver perch – A Resource Document

Detailed information concerning the species’ distribution and abundance in the past within the Murray Darling system is limited. In his notes of an expedition along the lower Murray between 1856/57, Blandowski (1857) noted the presence of silver perch, including illustrations of three different forms that “.. are all difficult to distinguish from each other.” The illustrations appear to include a gravid female, a juvenile and an adult. In a NSW Fisheries book, Tenison-Woods (1883) observed that silver perch were “... found in all rivers of the Murray system”. In their revision of Australian Therapontids, Ogiliby and McCulloch (1916) include the following quote from a fisheries officer in New South Wales - “The silver perch is one of the most plentiful and important fishes of the western waters of New South Wales, and occurs in large numbers in the and many of its tributaries”. In 1949/50, J. O. Langtry carried out an ecological survey in the Murray system; this work was compiled for publication by Cadwallader (1977). The report indicated a range of sites where silver perch were recorded from, with some indications of abundance, as well as some catch data. Between 1938 and 1942, 11 530 silver perch were recorded passing through the Euston-Robinvale fish ladder. Cadwallader (1977) noted that prior to Langtry’s work, there was little information concerning the distribution of freshwater fish in south eastern Australia. Despite the lack of detailed information, anectodal evidence suggests the species was previously quite common. General texts such as Roughley (1951) noted that the species ”... is fairly abundant in the Murray River and most of its tributaries.” Grant (1978) indicated that silver perch was “... commonly caught in the inland streams and waterholes west of the Great Dividing Range”, while Scott et al. (1980) indicated that the species is “.. one of the most abundant fishes in the River Murray system...”. However, Reynolds (1976) observed the decreasing abundance of common native freshwater species, including silver perch, in the Murray River in South Australia.

During a survey of the Murray River between Lake and Lake , Walker and Hillman (1977) noted that while they did not record silver perch, this species had been reported by local fishermen. During Langtry’s surveys in the late 1940s/early 1950s, he noted that silver perch occurred between Yarrawonga and Wangaratta, although there are no subsequent records on the Victorian Freshwater Fish Database. The Yarrawonga weir is likely to have prevented passage from the Murray River downstream. In Victoria, Cadwallader and Backhouse (1983) indicated that while silver perch was quite common in some areas, the species had recently experienced a significant reduction in range and abundance. In assessing the conservation status of freshwater species in northern Victoria, Brumley et al. (1987) considered silver perch to be locally abundant in lenthic and lotic sites in the Loddon River, while recognising the species was not widespread in northern Victoria. Brumley et al. (1987) indicated that there were probably fewer and smaller populations of silver perch than in the past, although they did not believe the species was threatened in the State.

Pierce (1988) noted that historically the species had been widespread throughout the Murray Basin, but experienced major, possibly natural, population fluctuations. The reasons for these fluctuations are not well understood. Poole (1984) indicated that prior to

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Silver perch – A Resource Document river regulation, the abundance of fish in the Murray varied according to floods and drought.

3.2 Present Distribution The natural distribution of silver perch has changed since European settlement, due both to the decline in its abundance and distribution, as well as to the translocation of fish into various river systems, impoundments and private dams. Merrick and Schimda (1984) indicate that silver perch is not known upstream of Chinchilla on the in southern Queensland, not beyond Bonshaw, New South Wales, on the in the north east, or beyond , New South Wales, on the Murray in the southeast, and Seven Creek and its junction within the in Victoria, to the south. It is apparent that the silver perch has suffered a significant decline in abundance and distribution within the Murray Darling Basin. A general map of the Murray Darling Basin including main tributaries is provided in Plate 3.1.

3.2.1 South Australia Limited information is available concerning the present status of silver perch in South Australia. Pierce (SARDI, pers. comm. 1998) believes the species can still be caught in most areas, although it has clearly declined in abundance. Surveys have indicated a dominant year class from the 1989 flood and no strong year classes since (B. Pierce, SARDI, pers. comm. 1998).

3.2.2 Victoria In Victoria, silver perch have been recorded from twelve river basins. These include eight basins where populations occur naturally - Upper Murray (Lake Hume stocking), , , Goulburn River, Campaspse River, Loddon River, Murray and the . Silver perch have been introduced into the other four river basins - Wimmera River, Yarra River, Werribee River and Corangamite. The majority of records are from the Goulburn River, Loddon River, Murray Riverina, and Mallee.

In Victoria, records concerning the distribution and abundance of freshwater fish species are held within the Freshwater Fish Database, DNRE. In terms of silver perch, no surveys have specifically been undertaken to assess this species’ distribution and abundance in the state. Therefore, information within the Freshwater Fish Database is a collation of records from a variety of Departmental surveys as well as miscellaneous other records. These include Langtry’s surveys of the Murray in 1949/52, a survey of Sevens Creek by Cadwallader (1979), surveys around the Shepparton area during the Carp project (Hume et al. 1983), a survey of northern Victoria by Brumley et al. (1987) to assess the conservation status of native species, surveys by other Departmental staff, inland angling information compiled by Tunbridge and Rogan (1976), Tunbridge and Rogan (1981), Tunbridge et al. (1991) and Tunbridge and Glenane (1982) as well as more recent surveys along the Murray River (Koehn 1996, Koehn and Nicol 1998).

Distribution and abundance information must be interpreted with care, taking into account gaps and concentrations in survey effort, survey methodology (e.g. nets used that

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Silver perch – A Resource Document may capture or fail to capture particular species, ages or sizes of fish), community composition (eg, some species may be far more abundant and dominant to catch) and the focus of surveys (stocking assessments, ecological surveys, presence/absence etc.).

3.2.3 New South Wales In New South Wales, Rowland et al. (1983) noted that while silver perch were found naturally in some impoundments, the majority of populations are small; they suggested this could be because of the absence of appropriate conditions for regular spawning and successful recruitment of larvae. The decline of silver perch is most effectively demonstrated by the comparison of numbers of silver perch caught at the top of the Euston fishway from 1939-1942 to 1987-1992. This indicates a decline of 93% in this area over the last 50 years (Mallen-Cooper and Brand 1992).

The Murray River below Torrumbarry currently contains one of the largest populations (Mallen-Cooper et al. 1995). They suggest this maybe the result of the long distance (528 km) of river between Torrumbarry and the next weir downstream which is at Euston (Lock 15). Between February 1991 and June 1993, Mallen-Cooper et al (1995) estimated 7479 silver perch moved through the Torrumbarry fishway per week. There have been recent angling reports of silver perch being quite abundant near Torrumbarry (G. Gooley MAFRI, pers. comm. 1997) as well as near Barmah (J. McKenzie, DNRE, pers. comm. 2000). In the last year, large numbers of small sized silver perch have been recorded moving up through the fishway at Torrumbarry (J. McKenzie, DNRE, pers. comm. 2000). Keenan et al. (1996) surveyed a number of rivers and impoundments in the Murray Darling Basin to investigate the genetics of several species including silver perch. They only detected large populations of silver perch along the Murray River near the Torrumbarry Weir, and in the in Queensland. A breeding population is also known from the Cataract near Sydney.

Recent surveys in the Murray Darling Basin indicate that silver perch numbers are currently extremely low throughout the Basin (Keenan et al. 1996). These have included surveys since 1990 to assess the impact of barriers on fish migration in the Darling River. Fish were sampled above and below the Bourke Weir; of the 9178 fish caught, only 7 silver perch were recorded, all downstream of the weir (Harris et al. 1992). A similar survey of fish above and below the Brewarrina Weir collected 922 fish; no silver perch were recorded (Mallen-Cooper and Thorncraft 1992). Sampling above and below the Main Weir at recorded 2819 fish, and again no silver perch were collected (Mallen-Cooper and Edwards 1991). Gehrke et al. (1996) surveyed nursery habitats in the Paroo catchment, the Darling River near Menindee, the near and the Murray River in the Millewa area. Catches of silver perch in these four catchments were extremely low. Of the more than 11 000 fish caught, only three were silver perch (Gehrke et al. 1995). The Rivers Survey in New South Wales has indicated that the species is in very low abundance in Darling/Barwon River (Harris et al. 1995). This extensive survey sampled 80 sites four times in two years. Only seven silver perch were recorded from the Darling region, and two fish (possibly stocked) in the Murray Region (Harris and Gehrke 1997).

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Silver perch – A Resource Document 3.2.4 Australian Capital Territory The distribution of silver perch in the ACT has recently been reviewed by Lintermans (2000). Lintermans (2000) noted that the ACT probably represented the upstream limit of the species' distribution in the Murrumbidgee catchment. Numbers in the Canberra region have declined from the mid 1980s. A migration of silver perch moving upstream to the lower reaches of the Murrumbidgee River has not been recorded since the late 1970s/early 1980s. There are no records of silver perch from the Paddys, Naas or Gudgenby rivers and there are anecdotal angler records from the in the 1940s and 1950s. ACT Parks and Conservation Service (1995) notes that silver perch is "…now less common in the Murrumbidgee River than they were 15 years ago."

3.2.5 Queensland Few surveys have been undertaken in Queensland on the distribution and abundance of freshwater fish in the Murray Darling Basin. Midgely (1989) surveyed areas within the Lake Eyre and Bullo River Systems, as well as areas within the Murray Darling Basin such as the Warrego, Condamine and Balonne rivers. Silver perch was considered common at sites along the Condamine River (near Condamine), Warrego River (near Charleville) and Boorara Creek (near Hungerford), and rare along the Ward River (near Charleville) and Warrego River (near Cunnamulla). Midgely (1989) also noted that while he did not catch silver perch at Macintyre Brook near Inglewood, locals reported the species as being present. Keenan et al (1996) recorded low numbers of silver perch in most sites they surveyed in Queensland. A recent survey of landholders was undertaken in southern Queensland and northern New South Wales; the survey sought opinions concerning changes to the natural environment, including perceived declines in fish numbers (Mottell 1995). The following opinions were provided concerning silver perch:

Table 3.1 Landholder opinions of the status of silver perch (taken from Mottell 1995).

River Year Last Seen Abundant Common In Decline Non Existent Barwon 1994 1 11 1 Birrie 1993 5 8 Bokhara 1993 3 10 7 1 Culgoa 1994 2 12 14 Narran 1994 21 29 15 2

In Queensland, Hamlyn and Thomas (1995) observed that while there is a concerted stocking program primarily in impoundments within the Murray Darling basin, silver perch appear to be becoming increasingly scarce in rivers. About 850 000 silver perch have been stocked within the Queensland Murray Darling Basin since 1984. Stocking groups have generally found that stocking fingerlings into rivers provides poor returns and such stockings are now not favoured (D. Moffatt, DNR, pers. comm. 1998). Recent surveys have indicated that relatively abundant natural populations still exist in the Paroo and Warrego catchments. Silver perch also occurs incidentally in the Condamine- Balonne and catchments (D. Moffatt, DNR, pers. comm. 1998). Silver

Freshwater Ecology, NRE & Murray Darling Basin Commission 7

Silver perch – A Resource Document perch appear to be largely restricted to areas <300 m elevation (D. Moffatt, DNR, pers. comm. 1988). Moffatt (DNR, pers. comm. 1998) notes that preliminary indications are that catches are only high during summer months.

3.3 Translocations Translocation is the deliberate movement of living organisms by humans from one area to another area. Silver perch have been translocated to various river systems, as well as impoundments and private dams in eastern Australia. Fish have been released into many eastern coastal river systems of New South Wales and south eastern Queensland and south western Western Australia (Merrick 1996).

3.3.1 Victoria In Victoria, few public waters have been stocked with silver perch. In 1979, 2000 silver perch were released into the Wimmera River, at Horsham, although by 1989 it was believed that the species was no longer present (Anderson and Morison 1989). In 1983, the Nagambie Angling Club released 3000 silver perch (from Hanwood fish farm at , New South Wales) in the Goulburn River and Nagambie (unpublished Fisheries file). In 1996/97, the Department of Natural Resources and Environment stocked three waters in Victoria with silver perch purchased from a private hatchery; 5000 fish were released into the Wimmera River east of Horsham, 2000 fish were released into near Altona, and 2500 fish into Bullen Merri Lake near Camperdown. Silver perch are currently not bred for release from the Department’s hatchery at Snobs Creek. There have been several private stockings in Victoria; for example Victoria Lake (200 fish in 1992) and International Village Lake (225 fish in 1992) near Shepparton.

3.3.2 New South Wales New South Wales annual reports from the 1915 to 1925 refer to the “rescue and transplantation” of silver perch from flooded river flats into dams and rivers. Fish were moved from the Murrumbidgee river flats to Burrinjuck (1916, 1919) and Temora Demonstration Farm Dam (1919), from flooded land near Bingagee into the Murrumbidgee (1919), from the Darling River at Menindie into Umberumberka Dam (1916), from the Murrumbidgee into the near Penrith and at Wallacia, and Cataract Reservoir and Prospect Observation Ponds (1916). Silver perch have been introduced into the , the Hunter River, supply dams in the Nepean River system, as well as numerous other locations (Harris and Battaglene 1989). Between 1976 and 1983, 427 000 silver perch were stocked in public waters in New South Wales (Rowland et al. 1983). Over the same period, more than one million golden perch Macquaria ambigua and silver perch fry have been sold to be stocked in private dams, and to be stocked in public waters by angling clubs, acclimatisation groups and government departments. Ingram (1993) noted that between 1972 and 1988/89, 2 122 000 silver perch fingerlings were released into the wild in New South Wales to enhance recreational fisheries. Rowland (1995d) notes that stocking of silver perch, as well as Maccullochella peelii peelii and golden perch, in the upper reaches of the Severn and Mole rivers in the has established good populations.

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Silver perch – A Resource Document Silver perch has also been routinely stocked in the by the Bingara Anglers’ Club Hatchery (Rowland 1995d).

Table 3.2 provides a summary of stockings of impoundments in New South Wales. Rowland (1995d) notes that silver perch stocking success in New South Wales has been patchy. Only and currently have good fisheries. It is apparent in a number of impoundments that survival can be good initially and that catches then decline. Rowland (1995d) suggests that survival may continue to be high, but that catchability of fish of two years and older may be low and variable.

Table 3.2 Summary of translocations of silver perch in New South Wales

Impoundment No. stocked No. stocked (1976- Comment 1994) Berrima Weir 32 300 (1982/83)1 191 6003 very few3 Burrendong Dam 54 700 (1978/79)1 113 7003 very few3 26 200 (1980/81)1 193 2003 very few3 (breeding population -K,W and S 1996) 92 000 (1981/82)2 107 0003 initially good, then marked decline in catches3 100 0003 very few3 Crookwell W. S. 15 400 (1981/82)1 68 000 (1975/76)1 68 0003 99 700 (1982/83)1 140 0003 initially good, then marked decline in catches3 Hume Weir 86 400 (1983) Lake Sooley 20 000 (1979/80)1 Lake Bathurst 21 700 (1979/80)1 Lake Hume 66 4003 very few3 Lake Mulwala 48 000 (1979/80)1 116 4003 few3 18 400 (1983)1 50 000 (1992) Lake Waldaira about 250 000 larvae stocked 1993, none recorded subsequently4 Lake Wyangan 12 500 (1976/77)1 30 000 (1980/81)1 21 400 (1981/82)1 Pindari Dam 50 0003 good fishery3 Marsden Weir 33 000 (1982/83)1 316 0503 good silvers3 Wyangala Dam 62 400 (1981/82)1 382 2703 very good silvers3 Rowland et al. (1983)1, Cadwallader (1983)2, Rowland (1995c)3 , Gehrke et al. (1996)4

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Silver perch – A Resource Document 3.3.3 Australian Capital Territory Cadwallader (1983) indicated silver perch were released into Lake Burley Griffin in 1975-77 and 1982 (considered unsuccessful) and into Lake Ginninderra (unsuccessful). Substantial numbers were released in Lake Burley Griffin and Lake Ginninderra in the late 1970s/early 1980s but few were caught by anglers. However, silver perch in Googong Reservoir has proven successful (Environment ACT 2000). Table 3.3 summarises known stockings within large impoundments in the ACT (ACT Parks and Conservation Service 1995). Lintermans (2000) noted there have been stockings of Lake Burley Griffin (1974-1983), Lake Ginninderra (1976-1982), Googong Reservoir (1983- 97), Cotter Reservoir (1987), Lake Burrinjuck, Lake George and Captain Flat Reservoir.

Table 3.3 Stockings of silver perch in the ACT

Impoundment Number stocked Years stocked Lake Burley Griffith 27 500 1981-83 Lake Ginnindera 23 000 1981-83 Googong Reservoir 100 000 1981-83 Googong Reservoir 26 000 1984-86 Googong Reservoir 75 000 1987-89 Googong Reservoir 15 000 1990-92 Googong Reservoir 30 000 1993-95 Googong Reservoir 10 000 1996-98 Cotter Reservoir 30 000 1987-89

3.3.4 Queensland In Queensland, silver perch have been translocated to the North East Division since 1986 to enhance recreational fishing in impoundments (Wager 1994). Hogan (1995) did not recommend the stocking of silver perch in impoundments. He observed that the regular stocking of silver perch has probably more a reflection of its ease of production rather than its success in developing a fishery. Since the ’s Recreational Fishery Enhancement Program commenced in 1986/87, 34% of all fingerlings stocked have been silver perch (Hamlyn and Thomas 1995).

There are currently at least eight hatcheries in Queensland which produce silver perch; they are stocked largely outside their natural range (M. Hutchison, DPI, pers. comm. 1998). The majority of stockings of silver perch have occurred in the Brisbane , followed by the Murray Darling, then the Burnett and Fitzroy (Table 3.4). In most impoundments where silver perch were stocked, numbers caught apparently decline once their diet changes with increasing size (Hogan 1995). Silver perch appear to change their dietary habits once they reach 30-35 cm in size, with plant material becoming predominant; these larger fish may therefore be more difficult for anglers to catch (Queensland Fisheries Management Authority 1996). There is no indication that those silver perch stocked in impoundments in Queensland spawn (Queensland Fisheries Management Authority 1996), and it is primarily a put and take fishery. Hogan (1995) also suspected recruitment would not occur in impoundments due to the high predation of larvae by the large number of predators including hardyheads, gudgeons and rainbowfish. Freshwater Ecology, NRE & Murray Darling Basin Commission 10

Silver perch – A Resource Document However, Rowland (NSW Fisheries, pers. comm. 1999) suggests this is more likely to be due to poor conditions for larvae in terms of food and water quality. Stocking productivity (i.e ratio of number of fingerlings stocked and the number of fish harvested) can vary significantly between impoundments (Hamlyn and Thomas 1995). Hogan (1995) lists a number of impoundments and river systems in the North East drainage division which have been stocked and where survival is either unknown or poor. Moffatt (DNR, pers. comm. 1998) notes that about 850 000 silver perch have been stocked within Murray Darling Basin waters since 1984. However, stocking groups have primarily found that stocking fingerlings into natural water bodies has provided poor returns and stocking is now not favoured. According to a summary of stocking of silver perch over five years up to June 1996, a total of 1 529 765 fingerlings had been stocked in the Brisbane region, 490 509 in the Murray Darling region, 81 666 in the Fitzroy region, 226 003 in the Burnett region and 55 500 in other miscellaneous areas (Hamlyn et al. 1997).

Queensland Fisheries Management Authority (1996) recommends that: • translocations should continue in the Boyne, Burnett and Burrum catchments; • translocations should continue in the Fitzroy catchment although the effect on leathery grunters Scortum hillii should monitored; • translocations should be reviewed in the Brisbane catchment, taking into consideration the cod Maccullochella peelii mariensis Recovery Plan; • the effects of translocations of silver perch on the Mary River cod should be considered in the Mary catchment. In 1994, Wager (1994) indicated that translocation of silver perch was no longer permitted in the Mary River Basin, since they may affect the survival of the Mary River cod. • translocations should be reviewed in the Logan and Albert catchments if Mary River cod is reestablished; • translocations in the Kolan catchment should be reassessed; • there should be no further translocations in the Burdekin catchment.

Table 3.4 Summary of translocations of silver perch in Queensland

Impoundment/River No. stocked - 5 No. stocked - 5 Comment years up to June years up to June 1994 1996 Barron (1100) Lake Tinaroo unsuccessful stocking3 Tully (1130) unknown success3 Burdekin (1200) unknown success3 Valley of Lagoons unknown success3 Pioneer (1250) failed stocking3 Fitzroy (1300)

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Silver perch – A Resource Document Bundoora Dam 1 0001 36 0001 4 0002 113 8761 77 6662 Glebe Weir 4 4001 Mount Morgan Dam 7 6001 Neville Hewitt Weir 14 5001 8 6501 Boyne (1330) Awoonga Dam 3 1251 poor survival3 Kolan (1350) Fred Haig Dam 5 0001 poor survival3 Burnett (1360) Bjelke Petersen Dam 52 4751 72 1112 32 3121 42 6922 Bundaberg Balance 1 2001 Barrage 8 4011 several sites, established, success unknown3 59 0001 81 2002 Claude Wharton Weir 31 5001 3 0001 Jones Weir 5 0001 Kilkivan Ring Tank 3 0001 40 0001 30 0002 Burrum (1372) 55 5001 55 5002 Brisbane (1430) 21 7501 20 0002 20 0001 50 0002 61 0001 111 0002 Lake Dyer 18 1741 11 3342 Lake MacDonald 45 2501 14 0002 Lake Samsonvale 212 4321 306 7512 20 5001 9 0002 43 3331 19 3332 570 7501 495 1272 809 3081 493 2202 Murray Darling Basin Beardmore Dam 21 0001 11 5382 Bonshaw Weir 11 2501 23 7502 Cecil Plains Weir 2 5001 12 0002 Chinchilla Weir 7 5001 17 5002 7 9001 2 0002 31 4001 56 4002 27 0641 58 8342 Dalby Weir 2 0001 Freshwater Ecology, NRE & Murray Darling Basin Commission 12

Silver perch – A Resource Document Gill Weir 12 1221 17 1222 55 9741 27 0342 Goondiwindi Weir 5 0001 16 0002 Gundi Lagoon 1 0002 Lemon Tree Weir 2 0001 12 1002 54 4001 78 1002 16 6672 Oakey Creek Weir 2001 16 8672 Storm King Dam 3 1251 10 2502 Surat Weir 8 8341 70 9432 Warra Weir 18 1821 18 1822 Yarramalong Weir 8 0001 24 2222

Hamlyn and Brooks (1995)1, Hamlyn et al. (1997)2, Hogan (1995)3

Keenan et al. (1996) recorded silver perch within the Lake Eyre drainage basin, in the Wilson River near Noccundra, and Coopers Creek near Currareva. Hamlyn (DPI, pers. comm. 1998) suggests farm dams within this drainage basin may well have been stocked with silver perch which have subsequently found their way into rivers during times of flood.

3.3.5 Other States In Western Australia, stocking of silver perch in rural, inland, farm dams has been permitted in recent years (Thorne and Brayford 1997). Following interest in further developing the commercial aquaculture of this species, guidelines have been developed. There has been some concern over the potential of introduced species such as silver perch to affect genetic diversity, the natural environment and biodiversity of native species, as well as to introduce diseases. Thorne and Brayford (1997) identify three categories for drainage basins. These are: • Drainage basins, or areas within drainage basins, in which silver perch farms and stocking are not permitted. • Drainage basins in which silver perch farms and stocking may be permitted, subject to conditions. • Drainage basins in which silver perch farms and stockings will be permitted, unless otherwise indicated by the Executive Director of the Fisheries Department.

Silver perch have also apparently been stocked into Clayton Bore which leads into Lake Eyre in central Australia (Unmack 1994).

Plate 3.1 The Murray Darling Basin

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Silver perch – A Resource Document

4 Conservation Status and Legal Protection The threatened classification of silver perch has changed over recent years, reflecting the growing recognition of its decline. In 1991, Lloyd et al. (1991) considered silver perch to be 'common' within the Murray Darling system. By 1995 silver perch was considered 'potentially threatened' (Jackson 1995). In 2000 silver perch remains classified as 'potentially threatened' according to the ASFB classification, while it is considered 'vulnerable' according to the IUCN categories (ASFB 2000). The recovery team for silver perch suggests that silver perch may satisfy the criterion of 'critically endangered'.

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Silver perch – A Resource Document The classification of silver perch has also varied in Victoria in recent years. The species was considered 'vulnerable' in Victoria from 1990 onwards (Koehn and Morison 1990, CNR 1995) before being classified as 'critically endangered' in 1999 (NRE 1999). In Victoria, silver perch is listed under the Flora and Fauna Guarantee Act 1988. The species is considered 'insufficiently known' in Queensland (Wager 1993). The species has not received a formal threatened classification in New South Wales or South Australia.

Commercial fishing In New South Wales, commercial fishermen introduced a voluntary ban on commercial landing of silver perch in 1993. In South Australia, many commercial fishermen have been voluntarily releasing silver perch for about ten years. In 1997, silver perch was declared a protected fish in South Australia by regulations under the Fisheries Act 1982. In Victoria, silver perch has been listed under the Flora and Fauna Guarantee Act 1988 meaning that the species cannot be taken without a permit. There is no commercial fishing industry for silver perch in Queensland.

Recreational fishing Table 4.1 outlines recreational fishing regulations within states in the Murray Darling Basin. Regulations are currently being considered in Victoria including closed seasons, bag limits and minimum size limits.

Table 4.1 Recreational fishing regulations for silver perch State Size Limit (cm) Bag Limit Queensland 30 • Combined total of 10 fish of silver perch, welch’s grunter and barcoo grunter New South Wales 25 (impoundments only) • Five fish (impoundments) • No take (rivers) Victoria 25 • Bag limit of 5 in lakes and impoundments north of the Great Dividing Range • Silver perch must not be taken from all other waters north of the Great Dividing Range (excluding the Wimmera Basin) • Bag limit of 5 in all waters south of the Great Dividing Range (including the Wimmera Basin) ACT Fully protected Fully protected South Australia Fully protected Fully protected

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Silver perch – A Resource Document 5 Biology and Ecology The following information on the biology and ecology of silver perch is summarised at the end of this section (Table 5.1)

5.1 Growth, Longevity and Maturity Silver perch usually reach between 350 and 400 mm in length and weight between 0.75 and 1.5 kg (Merrick and Schmida 1984). There have been records of fish reaching a length of 610 mm and a fish weighing 7.7 kg “... has been seen..” (Lake 1959). Lake (1967d) noted that fish up to 3.2 kg were not uncommon. In the , Beveridge (1883, in Roberts and Sainty 1996) noted silver perch “… from 10 lb to 2 lb (4.5 kg to 0.9 kg) could be hauled from lagoons...”. In 1915, Mr Anderson, a fisheries officer in New South Wales observed silver perch “... usually attains a weight of 5 lb (2.3 kg) although much larger specimens have been taken”. Between 1939-41, a local fisherman, Mr McKenzie recalled catching silver perch up to 3.2 kg in , New South Wales (Trueman and Lucker 1983).

Some original data from Langtry’s work (Cadwallader 1977) in the late 1940s-early 1950s was reviewed; a sample of 567 fish had a mean length of 413.6 mm (median 411.5 mm) ranging from 226.1 mm to 551.2 mm. 68% of fish caught measured over 400 mm. A sample of 542 fish had a mean weight of 1.03 kg (median 0.99 kg) ranging from 0.15 kg to 3.9 kg. Four and five inch gill and drum nets were used in Langtry’s study, which would tend to bias the catch towards larger fish. In a recent study in the Murray River, the vast majority of silver perch collected (n=394) measured between 100 and 400 mm (Mallen-Cooper et al. 1995). In Victoria, a small amount of data (n=141- 150) was reviewed for weights and lengths of silver perch; 6.4% of fish measured over 400 mm (ranging from 95 mm to 450 mm) and 8.7% weighed over 1 kg (ranging from 0.018 kg to 1.75 kg). While a range of equipment was used in these surveys (electrofishing, light traps, fyke nets, drum nets, gill nets) that would enable the capture of small fish, it may imply to some degree that fewer large silver perch tend to be caught nowadays.

Otoliths are commonly used to age fish for annual and daily increment determinations (Anderson 1990). Mallen-Cooper et al. (1995) found otolith rings were accurate annual markers for silver perch, with three independent readers agreeing on the majority of age determinations. They found that annual rings were largely formed between October and December. Otolith weight appeared to be closely correlated with fish age, and was almost independent of fish weight (Mallen-Cooper et al. 1995). Scale growth patterns are also used as a method of aging silver perch (Willett 1993), and are discussed in the aquaculture section.

Growth data indicates that once silver perch reach five years of age, growth slows dramatically. Mallen-Cooper et al. (1995) noted females had slightly slower growth rates than males although they reached a greater maximum size. Variations in growth rates and maximum size of silver perch were recorded between areas. For example, fish in the Warrego River had a significantly slower growth rate and smaller maximum size Freshwater Ecology, NRE & Murray Darling Basin Commission 16

Silver perch – A Resource Document compared to those in the Murray River. Silver perch in impoundments had greater growth rates than silver perch in rivers. A sexually mature silver perch from the Murray River grew from a mean weight of 765 g to 1.4 kg in nine months when on an artificial diet in earthern ponds at the Grafton Research Centre (S. Rowland, NSW Fisheries, pers. comm. 1999).

While little is known of the longevity of silver perch, Mallen-Cooper et al. (1995) notes the species appears to be long-lived. The oldest silver perch collected by Mallen-Cooper et al. (1995) in the Murray River was 17 years of age and measured 424 mm in length, while the oldest specimen was collected from Cataract Dam, being 27 years of age, measuring 440 mm in length and weighing 1.2 kg. Mallen-Cooper et al. (1995) indicated that a fish could be 15 years old, measure 400 mm and only weigh 1.4 kg.

Lake (1967d) noted males reached sexual maturity in the second year, females in the third year. Male and female silver perch cultured intensively in earthen ponds also reached sexual maturity at two and three years respectively (S. Rowland, NSW Fisheries, pers. comm. 1999). However, based on observations of mature gonads, Mallen-Cooper et al. (1995) considered males to reach maturity at about three years and females at about five years. The mean minimum size of fish at maturity was 263 mm. Merrick and Schmida (1984) stated females mature at 340 mm standard length, males at 233 mm standard length. Further work is required to explain this variation in age of maturity.

Mallen-Cooper and Stuart (in prep) suggested the variation in growth rates and maximum sizes of silver perch indicates that growth is opportunistic and flexible. They observed that the plasticity of life history characteristics and the extended reproductive season are likely to be evidence of adaptations to different latitudes and local habitat characteristics which may increase survival.

5.2 Diet There is little information available regarding the diet of silver perch in the wild. Cadwallader (1979) primarily recorded insect larvae in stomachs of silver perch from Sevens Creeks in Victoria, while Merrick (1996) indicated silver perch eat small aquatic insects, molluscs, earthworms and green algae. Dietary analysis of adult fish from the Murray River in June 1996 revealed algae to be the main component of the diet (Koehn, DNRE, unpublished data). Rowland (NSW Fisheries, pers. comm. 1998) notes that it is unclear whether silver perch digest algae either partially or completely. Much algae appears to be reasonably intact in the intestine and as it leaves the anus. Silver perch possibly only digest fauna which live on the algae. Burchmore and Battaglene (unpublished data, cited in Allan 1995) examined the stomach contents of 917 silver perch finding that 32% comprised algae, with the proportion of algae and other plant material increasing as fish grew. Allan (1995) indicated that as silver perch grow, the proportion of algae and other plant material in their diet increases. Queensland Fisheries Management Authority (1996) notes that at sizes of 20-25 cm, silver perch feed on a wide range of organisms with plant material forming a small proportion of the diet. Once fish reach 30-35 cm in length, their diet apparently changes, with plant material becoming

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Silver perch – A Resource Document predominant. At this larger size, silver perch seemingly become more difficult for recreational fishermen to catch. Lintermans (2000) states that reports that silver perch become mainly herbivorous once they reach 25 cm are incorrect, at least based on data from populations in Googong Reservoir where there was little change in diet with fish size.

The diet of silver perch in artificial rearing ponds has been investigated. Lake (1967d) recorded larvae feeding on phytoplankton and zooplankton, while adults were omnivorous feeding on zooplankton, shrimps, filamentous algae and aquatic plants. Early feeding larvae feed on small rotifers, algae, chironomid larvae and small (Thurstan 1991).

Barlow et al. (1986) investigated the diet of silver perch reared in farm dams, primarily to determine whether this species and golden perch would be appropriate species to use in polyculture. Polycultures can increase the production of fish in farm dams if species which are stocked have complementary feeding habits. Diets were compared in monocultures, and polycultures with golden perch. In monocultures, silver perch fed predominantly on zooplankton, while in polyculture silver perch ate zooplankton and chironomid larvae. Barlow et al. (1986) noted that previous observations of diet (e.g. Merrick and Schmida 1984) had not indicated such a high component of zooplankton. Thurstan and Rowland (1995) indicated that zooplankton is the most important component of the silver perch larval and post larval diets, with a succession of dominant zooplankton species occurring in rearing ponds. Silver perch have a small, terminal, mouth, with premaxillary teeth which can rasp material off solid surfaces (Barlow et al. 1986). When the gill arches come together, the villiform (velvety) teeth on the gill rakes can sieve material; a method to collect zooplankton.

The diet of silver perch in fertilised aquaculture ponds was dominated by chironomid larvae, Daphnia and calanoid copepods (Warburton et al. 1998). The selection of planktonic prey was related to prey densities. While silver perch were dietary generalists, feeding on a wide range of insect and zooplankton foods, individuals could show feeding patterns that were highly specialised on particular prey types.

Barlow et al.(1986) considered silver perch to be an omnivore primarily eating zooplankton, insects and microscopic plants and (predominantly algae, protozoans and rotifers) which live on or in close association with submerged plants and other material. The species has quite a long intestine which is suggestive of an omnivorous diet. The range of water conditions in which silver perch are recorded further supports the observation that it is an omnivore. The available food in rivers and streams differs from that in lakes and large bodies of standing water, due to factors such as current speed, water temperature, substrate type, concentrations of dissolved substances and many other characteristics (Williams 1980).

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Silver perch – A Resource Document 5.3 Habitat preference Little detailed information is available concerning the habitat preference of silver perch. The species occurs in rivers and large streams, as well as lakes and impoundments. Rowland (1995b) noted that the species occurs in both cooler, clearer, upper reaches of the Murray Darling River system with gravel beds and rocky substrates, and the turbid, slow flowing rivers in the west and north. While silver perch are found in a range of water conditions, Merrick and Schmida (1984) noted they prefer fast flowing waters, particularly where there are rapids and races. Cadwallader and Backhouse (1983) indicated open waters were preferred to those that were heavily snagged. Allen (1989) observed the species to be frequently seen below weirs and rapids. Predation rates of fish are higher immediately below weirs, as is angling pressure (Harris et al. 1992).

In Sevens Creek, Victoria, Cadwallader (1979) recorded silver perch in situations where cover was provided by debris and occasionally by stands of Phragmites and where the water was very turbid. In New South Wales, Llewellyn (1983) observed the species to be common in turbid, standing and slow flowing waters, often schooling in faster flowing water. Captures of silver perch during Murray River surveys in June 1996 mainly occurred in open waters off sandy beaches (J. Koehn, DNRE, unpublished data). Callanan (1985) recorded juvenile silver perch in the warm waters of side channels of the Warrego River, and noted that in these areas there was a lack of shade, shelter and plant life. In Queensland, Moffatt (DNR, pers. comm. 1998) notes that silver perch appear to be largely restricted to regions of less than 300 m in elevation. The dominant habitat in these areas is slow flowing and highly turbid lowland rivers with braided channels. Small juveniles appear to be common in isolated riverine pools. Silver perch is one of the only larger native fish species to appear near the water surface (Lake 1967c).

5.4 Movement Silver perch is a potamodromous species, migrating entirely in freshwater. It was previously thought that only adults undertook extensive upstream movements, particularly during times of flood, while eggs, larvae and juveniles drifted downstream. During Reynold’s (1983) study only a small number of tagged adult silver perch were recaptured; most moved about 40 km upstream, while one fish moved 570 km upstream in 19 months, and another moved 110 km. Detailed research has recently been carried out by Mallen-Cooper et al. (1995) on the movement of silver perch in the Murray River. They found that fish of a wide range of ages move upstream - 87% of the silver perch migrating upstream were immature, all being at least one year old. These fish generally measured between 100 and 400 mm in length. Mallen-Cooper et al. (1995) noted that Reynold’s (1983) work, which suggested upstream movements of silver perch were made only by mature fish, did not include tagging of small fish. Mallen-Cooper et al. (1995) observed a strong seasonality in upstream movement, with differences detected between immature and mature fish. While immature fish moved upstream from October to April, mature fish moved over a briefer time period from November to February. From 1938 to 1942, J. O. Langtry (Cadwallader 1977) recorded silver perch moving through the Euston-Robinvale fish ladder between October and May. Mallen-Cooper et al. (1995) suggested the season of migration would most likely be longer in the north of basin, Freshwater Ecology, NRE & Murray Darling Basin Commission 19

Silver perch – A Resource Document being influenced by daylength and/or water temperature. Spawning patterns of species probably differ between catchments because climatic influences vary between catchments in the Murray Darling River system (Gehrke 1991).

Migration of immature and mature fish commenced once water temperatures rose above 20oC (Mallen-Cooper et al. 1995). As water temperatures declined in late summer and autumn, immature fish continued to move upstream until the water temperature reached 16oC; no upstream movement occurred with water temperatures below 16oC. Immature fish only moved upstream during daylight. During 1991/92 few silver perch moved upstream on very low flows (i.e. below 3500 ML/day), high numbers moved on quite low flows (i.e. 4000-4500 ML/day), and significant numbers moved on flows of 6000- 8000 ML/day which represented small rises in river levels (Mallen-Cooper et al. 1995). In 1993, few silver perch moved at flows lower than 10 000 ML/day, with larger numbers moving during flows of 10 000-12 000 ML/day. Upstream migration of large numbers of silver perch occurred during small rises in the river, with small daily increments in flow. The highest number of fish moving upstream were recorded when the river level rose or fell between 1 and 10 cm over 24 hours on the crest of small rises. This study provides valuable information for a section of the Murray River. How flows vary within the Murray Darling Basin due to river regulation, and the possible impacts on the movement of silver perch need to be studied further. Mallen-Cooper (1993) noted that the number of small floods (5000-10 000 ML/day) occurring in the Murray River over the last 50 years has declined by half (Close 1990).

Mallen-Cooper et al. (1995) emphasised that the reasons for movement of silver perch are not well understood. Energy is expended by migrating upstream, so presumably there is some benefit to fish in undertaking this movement. Silver perch may move upstream to optimise feeding, to enhance colonisation, or to compensate for the downstream drift of pelagic eggs and larvae (Mallen-Cooper et al. 1995). The apparent lack of movement by those fish less than one year old may be because their existing habitat provides adequate food and shelter. Also their swimming abilities would be poor due to their small size, possibly increasing their susceptibility to predation (Mallen-Cooper et al. 1995). The energy required for small fish to swim upstream against the current may be too high to be justified.

Mallen-Cooper et al. (1995) noted that there was indirect evidence that adults move upstream prior to spawning. Commercial fishing information indicates that silver perch are caught in drum nets as they move upstream. Advantages of adult fish moving upstream to spawn may be to provide a longer period of exposure to flood, to ensure dispersal along the length of a river, or to help protect young from predation (Mallen- Cooper et al. 1995). Richardson (1994) suggested the upstream migration of mature fish to spawn may be a strategy to heighten the survival of larvae. As the eggs move downstream during floods, they have time to develop and hatch into larvae. Once individuals develop, they begin to feed and can swim against the current.

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Silver perch – A Resource Document 5.5 Reproduction

5.5.1 Spawning cues Cadwallader and Lawrence (1990) considered silver perch to have a reproductive strategy geared to the extensive floods which would have occurred in the Murray-Darling system in the past. Lake (1967a) noted the species requires a rise in water level, as well as water temperatures of about 23oC to induce spawning. However, Thurstan and Rowland (1995) indicated that success can be achieved once bottom temperatures reach 21oC. Such water temperatures would usually occur from mid-spring to late summer. Rowland (NSW Fisheries, pers. comm. 1998) induced spawning of viable, high quality, eggs (and sperm) at an ambient water temperature of 17oC in early spring. However, there was a high mortality of eggs and larvae incubated at 17oC, whereas survival of eggs from batches incubated at 20-21oC was high.

Silver perch will spawn successfully in impoundments (Hogan 1995). A wide range of year classes has been recorded from Cataract Dam indicating successful reproduction. Lake (1967d) recorded spawning occurring in October and November, although it could be delayed until late summer if conditions were unfavourable. Rowland (1995c) notes that adults are stay in an advanced stage of gonadal development until at least late January or February. Hormone induced spawning can artificially extend the spawning period of silver perch to February-April (Guo et al. 1993, Thurstan and Rowland 1995).

5.5.2 Spawning behaviour Lake (1967c) noted that when in their natural environment, silver perch will spawn in flooded backwaters of low gradient and meandering rivers. In 1915, Mr Anderson, a fisheries officer in New South Wales, provided an early observation of breeding behaviour; a school of between 50 and 70 fish was seen moving around in a series of eddies, some feeding at the surface (NSW Fisheries 1915). The majority of larger fish, which he presumed were females, were located in the middle while the smaller fish swam around them. Suddenly all the fish moved into the centre, splashing the water. The water then developed a white opaque tinge; the milt of the male fish. This behaviour was observed five or six times at periods of twenty to thirty minutes. Merrick and Schimda (1984) note spawning can occur at the water’s surface to about 4 m in depth, where water flows over a gravel or rock rubble substrate. During spawning, Lake (1967b) observed fish exhibiting vigorous activity at the water’s surface, sometimes with several males chasing a female. Thurstan and Rowland (1995) note the aggression displayed during spawning, with fish experiencing damage to scales and fins. Lake (1967b) observed some individuals died following spawning, while Rowland (1984) recorded a mortality of 13% within two days of spawning.

5.5.3 Fecundity and spawning Spawning was usually recorded in late afternoon and before sunset. Most eggs were released at the one spawning. Lake (1967d) considered silver perch to have a high fecundity. Merrick (1996) indicates a female can produce over 300 000 eggs. A 1.8 kg female has been recorded producing 500 000 pelagic eggs (Merrick and Schmida 1984). Freshwater Ecology, NRE & Murray Darling Basin Commission 21

Silver perch – A Resource Document Langtry (unpublished data, 1949-52) estimated a 2.4 kg Silver Perch produced 718 416 eggs, by extrapolating the number of eggs counted in a weighed subsample. Eggs are non adhesive, spherical, colourless, pelagic and semibuoyant. They measure between 2.5 and 3.0 mm in diameter when water hardened (Thurstan and Rowland 1995). Lake (1967c) noted that the large perivitelline spaces within silver perch eggs provide protection against damage and would enable a better survival in flowing water conditions. Cadwallader (1978) noted that pelagic eggs are suited to quite tranquil flood-spread waters rather than fast water currents. Silver perch eggs were collected in drift nets in the Murray River downstream of Lake Mulwala in early December 1996 (J. Koehn, DNRE, unpublished data). Characteristics of the eggs may also help in gaseous exchange, which may be beneficial when oxygen levels in water are low. There is no apparent parental care of eggs following spawning (Lake 1967b). Lake (1971) noted the similarity between silver perch and golden perch eggs, although he indicated silver perch eggs are more dense and will sink if there is no water movement.

5.5.4 Larval growth In rearing experiments, Lake (1967c) observed larvae to hatch rapidly, within 30 to 31 hours at temperatures of between 26 and 27 oC; they are in an early stage of development and measure about 3.6 mm. In hormone-induced spawning experiments, Rowland (1984) recorded hatching to commence 28-31hours after spawning, at temperatures of 24-25oC. Similarly, Guo et al. (1993) observed spawning taking place 34 hours after fish were injected with hormones in temperatures of between 24-26oC. Newly hatched larvae tend to sink, although they can make intermittent movements up to the water surface (Lake 1967b). Thurstan (1991) noted the similarities between silver perch larvae and marine fish larvae, both being a small size and poorly developed at the time of first feeding. Larvae are free swimming at five days and commence feeding by day six (Lake 1967b); while Guo et al. (1993) observed first feeding to occur at four days after hatching. While larvae of golden perch and Murray cod appear to use structures to provide cover and shelter, silver perch larvae rarely use cover and spend much time swimming in groups (Thurstan 1991). By the eighteenth day, larvae have developed into juvenile fish, and measure approximately 11 mm. In artificial rearing ponds, Rowland et al. (1983) recorded fry measuring 30 mm within five weeks. Also under pond rearing conditions, Thurstan (1991) observed a linear relationship in growth of fry. Clearly fish growth will vary depending on conditions such as food availability.

5.5.5.Phototactic Responses Gehrke (1990) investigated the ability of silver perch larvae to detect and respond to gradients of light, depth, flow and leachates from River Red Gum Eucalyptus camaldulensis wood. The response of larvae to such variables may influence their distribution patterns in floodplain habitats. Batches of silver perch larvae showed a significant variation in their response to gradients. The larvae also showed a significant positive response to light, and did not seek shelter in shade. This corresponds to Thurstan’s (1991) observation that larvae rarely use cover. Larvae may respond to light in different ways according to its intensity; this would assist in keeping a position in the

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Silver perch – A Resource Document water column. Larvae varied in their response to flow, some drifting downstream while others actively maintained their position (Gehrke 1990).

While light traps can be used to collect small fish, Gehrke (1994) noted little work has been carried out in relation to the characteristics of the light source which attracts the fish. With the intention of developing an optically efficient light trap to capture fish larvae, Gehrke (1994) investigated whether the phototactic behaviour of silver perch larvae is equally affected by changes in light intensity at different wavelengths. Silver perch larvae showed a degree of positive phototaxis in all light gradients which became stronger at greater light intensities. This response was not influenced by the dominant wavelength. The eyes of silver perch larvae were responsive to visible radiation by 10-12 days old. The highest sensitivity recorded at yellow-orange wavelengths can be explained: in the typically turbid inland waters in south eastern Australia, the visible radiation is dominated by these wavelengths. This suggests silver perch larvae are well adapted to their environment for this parameter.

Table 5.1 Existing biological information for silver perch Bidyanus bidyanus Inducement to spawn Rise in water level, rise in water temperature1. Spawning temperature 21oC2, 23oC1 Spawning period October to November, can be delayed until late summer if conditions unfavourable3, to February-April artificially2. Age at sexual maturity Males 2 years, females 3 years3, (Rowland pers. comm.). Males 3 years, females 5 years according to mature gonads4. Max. known age Long-lived. 17 years (from Murray River), 27 years (from dam)4. Fecundity High. Around 300 000 eggs., 1.8kg female can produce 500 0005 Spawning site Flooded backwaters of low gradient and meandering rivers6. Spawning behaviour Can school, with females surrounded by males, vigorous activity at water’s surface, some aggression and damage or death can result2 (Anderson, pers. obs.). Parental care No apparent parental care of eggs following spawning7 Egg description Non-adhesive, pelagic, semi-buoyant. Diameter of eggs 2.5-3.0mm when water hardened2 . Min. time to hatching Rapid hatching, within 28-31hours, at temperatures of 24-27oC7,8 Length of larvae at 3.5-3.7mm7. hatching Age at free swimming 5 days, commence feeding at 4-6 days7,9. Age & length at end of 18 days7, 9.5-12.5mm7. larval development Larvae behaviour Rare use cover and spend much time swimming in groups10 Habitat preferences - Wide range of habitats, from cooler, upper reaches with gravel beds and rocky adults substrates, to turbid, slow flowing water11, prefer fast flowing water, particularly where rapids and races5, prefer open water to heavily snagged12. Behaviour Schools, spends time close to water’s surface. Diet - adults Omnivore. Small aquatic insects, molluscs, earthworms, green algae13. Zooplankton, shrimps, filamentous algae, aquatic plants (in rearing ponds)3. Diet - juvenile Crustaceans and zooplankton. Proportion of algae and other plant material in diet increases as fish grow14. Diet - larvae Phytoplankton, zooplankton (in rearing ponds)7. Small rotifers, algae, chironomid larvae and small crustaceans (in rearing ponds)10. Movement - life stage Potamodromous. Upstream movement of adults and juveniles4.

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Silver perch – A Resource Document Movement - season Juveniles from October to April, adults from November to February4. May vary over basin. Movement - cues Migration occurs once water temperature above 20oC and rises in water levels4. 1Lake 1967a, 2Thurstan & Rowland 1995, 3Lake 1967b, 4Mallen-Cooper et al. 1995, 5Merrick & Schmida 1984, 6Lake 1967c, 7Lake 1967d, 8Rowland 1984, 9Guo et al. 1993, 10Thurstan 1991, 11 Rowland 1995, 12Cadwallader & Backhouse 1983, 13Merrick 1996, 14Allan 1995.

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Silver perch – A Resource Document

Section 2 - Review of Threats

6 River Regulation River regulation and water abstraction represent some of the most wide ranging and significant changes, influencing the vast majority of the Murray Darling Basin. Water resource development has detrimentally affected the riverine environment as well as surrounding riparian and floodplain habitats. There was a dramatic increase in water use from the 1950s onwards and demands on water continue to increase. It is likely to have played a key role in the decline of silver perch, since there are a wide range of significant associated impacts to the aquatic environment. The effects are interrelated and complex and vary between areas. Three key impacts of river regulation are discussed separately below: • Changes to Flow Regimes • Changes to Temperature Regimes • Barriers to Movement

6.1 Changes to Flow Regimes

River regulation has altered natural flow regimes and key hydrological processes including the magnitude, timing, frequency and duration of floods, rates of rise and fall of water levels, changes in seasonality of flow patterns and changes to floodplain .

River regulation may affect silver perch in a variety of ways, in relation to the quality and availability of habitats for spawning as well as survival. Silver perch have been recorded in floodplain habitats such as backwaters and billabongs. These habitats have declined as a result of river regulation; how important such habitats are to silver perch is not known.

Most attention has been focussed on the possible impact of river regulation to the spawning success of silver perch since spawning is at least partially initiated by rises in water level. Since there is some evidence that adults move upstream prior to spawning, river regulation may have also influenced the movement patterns of adults. The range of flow conditions under which spawning occurs and the associated spawning success of silver perch requires further research. While it has previously been suggested that silver perch require floods to spawn, research by Mallen-Cooper et al. (1995) observed stronger year classes when flows were contained within river channels. Recruitment of silver perch may be more localised and opportunistic than previously believed and fish may spawn both during inchannel flows and during large floods (Mallen-Cooper and Stuart, in prep). Comparisons of spawning success over a range of flow conditions is required to assist in determining the impact of changes to flow regimes to silver perch. Whether there are variations across the species' range also requires investigation.

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Silver perch – A Resource Document The importance of habitats such as floodplains, floodplain wetlands and main river channels to all life history stages of silver perch is currently unknown. The significance of different habitats for spawning and recruitment, as well as what are key features of these habitats also requires investigation. Without this knowledge it is difficult to determine how the range of flow characteristics which have changed as a result of river regulation have affected silver perch. For example, the effect of changes to flow regimes on the survival of eggs and larvae is not known. In some situations, it is possible eggs and larvae may settle out within the impounded waters of weir pools and other low flow areas where they may be subject to poor water quality and restricted downstream movement. Alternately, flows contributing to greater flows at or near channel capacity occur more frequently than in the past and it is possible eggs and larvae are washed long distances downstream where they may be physically be damaged moving over barriers.

6.1.1 Background While the Murray Darling Basin experiences a variety of climates, it is primarily a semi arid system. Flow variability is high, particularly in the drier and temperate regions of the Murray Darling Basin (Humphries et al. 1999). Rivers such as the Murray are subject to periods of both drought and flood conditions. Rainfall patterns vary across the Murray Darling Basin with peak rainfalls occurring during winter-spring in southern areas and during summer in the north. Streamflow is more variable in western areas with flows being determined by summer monsoons in south-east Queensland. While 86% of the Murray Darling Basin contributes almost no runoff into rivers except during floods, the upper Murray River, Murrumbidgee and Goulburn rivers contribute over 45% of the total runoff.

This flow variability led to the establishment of numerous storages and regulatory structures on rivers and streams to ensure a more reliable water supply for domestic consumption, industry and agriculture. The Murray Darling Basin is the most regulated Australian drainage division (Walker 1981) and agriculture is the dominant economic activity. The Murray Darling Basin supports about 75% of Australia’s irrigation agriculture, and 95% of the water diverted is used for agriculture.

There has been a dramatic increase in water use within the Murray Darling Basin from the mid 1950s onwards. A key impact has been reduced flows reaching terminal points, with the median annual flows to the river mouth having declined to 21% of the natural flow (MDBC 1995). The probability of the lower reaches of the Murray River experiencing drought like flows has increased from 5% of years to over 60% of years. An audit on water use in 1995 found that diversions had increased by 8% between 1988/9 and 1993/4 (MDBC 1995). Of the diversions over half were in New South Wales (57%), followed by Victoria (34%) and South Australia (5%). Growth in diversions had been greatest in Queensland (89%) and between 4.1 and 6.7% for the other states. Diversions were continuing to increase at more than 1% per year and there was a potential for increases in diversions if all existing water entitlements were fully developed. It has been recognised that in areas such as north-western New South Wales flows are overcommitted. For this reason and because of the significant growth in the cotton Freshwater Ecology, NRE & Murray Darling Basin Commission 26

Silver perch – A Resource Document industry the number of large privately owned farm storages which harvest water from off allocation flows is also increasing dramatically (Allan and Lovett 1997).

Under natural conditions, flow patterns varied along the Murray River with peak flows in the upper Murray occurring in winter-spring and subsequently reaching South Australia in spring and early summer. An assessment of the effects of river regulation and water abstraction along the Murray River demonstrates how there are different impacts in different areas. Maheshwari et al. (1995) note that the monthly and annual average flows are now much lower than under natural conditions. The magnitude of average annual floods has also declined although large floods are not significantly affected. A key feature of the upper Murray River such as at Albury is the reversal of the flow regime; while under natural conditions flows were highest in spring and lowest in autumn, they are now usually highest in autumn and lowest in winter. In the past, March was a time of low flow while it now often runs at channel capacity (MDBC 1995). Further downstream at Yarrawonga the reversal of flow seasonality is not as apparent and there is a more uniform flow pattern between seasons, although average flow and flow peaks have been reduced (MDBC 1995). A similar pattern is apparent for the at . At Euston and further downstream to Wentworth, the seasonal flow pattern is similar to the natural situation although the sizes of flow have been greatly reduced. The lower Murray River in South Australia is now a system of cascading weir pools which receive and store water and sediment from upstream (Thoms and Walker 1993).

The Barwon-Darling River upstream of Menindee is often described as unregulated although about 40% of the average annual natural flow is diverted from the river and its tributaries (Thoms et al. 1996). There are also 17 weirs controlling river flow in this region. The proportion of water diverted from the Barwon-Darling river has also increased greatly since the 1960s. Water contributions from major tributaries such as the Gwydir, Namoi and Macquarie are now largely controlled by dams in their headwaters.

Thoms et al. (1996) indicate the following hydrological changes within the Barwon- Darling system: • Decreased annual and daily volumes • Increased rates of flood recession • Decreased rate of rise due to pumping, adding to river level stability • Decreased flood duration, particularly low flows • A marked change in the character of flood frequencies

In addition, within the lower Darling the natural flow pattern has also changed significantly. Under natural conditions there were generally two annual flood peaks (during March and September). Under current conditions there is only one peak flow in December-January and flows fall rapidly in February and March, remaining at a constant low flow until October (Thoms et al. 1998).

Other river systems within the Murray Darling Basin have also experienced significant changes. Systems such as the Goulburn, Broken, Campaspse and Murrumbidgee

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Silver perch – A Resource Document experience significantly reduced flows in average years with drought like flows occurring much more frequently (MDBC 1995). As with many other river systems however, flows during the largest floods have not changed greatly. In the Campaspe system, there has been a reduction in winter flows and an increase in summer flows. In the Lachlan system, the volume of water reaching the terminal point has been reduced by about half, with reductions in frequency of all but large floods. The Gwydir River system now experiences a reduced total volume of flow and reduced frequency of large to medium size floods. There has also been an increased frequency of moderate flows during the summer irrigation period and increased frequency of low flows. Private off stream storages have also increased greatly in recent years. In the system, the seasonal pattern remains similar to natural conditions although annual median flows are lower. The flow regime is also now more uniform. In the system, there have been reductions in the magnitude of floods, in amounts of water reaching floodplains, in frequency of moderate sized floods and in no flow periods although an increased frequency of low flow periods in summer.

Queensland rivers within the Murray Darling Basin are characterised by a relatively dry climate and intermittent flows. The natural wet season occurs from September to March. In contrast to most other rivers within the Murray Darling Basin, the Condamine- system is relatively unregulated, with less than 30% of the length of the Condamine, Balonne and Culgoa channels influenced by river regulation (NRE, in press). However, there have been significant increases in water resource development in this area in the last ten years and extraction and harvesting of overland flows has affected the natural flow regime of this river system (NRE, in press). There are a number of water storages and instream structures within the system although most are small by comparison to other parts of the Murray Darling Basin. Most water is extracted by harvesting of natural flows. Comparisons to changes in flow regimes in a number of regulated rivers in New South Wales indicate the changes are much greater in New South Wales. An overall assessment of environmental flows considered the upper and lower Condamine and upper Balonne generally good to fair, while the lower Balonne was poor to fair (NRE, in press).

6.1.2 Management Conflicts are always likely to exist between competing water users as it is a finite resource. The need to find a balance between consumptive use (irrigation, domestic and industrial) and environmental use has only been acknowledged quite recently and is still being addressed. The National Principles for the Provision of Water for Ecosystems (ARMCANZ 1996) outlines principles to follow for management of flows. In 1994, the Council of Australia Governments (COAG) developed a water reform package which specified that appropriate water allocations were to be made to the environment to enhance and restore health of river systems. Koehn and Nicol (in press) note that altered flow regulation practices to enhance the restoration of native fish populations in the Murray Darling Basin should be given a high priority. The Murray Darling Basin Commission Ministerial Council also decided to introduce a moratorium on further increases in diversions of water and to cap future levels. The Cap restricted volumes of diversions to those associated with 1993/4 levels. The Cap aimed to maintain and where

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Silver perch – A Resource Document appropriate improve existing flow regimes in Murray Darling Basin rivers to protect and enhance the riverine environment, as well as achieve sustainable consumptive use by developing and managing water resources to meet ecological, commercial and social needs (MDBC 1996). Several reviews of the Cap have indicated that there is a high level of commitment (MDBC 1997). A review in 1998 found that South Australia and Victoria were within the Cap, while in New South Wales, the Lachlan River had exceeded the Cap and diversions in the Barwon-Darling and Border rivers were likely to exceed it. In Queensland, the Water Allocation Management Plans (WAMPs) for the Condamine- Balonne and Border catchments are still to be finalised. Other management processes being implemented include water trading which involves buying and selling water entitlements or allocations.

There are also associated programs which recognise the need to improve water management. The Floodplain Wetlands Management Strategy (MDBC 1998) aims to ‘maintain and where possible enhance floodplain ecosystems in the Murray Darling Basin’. This includes the objective to evaluate and manage river flow regimes and water allocations to maintain, restore and enhance floodplain wetlands. There are also relevant state plans, such as the New South Wales Wetland Management Policy (DLWC 1996), which recognise the need for restoring hydrological cycles. An example is a plan for the Macquarie Marshes which aims to 'maintain and restore where possible an equitable and environmentally appropriate distribution of water within the marsh' (MMCC 1997). There have been allocations of water for environmental purposes such as the Barmah-Millewa forest to maximise the potential for bird and fish breeding and tree growth.

Thoms et al. (1998) note that flow management plans need to be developed for all major river systems to maximise environmental benefits and also meet the needs of existing users. Such plans are being developed for the Murray River (Thoms et al. 1998) and the Barwon-Darling river system (Thoms et al. 1996) using the expert panel approach. A key aim for improved management of water is to protect the range of flows which occurred naturally and reinstate more natural flow regimes. Walker and Thoms (1993) suggested the restoration of a more natural balance of flows may be unrealistic because of users needs other than the environment. Constraints to be considered in implementing environmental water requirements include physical constraints on operation of structures such as dams and weirs, as well as consumer demands (Jensen 1996). Pressey (1986) recommended periodic drying out of wetlands by manipulation of weir pool levels and the use of regulators. Jensen (1996) emphasised the need for extensive community involvement in determining flows because of the direct impacts on the community.

Pusey et al. (1998) summarise the methodologies of the range of methods used to assess environmental flows. These vary from simplistic assessments of hydrological data to determine minimum and flushing flows to complex modelling procedures associating changes in river discharge with geomorphological and ecological responses. Jensen (1996) notes that flushing flows for the management of algal blooms and to improve water quality do not benefit the riverine environment since they are generally too small

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Silver perch – A Resource Document and brief to flow overbanks and fill wetlands. Lack of detailed historical data make difficult to understand baseline conditions, particularly in very variable systems such as Murray Darling Basin. In Queensland, most of gauging stations in Condamine-Balonne have only been established since the 1960s and there have been increasing levels of water development since this time (NRE, in press). NRE (in press) emphasised that the precautionary principle should be applied with a conservative approach taken to the allocation and diversion of water.

6.1.3 Assessment of the significance of the threat While the degree of regulation within rivers in the Murray Darling Basin varies, the creation of storages and regulatory structures on rivers in the Murray Darling Basin has had a significant effect on flow regimes. Flow is a governing influence in rivers, and is particularly crucial in semi arid rivers which experience greater extremes and more unpredictable flow variations both spatially and temporally (Walker and Thoms 1993). Flow must be considered in relation to its lateral and longitudinal movement within the landscape. Changes to flow regimes have affected the aquatic environment as well as the riparian zone and floodplain habitats in a number of significant ways. Streamflow plays a role in physical processes of erosion, deposition and sediment transport (NRE, in press). Different aspects of flow regimes may be important for different aspects of river ecology.

The impacts of river regulation and water abstraction within the Murray Darling Basin are not yet fully understood, and there is likely to be a lag time before the ecological responses to changes in flow regimes become apparent. For example, Thoms and Walker (1993) observed the physical responses of the environment to weir establishment were still occurring after 70 years. Gehrke et al. (1995) observed a trend of reduced fish species diversity in increasingly regulated catchments which may be attributed to desynchronising environmental and reproductive cycles. Welcomme (1994) argues that river regulation may favour the establishment of more generalist introduced species which do not have reproductive cues closely tied to seasonal flow regimes. Fauna are likely to have evolved to cope with unpredictable and variable conditions within the Murray Darling Basin and such conditions have changed to become more stable. This may have implications of the future composition of stocks; i.e. more uniform environmental conditions may select for more uniform individuals within a population.

Changes to occurrence and patterns of flows Changes to flow regimes include reversal of flow regimes, increased rates or rise and fall of water and more frequent periods of water running at channel capacity. Reversal of flow regimes now occurs in some parts of the Murray Darling Basin, such as in the upper Murray River. Under natural conditions, flows were low in late spring and summer while periods of high flow now occur during this time. Humphries et al. (1999) argue that the use of rivers to pass irrigation water during periods when flows would normally have been low has undoubtedly adversely affected those species which use in channel habitats for spawning and rearing. River regulation has resulted in more constant flows in many parts of the Murray Darling Basin as well as running at or near channel capacity more frequently. These may cause instability of riverbanks, increase erosion and sedimentation

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Silver perch – A Resource Document and increase groundwater problems (MDBC 1995, Thoms et al. 1998). Changes to rates of rise and fall of water levels could also potentially cause stranding of aquatic fauna in floodplain wetlands and below weirs.

Increased uniformity of flows and establishment of weir pools Many areas within the Murray Darling Basin now experience more uniform flows and more stable water levels, which has reduced the complexity of habitats. For example, 55% of the river length in the Murray River below Lake Hume consists of weir pools (Pressey 1986). While there has been an introduction of daily level fluctuations in reaches immediately below weirs (Walker et al. 1992), areas above these pools can experience far more stable water levels. Pressey (1986) noted that while permanent pools could be beneficial in providing drought refuges, there is some evidence that permanently inundated areas are less productive that those which experience fluctuations in water level and periodic drying. The establishment of weir pools can also change channel morphology, with erosion of benches immediately downstream of weirs and in middle and lower sections of weir pools occurring in South Australia (Thoms and Walker 1993).

Reduced flows and loss of floodplain wetlands An overriding effect of river regulation and water abstraction in most catchments within the Murray Darling Basin has been a reduction in overall flows. This has affected both the riverine environment as well as associated riparian zones and floodplain wetlands.

Floodplain wetlands are the most predominant wetland type within the Murray Darling Basin. They are depressions of floodplains of rivers, creeks and tributaries and include backwaters, billabongs, cowals, lagoons, marshes and swamps (MDBC 1998). They may be permanent or temporarily inundated. Floodplain wetlands enhance water quality by filtering sediments and reusing nutrients, absorbing and releasing floodwaters and are a source of organic matter for rivers (MDBC 1998). Pressey (1986) identified four categories of floodplain wetlands of the Murray River and its anabranches based on whether they were connected to the river at minimum and maximum regulated flows.

It is estimated that up to 50% of the area occupied by wetlands in Australia has declined since European settlement (MDBC 1998). Changes have occurred as a result of regulation and diversion of water; some areas have been drained and degraded by clearing and grazing, some are now filled either more or less frequently, at different times of year and for different periods than in the past. Changes in timing of inundation can affect drying and wetting cycles within wetlands and associated nutrient recycling patterns. Areas such as the Barmah-Millewa forest are now flooded less in spring and dry out less in autumn. Dry phases allow material to decompose and accumulate and then during wet periods such as floods nutrients are released from soils and enter the water (NRE, in press). In parts of the Murray Darling Basin, there has been a reduced frequency of flooding periods and decreased linkages between rivers and floodplains. The construction of levees has also contributed to the reduction in frequency and depth of inundation of wetlands (Allan and Lovett 1997). Prior to river regulation many wetlands were once ephemeral. Low lying wetlands located upstream of weirs and impoundments

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Silver perch – A Resource Document are now permanently wet, as are wetlands kept high during summer and autumn months for irrigation needs. About 35% of the intermittently flooded wetlands between Lake Hume and Wellington along the Murray River have disappeared, while there has been a 55% reduction of the Barmah Forest wetlands (Pressey 1986). There have also been significant reductions in wetlands associated with the Murrumbidgee, Lachlan, Macquarie and Gwydir river systems and unknown reductions in the intermittently flooded wetlands in Queensland (MDBC 1998). Kingsford (1999) argues that those rivers in the Murray Darling Basin which terminate in wetlands have been most affected by water extraction since the amount of water reaching them has declined significantly.

Reduced flows may also affect the ability of fish to move upstream and cross barriers such as weirs (MDBC 1995). This issue is discussed in section 6.3 Barriers.

Flood pulse concept versus the low flow recruitment concept The significance of flow regimes and floodplains in the reproduction of many native fish species including silver perch within the Murray Darling Basin is not fully understood and requires further investigation. The flood pulse concept has commonly been discussed in the past, although more recently a low flow recruitment model has been suggested (Humphries et al. 1999).

The flood pulse concept argues that overbank flows which inundate floodplains produce abundant food for larvae and the subsequent recruitment of fish species (Junk et al. 1989). This model was developed overseas for temperate rivers and anadromous fish. In Australia, Gehrke (1991) outlined the four stages of the flood recruitment model; floodplain habitats are inundated and nutrients are released from soil and debris into the aquatic environment; an increase in primary production follows resulting in a plankton bloom; mature fish, fertilised eggs and larvae enter this floodplain habitat feeding on the abundant food source; as floodwaters receded the fish and young return to the river. While Junk et al. (1989) indicated that the flood pulse concept is not easily applied when pulses are variable, Walker et al. (1995) suggest opportunism and flexibility of life history attributes are adaptations to unpredictability. Humphries et al. (1999) argue that the suggestion that inundation of floodplains is an important cue for fish spawning and larval and juvenile survival for Murray Darling Basin fish species has been largely based on hatchery based pond breeding rather than field experiments. The experiments of Lake (1967a) however were in small ponds as well as larger ponds described as 'flood ponds'. Lake (1967a) noted that experienced fishermen have long maintained that larger fish within the Murray Darling Basin spawn during floods and that delays in flooding from spring to autumn leads to a delay in spawning. Mackay (1973) argued golden perch required floods to trigger spawning and that when floods fail little spawning occurs. Reid et al. (1997) also observed that major peaks in commercial catches of golden perch in New South Wales may have been attributable to large floods. Reynolds (1976) noted that peaks in commercial catch of golden perch could be related to good spawnings and good flooding periods. Humphries et al. (1999) emphasise that no Murray Darling Basin fish species have been recorded specifically spawning on floodplains and no larvae have been recorded from seasonally inundated floodplain habitats. They note that recent research

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Silver perch – A Resource Document has suggested that some species may spawn during low flow periods in channel and propose a low flow recruitment model where some species spawn during low flow periods where adequate food resources are available. Humphries et al. (1999) argue that since climatic conditions within the Murray Darling Basin are variable, species may have evolved a range of strategies and recruitment styles. Periods of high flows and high water temperatures vary between parts of the Murray Darling Basin. For example, high flows in south-eastern parts of Murray Darling Basin occur in mid winter/early spring and do not coincide with warmer temperatures. In the lower Murray and Darling rivers and their major tributaries floods tend to occur during summer when temps are high (Humphries et al. 1999). Puckridge et al. (1998) argue that the flood pulse concept can be extended to address hydrological variability and incorporate differences among groups of rivers from different climatic areas.

It has generally been believed that inundation of floodplains provides an abundant food source for larvae due to high primary and secondary production as a result of dissolution of organic material. Humphries et al. (1999) argue that further data is required to compare densities of microinvertebrates in floodplains and river channels, to determine whether blooms after flooding are consistent throughout the Murray Darling Basin and whether zooplankton densities are sufficient for larvae. Humphries et al. (1999) note that floodwaters probably wash food, including terrestrial insects, into river and provide food source. They argue that there is evidence to suggest that microfauna in habitats such as backwaters in lowland rivers may be sufficient for larvae. Flood pulses within a river channel also release nutrients from sediments and detritus (Walker et al. 1995). Mallen- Cooper and Stuart (in prep) argue that the presence of larvae of bony herring Nematalosa erebi, Australian smelt and western carp gudgeon during non-flood flows in the lower Murray River (Puckridge and Walker 1990) supports the suggestion that there was adequate food available within the main river channel at this time.

Impact of changes to flow regimes to silver perch Changes to flow regimes may have affected the distribution and abundance of silver perch in a variety of ways, in relation to the quality and availability of habitats for spawning as well as survival. While the habitat requirements of silver perch are not well understood, the loss of floodplain wetlands including backwaters and billabongs where silver perch can occur represents a restriction of available habitat. How important such habitats are to silver perch is not known. Silver perch have been described as spawning in flooded backwaters of low gradient and meandering rivers (Lake 1967c). Further research is required to understand the types of habitats in which silver perch spawn and their key habitat characteristics.

Most attention has been focussed on the possible impacts of changes to flow regimes to the spawning success of silver perch. Spawning is initiated by rises in water level as well as increases in water temperatures. There is indirect evidence that adults move upstream prior to spawning, possibly to compensate for the downstream drift of eggs and larvae (Mallen-Cooper et al. 1995). Thus it is possible that changes in the patterns and timing of

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Silver perch – A Resource Document flow could affect movement patterns of silver perch as well as spawning behaviour and spawning success.

While spawning of silver perch is considered to be initiated by rising water levels, the range of conditions under which spawning occurs and the associated spawning success requires further research. Gehrke (1991) suggested silver perch may be an obligatory flood spawner which would require floodplain inundation to not only stimulate spawning but also to assist in larvae survival. Rowland (NSW Fisheries, pers. comm. 1998) considers this unlikely and perhaps is the case for golden perch. Mallen-Cooper et al. (1995) note that the flood-recruitment model suggests that appropriate recruitment conditions are produced as high spring flows inundate floodplains. Those spring flows which do not inundate floodplains and are contained within the river channel would presumably result in poor recruitment. However, research in the Murray River at Torrumbarry by Mallen-Cooper et al. (1995) observed the reverse to occur for silver perch. Clear year classes were recorded when flows were contained within the river channels, while weak year classes were recorded when high flows inundated the low lying floodplains. This may suggest that small floods produce the appropriate food while large floods do not. They questioned whether alteration to the environment has changed floodplain conditions and suitability. Mallen-Cooper and Stuart (in prep) observed that there was high variability within the flows contained in the river channel when the strong year classes were observed and suggested these may affect the river ecosystem, contributing to small increases in plankton and invertebrate food abundance. Mallen- Cooper et al. (1995) emphasised the need for further research into the recruitment patterns of silver perch, noting that their study only involved one part of the Murray River. They suggested that recruitment may be more localised and opportunistic than was previously believed. Mallen-Cooper and Stuart (in prep) suggest that silver perch may spawn during both inchannel flows and during large floods. Such a reproductive strategy would enable the exploitation of the more frequent inchannel flow events where some level of recruitment could be sustained, while during large floods larval survival may be much greater (Mallen-Cooper and Stuart, in prep).

Changes in flow regimes may have not only affected the initiation of spawning by silver perch but also the survival of eggs and larvae and their patterns of distribution. Humphries et al. (1999) argue that while species such as silver perch produce large numbers of eggs, it is likely mortality of eggs and larvae is very high. The eggs of silver perch are non adhesive, pelagic and semibuoyant. Cadwallader (1978) suggested that pelagic eggs are suited to quite tranquil flood spread waters rather than fast water currents. Silver perch eggs have been collected in drift nets in the Murray River downstream of Lake Mulwala (J. Koehn, DNRE, unpublished data). The survival of silver perch eggs under different flow and water quality conditions requires investigation. Lake (1967c) noted that silver perch eggs had large perivitelline spaces which could provide them with protection against damage and would enable better survival in flowing water conditions. Lake (1967c) also suggested that the characteristics of the eggs could possibly assist in gaseous exchange which could be significant in waters with low dissolved oxygen. While silver perch eggs hatch rapidly and larvae are free swimming at

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Silver perch – A Resource Document five days, they may be susceptible to poor water conditions during this time. There is the potential for eggs as well as larvae to settle out in weir pools which are common in many rivers within the Murray Darling Basin. This may be detrimental if pools stratify and dissolved oxygen levels are low, and may also represent a barrier to populations of silver perch. Humphries et al. (1999) argue that spawning and recruiting during low flows may have disadvantages, with the potential for pools to stratify and dissolved oxygen levels to fall. Lake (1971) noted that silver perch eggs are similar to those of golden perch although there are slightly denser and will sink if there is no water movement. Pollard et al. (1980) suggested that eggs and larvae will not survive in waters with anoxic lower layers, a feature which is more prevalent towards the northern limit of this species distibution. They suggested that this may explain why silver perch does not have the same extensive northward distribution as golden perch.

Changes to flow regimes have also meant that some rivers may run at channel capacity more frequently particularly when supplying irrigation waters during summer months. There is the potential for eggs and larvae to be washed greater distances downstream than in the past which may have longer term implications on the species' distribution in the upper reaches of rivers because of barriers restricting the upstream movement of juveniles and adults. Eggs and larvae may also be physically damaged by downstream movement across barriers. Increased rates of rise and fall of water levels may also cause eggs and larvae to be stranded in floodplain habitats where they may be susceptible to desiccation and poor water quality.

Survival and growth of larval fish is likely to be closely linked to food availability. Since larvae are small at first feeding, they are restricted in the size of prey which could be eaten and feed on rotifers and small planktonic crustacea (Humphries et al. 1999). These food sources can occur in greater densities in slow flowing and still habitats.

Some research has been carried out on the behaviour of silver perch larvae. Larvae showed a significant positive response to light, do not seek shelter in shade and rarely use cover (Gehrke 1990, Thurstan 1991) Larvae varied in their response to flow, some drifting downstream while others actively maintained their position (Gehrke 1990). The usage of different habitats such as slow flowing billabongs, floodplains and main river channels by silver perch larvae, as well as their behaviour, growth and survival within these habitats requires investigation.

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Silver perch – A Resource Document

6.2 Changes to Temperature Regimes Cold water pollution is likely to have played a role in the decline of silver perch in some river reaches within the Murray Darling Basin which have impoundments that release water from lower levels below the hypolimnion. This problem has generally been underestimated in the past. Recent estimates indicate that lengths of river potentially affected by cold water pollution can reach up to 400 km downstream; these represent a significant proportion of river reaches within the Murray Darling Basin. Areas which are affected by cold water pollution include downstream of on the Murray River, Burrinjuck Dam on the Murrumbidgee River, Wyangala Dam on the Lachlan River, Burrendong Dam on the Macquarie River and Lake Eildon on the Goulburn River. All these reaches have experienced declines in silver perch.

Silver perch is a 'warmwater' native species. Lake (1967a) noted the species spawns once water temperatures reach about 23oC. While spawning can be induced at lower temperatures, recent research has indicated that temperatures below 22oC may adversely affect hatching success and survival (T. Ryan, DNRE, pers. comm. 2000). Since temperatures downstream of some dams can be suppressed by up to 15oC, there is clear potential for spawning success to be restricted in particular areas which could potentially cause the decline and disappearance of populations downstream of impoundments where cold water pollution occurs.

Silver perch are a mobile species and it is not known whether they avoid colder water habitats. Mallen-Cooper et al. (1995) observed migration of juvenile and adult fish to occur once water temperatures rose about 20oC. Therefore there is the clear potential for cold water pollution to also affect the species' movement patterns. This may have further implications to the species' reproductive success if upstream movement is linked to spawning requirements.

Reduced water temperatures may affect other aspects of the species' biology including general metabolic functioning, feeding, maturation and growth rates. It may also indirectly affect the species by reducing available food sources through lowered productivity of the ecosystem. Reduced growth rates caused by lower temperatures may make juveniles susceptible to predation for longer periods and may increase the species' susceptibility to diseases. Whether eggs, larvae and juvenile fish are more sensitive to lower temperatures than adults requires investigation. Some recent research by NSW Fisheries has indicated that colder water temperatures can be detrimental to both the growth and survival of juveniles silver perch (Lugg 1999).

6.2.1 Background The widespread modification of rivers within the Murray Darling Basin since European settlement has resulted in changes to water temperature in particular river reaches. These temperature changes have generally involved reductions in water temperatures during warmer months, but some other factors may also lead to temperature increases. To date, most attention has been focused on the effect of reductions in water temperature. The Freshwater Ecology, NRE & Murray Darling Basin Commission 36

Silver perch – A Resource Document construction of large, deep impoundments on many rivers within the Murray Darling Basin has profoundly influenced temperature regimes along many stretches of these rivers. In the majority of cases, water is released from below the thermocline near the bottom of these dams during summer for irrigation needs and is usually colder than the surrounding water. This water can be lower in oxygen, high in ammonia, have strong odours of hydrogen sulphide and can result in the release of nutrients from sediments which can facilitate the development of algal blooms. Varying levels of thermal stratification and deoxygenation has been noted in numerous large dams including Dartmouth, Eildon, Blowering, Burrinjuck, Windamere, Burrendong, Wyangala, Carcoar and Pindari. Temperature stratification can also occur in ponded waters within rivers as a result of groundwater intrusion and/or general lack of mixing.

Determining the significance of this problem requires documentation of the specific differences in temperature downstream compared to upstream and the actual distances downstream of large impoundments that are affected by lower temperatures. Lugg (1999) notes that cold water pollution is more complex than a simple reduction in water temperature. Other implications include variation from natural temperatures throughout the year, a delay in summer peak, the elimination of the rapid rise in temperature during spring and a significant reduction in the difference between annual maximum and minimum temperatures.

Lugg (1999) notes that while temperature suppression of up to 15oC can occur, it usually ranges between 8 and 12oC. In the Murray River, Walker et al. (1978) found that water temperatures above Hume Dam ranged between 7-24oC, while those below the dam ranged between 10-21oC. There was also a one month lag in the seasonal pattern. While the intensity of the thermal stratification varied over time, at the peak of stratification there was a thermocline and surface waters ranging between 3-8oC. More recently, Lugg (NSW Fisheries, pers. comm. 1999) indicates that temperature peaks above Hume Dam occur in mid January, while they peak below the dam in mid March, indicating a two month delay.

Cadwallader and Backhouse (1983) indicated that water released in summer from Lake Eildon on the Goulburn River tended to be 10-15oC lower than inflowing water. Mackay and Shafron (1988) stated that water temperatures downstream of Lake Eildon were 8oC lower and that reductions in temperature occurred for about 100 km downstream to the Goulburn Weir. Further work is required to clarify the magnitude of the decrease in water temperature and the distance downstream affected by Lake Eildon. Water temperatures downstream of Lake Dartmouth can be up to 15oC lower, following large releases in summer (Shafron et al. 1990). In the Macquarie River, water temperatures downstream of Burrendong Dam are apparently 10oC lower than upstream, and they do not recover to ambient conditions until as far downstream as Warren, a distance of well over 350 km. When Burrendong Dam filled recently and water was released over the spillway, it was 10oC warmer than the water being released from the bottom of the dam (D. Love, DLWC, pers. comm. 1999).

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Silver perch – A Resource Document Since 1990, NSW Department of Land and Water Conservation has monitored river temperatures above and below Burrinjuck Dam on the Murrumbidgee River and Blowering Dam on the River, which flows into the Murrumbidgee River (Keenan and Buchan, unpublished data). The maximum temperature reduction immediately below Burrinjuck Dam was 7-8oC and 13-16oC below Blowering Dam (A. Lugg, NSW Fisheries pers. comm. 1999). Maximum temperature depletions occurred in mid summer, with intermediate effects in spring and autumn and a minimal effect of <1oC by mid winter.

In comparison to other parts of the Murray Darling Basin, Queensland waters are less regulated and there are fewer large instream impoundments. Recent investigations of water temperatures in Leslie and Beardmore dams (which are moderate in size) indicate that there is quite good mixing of waters and little variation in temperatures from top to bottom (D. Moffatt, DNR, pers. comm. 1999). The increasing trend of irrigated agriculture for crops such as cotton in Queensland may lead to more, larger dams being built. NRE (in press) notes that thermal pollution is not likely to be as severe in Queensland in comparison to other parts of the Murray Darling Basin. There is however some evidence of thermal pollution in some storages.

Thoms et al. (1998) indicated that the effect of low level storages such as the Menindee Lakes and Yarrawonga Weir has not been addressed, although there are suggestions that there may be occasional impacts on the lower Darling River. In the Murrumbidgee River, temperature depletions of about 1-2oC maximum occur below weir pools such at Golgeldrie, Hay, Maude and Redbank. The recovery distances in summer are unknown (Keenan and Buchan, unpublished data).

It appears that the extent of cold water pollution has been underestimated in the past. For example, the Murray Darling Basin Environmental Resources Study (MDBC 1987) provided some estimates of the distances downstream of dams that were affected by cold water releases. These include about 60 km downstream of Blowering Dam on the , 50 km downstream of Copeton Dam on the Gwydir River, and about 100 km downstream from Burrendong Dam on the Macquarie River. Subsequent, more detailed studies have indicated that effects of cold water discharge are detected much further downstream.

Regulated rivers tend to regain ‘natural’ characteristics as the distance downstream of a dam increases. The distances downstream of dams that are affected by cold water are likely to vary between rivers depending on many factors including flow regimes. The differences in temperature of bottom water releases compared to other water in the river (e.g. upstream) are likely to vary as well. For example, characteristics of dams such as depth and degree of thermal stratification, as well as timing and magnitude of water releases vary. Walker et al. (1978) noted that while Lake Hume affected water temperatures downstream, Lake Mulwala, which is shallower and has a rapid flushing rate, did not show persistent thermal stratification. Lake Hume is much larger than Lake Mulwala, with capacities of 3 038 000 ML and 118 000 ML respectively.

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Silver perch – A Resource Document

NSW Fisheries has recently analysed water temperature data for parts of the Murrumbidgee, Murray, Tumut, Edwards and rivers: records have been compared from the mid 1980s onwards for the Murray River, while the records for the other rivers are for the 1990s onwards. This data has been graphed with overlays of the minimum temperatures required for spawning for Murray cod, silver perch, golden perch and freshwater catfish Tandanus tandanus. In terms of silver perch in the Murray River, water temperatures immediately downstream of Hume Dam do not reach required spawning temperatures. As distance downstream of the dam increases, temperatures begin to recover and overlap in suitable spawning temperatures improves; there is some overlap at Yarrawonga (233 km) and reasonable overlap at Torrumbarry (587 km), Barham (701 km) and Swan Hill (823 km). It should be noted that the unregulated Ovens River enters the Murray upstream of Yarrawonga, which highlights the effect of a more natural flow regime. There is a similar pattern for the Murrumbidgee River, with no overlap in suitable temperatures immediately downstream of Burrinjuck Dam and at (113 km), minimal overlap at Wagga (240 km), reasonable overlap at Golgeldrie (481 km) and downstream of Weir there is significant overlap (1 053 km). Recent predictive work has indicated that the decreases in water temperature in the Murrumbidgee River occur to at least Balranald, thus apparently affecting the entire regulated river (A. Lugg, NSW Fisheries, pers. comm. 1999).

6.2.2 Management There are a range of options for reducing or preventing cold water pollution including constructing variable or multi level offtakes on dams which can control where in the water column water is released from, using aerators to increase the depth of the surface layer, and introducing siphons over the dam wall to enable mixing of water. Few large impoundments currently have such management options. Lugg (1999) notes that the range of potential alternatives is currently being assessed in New South Wales. Lugg (1999) indicates that constraints such as the presence of blue-green algae in surface layers can complicate management solutions. In New South Wales, Windamere Dam on the , the enlarged Pindari Dam on the Macintyre River, and Spilt Rock Dam on the Manila River have multi-level offtakes. While water is released from a depth of 60 m at Burrendong Dam, water is released from a depth of between 8 and 15 m at Windamere Dam (D. Love, DLWC, pers. comm. 1999). Burrinjuck Dam on the Murrumbidgee River has a multi-level offtake which is partially effective and has surface-release sluice gates (Cantlon and Blanch 1999). Multi-level offtakes need to be monitored and maintained to ensure they enhance the water quality downstream of dams. Cantlon and Blanch (1999) note that the NSW government has identified six storages which require multi-level offtakes: Wyangala, Blowering and Burrendong are considered highest priority, while Copeton, Keepit and Carcoar are a lower priority. Wyangala has the capacity to draw some water from a mid level outlet (Lugg 1999). The installation of variable-level offtakes has been recommended for on Dartmouth and Hume dams was recommended (Thoms et al. 1998). Copperlode Dam is a relatively small impoundment (44.5 GL) with an area of 3.1 km2. A large compressor is utilised on site near the dam

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Silver perch – A Resource Document wall to pump air at depth to break down stratification and thereby maintain adequate water quality for domestic purposes in Cairns (T. Ryan, DNRE, pers. comm. 2000).

Cold water pollution is one of a large number of problems to the aquatic environment, and coordinated management of all issues is required. Management actions such as restocking stretches of river downstream of large dams are unlikely to enable self- sustaining populations to develop if cold water releases consistently occur.

A cold water pollution forum, organised by the World Wide Fund For Nature Australia and the Inland Rivers Network, was held in June 2001. Participants included ecologists, engineers, wter users, water resource managers, anglers, environmentalists and community representatives. Recommendations which came from this workshop included that: • All jurisdictions within the MDB recognise cold water pollution as a serious issue • Thermal pollution, in conjunction with blocakge of fish passages and loss of environmental flows should be listed as a threatening process under Federal and State legislation • The extent of the problem be recorded and mapped • All jurisdictions in the MDB should review licensing arrangements and procedures to ensure these are providing the necessary regulatory framework to support the mititgation of thermal pollution; and • All jurisdictions should review existing natural resource management programs and policies within the MDB to ensure that the mitigation of thermal pollution is regosnied as an integral part of river restoration activities.

6.2.3 Assessment of the Significance of the Threat It is generally recognised that lower water temperatures downstream of large, deep impoundments can result in changes to fish communities. Overseas examples of the effects of decreased water temperatures on aquatic biota include: lower abundances and diversity of larval fish downstream of dams (Wolf et al. 1996), reduced aquatic biodiversity compared to unregulated systems (Stanford and Hauer 1992), reductions in prolonged swimming ability of juvenile fish (Childs and Clarkson 1996) and decreased growth rates (Schaugaard and Crowl 1994).

Reduced water temperatures have the potential to affect aquatic biota in several ways. There may be a reduction on metabolic and biochemical rates, influencing all processes within a river including photosynthesis, degradation, respiration and bacterial production (Thoms et al. 1998). Colder temperatures may affect the general metabolic functioning, feeding, maturation and growth rates of fish and invertebrates as well as their ability to reproduce successfully. Eggs, larvae and juvenile fish may be more sensitive to lower temperatures than adults (Elliot 1981). The resistance of fish to diseases and pollution may also be affected due to the stress placed on organisms by temperature extremes. Since water temperature can be one cue which stimulates movement of many fish species, cooler water temperatures may also affect migratory patterns. More indirect effects of reduced water temperatures may include reduced growth and survival of food Freshwater Ecology, NRE & Murray Darling Basin Commission 40

Silver perch – A Resource Document resources such as macrophytes and macroinvertebrates. Reduced growth rates may leave juvenile fish susceptible to predation for longer periods.

There is relatively little information available on the tolerance of Australia aquatic organisms to temperature changes. In aquaculture, temperature affects all aspects of fish biology including reproduction, appetite, digestion and growth (S. Rowland, NSW Fisheries, pers. comm. 1999). Rates of chemical and biological reactions approximately double for every increase of 10oC in temperature. Research is required to fully understand the impact on aquatic biota of reduced water temperatures (and other associated water quality problems) released from large impoundments. Those species which have a narrow temperature tolerance range are likely to be worst affected. Species which require specific temperatures to spawn at certain times of the year may fail to breed downstream of dams, possibly leading to local extinctions over time. The amplitude of temperature change may be a significant issue, since some species appear to tolerate a greater range of temperatures if they go through acclimatisation periods rather than dramatic changes in temperature. It is possible that mobile fish species may react to changes in water temperature by moving away to seek preferable areas. Baxter (DNRE, pers. comm. 1999) indicated that golden perch stocked in the Goulburn River have been observed to swim away from colder waters.

In Australia, there are examples where native species have been replaced by exotic species that may be more suited to the altered temperature regimes. For example, downstream of the on the Mitta Mitta River, the ‘warmwater’ species trout cod Maccullochella macquariensis, Murray cod and macquarie perch Macquaria australasica have disappeared and the exotic brown trout Salmo trutta are now dominant (Koehn et al. 1997). There have also been changes in macroinvertebrate fauna, with a reduction in ‘summer warm’ species and a lack of seasonality in species composition. Koehn et al. (1997) note that effects of these reduced water temperatures may be influencing the aquatic fauna downstream to Lake Hume (a distance of over 50 km) and possibly further, into the Murray River. A similar reduction in fish abundance and diversity was observed by Thorncraft and Harris (1996) in the Bell River which is close to the with the Macquarie River that has been significantly affected by the release of cold water from Burrendong Dam.

Some research has been undertaken specifically on the temperature tolerances of silver perch. Lake (1967d) noted that juvenile silver perch (100 mm) could tolerate low temperatures of 2oC for several days and died following prolonged exposure to such temperatures. This tolerance followed considerable acclimatisation, with death occurring if temperature changes were rapid. More recently, research has been undertaken on the effect of cold water pollution on juvenile silver perch (40-60 mm) at Burrendong Dam. Fish were kept in water channels of two different temperature regimes; one of warmer water (18-24oC) and the other of colder water (12-14oC). After 31 days, the fish in the warmer water grew an average of 25% in length and 112% in weight, while those in colder water grew an average of 2% in length and 16% in weight. While all fish in the warmer water survived, only 25% of those in colder water survived (Lugg 1999). This

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Silver perch – A Resource Document provides clear evidence that colder water temperatures can affect both the growth and survival of silver perch.

Temperature has been demonstrated to affect the survival of silver perch eggs. Rowland (NSW Fisheries, pers. comm. 1998) induced spawning of viable, high quality eggs (and sperm) at an ambient water temperature of 17oC in early spring. There was however a high mortality of eggs and larvae incubated at this temperature, whereas survival of eggs incubated at 20-21oC was high. Further research is currently being undertaken on the effect of cold water pollution on egg survival (T. Ryan, DNRE, pers. comm. 2000). Initial trials on fertilised eggs indicate that temperatures below 22oC may adversely impact hatching success and survival. This indicates a relative cold water tolerance in comparison to other species tested such as freshwater catfish and Murray cod which are impacted by temperatures below 16oC and 19oC respectively. Golden perch eggs showed a similar tolerance to silver perch (T. Ryan, DNRE, pers. comm. 2000). Further trials are intended to monitor feeding, growth and activity of juvenile silver perch. Once these trials are completed, they will provide valuable information to determine whether particular decreases in water temperature downstream of impoundments actually entirely prevent spawning or reduce spawning events and their success.

6.2.4 Increases in Water Temperature Riparian vegetation provides varying levels of shade to streams and rivers, and it presumably influences water temperatures to some degree. It is likely that removal of riparian vegetation could cause some increases in water temperatures, although little research has been undertaken concerning the extent of these changes or their possible significance to aquatic biota.

Increases in water temperatures may be most significant in cases where a river or stream has become a series of pools during drier periods of the year. Small pools may experience relatively high temperatures for particular periods. Prior to river regulation, rivers such as the Murray would have experienced periods of drought where stretches became a series of pools; however the variations in temperature may now be more extreme. There are examples from overseas where increased water temperatures have caused mortalities of aquatic fauna (Voelz et al. 1994). In Victoria, during a drought year in 1982/3, a school of Australian grayling Prototroctes maraena in the Dargo River was observed dying in a pool of water with a surface temperature of 26oC and a bottom temperature of 24oC. Dissolved oxygen readings were 4 mg/L at the bottom and 6 mg/L at the surface (B. O’Connor, DNRE, pers. comm. 1999). This observation may indicate that increased water temperatures are detrimental to this species, although further research is required.

Limited experiments have been carried out on the temperature tolerances of juvenile silver perch. They were able to tolerate temperatures of 37oC for a few hours although mortalities were recorded at 38oC (Lake 1967d). While increased water temperatures probably occur in isolated pools in particular rivers and streams in the Murray Darling Basin during periods of low flow, it seems unlikely that increased temperatures has played a significant role in the decline of silver perch.

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Silver perch – A Resource Document

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Silver perch – A Resource Document

6.3 Barriers Silver perch undergo upstream and downstream movements during their life cycle. Therefore it is likely that barriers have had a detrimental effect on the species, although quantifying the role barriers may have played in the species decline is difficult. In particular, it is difficult to separate the effects of other associated impacts of river regulation such as changes to flow regimes. A recent inventory of barriers within the Murray Darling Basin indicates there is a substantial number of barriers of varying types.

Significant numbers of adults and juvenile silver perch are now known to undergo upstream movement. This movement is seasonal and is influenced by increasing water temperature and rising water levels. Upstream movement of the largest numbers of silver perch in a study at Torrumbarry weir occurred on small rises in water level. Monitoring of several fishways indicate that adult and juvenile silver perch are able to negotiate these structures, including higher water velocities than expected (2.25-2.70 m/s). Inland fishways are now designed with a maximum water velocity of 1.8 m/s and so if operated effectively should provide for movement of silver perch. Whether fishways operate effectively over a range of flows, particularly low and medium flows is likely to be a key issue for this species.

While the reasons for upstream movement are not well known, they may be for dispersal and access to feeding areas and to compensate for downstream drift of eggs and larvae. Barriers may limit or prevent access of adults and juveniles to upstream habitats that could result in extinctions in particular stretches of river.

Silver perch eggs are semi buoyant, non adhesive and pelagic. They hatch in about a day and larvae are poorly developed. While little is known of the movement of eggs within rivers, they have been recorded in drift nets downstream of Lake Mulwala. Their movement along a river is likely to be influenced by flow characteristics, since the eggs require water movement to remain in suspension. Larvae appear to be relatively poor swimmers until free swimming at about five days old. They rarely use cover and show a positive response to light. They exhibit a variable response to flow with some maintaining position against the flow and some drifting downstream. The nursery habitats of silver perch are not well understood.

It is unlikely that any upstream movement of eggs and larvae occurs. Whether downstream movement across barriers has an effect on eggs and larvae is unknown, although it may cause physical injury and/or mortalities. Eggs and larvae may also settle out in the low flow areas immediately above barriers. Recent research suggests larvae of golden perch may settle out in weir pools (C. Schiller, NSW Fisheries, pers. comm. 2000) which could can act as a major barrier to downstream movement. Since significant numbers of silver perch appear to spawn on low to medium flow events, eggs and larvae may frequently settle out behind barriers. Any detrimental effects of this on eggs and larvae are unknown. Since eggs hatch rapidly, deaths from sediment may not be a key issue. If eggs settle into deep water behind barriers they may experience anoxic Freshwater Ecology, NRE & Murray Darling Basin Commission 44

Silver perch – A Resource Document conditions and lethal gases such as H2S. Effects on the survival of larvae may depend on whether food sources in pools above barriers are adequate.

Mallen-Cooper et al. (1995) suggested that the length of river unimpeded by barriers may be a key issue for silver perch. For example, one of the reasons that an abundant population has been identified between Torrumbarry and Euston may be that there is sufficient distance for larvae to develop and swim against the current and not pass over a barrier. Priority should be given to providing access for long stretches of river without barriers to movement.

6.3.1 Background It can be presumed that all native fish species undertake some movement; this may be minor or localised movement within their general habitat or larger scale movements such as migrations. Fish may move to seek food and shelter, for reproduction or for dispersal. Movement may be up and down rivers as well into anabranches, channels and onto floodplains. Migrational movement may be stimulated by seasonal or diurnal cycles and by changes in water flows and water temperatures. Mallen-Cooper (in press) observed that all 33 species of fish within the Murray Darling Basin require some degree of free passage and of these 14 undertake large scale movements.

Since European settlement numerous barriers to fish movement have been created including dams, weirs, levee banks, fords, culverts, stream gauging stations and road crossings. They can physically prevent passage or create areas of high water velocity which fish cannot negotiate successfully. Structures may range in their significance as a barrier, depending on their height and design. For example, low weirs may be frequently drowned out. Some weirs may be dismantled or have their gates lifted in high flows. Assessments of the significance of barriers to fish species need to consider whether barriers provide passage at times of year when fish may be migrating. Others such as large dams can represent impenetrable barriers unless they have structures to provide for fish passage. The number of barriers and distance between them needs to be considered in terms of the continuity of movement along a river.

The majority of large dams within the Murray Darling Basin were built from the 1960s onwards with increasing development of irrigated agriculture. Main locks and weirs were established on the Murray River between the 1920s and 1940. Numerous other weirs and other structures have been established throughout the Murray Darling Basin since settlement according to demands for water use.

It has been recognised for some time that dams and weirs can represent barriers to the movement of native fish species. Between 1937 and 1989, 22 fishways were constructed on dams within the Murray Darling Basin (Mallen-Cooper et al. 1995). In the past most fishways were poorly built, using inappropriate designs and were generally not maintained, thus providing limited passage for fish (Thorncaft and Harris 1999). Many early fishways were based on northern hemisphere designs for salmonid fish and proved to be ineffectual for our less agile native fish species. In the past fishway designs have

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Silver perch – A Resource Document been developed to operate over quite a narrow range of water levels. Peterie and Blanch (1999) estimate that only about 1% of barriers within the Murray Darling Basin have fishways.

6.3.2 Management Thorncraft and Harris (1997) noted that in Australia we are now able to develop fishways for new weirs, alter existing fishways and retro-fit fishways to old weirs. There are numerous types of fish passage options including pool, Denil, locks, trap and transport, vertical-slot fish ladders, rock ramp and bypass types. Recent research has been undertaken to design fishways which are more suitable for native fish species within the Murray Darling Basin. Mallen-Cooper (1997) discussed different types of fishways in relation to different flow patterns. Harris (1997) observed that constructing vertical-slot and Denil fishways are expensive and that cheaper solutions need to be found. Rock- ramp fishways and bypass channels are two such options. Rock-ramp fishways have potential to work well during low flows and vertical-slot fishways work at lower flows compared to Denil fishways (Mallen-Cooper 1997). Vertical-slot fishways can be effective for structures of below about 6.5 m in height. Lock fishways, such as the one built at Yarrawonga weir, may be appropriate for higher barriers. MDBC (in prep.) argues that appropriate research and monitoring is required as a component in the construction of fishways.

Mallen-Cooper (1997) emphasised the need to recognise the importance of local and regional hydrology and variable flows to develop the most suitable fishway designs so that they work for low and high flows. Design of fishways need to consider not only the water velocities which can be negotiated by the fish but also the time taken for a fish to ascend the entire fishway and the diel movement patterns of each species (Mallen-Cooper et al. 1995). Harris et al. (1992) note that the design of fishways need to take into account the upstream movement of juvenile fish, which may have poorer swimming abilities.

In recent years there has been a concerted effort to establish a database of instream barriers in all states of the Murray Darling Basin with the exception of South Australia. This database provides an important summary of barriers to fish movement, including location and characteristics of barriers, and gives a broad view of the current situation. Definitions of barriers differ between states and many are small structures on minor streams that may not be significant barriers (MDBC, in prep.). The database includes a total of 3 651 barriers: 651 in Queensland, 1 768 in New South Wales and 1 232 in Victoria. This inventory indicates that previous assessments of the number of barriers have been underestimates; for example Mallen-Cooper et al. (1995) indicated that there were currently over 1 500 dams and weirs that did not have fishways in the Murray Darling Basin.

A MDBC Fish Passage Reference Group was established in 2001 with representatives from all relevant states. This group coordinates fish passage issues across the Basin, including providing advice on fishway designs, ensuring that appropriate fishway designs are used with an emphasis on river ecology and its relationship with fish passage. It is

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Silver perch – A Resource Document proposed to establish fishways on the locks and barrages from the Murray mouth to Hume Dam. The program is currently in its planning phase determining appropriate fishway designs, which fishways will be established over the next five years and an appropriate monitoring schedule.

Queensland While most existing barriers in Queensland do not have fishways, there is now provision under the Fisheries Act 1994 for the Department of Primary Industries to direct building of fishways on any barriers which are constructed across a waterway (Jackson 1997). There is a set procedure for assessing the requirements and feasibility of incorporating a fishway on such structures (Jackson 1997). Modifications are currently being made to some existing fishways, and a program has commenced on fishway design, construction and monitoring. A Fishway Co-ordinating Committee was established in 1990 to review existing and proposed fishways and to determine economical designs for rectification/implementation.

A breakdown of the number of barriers in Queensland by river basin indicates that the highest number occur in the Condamine-Culgoa (355) and the Border (216) with far fewer in the Warrego (43), Moonie (35) and Paroo (2) (MDBC in prep.). The majority of barriers are classified as farm dam/weir (47%) and fixed crest dam/weir (48%) with the remainder being stream gauging weirs, gated dams, culverts and tanks on a watercourse. The list of 18 priority sites where new or improved fishways are required include five on the Condamine River, three on the Balonne River, three on the Macintyre River, three on the Dumaresq River, and one each on the Paroo, Warrego, Moonie and Maranoa rivers. To date, most fishway programs in Queensland have focused on coastal rivers due to concerns over significant angling and commercial fisheries such as Barramundi and Sea Mullett. Overall there are about seven vertical slot fishways and three coastal fish locks in Queensland.

New South Wales The Fisheries Management Act 1994 gives the Minister authority to require the construction of works that enable fish to pass through or over dams, weirs or . New South Wales recently established a State Fishways Program, administered primarily by the Department of Land and Water Conservation and NSW Fisheries. A draft priority list was developed in 1992 that listed barriers by drainage basin and type and indicated priorities, types of fishway design recommended and whether negotiations had commenced. The current inventory includes licensed weirs and Departmental weirs; the number of unlicensed weirs is unknown. A Weir Review Committee assesses proposals for refurbishing old weirs or building new weirs. Within the Murray Darling Basin, the list included 11 sites which were considered the most urgent, and a further 90 of lower priority. More recently, a draft Murray Darling Basin report on barriers to fish migration lists 10 priority sites.

A breakdown of the number of barriers in New South Wales by river basin indicates that the highest number occur in the Murrumbidgee (433), the Lachlan (350) and the

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Silver perch – A Resource Document Macquarie-Bogan (342), followed by the Murray Riverina (150) and the Namoi (116) with fewer in the Gwydir (82), the Border (76), the Warrego (57), the Castlereagh (41), the Darling (36), the Balonne-Condamine (31), the Upper Murray (19), the Paroo (16), the Benanee (10) and the Moonie (7) (MDBC in prep.). The majority barriers are classified as by-wash dams (39.9%), and fixed crest dam/weir (22.2%), over-shot dam (9.6%) and block dams (8.7%), gated dam/weir (5.7%) and a mixture of other smaller structures. A status report on fishways and fish passage is also being prepared for New South Wales (Thorncraft and Harris in prep).

In New South Wales, six fishways have been built since 1985 within the Murray Darling Basin. These include vertical-slot designs ( on the Macintyre River, Barraba on the and Torrumbarry Weir on the Murray River), rock ramp and lock designs. Varying levels of monitoring of the effectiveness of these structures has been undertaken and is planned for the future. While the Torrumbarry fishway on the Murray River has proved successful, there are some operational problems at other sites such as Yarrawonga and Goondiwindi (C. Lay, NSW Fisheries, pers. comm. 1999).

Torrumbarry weir was considered the most significant barrier to fish movement on the Murray River and a vertical slot fishway was constructed in 1989-91. An assessment of the fishway by Mallen-Cooper et al. (1995) indicated that it is successful, with fish migrating up to the weir, finding the fishway entrance and successfully negotiating it. Monitoring of this fishway is ongoing and it is regarded as one of the most successful fishways in New South Wales (C. Lay, NSW Fisheries, pers. comm. 1999). Monitoring is intended for the vertical slot fishway on the Macintyre River at Boggabilla; operation of this fishway is dependent on management of the upstream weir pool (C. Lay, NSW Fisheries, pers. comm. 1999). No monitoring of the vertical slot fishway on the Manilla River at Barraba has been undertaken.

There have been low numbers of fish using the rock ramp fishway on the Bell River (Thorncraft and Harris 1996), and modifications are required for the rock ramp fishway at Goondiwindi on the Macintyre River (C. Lay, NSW Fisheries, pers. comm. 1999).

A lock fishway was established at Yarrawonga in 1995, being the first of its type in the Murray Darling Basin. Monitoring has indicated it is currently not operating effectively, although seven species and a range of size classes of migratory fish have been recorded to move through the fishway (Thorncraft and Harris 1997). The fishway is considered to have potential to provide effective fish passage and modifications are planned for the near future to improve its operation (C. Lay, NSW Fisheries, pers. comm. 1999). A trap and truck operation is currently being used as an interim measure.

Assessments of fishways at Euston and Murtho on the Murray River have indicated that they are inefficient at allowing passage of native fish (Mallen-Cooper and Brand 1992). Characteristics such as high water velocities and turbulence have meant that most native migratory species were restricted or prevented in their ability to move up through the fishway. Monitoring of the fishway at Brewarrina on the Darling River indicated it was

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Silver perch – A Resource Document also ineffective in providing fish passage (Mallen-Cooper and Thorncraft 1992). The Main Weir of the Menindee lakes on the Darling River is considered the most persistent barrier to fish migration on this river (Mallen-Cooper and Edwards 1991) and is considered a priority for a fishway. Harris et al. (1992) also recommended the construction of a vertical slot fishway on the Bourke Weir on the Darling River.

A proposed recommendation has recently been made by the NSW Fisheries Scientific Committee to list "Installation and operation of instream structures that modify flow" as a key threatening process under the Fisheries Management Act 1994.

Victoria There are a number of legislative acts within Victoria which relate to instream works and protection of fish movement (e.g. Conservation, Forests and Lands Act 1987, Flora and Fauna Guarantee Act 1988, Water Act 1989). A Fishway Implementation Committee, primarily comprising Department of Natural Resources and Environment staff, is responsible for setting priorities and work programs for fishway construction.

An inventory of fish barriers has been completed for Victoria which includes Murray Darling Basin catchments (with the exception of the Mallee basin) and the south east coast division (McGuckin 1999). A breakdown of the number of barriers in the basin indicates that the highest number occur in the Loddon (317) and the Goulburn (306), followed by the Ovens (156) and the Campaspse (137) with fewer in the Broken (96), Upper Murray (89), Kiewa (64), Avoca (39) and the Wimmera (28) (MDBC in prep.). The majority of barriers are classified as gauging stations (47% weirs, 15% natural), followed by dams/weirs (17%), fords (13%), waterfalls (5%) and culverts (3%). The priority list of seven sites requiring fishways includes four in the Broken basin and one each for the Loddon, Ovens and Goulburn basins (MDBC in prep.). To date, assessments of the effectiveness of fishways on the Broken Creek have been inconclusive, although there is some anecdotal information that fish are moving through them (T. O'Brien, DNRE, pers. comm. 1999). There has been a focus on completing fishways within the Broken catchment (seven to date) to provide a combined effect to improve fish passage in the entire Broken basin below Benalla (T. O'Brien, DNRE, pers. comm. 1999).

South Australia There is currently no formal state fishway program in South Australia. However, some work is currently being undertaken on modification to the barrages at the mouth of the Murray River. There are a number of locks along the Murray in this state which may be barriers to some degree although many can provide for some fish movement during higher flows (J. Winwood, formerly SA Recreational Fishing Council, pers. comm. 1999). Winwood also indicated that another main concern is the barriers between the river channels and adjoining floodplain habitat.

The recent report to the Parliament of South Australia (1999) noted that existing fish ladders are ineffective in allowing fish to move easily past locks. The committee recommended that alternative fish bypass systems should be investigated and encouraged

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Silver perch – A Resource Document projects such as the Barrage Fish Passage project at Lake Alexandrina. This project aims to place fish gates into the barrages. The report noted that fish are unable to readily migrate upstream and that “Witnesses before this Inquiry constantly referred to a need for fish passages and fish ladders to assist this migration”.

Prioritisation for works The draft Murray Darling Basin Commission report on barriers to fish migration lists priority barriers within the Murray Darling Basin which require new or improved fishways; these include 18 for Queensland, 10 for New South Wales and seven for Victoria. The report emphasises that the list is provisional and that further on ground assessments are needed. It was also noted that provision of fish passage should be encouraged on other barriers.

MDBC (in prep.) argues that strategically placed fishways can significantly improve fish movement by opening up long distances along a river. Priority barriers for action have been identified taking into consideration of issues such as the following: • Presence of native fish species • Condition of the river system • Characteristics of the barrier • Potential to enhance commercial and recreational fishing • Proximity to major river systems • Potential for recruitment from existing fish populations • Other relevant existing programs

The appropriate management of fish barriers includes establishment of fishways, removal of barriers that are no longer in operation and altering water regulation systems to provide adequate flows to drown out barriers at appropriate times of year (MDBC in prep.).

6.3.3 Assessment of Significance of the Threat Mallen-Cooper (in press) notes that the effect of dams and weirs on fish migrations has frequently been cited as one of the main reasons for the significant decline in range and abundance of native freshwater fish species. The recent inventory of barriers indicates that there is a large number of barriers throughout the Murray Darling Basin although they vary in their significance to fish movement. Few significant barriers however specifically provide for fish passage. Significant barriers have the potential to cause local extinctions upstream, isolate populations and alter fish community structure.

Impact of barriers on silver perch An assessment of the significance of barriers on silver perch and the possible role barriers may have played in the species needs to consider the movement patterns of different life stages of this species.

Adults and juveniles

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Silver perch – A Resource Document Silver perch is a potamodromous species, migrating entirely in freshwater. Initial research on the movement of adult silver perch was undertaken by Reynolds (1983) who tagged a total of 660 adult fish. Of 26 recaptured fish, 73% moved more than 10 km with similar numbers moving upstream and downstream, while 12% had moved less than 10 km up or downstream and 15% had not moved. The greatest distance moved by a fish was 570 km upstream in 19 months; another fish moved 110 km. Reynolds (1983) noted that silver perch negotiated some of the locks along the Murray River, which were open in high flows and floods and closed in low and normal flows.

Mallen-Cooper et al. (1995) monitored the movement of silver perch through the Torrumbarry Fishway on the Murray River; further work is required in other areas to determine whether similar patterns are observed. They recorded both adult and juvenile fish moving upstream seasonally, and migration occurred when water temperatures and water levels increased. It was previously thought that only adults undertook extensive upstream movements, particularly during times of flood, while eggs, larvae and juveniles drifted downstream. The majority (87%) of the silver perch migrating upstream were immature, all being at least one year old. These fish generally measured between 100 and 400 mm in length. Mallen-Cooper et al. (1995) observed a strong seasonality in upstream movement, with differences detected between immatures and matures. While immature fish moved upstream from October to April, mature fish moved over a briefer time period from November to February. From 1938 to 1942, J. O. Langtry (in Cadwallader 1977) recorded silver perch moving through the Euston-Robinvale fish ladder between October and May. Mallen-Cooper et al. (1995) suggested the season of migration would most likely be longer in the north of basin, being influenced by daylength and/or water temperature.

Migration of immature and mature fish commenced once water temperatures rose above 20oC (Mallen-Cooper et al. 1995). As water temperatures declined in late summer and autumn, immature fish continued to move upstream until the water temperature reached 16oC; no upstream movement occurred below 16oC. Immature fish only moved upstream during daylight. During 1991/92 few silver perch moved upstream on very low flows (i.e. below 3500 ML/day), high numbers moved on flows of 4000 to 4500 ML/day, and significant numbers moved on flows of 6000-8000 ML/day which represented small rises in river levels (Mallen-Cooper et al. 1995). In 1993, few silver perch moved at flows lower than 10 000 ML/day, with larger numbers moving during flows of 10 000-12 000 ML/day. Upstream migration of large numbers of silver perch occurred during small rises in the river, with small daily increments in flow. The highest number of fish moving upstream were recorded when the river level rose or fell between 1 and 10 cm over 24 hours on the crest of small rises. Mallen-Cooper (1993) noted that the number of small floods (5000-10 000 ML/day) occurring in the Murray River over the last 50 years has declined by half. Clear year classes were recorded when flows were contained within the river channels, while weak year classes were recorded when high flows inundated the low lying floodplains.

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Silver perch – A Resource Document Mallen-Cooper et al. (1995) emphasised that the reasons for movement of silver perch are not well understood. Mallen-Cooper et al. (1995) noted that there was indirect evidence that adults move upstream prior to spawning. Commercial fishing information indicates that silver perch are caught in drum nets as they move upstream. However, Mallen-Cooper et al. (1995) cited the example of Cataract Dam near Sydney as indicating that migration is not essential for spawning of the species. Energy is expended by migrating upstream, so presumably there is some benefit to fish in undertaking this movement. Silver perch may move upstream to optimise feeding, to enhance colonisation, or to compensate for the downstream drift of pelagic eggs and larvae (Mallen-Cooper et al. 1995). Richardson (1994) suggested the upstream migration of mature fish to spawn may be a strategy to heighten the survival of larvae. As the eggs move downstream during floods, they have time to develop and hatch into larvae. Once individuals develop, they begin to feed and can swim against the current. The apparent lack of movement by those fish less than one year old may be because their existing habitat provides adequate food and shelter. Also their swimming abilities would be poor due to their small size, possibly increasing their susceptibility to predation (Mallen- Cooper et al. 1995). The energy required for small fish to swim upstream against the current may be too high to be justified.

Swimming ability of silver perch and negotiation of barriers Mallen-Cooper (1994) investigated the swimming abilities of adult golden perch and silver perch in an experimental vertical slot fishway. The results for silver perch (258 mm +/-10 mm) were inconclusive, either because the fishway was unsuitable for the species or the fish were not motivated to swim upstream. Mallen-Cooper (1994) had observed silver perch to move through vertical slot fishways, and suggested that in this experiment fish may not have been motivated to move. This suggestion is supported by the observation that the fish were not in a spawning condition. At water velocities of 2.0 m/s, the swimming abilities of silver perch were similar to golden perch, while above this silver perch exhibited a varied response with many not attempting to move through the fishway. The maximum velocity negotiated by both golden perch and silver perch was 2.02-2.63 m/s, representing 7.8-10.2 lengths/s for silver perch. Mallen-Cooper (1994) considered 1.83 m/s to be a suitable maximum water velocity for golden perch and silver perch.

Subsequent to this work, a fishway was constructed on the Torrumbarry Weir, designed for a water velocity of 1.8 m/s. Mallen-Cooper et al. (1995) monitored the movement of both native and introduced fish through this fishway. Between February 1991 and June 1993, a total of 5342 silver perch passed through the fishway (monitoring 5 days a week). The high numbers moving through the fishway and the absence of accumulations of silver perch below the weir indicated that the fishway was effective. It was found that 95% of silver perch (majority 100-400 mm) could move through the fishway at 2.25- 2.70 m/s, a higher water velocity than expected. Small silver perch (80-120 mm) were sometimes observed ascending the entire fishway. Harris et al. (1992) note that the design of fishways need to take into account the upstream movement of juvenile fish, which have a poorer swimming ability. They have observed fish to move within the

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Silver perch – A Resource Document slower flowing water at the edges of a river channel, rather than the faster flows in mid channel.

Mallen-Cooper and Brand (1992) investigated the effectiveness of fishways at Murtho (Lock 6) and Euston (Lock 15) on the Murray River. They noted the high water velocities and turbulence prevented or restricted the passage of most fish species upstream through these fishways. Maximum water velocities for Euston and Murtho were 2.4 m/s. During their study only small numbers of silver perch were recorded, making results for this species inconclusive. Mallen-Cooper and Edwards (1991) monitored fish passage from the Main Weir at Menindee Lakes, the largest weir on the Darling River representing the greatest barrier to fish migration in this river. They observed that fish passage through the weir only occurred when the gates were lifted clear of the water. While large numbers of golden perch were recorded, no silver perch were caught. Mallen-Cooper and Edwards (1991) suggested this may be because silver perch were not stimulated to spawn, they were in low numbers, or may have passed through the weir prior to the monitoring period. Other studies which have monitored fish passage through weirs at Bourke (Harris et al. 1992) and Brewarrina (Mallen-Cooper and Thorncroft 1992) have either failed to record silver perch or recorded very low numbers. While the fishway at the Yarrawonga weir is not operating effectively and requires modification, some silver perch have been recorded successfully using the fishlift (Thorncraft and Harris 1997).

Eggs and Larvae Silver perch produce non adhesive, semi buoyant, pelagic eggs. Lake (1967c) noted that the large perivitelline spaces within the eggs provide protection against damage and would enable a better survival in flowing water conditions. He also suggested that the characteristics of the eggs could possibly aid in gaseous exchange which could be relevant in waters low in oxygen. Cadwallader (1978) considered pelagic eggs to be suited to quite tranquil floodspread waters rather than fast water currents. Lake (1971) indicated silver perch eggs are denser compared to golden perch eggs, and will sink if there is no water movement. Silver perch eggs have been collected in drift nets in the Murray River downstream of Lake Mulwala in early December (J. Koehn, unpublished data).

The eggs hatch rapidly within a day or so and are at an early developmental stage. Newly hatched larvae are incapable of swimming freely and will tend to sink; however they are able to make intermittent movements up to the water surface then sink (Lake 1967b). Thurstan and Rowland (1995) observed larvae to spend the majority of time hanging vertically with infrequent swimming actions. Larvae are free swimming at about five days (Lake 1967b) and commence feeding when the yolk is completely absorbed at six days (Thurstan and Rowland 1995). In aquaculture operations, Thurstan and Rowland (1995) note that prolonged vigorous turbulence can damage larvae which are weak swimmers. Larvae appear to rarely use cover and do not seek shelter in shade (Gehrke 1990, Thurstan 1991).

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Silver perch – A Resource Document There are two possible impacts that barriers may have on eggs and larvae. Eggs and larvae may potentially be physically damaged as they move downstream over barriers. At times of lack of flow above a barrier, eggs and/or larvae may sink; this may prevent their drift further downstream and result in a degree of fragmentation of populations between barriers. Eggs which settle out may be detrimentally affected by sedimentation causing mortalities due to oxygen deprivation. Since silver perch eggs hatch in less than two days, this may not be a significant issue. Schiller (NSW Fisheries, pers. comm. 2000) has recorded some evidence of Murray cod larvae settling out in weir pools. Since silver perch larvae are not strong swimmers until about five days old, still water may cause a proportion of the larvae to settle out. Whether this is detrimental may depend on the available food source and water quality within the still water above the barrier. Since silver perch spawn on small rises in water level, it is possible that settling out of eggs and larvae above barriers occurs frequently.

Mallen-Cooper et al (1995) suggest that length of river may be an important issue to species such as silver perch. They observed that one of the reasons why a good population of silver perch remained between Torrumbarry and Euston may be that this stretch measured 528 km in length. If silver perch spawned below Torrumbarry in flows up to 10 000 ML/day then eggs and larvae may drift about 300 km downstream. They noted that bankful floods of 60 000 ML/day would carry larvae over Euston weir. Sufficiently long stretches of river would provide the larvae with enough time to develop and swim against the current and not pass over weirs. Mallen-Cooper et al. (1995) noted that low-level weirs have severely fragmented rivers within the Murray Darling Basin; the most common length of river reach of 0-50 km in length with few longer than 300 km.

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Silver perch – A Resource Document

7 Introduced Species McDowall (1996) note that 25 species of fish have been introduced into Australia's freshwater environment. The exact impact many of these species have had on native fish is not well understood. Two of the most significant introductions, carp Cyprinus carpio and redfin are discussed in detail below, while several others are briefly reviewed.

7.1 Carp Carp began their rapid expansion throughout the Murray Darling Basin in the early 1970s and are now widespread and abundant in most areas. While their exact impact on the aquatic environment is not yet clear, perceived problems associated with carp include increased water turbidity and siltation, decreased macrophyte biomass and diversity, increased water nutrient loads and algal concentrations, reduced native fish and macroinvertebrate numbers and diversity and erosion of streambanks.

There is no direct evidence that carp have caused a decline in the abundance and distribution of any native fish species within the Murray Darling Basin, including silver perch. Carp are widespread and common throughout most of the basin and silver perch has declined throughout the majority of its range. Carp began to spread throughout the basin from the early 1970s onwards. A consideration of trends in commercial catch data for silver perch indicates a relatively consistent pattern of decline in New South Wales from the early 1960s onwards and in South Australia from the early 1980s onwards. Thus silver perch had begun to decline in some areas before the appearance of carp.

Since silver perch and now carp have broad distributions within the basin, occurring in a wide range of habitats, there is the potential for overlap in niches. Both species could be considered omnivores, feeding on a wide range of items. There is some overlap in diet with both species feeding on some of the same classes of zooplankton and macroinvertebrates such as copepods and chironomid larvae. Filamentous algae has also been recorded in both species' diet. While there is some potential for overlap in diet, it is not known whether food is a limiting resource. Since the diet of carp consists predominantly of benthic invertebrates, it seems unlikely that the species directly predates on silver perch. Dick (1997) notes that there have been no reports of carp eating young fish or fish eggs. There have been no specific records of silver perch being taken by carp.

While silver perch can occur in areas which have high turbidities, the species' tolerance to turbid water is not known. Increased turbidities may result in increased sedimentation. Silver perch eggs are semi buoyant, pelagic and hatch rapidly. Sedimentation may be of concern if eggs settle out in still water habitats such as in weir pools. Larvae may also sink in still water habitats and sedimentation may also be detrimental to them until they are free swimming. The reduction in aquatic vegetation which can result from increased turbidity and sedimentation may be detrimental to silver perch since aquatic vegetation and filamentous algae are components of the species' diet. It is not known whether silver perch digest algae either partially or completely since much algae appears to be Freshwater Ecology, NRE & Murray Darling Basin Commission 55

Silver perch – A Resource Document reasonably intact in the intestine and as it leaves the anus. Silver perch possibly only digest fauna which live on the algae (S. Rowland, NSW Fisheries, pers. comm. 1998). Whether aquatic vegetation provides shelter for larval and juvenile silver perch is also unknown. Since larvae do not appear to seek shelter in shade and rarely use cover (Gehrke 1990, Thurstan 1991), aquatic vegetation may not be an important habitat component for this life history stage. Habitat requirements of juveniles is not well understood.

The tolerance of silver perch to increased nutrient loading and algal concentrations in the water column has not been documented. Algae can apparently make up a significant component of the species' diet and this can increase as fish grow. Whether the algae recorded within the diet is consistent with the types of algae recorded in blooms requires investigation.

7.1.1 Background Carp is one of the world's most widely distributed freshwater fish. Three strains have been introduced into Australia: an ornamental ‘Prospect’ strain near Sydney in 1850-60, the ‘Boolara’ strain for aquaculture in Victoria in 1961 and the ‘Singapore’ strain of koi in the Murrumbidgee in 1987 (Brumley 1996). The Boolara strain subsequently escaped in the Murray Darling Basin, interbreeding with the koi strain and producing a stock of broad genetic composition (Brumley 1996).

Carp are now widespread throughout the Murray Darling Basin and have become one of the most common and abundant species in many areas. Their spread has been assisted by floods (e.g. 1974/5, 1993) and by further introductions for use as live bait or by anglers. By 1979 Pribble (1979) indicated carp occurred in the Murray River in South Australia to the river mouth, throughout most of the Murray, Darling, Murrumbidgee and Lachlan rivers in New South Wales, most of the drainage rivers in Victoria and was spreading upwards into the major rivers within Queensland. In Victoria, surveys of northern Victoria between 1979 and 1983 noted carp and goldfish Carassius auratus were both more abundant than any native fish species (Brumley et al. 1987). Roberts and Tilzey (1996) note that recent surveys in New South Wales and Victoria indicate that carp are now over 80% of total fish biomass within the Murray Darling Basin, and up to 96% in some areas. The recent New South Wales Rivers survey found that carp were the dominant fish species in the Murray and Darling River systems representing 40% and 11.4% of the total catch respectively. Gehrke et al. (1995) found that carp dominated the fish community in the Murray and Murrumbidgee catchments and were also abundant in the Paroo and Darling catchments. Carp have been present in some Queensland rivers of the Murray Darling Basin since the 1970s. A survey by Midgley (1989) considered carp to be common at two sites in the Warrego and basins reasonably close to the border, while they were not recorded at a further six sites in the Warrego, Condamine- Balonne and Border catchments. Carp now occur in all major river systems in Queensland, with their progress hampered by the presence of weirs e.g. at Millmerran on the Condamine-Balonne system (D. Moffatt, DNRE, Queensland, pers. comm.). Recent surveys in Queensland ranked carp as the fourth most abundant species caught.

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Commercial catch data from New South Wales indicate a dramatic increase in catches of carp from the early 1970s, reaching a peak in 1977/8 of 548 tonnes and then declining to 141 tonnes in 1995/96 (Reid et al. 1997). In South Australia catch from 1976 onwards has ranged from 206 tonnes in 1977/78 gradually increasing to a peak of about 1150 tonnes in 1991/92 and then gradually declining to 676 in 1997/98 (Cadwallader 1985, Pierce and Doonan 1999). In Victoria, catch has fluctuated from about 100 tonnes in 1971/72 to a peak of about 600 tonnes in 1990/91 declining to about 400 tonnes in 1994/95 (A. Baxter, DNRE, pers. comm.).

Ecology Koehn et al. (in press) provide a recent summary of biology and management of carp within Australia. Carp have a broad ecological niche and are high adoptable because of their high fecundity, rapid growth, longevity, tolerance to a range of water quality conditions, ability for rapid dispersal and flexible omnivorous diet (Harris 1996). They are tolerant to poor water quality, low oxygen levels, turbid water, moderate salinities and have a higher tolerance to toxicants compared to many other species (Koehn et al. in press).

Carp can take food from within the water column and are also benthic feeders sucking in food and sediment and filtering out unwanted sediment. They are omnivores, and their diet varies between areas and seasons and probably reflecting food availability (Koehn et al. in press). In Australia, carp have been recorded feeding on microcrustaceans (cladocerans, copepods, ostracods, decapods), aquatic insect larvae (chironomids, corixids), molluscs, terrestrial insects, plant material including seeds and filamentous algae (Koehn et al. in press). Koehn et al. (in press) notes that juveniles feed exclusively on zooplankton and as they grow begin to also feed on macroinvertebrates.

Spawning usually occurs in late spring or early summer, at temperatures ranging between 17-25oC (Brumley 1996). Eggs are laid on aquatic vegetation, often in shallow areas. Koehn et al. (in press) observed that the species is opportunistic in terms of spawning and recruitment, occurring in suitable habitats which provide resources for juveniles.

Carp can occur in a wide range of habitats, although are often found in low altitude, slower flowing and still waters, and are rarely found in cool fast flowing waters. In the New South Wales Rivers survey, Driver et al. (1997) observed carp to be strongly associated with lower altitudes, with lowland areas providing suitable slow flowing spawning and nursery habitats such as floodplains, backwaters, shallow river edges and billabongs. Carp were recorded in all inland sites below 500 m ASL (Harris and Gehrke 1997). Driver et al. (1997) suggested that the absence of carp from montane sites may have been due to lack of suitable habitat as well as the presence of barriers blocking upstream dispersal. Koehn and Nichol (1998) observed carp to prefer slower water velocities close to the river bank.

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Silver perch – A Resource Document Mallen-Cooper et al. (1995) observed that movement of carp appeared to be stimulated by increasing water temperature rather than flow, and that movement did not seem linked to spawning suggesting it facilitated colonisation. Radiotracking indicated that carp could move large distances up and downstream in the Murray River (Koehn and Nichol 1998).

7.1.2 Management There has been no coordinated management to control carp in the past within the Murray Darling Basin, with limited control programs occurring in particular areas usually by state fisheries agencies. Harris (1996) argues that integrated management programs of restoration which deal with catchment management, flow allocation, pollution abatement, habitat reconstruction and restoration of connectivity all have the potential to assist in the control of carp, although control of carp is also needed. It is argued that rehabilitation of native fish communities will exert a greater competition and predation pressure on carp.

Roberts and Tilzey (1996) and Koehn et al. (in press) outline the range of possible methods of controlling carp: poisoning, habitat modification, exclusion netting, commercial and recreational fishing, native predator enhancement, chemical repellents, fertility control and biological control. It is generally accepted that complete eradication of carp on a national scale is unrealistic using current technology. The extent to which reducing numbers helps solve problems caused by carp depends on the scale of harvest and population dynamics (Thresher 1996). Roberts and Tilzey (1996) note that control programs to date have been local or regional, and that addressing entire river systems has been expensive as well as impractical.

Renewed concern over the effect of carp on the aquatic environment led to the creation of the National Carp Task Force in 1996 which aims to develop a natural resource management strategy dealing with research, education, commercial exploitation and legislation. A Carp Control Coordinating Group has also been established with representatives from state fisheries agencies and the commonwealth. A key task of this group was to prepare a national management strategy to provide direction and focus for a coordinate national approach; a draft has been released for comment (MDBC 2000). MDBC (2000) argues that integrated pest management requires a range of approaches and techniques. A Fish Rehab 2001 project is currently looking at point source management of carp in Barmah Millewa Forest.

7.1.3 Assessment of Significance of the Threat Potential impacts of introducing new species to a fish community include competition, predation, changes in food web structure, introduction of diseases and parasites, alteration to habitat and hybridisation. In relation to carp, MDBC (1999) argues that there is little doubt that this species has had an impact on the aquatic environment, particularly where it occurs in large numbers. However, the exact impact the species has had is hard to determine, being difficult to separate from numerous other problems of habitat degradation. In general, the species has a bad public image and has frequently been blamed for habitat degradation. MDBC (1999) argues that the decline of native species

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Silver perch – A Resource Document and changes to the environment including river regulation were well advanced prior to the dramatic increases in carp numbers.

Prior to the dramatic spread of carp throughout the Murray Darling Basin, a Victorian government report was prepared in 1961/62 which considered the survival potential of the species and its possible impact on the aquatic environment (State Development Committee 1961/2). While conflicting views were provided, concerns were raised about the potential for carp to compete for space and food with native species, how their feeding may increase turbidity and destroy weedbeds. The report noted that a: “.. [scientist] has suggested that any adverse effects resulting from the introduction of European Carp into Victorian waters could be checked by the propagation on a larger scale of Murray cod or other fish of prey. On the other hand, he has given evidence that continuous pollution of streams, while favouring the survival of more resistant species such as European Carp, will result in the disappearance or extermination of the more sensitive species of fish, to which group most of the fish of prey belong.”

Roberts et al. (1995) note that while the effect of carp on aquatic ecosystems in Australia has not been fully assessed, it may be substantial given its wide distribution and abundance. It has been argued that carp have taken advantage of a degraded environment, since the species has a broad ecological tolerance. The spread of carp may have been facilitated by the disturbance and alteration of riverine habitats. Driver et al. (1997) conclude that changes to flow and water temperature has reduced the abundance of native species and increased the abundance of carp. There is evidence that regulation of flow regimes associated with lowered species diversity and higher relative abundance of carp (Gehrke et al. 1995). The results of the New South Wales Rivers Survey and work by Gehrke et al. (1995) noted the association between fish communities dominated by carp and more regulated rivers. Driver et al. (1997) observed that flow regulation and agricultural activities lead to higher carp biomass densities. River regulation may have favoured carp migration compared to native species and probably facilitated their rapid invasion of the Murray Darling Basin.

King (1995) reviewed the types of ecological damage attributed to carp and the evidence from overseas and Australia. Ecological damage attributed to carp include: - increased water turbidity and siltation - decreased macrophyte biomass and diversity - increased water nutrient loads and algal concentrations - reduced native fish numbers and diversity - decreased macroinvertebrate numbers and diversity - erosion of streambanks

The majority of research on carp in Australia has focussed on pond experiments (MDBC 1999). Research has indicated that carp do increase water turbidity and dissolved nutrient levels and displace shallow rooted aquatic macrophytes (Roberts and Ebner 1997). Evidence concerning effects on macroinvertebrates, phytoplankton and bank erosion is anecdotal.

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Increased water turbidity and siltation Roberts et al. (1995) note increased turbidity may: - reduce the amount of light reaching the bottom thus influencing water temperature. - result in reduced photosynthesis in aquatic plants and reduced growth - restrict foraging by visual feeders - reduce survival of fish eggs and larvae - alter breeding behaviour

While overseas experiments have found a positive relationship between carp density and turbidity, Australian studies have provided conflicting results. Studies by Hume et al. (1983) and Fletcher et al. (1985) did not find correlations between carp density and turbidity, although experiments in irrigation drains (Bowmer et al. 1994) and artificial ponds (Roberts et al. 1995) did show associations. King et al. (1997) observed high densities of carp to increase turbidity in billabongs, although noted that other factors also affected turbidity (e.g. sediment type, wind and water runoff, presence of aquatic vegetation). Roberts et al. (1995) observed short term changes in water quality with an increase in suspended sediment and thus turbidity at high densities of carp (510 kg/ha). There was less of an increase in ponds when plants were present. The presence of macrophytes may modify the ability of carp to disturb sediments (King et al. 1997). Robertson et al. (1997) found resuspension rates increased with increasing carp densities. They also noted that carp could have an indirect effect on settlement of particles by the loss of aquatic vegetation from littoral areas which would expose sediments to more disturbance by wind.

Many Australian rivers now tend to be turbid in appearance because of soil characteristics where particles stay in suspension. Fletcher et al. (1985) noted that changes in turbidity could be related to hydrological conditions, with some evidence that turbidity increased with increasing water levels. Sites such as the Goulburn River had widely fluctuating turbidities before and after the arrival of carp (Fletcher et al. 1985).

Decreased macrophyte biomass and diversity Fletcher et al. (1985) noted that carp are said to affect aquatic vegetation by: - consuming the vegetation - reducing photosynthesis (due to increased turbidities) - uprooting it when feeding on invertebrates - disturbing it when spawning upon it

Koehn et al. (in press) notes that while carp do eat plant material, it is ingested incidentally and in the absence of other food items. Fletcher et al. (1985) found plant material was a minor component of the diet.

Roberts (1995) concluded that in Australia, under conditions such as high carp densities and low food availability, particular species of aquatic vegetation could be eliminated. King (1995) notes that overseas experiments have found that carp affect the density and

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Silver perch – A Resource Document diversity of macrophytes, particularly soft leaved and shallow rooted species. In enclosure experiments, Crivelli (1983) found that the feeding activities of carp caused the destruction of aquatic vegetation, with some plants being uprooted. Crivelli (1983) suggested the greatest damage to aquatic vegetation would be done in shallow water where carp preferentially feed and spawn. Damage to aquatic plants could vary between species and could be affected by factors such as susceptibility to uprooting and soil type, phenology and timing of seed production (Crivelli 1983).

In a study of billabongs of the Goulburn River, Fletcher et al. (1985) suggested carp may destroy less robust aquatic vegetation. For example, Potamogeton species were absent from some sites where carp occurred, while more robust plants (e.g. Cumbungi, Phragmites, Juncus spp.) and those which formed dense mats (Myriophyllum propinquum, Ludwigia peplides) were present. Fletcher et al. (1985) suggested a critical threshold level of 450 kg/ha, above which carp may cause increasing damage to aquatic vegetation. In experimental ponds, Roberts et al. (1995) found that high densities (mean stocking rate of 510 kg/ha) of carp directly affected two of five plant species, these being the submergent Vallisneria sp. and Chara fibrosa possibly because they are less well anchored compared to emergent species. An experiment in irrigation drains found that there was new growth of macrophytes once carp had been removed (Bowmer et al. 1994). Robertson et al. (1995) did not observe a recovery of macrophytes in their billabong experiments and suggested this could have been due to a number of factors.

Robertson et al. (1997) raised the issue of what happens once macrophytes are removed from a waterbody: suggesting there may be a shift to an alternative stable state in which submerged and emergent vegetation will not reestablish even if the cause of the loss is removed.

Increased water nutrient loads and algal concentrations King et al. (1997) found that phytoplankton blooms could be influenced by carp, although noted that the mechanisms which promote higher phytoplankton levels are not clear. King et al. (1997) also noted that overseas research has indicated that carp can increase phytoplankton biomass by: - reducing grazing of phytoplankton by zooplankton - reducing macrophytes which would compete for nutrients - resuspending sediments which could release nutrients - directly increasing nutrient concentrations by excretion

High densities of carp may reduce the standing stock of zooplankton to such a degree that the zooplankton is unable to suppress algal growth by grazing (Koehn et al. in press). Gehrke and Harris (1994) indicate that since carp expansion within the Murray Darling Basin there has been a significant decrease in the abundance of aquatic vegetation and a shift to greater algal production. Gehrke and Harris (1994) argue that carp probably contribute to increasing the availability of nutrients more than other species by excretion, resuspension of sediments and macrophyte destruction. In billabong experiments, Robertson et al. (1995) found a positive relationship between carp biomass and

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Silver perch – A Resource Document phytoplankton biomass. King (1995) notes that overseas studies have found carp may increase the concentration of phytoplankton with the increase of nutrients from direct excretion the most likely pathway. King et al. (1997) indicate that carp affects on nutrient release from sediments was not yet known in an Australian context. In their experiments in billabongs, King et al. (1997) did not find a clear relationship between carp, nutrients and phytoplankton levels. Meredith et al. (1995) did not find a correlation between increasing carp numbers and increasing sedimentary phosphorous concentrations. Roberts et al. (1995) note that benthivorous fish such as carp can turn over a large amount of substrate and so make a significant contribution to nutrients in the water column by resuspension or by excretion. Their experiments did not provide evidence that carp acted as a nutrient pump and that it was instead a nutrient sink. King et al. (1997) note that resuspension of sediments by carp may lead to an increase in nutrients within the water column which are available to algae.

In billabong experiments, Robertson et al. (1997) found that carp had a significant detrimental effect on biofilm development which could affect trophic and nutrient pathways in billabong environments. Altering carp biomass did not significantly affected algal biomass on sediment surface, decomposition rate of macrophyte detritus or sediment solid-phase nutrients or nutrient ratios (Robertson et al. 1997). Robertson et al. (1995) found that carp had a detrimental effect on the development of epiphytic algae and suggested this was probably due to decreased light availability for epiphytes because of increased turbidity. Richardson et al. (1990) (cited in King 1995) suggested that carp caused a reduction in occurrence of filamentous algae in experimental tanks although had little impact on the total mass of biofilm.

Reduced native fish numbers and diversity There is currently a lack of understanding on the direct and indirect interactions between carp and native species, or about population dynamics of carp in the wild in Australia such as age structure and growth rates (Roberts and Tilzey 1996). There is no direct evidence that carp have caused a reduction in native fish abundance and diversity. However, declining fish densities and increasing carp densities from surveys over time have been apparent on a broad as well as local scale. Whether the decline of particular native species was a direct result of increases in carp is yet to be proven. Commercial catch data from New South Wales show increases in carp take while many native species have declined. However, Derwent (1994) notes that drought/flood cycles and fishing effort have had a significant impact on species such as Murray cod and golden perch.

While carp diet may overlap with many native species including Australian smelt, western carp gudgeon, flat-headed galaxias Galaxias rostratus (Hume et al. 1983) it is not known whether direct competition occurs or whether food is a limiting resource.

Decreased macroinvertebrate numbers and diversity Robertson et al. (1995) note there have been overseas experiments which indicate that high carp densities can decrease the diversity and density of benthic macroinvertebrate communities by predation or indirectly by habitat changes. Richardson et al. (1990)

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Silver perch – A Resource Document (cited in King 1995) argued that carp caused a decline in filamentous algae which resulted in a decline in particular larvae associated with this algae. In billabong experiments in Australia, Robertson et al. (1995) found that changes to carp biomass affected the community structure of invertebrates. While species richness of epifauna did not change, and densities declined with a high biomass of carp. The epifaunal community was dominated by corixids and chironomids. Changes to carp biomass did not affect infauna density.

Erosion of streambanks Roberts and McCorkelle (1995) note that erosion can be influenced by factors such as bank slope and soil characteristics and that biological factors such as stock, yabbies and carp have not commonly been considered. However, landholders and water managers often believe that carp contribute to bank erosion of irrigation channels. A study of irrigation supply channels found that undercutting and bank slumping occurred along river banks in areas where carp occurred and had been excluded and indicated that the impacts could not specifically be attributed to carp (Roberts and McCorkelle (1995). Roberts and McCorkelle (1995) found that site characteristics appeared to be a stronger factor in bank instability.

Impact of carp on silver perch There is no direct evidence that carp have caused a decline in the abundance and distribution of any native fish species within the Murray Darling Basin, including silver perch. Carp are widespread and common throughout most of the basin and silver perch has declined throughout the majority of its range. Carp began to spread throughout the basin from the early 1970s onwards. A consideration of trends in commercial catch data for silver perch indicates a relatively consistent pattern of decline in New South Wales from the early 1960s onwards and in South Australia from the early 1980s onwards. Thus silver perch had begun to decline in some areas before the appearance of carp.

Since silver perch and now carp have broad distributions within the basin, occurring in a wide range of habitats, there is the potential for overlap in niches. Both species can be considered midwater, schooling species but any competition for habitats is unknown. Both species could be considered omnivores, feeding on a wide range of items. There is some overlap in diet with both species feeding on some of the same classes of zooplankton and macroinvertebrates such as copepods and chironomid larvae. Filamentous algae has also been recorded in both species' diet. While there is some potential for overlap in diet, it is not known whether food is a limiting resource. Since the diet of carp consists predominantly of benthic invertebrates, it seems unlikely that the species directly predates on silver perch. Dick (1997) notes that there have been no reports of carp eating young fish or fish eggs. There have been no specific records of silver perch being taken by carp.

While silver perch can occur in areas which have high turbidities, the species' tolerance to turbid water is not known. Increased turbidities may result in increased sedimentation. Silver perch eggs are semi buoyant, pelagic and hatch rapidly. Sedimentation may be of

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Silver perch – A Resource Document concern if eggs settle out in still water habitats such as in weir pools. Larvae may also sink in still water habitats and sedimentation may also be detrimental to them until they are free swimming. The reduction in aquatic vegetation which can result from increased turbidity and sedimentation may be detrimental to silver perch since aquatic vegetation and filamentous algae are components of the species' diet. It is not known whether silver perch digest algae either partially or completely since much algae appears to be reasonably intact in the intestine and as it leaves the anus. Silver perch possibly only digest fauna which live on the algae (S. Rowland, NSW Fisheries, pers. comm. 1998). Whether aquatic vegetation provides shelter for larval and juvenile silver perch is also unknown. Since larvae do not appear to seek shelter in shade and rarely use cover (Gehrke 1990, Thurstan 1991), aquatic vegetation may not be an important habitat component for this life history stage. Habitat requirements of juveniles is not well understood.

The tolerance of silver perch to increased nutrient loading and algal concentrations in the water column has not been documented. Algae can apparently make up a significant component of the species' diet and this can increase as fish grow. Whether the algae recorded within the diet is consistent with the types of algae recorded in blooms requires investigation.

The issue of diseases and parasites which carp may have introduced into the aquatic environment is discussed in section 12 Diseases.

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7.2 Redfin Redfin were introduced into Australia over a century ago and occur across much of the Murray Darling Basin. They are however absent from rivers in northern New South Wales and Queensland since the species has an upper thermal limit of 30-31oC.

The impact of redfin on silver perch is unknown. There is no direct evidence to indicate that redfin have caused a significant decline in silver perch in areas of the Murray Darling Basin where they occur together. In addition, silver perch have declined in some parts of the Murray Darling Basin such as Queensland where redfin are not present. Redfin are mainly found in slow flowing areas and prefer areas with abundant aquatic vegetation. While the habitat preferences of silver perch are poorly understood, they occur in a wide range of habitats and so it is likely that both species' habitats overlap to some extent. It is not known whether redfin and silver perch compete for resources such as food although it is possible since there is a degree of dietary overlap with both species feed on a range of items such as zooplankton and insect larvae.

While it is not known whether redfin prey on juveniles, larvae or eggs of silver perch, it is a distinct possibility. The reduced survival of juvenile silver perch in an impoundment where redfin were present (Harris et al. unpublished data in Faragher and Lintermans 1997) suggests that predation in confined situations may influence population numbers of silver perch.

7.2.1 Background Redfin was first introduced into Australia in the 1860s, and is now widespread in eastern Australia in central and southern New South Wales, Victoria, South Australia and Tasmania (McDowall 1996). The species is primarily found in cooler waters and its upper thermal limit of 30-31oC (Weatherley 1963) prevents it from spreading effectively through much of the Darling River in northern New South Wales. The species distributional limit therefore reflects an environmental gradient (Gehrke et al. 1995). Although introduced into Queensland in the late 1800s, the species has never thrived there (Grant 1987). Moffatt (DNR, pers. comm. 1999) is not aware of any existing populations in Queensland, although they may occur occasionally in parts of the Macintyre River system. Floods and releases of fish have facilitated the species' spread, while its sedentary behaviour may limit its dispersal (Cadwallader and Backhouse 1983). Redfin are generally found in low altitude waters, and Cadwallader and Backhouse (1983) observe they are absent from highland streams with steep gradients and high water velocities. The recent New South Wales Rivers Survey recorded lower numbers from montane sites (Faragher and Lintermans 1997). Redfin are mainly found in slow flowing waters and prefer areas with abundant aquatic vegetation or other cover such as woody debris and rocks. Low temperatures of 11-12oC are required for spawning (Cadwallader and Backhouse 1983) and eggs are laid in ribbons often in weed beds. Redfin are primarily carnivorous, feeding on zooplankton, crustaceans, insect larvae and fish (Cadwallader and Backhouse 1983). In a Western Australian river, Pen and Potter (1992) Freshwater Ecology, NRE & Murray Darling Basin Commission 65

Silver perch – A Resource Document found that their diet changed with age: fish <50 mm fed primarily on planktonic crustaceans, those between 50-120 mm switched to benthic invertebrates, while those >120 mm fed on decapods and fish.

It is difficult to determine exactly how redfin have fluctuated in distribution and abundance over time in the Murray Darling Basin due to the lack of detailed records. Commercial catch data provides some indications, although they must be treated with caution due to likely variations in effort. Catches of redfin can also be very seasonal (A. Baxter, DNRE, pers. comm. 1999). The New South Wales commercial catch data from 1949 onwards indicates fluctuations in catch with peaks in the early 1950s and mid 1970s, with a clear decline from the late 1970s onwards. The recent New South Wales Rivers survey recorded redfin as the forth most abundant of six alien species, with at average percentage of total catch of 2.8% for sites in the Darling region and 5.7% of total catch for sites in the Murray region (Faragher and Lintermans 1997). Commercial catch data from South Australia from between 1976/77 and 1990/91 also indicates fluctuations in catch, with relatively low catches through the 1970s and early 1980s, a significant increase from 1983 onwards with a peak in 1988/90 followed by a noticeable decline. In Victoria, redfin used to form a substantial part of commercial fishing operations (Cadwallader 1985). Baxter (DNRE, pers. comm. 1999) notes that there was an apparent surge in numbers in Victoria in the 1950s and 1960s when a number of new impoundments were built, although numbers have now declined and tend to fluctuate between years. While there is commercial catch data from 1978 onwards, there are concerns the accuracy of some data due to incorrect returns. However, available data again shows fluctuations in catch, with higher catches between 1978/77 to 1985/86 including a peak in 1982/83, then a clear decline from 1986/87 onwards.

It is suspected that the decline of redfin is to some degree due to its susceptibility to the EHN virus; a number of fish kills have been recorded in areas of Victoria and New South Wales. The EHN virus is discussed in section 12 Diseases. It is possible that carp are detrimental to redfin, in terms of loss of aquatic vegetation in which they tend to spawn and possibly feeding on the ribbons of eggs within this vegetation (A. Baxter, DNRE, pers. comm. 1999).

7.2.2 Management There is no specific overall management program to control redfin within the Murray Darling Basin. Some restocking programs for native fish species may undertake limited control programs.

7.2.2 Assessment of Significance of the Threat Information on the impact of redfin on other species is largely anecdotal and speculative (Faragher and Lintermans 1997). Cadwallader and Backhouse (1983) argue that since the species is predatory and can be a profilic breeder, it is generally considered to be detrimental to native species. They suggest redfin compete for food and space with Murray cod and golden perch and may have been involved in the decline of macquarie perch at Lake Eildon. However redfin can be prey for species such as Murray cod and the

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Silver perch – A Resource Document exact relationships between redfin and larger native species are unknown (Cadwallader and Backhouse 1983). Roughly (1951) observed that following the release or escape of redfin into the Murray River system, "… they have increased enormously and, in competition with the Murray cod and other indigenous species, they have done serious damage to the freshwater fishery”. He also suggested redfin was in direct competition for food with Murray cod, golden perch and silver perch, and that it was possible that redfin ate Murray cod eggs. Cadwallader (1978) observed redfin had a similar diet to larger native species such as Murray cod and golden perch. Cadwallader (1978) compared the proportions of catches of redfin and ‘Murray fish’ between 1919 and 1949 from the Kerang Lakes area, observing that when large numbers of redfin were caught, the ‘Murray fish’ numbers were low. However, he emphasised the need to interpret this information with caution.

Redfin can establish large populations of stunted fish in small waterbodies (Cadwallader and Backhouse 1983). It has been suggested that smaller native species such as pygmy perch, rainbowfish and western carp gudgeons could suffer significant levels of predation particularly in small enclosed water bodies (Cadwallader and Backhouse 1983). There is some evidence of redfin eating smaller native species such as Edelia vittata and Bostockia porosa in Western Australia (Pen and Potter 1992), western carp gudgeon and gambusia Gambusia holbrooki (Faragher and Lintermans 1997) and southern pygmy perch Nannoperca variegata (S. Saddlier, DNRE, pers. comm. 1998). Pen and Potter (1992) argued that if major food sources of redfin declined in particular river systems, redfin could potentially then focus on certain fish species which could then be eliminated from such areas. It has been suggested that the absence of Edelia vittata from parts of the Murray River in Western Australia could be due to the spread of redfin into this area (Hutchinson 1991). There have also been observations of redfin feeding on recently released trout fingerlings (Moy 1974, Baxter et al. 1985).

Impact of redfin on silver perch The impact of redfin on silver perch is unknown. There is no direct evidence to indicate that redfin have caused a significant decline in silver perch in areas of the Murray Darling Basin where they occur together. In addition, silver perch have declined in some parts of the Murray Darling Basin such as Queensland where redfin are not present. Redfin are mainly found in slow flowing areas and prefer areas with abundant aquatic vegetation. While the habitat preferences of silver perch are poorly understood, they occur in a wide range of habitats and so it is likely that both species habitats overlap to some extent. It is not known whether redfin and silver perch compete for resources such as food although it is possible since there is a degree of dietary overlap with both species feed on a range of items such as zooplankton and insect larvae. The significance of such a dietary overlap would depend on whether food is a limiting resource, an issue which has not been investigated in detail within the Murray Darling Basin.

While it is not known whether redfin prey on juveniles, larvae or eggs of silver perch, it is a distinct possibility as redfin are a highly piscivorous species. The reduced survival of juvenile silver perch in an impoundment where redfin were present (Harris et al.

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Silver perch – A Resource Document unpublished data in Faragher and Lintermans 1997) suggests that predation in confined situations may influence population numbers of silver perch. Faragher and Lintermans (1997) note that where threatened fish species are being stocked, the presence of large numbers of redfin may play a substantial part in the ultimate success or failure of a reintroduction.

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7.3 Other Introduced Fish Species

Gambusia Gambusia is widespread and common throughout the Murray Darling Basin. The species appears to have first been introduced into Australia for use in aquaria and was subsequently released into the wild to control mosquitos (McDowall 1996). Gambusia, which can reproduce rapidly, is often abundant in warm and slow flowing waters along margins and edges of aquatic vegetation. It appears relatively tolerant to a range of temperatures and salinities (McDowall 1996). It has been suggested that gambusia represent a threat to other small fish species (McDowall 1996). Cadwallader and Backhouse (1983) state gambusia eat fish eggs and young fish and attack larger fish by nipping their fins. Ivantsoff and Aarn (1999) note that while it has been suggested that gambusia prey on eggs and larvae of rainbowfish (family Melanotaeniids), they were not aware of any evidence of predation of fish other than cannibalised juveniles.

It is not known whether gambusia represent a significant threat to silver perch and it seems unlikely that they have played a key role in the species' decline. However, predation of juveniles, larvae and eggs is a possibility in areas where redfin occur in high numbers. Silver perch have semi buoyant, pelagic eggs which could potentially be eaten by gambusia, particularly if they occur along the margins and edges of aquatic vegetation and shallow areas. Larvae may also be susceptible to predation if they were to occur in such habitats.

Trout Brown trout and rainbow trout Oncorhynchus mykiss have restricted distributions within the Murray Darling Basin. They are primarily restricted to cooler waters, primarily in highland areas in northern New South Wales and southern Victoria (Davies and McDowall 1996). The rainbow trout tends to be more restricted in distribution and abundance. These species have been implicated in the decline of a number of species in Australia, primarily because of predation (Cadwallader 1996). However it is very unlikely they have played an important role in the decline of silver perch since their distributions do not overlap to a significant degree.

Tench Tench Tinca tinca occur predominantly in the Murray River and Murrumbidgee rivers. While the species was once a commercially fished species in the Murray River, it apparently declined with increases in carp in the 1970s (Brumley 1996). The impact of tench on native species in Australia has not been investigated in detail. It seems unlikely to have played a significant role in the decline of silver perch since its distribution only represents a small portion of that occupied by silver perch.

Goldfish Goldfish is widespread throughout much of the Murray Darling Basin and commonly occurs in still and sluggish waters (Brumley 1996). While this species occurs in large

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Silver perch – A Resource Document numbers in many areas, as is the case with carp, it is generally believed that it is of less concern to the aquatic environment. There have been no specific studies on the impact of goldfish on the aquatic environment in Australia and its effect on silver perch is unknown but is not considered to be substantial.

Weatherloach Weatherloach Misgurnus anguillicaudatus, an aquarium species, has been recorded at a number of sites within the Murray Darling Basin since 1984. It is found in slack water with sand or mud substrates where it burrows for protection and to aestivate (Lintermans and Burchmore 1996). Given the difference in habit and habitats, it is very unlikely that weatherloach has played any role in the decline of silver perch in the past. However, its impact in the future if it spreads widely throughout the Murray Darling Basin is unknown.

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8 Water Quality This section on water quality discusses sedimentation, salinity, algal blooms and agricultural chemicals in relation to their possible role in the decline of silver perch.

8.1 Sedimentation The presence of sediments is a natural component of rivers and many rivers within the Murray Darling Basin are naturally turbid. However, human activities have increased sediment levels in many rivers. Since much material comes from diffuse land runoff from sources such as agriculture, this is a widespread issue throughout the Murray Darling Basin. Increased sediment levels may potentially be detrimental to silver perch when in suspension or when deposited, although how the species is affected is not well understood. Silver perch has been recorded in a range of habitats, including areas with high turbidities.

Silver perch produce large numbers of pelagic eggs which are semi buoyant. Eggs have been recorded drifting in the water column, although they may sink in areas where there is no water movement. Thus deposited sediments may be detrimental to eggs and larvae of silver perch in still water habitats such as backwaters, floodplains and weir pools. The majority of sediment is transported during high flow events. If these events and subsequent settling out of material occurs when silver perch spawn and eggs and larvae settle in still waters, reproductive success may be reduced. Deposited sediment may reduce gas exchange and inhibit development of eggs, larvae and juveniles. High mortalities of silt-covered eggs of other species of freshwater fish have been recorded (Koehn, DNRE, unpublished data).

It is not known whether increased sediment levels interfere with the respiration, feeding efficiency, growth, behaviour, food availability, reproduction or survival of silver perch. It is also not known whether the species displays avoidance behaviour to areas with high sediment levels.

Algae appears to make some component of the diet of silver perch, with observations that the proportion of algae and other plant material increasing as fish grow. It is not clear whether silver perch digest algae either partially or completely. Sedimentation may be detrimental to silver perch by affecting the abundance of phytoplankton and aquatic macrophytes. It may also affect the abundance of other components of silver perch's diet such as zooplankton and insects which can be associated with aquatic macrophytes.

8.1.1 Background The presence of sediments, derived from the bed and banks, is part of the normal geological process in rivers and streams. Sediments can also come from external sources such as runoff from surrounding land. Sediments which occur in aquatic environments can be separated into three phases: suspended (particles in the water column), deposited (particles lying on the streambed) and hyporheic (particles within the matrix of the stream bottom) (Metzeling et al. 1995). Particles may move between these phases depending on Freshwater Ecology, NRE & Murray Darling Basin Commission 71

Silver perch – A Resource Document changing physical characteristics in streams. Sediments within a stream will move down the system in cycles of suspension and deposition until stopped by a barrier and eventually reaching the sea or estuary (Metzeling et al. 1995). Flushes of high concentrations of sediment can occur following severe events such as floods. Research indicates that up to 70-90% of material is transported during high flow events (ANZECC and ARMCANZ draft 1999). Even after sediment inputs return to normal, redistribution and transport of deposited sediment within a stream can continue for years causing continued disturbance (Campbell and Doeg 1989). Fine sediments are able to penetrate deep into spaces within the streambed and their removal may require major floods sufficient to disturb the streambed (Davey et al. 1987).

Turbidity is a measure of water clarity and reflects the presence of suspended and colloidal matter including silt, clay, finely divided organic and inorganic matter, plankton and other microscopic organisms. Turbidity is strongly influenced by river flows and surface runoff. While most Australian inland waters are turbid and may have been this way prior to European settlement, there seems little doubt that most waterbodies have experienced increasing turbidity following extensive clearing (ANZECC and ARMCANZ draft 1999). Human activities including land clearance, grazing, cropping, forestry, dam construction, river regulation, weir desilting, mining and gravel extraction, roading, sewage outfall and industrial wastes etc have increased the amount of sediment within the aquatic environment. The majority of suspended particulate matter comes from diffuse land runoff due to soil erosion with some from point sources (ANZECC and ARMCANZ draft 1999). Wood and Armitage (1997) note that the impact of sediment associated with agriculture is typically more significant compared to forestry. Bank erosion can be a feature of rivers particularly in areas associated with large water diversions; this erosion may result in loss of in-channel features. The nature of the river and surrounding environment strongly influences the volume of sediment transported to a river, the degree of sedimentation and its impact on the aquatic community (Wood and Armitage 1997). Weirs have affected sediment transport within rivers, with erosion occurring below weirs and being transported downstream to behind the next weir (Walker et al. 1992).

The relationship between turbidity and suspended solid concentrations are site specific (Gippel 1989). The OCE (1988) provided an index rating scale with <30 mg/L considered excellent and >60 mg/L as degraded. ANZECC and ARMCANZ (draft 1999) suggest interim trigger levels for assessing possible risks of adverse effects: these are 10 NTU or 6 mg/L suspended particulate matter for lowland rivers. Turbidity levels can vary significantly between areas and over time. The Murray River is comparatively clear while the Darling River has a high level of suspended fine clay (Walker et al. 1992). Turbidities in the Darling River during a drought were about 25 NTU although increased twenty-fold during flooding (Shafron et al. 1990). The Darling River has fine clay particles which remain in suspension. In the Murray River, there is a marked increase in turbidity levels with distance downstream, increasing rapidly to over 50 NTU below the confluence with the Darling River (http://www.mdbc.gov.au). Turbidities increased from <3 NTU in headwaters of the Murray River to about 70 NTU in the lower reaches. Levels often

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Silver perch – A Resource Document exceed 100 NTU in South Australia for periods of several months. There was a fall when the Murray River enters to weir pool at Euston possibly because of differences in suspended sediment types with coarse sands of the mallee plains compared to fine clays in the riverine plains.

8.1.2 Management There are a range of management options which can reduce sediment input into rivers. These include sediment traps, stabilising river banks, introducing instream devices such as groins, fencing off riparian vegetation, retaining buffer strips and revegetating riparian zones, improved agricultural practices to minimise soil erosion and runoff, improved roading and sealing of roads, alternative methods of weir cleaning, retaining snags in rivers etc. Numerous water quality monitoring programs are undertaken throughout the Murray Darling Basin which also enable identification of point sources of sediment.

8.1.3 Assessment of Significance of the Threat Suspended material may affect the aquatic environment when in suspension or when it settles out onto the riverbed. The impact of increased sediment levels will depend on the sources, types and concentrations of sediment, patterns of transport and deposition and the length of time sediment levels are elevated. Wood and Armitage (1997) note that it is difficult to determine the exact impact of diffuse sources of sediment such as agriculture, since there is often a lack of data on natural baseline conditions. What natural levels of sediment are and how they change can be significant when considering impacts.

In suspension, material reduces light penetration and so affects primary production causing decreased photosynthesis and growth rates of phytoplankton and aquatic macrophytes. Suspended material can cause scouring of algae from stream beds thereby reducing biomass. This material can also absorb nutrients and toxic compounds.

Once the material settles out it can affect benthic organisms and their habitats. Sedimentation can result in a much more uniform habitat, covering material such as woody debris and filling interstices, scour holes and pools. Campbell and Doeg (1989) consider sediment which settles on or penetrates into the streambed to be of more concern than suspended sediment and noted it could lead to long term deleterious changes in fish and invertebrate populations. Severe increases in sediment may smother an entire riverbed and alter channel morphology. It can lead to reductions in species diversity, reduction in biomass and changes in species composition. Macrophytes may be affected by smothering of deposited material or by reducing sites for attachment. Wood and Armitage (1997) note there have been few studies on effects on macrophytes.

Wood and Armitage (1997) summarise the possible effects of increased sediment in rivers on invertebrates: • Substrate composition may be altered which can change its suitability as a surface for particular taxa. • Sediment deposition or substrate instability can lead to increased invertebrate drift • Deposition of silt on respiratory structures can affect respiration Freshwater Ecology, NRE & Murray Darling Basin Commission 73

Silver perch – A Resource Document • Filter feeding may be reduced because of the increased suspended sediment concentrations, food value of periphyton may be reduced and density of prey items may be reduced.

Benthic invertebrate communities can recover from short term events of increased particulate matter, however continuous inputs of high levels of sediment can change faunal assemblages. Doeg and Milledge (1991) found that addition of suspended sediment at mean concentrations of 133 mg/L (compared to a control of 20 mg/L) resulted in a seven-fold increase in the total number of invertebrates drifting. They observed that taxa can vary in their sensitivity to suspended sediment with most of those drifting being those which prefer undisturbed cool clean well oxygenated water. Drift is a primary response of invertebrates to the onset of stressful conditions (Campbell and Doeg 1989). Blyth et al. (1984) observed that increases in sediment caused reductions in some species such as those which graze open clean surfaces while others such as oligochaetes increased in numbers. Suspended sediment would be expected to have a greater effect on filter feeders by clogging feeding apparatus (Campbell and Doeg 1989). Smothering of streambeds may also influence the decomposition and availability of detrital material with subsequent impacts on food availability.

Fish Metzeling et al. (1995) noted that sedimentation has been identified as a major cause of loss of fish habitat and breeding grounds. While it has yet to be directly proven as a cause for the decline in range and abundance of many fish species, there are legitimate concerns considering the behaviour of sediments and the life histories of many fish species (Metzeling et al. 1995).

The range of possible impacts of increased sediment levels on fish include: • Reduction in oxygen uptake by coating and clogging of gills and mechanical and abrasive damage. This could cause death if levels are high. • Reduction of suitable habitat for shelter, spawning and rearing. Fish may lay eggs between particles or need silt-free surfaces for attachment of adhesive eggs. Spaces in between particles may be important rearing habitat areas for juveniles and small fish. Habitat variability may be reduced including smothering of woody debris, filling in of pools and scour holes. • Hindered development and survival of fish eggs, larvae and juveniles which are generally considered more susceptible than adults. Coating may reduce gas exchange and development. • Alteration of diet by changes in populations of prey species. There may be reductions in abundance of food including macrophytes, algae and benthic invertebrates. • Reduction in feeding efficiency particularly for visual feeders. • Reduction in growth rates. • Altered behaviour including avoidance behaviour. • Increased stress and incidence of disease • Modification of natural migration patterns

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Silver perch – A Resource Document Most research undertaken on the effects of increased sediments in aquatic environments has focused on northern hemisphere salmonids. Metzeling et al. (1995) note that there is no reason that Australian species would not display similar effects although at what levels effects would occur is not known. ANZECC and ARMCANZ (draft 1999) note that it is difficult to extrapolate overseas data since numerous factors are involved (e.g. type and natural levels of suspended material, dissolved oxygen concentrations, water temperatures).

There have been a few relevant studies in Australia and New Zealand. In New Zealand, increased water turbidity caused reduced feeding rates and avoidance behaviour in the banded kakapo Galaxias fasciatus. In New South Wales, increased mortalities of freshwater blackfish Gadopsis marmoratus were recorded after sediment increases attributed to logging activities. ANZECC and ARMCANZ (draft 1999) note that it is difficult to separate the effects of other activities in such results.

In Victoria, increased suspended sediments occurred in Armstrong Creek following the draining of a weir before desilting. Doeg and Koehn (1994) recorded reductions in freshwater blackfish numbers at sites downstream with the greatest impact closest to the weir. The suspended sediment concentration of 4610 mg/L was above the level found to be lethal to adults of many species. Subsequent flushes of sediment were probably responsible for further reductions. Doeg and Koehn (1994) suggested recovery would take a long time and would depend on recruitment. They considered it unlikely that the population would return to the same levels since the species had short home range and did not undertake large scale movements. Impacts appeared greater for younger year classes which corresponds to other studies on Australian freshwater fish species which have shown juveniles are generally more susceptible than adults. Freshwater blackfish require clean sites for deposition of their adhesive eggs and are particularly susceptible to smothering by sediment (Metzeling et al. 1995).

Campbell and Doeg (1989) note that adult fish can survive quite high levels of suspended inorganic sediment although these high levels can interfere with behaviour. Doeg and Koehn (1994) indicate that avoidance behaviour has been observed for some species at relatively low levels of suspended sediment of <500 mg/L. There are also reports of dead and dying freshwater blackfish, short-headed lamprey ammocoetes Mordacia mordax and Australian smelt in the Thomson River due to high suspended solid levels: concentrations of 190-200 mg/L and up to 2000 mg/L had been recorded (Tunbridge in Doeg and Koehn 1994). Significant mortalities of some species including common galaxias Galaxias maculatus have been recorded at levels of <3500 mg/L (Koehn and O’Connor unpubl. data). In laboratory trials, there was a 100% mortality of fertilised eggs of broad- finned galaxias Galaxias brevipinnis and spotted galaxias G. truttaceus when covered with silt while those not covered with silt all survived (Koehn and O’Connor, unpubl. data). Higher mortalities of the adhesive eggs of macquarie perch were recorded in laboratory trials when they were covered with silt (Koehn and O’Connor, unpubl. data). The decline of macquarie perch has partly been blamed on sediment since this species lodges its eggs among stones and gravel (Metzeling et al. 1995).

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Impact of sedimentation on silver perch The significance of sedimentation as a threat to silver perch is not known. Silver perch occur in a wide range of habitats, including turbid waters. In aquaculture ponds, Rowland (1995) observed that unless turbidity is extreme (>20 000 mg/L), it is not harmful to fish and has little influence on production in ponds.

Silver perch produce large numbers of pelagic eggs which are semi buoyant and have been recorded drifting in the water column. Eggs may sink if there is no water movement. Thus deposited sediments may be detrimental to eggs and larvae of silver perch in still water habitats such as backwaters, floodplains and weir pools, possibly affecting affecting gas exchange and development.

Algae appears to make some component of the diet of silver perch, with observations that the proportion of algae and other plant material increasing as fish grow. Sedimentation may be detrimental to silver perch by affecting the abundance of phytoplankton and aquatic macrophytes. It may also affect the abundance of other components of silver perch's diet such as zooplankton and insects which can be associated with aquatic macrophytes.

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8.2 Salinity While the role increased salinity levels have played in the decline of silver perch is unknown, it is possible not to have been significant in a basin-wide context in the past. A reasonable amount of research has been undertaken on the salinity tolerance of silver perch eggs, larvae and juveniles. This research has included direct and slow acclimitization trials. The research indicate that early life history stages are most sensitive to high salinity levels, which is a general trend observed in many fish species. Silver perch appears quite tolerant to high salinity levels, and may be more sensitive to other water quality factors such as low dissolved oxygen. The reasonable tolerance of silver perch to high salinity levels may reflect the species having evolved in an environment which experienced naturally high salinity levels. Prior to European settlement and land clearance, salinity levels in rivers within the Murray Darling Basin may have experienced large fluctuations in salinity; especially during periods of very low flow when pools with higher salinity levels formed.

Whether silver perch experiences sublethal responses to salinity has not been well studied and requires investigation. Whether high salinity levels have other detrimental indirect impacts of silver perch is also unknown. High salinity levels may affect food sources such as invertebrates, algae and macrophytes, as well as habitat complexity and quality of instream habitat. High salinity levels may also cause fish to experience stress which may make them more susceptible to infections.

Salinity has been recognised as a major problem in the Murray Darling Basin for some time. Although salinity levels vary widely between areas, the Salinity Audit (MDBC 1999a) indicates that levels are likely to increase significantly in many tributaries in the future becoming a far more significant environmental problem.

8.2.1 Background Rising salinity levels in rivers and terrestrial habitats has been recognised as a problem since European settlement in many areas of Australia. Salinity has been considered one of the major issues confronting the Murray Darling Basin. MDBC (1999) noted that rising water tables and soil salinity occurred in irrigation areas before dryland areas. In dryland areas, watertables have risen because of land clearance and establishment of agricultural crops bringing saline water to the surface and into rivers and streams. Water usage for irrigation also adds to the salinity problem since it also causes rising watertables and reduces dilution flows. Smith (1999) indicated that there is general agreement that approximately a third of salt load currently transported by the lower Murray River is due to habitat clearance and agriculture. Salt enters rivers by irrigation channels, overland flow and intrusions and seepage from groundwater sources (Ryan and Davies 1996). Increased salt load comes largely from the addition of saline groundwater to river systems rather than through direct surface runoff. Groundwater discharge represents cumulative impact of both dryland and irrigation salinity. Salinity problems are more likely in wetland areas where there is a terminal basin rather than a flow through system which allows for flushing of saline water. Freshwater Ecology, NRE & Murray Darling Basin Commission 77

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The creation of locks and weirs in the system over the last 100 years has resulted in an increase in the mean salinity but a decrease in the range of salinities (Williams and Williams 1991). Walker (1985) notes that about a quarter of the salt which enters rivers within South Australia is contributed by weirs, with saline groundwater downstream being forced into the river by the hydraulic pressure of impounded water.

8.2.2 Management The Salinity and Drainage Strategy prepared in the late 1980s provided a framework for the coordinated management of salinity in the Murray River and land salinisation and waterlogging in Murray Darling Basin particularly in the Murray and Murrumbidgee rivers. A specific objective was to reduce the salinity in the Murray River at Morgan. MDBC (1999) notes that while there has been a decrease in salinity levels since the strategy period, there has been a long term overall increase in average river salinities. Improvements to salinity levels have been made by the construction of salt interception schemes, the implementation of salinity and land and water management plans by states and adoption of best practices in irrigated agriculture. The Salinity and Drainage Strategy is currently being reviewed.

A salinity audit was undertaken to determine the current and future threats of salinity within the Murray Darling Basin (MDBC 1999a). Salinity levels vary widely between areas. Key points from the audit are that: • trends in future salt loads more severe than previously thought. • salt mobilisation is occurring in all major river valleys and will double in the next 100 years. • average river salinities will rise significantly and will be above desirable thresholds for domestic and irrigation water supplies in many tributaries and exceed critical levels in some areas. • our ability to estimate land areas affected by future salinity is inadequate and our current understanding of environmental impacts is inadequate. • future salt loads will shift from irrigation induced sources to dryland catchment sources.

The Salinity Audit (MDBC 1999a) noted that while it was previously recognised that river salinities were a problem in the lower Murray River, it has now been found that levels are rising in tributaries of the Murray Darling system. The report indicates that dryland areas particularly the Mallee in Victoria and South Australia will be a dominant source of salt in the future. Also there will be significant rises in river salinity in the Macquarie, Namoi, Lachlan, Castlereagh and Bogan rivers in New South Wales and the Condamine-Balonne, Border and Warrego rivers in Queensland. High salinities already occur in the Loddon and Avoca rivers in Victoria (MDBC 1999a). By 2020, salinities of over 800 EC are estimated by the Avoca and Loddon rivers in Victoria, the Bogan, Macquarie, Namoi rivers in New South Wales and the Warrego, Condamine-Balonne and Border rivers in Queensland.

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Silver perch – A Resource Document 8.2.3 Assessment of Significance of the Threat Williams (1987) notes that little is known of the extent to which aquatic community structure and function is affected by salinity. It is generally accepted that large increases in salinity are detrimental to the aquatic environment. However, the nature of the impact of small rises in salinity and the maximum amount that salinity can rise without affecting ecological processes are uncertain. It may be difficult to distinguish the effects of salinity from other environmental impacts such as river regulation, changes to aquatic communities may be subtle and occur gradually and before and after comparisons may not be possible (Williams 1987). In general, in wetlands and streams increased salinity is associated with a decrease in species richness and diversity. There is the potential for changes to community composition to occur, with the loss of salt sensitive species which are replaced with fewer salt tolerant species. The trend of declining species diversity and richness has been documented for invertebrates, algae, fish and aquatic macrophytes. Walker (1985) noted that there is no strong evidence to indicate that aquatic flora and fauna are affected by saline water in South Australia, other than in floodplain areas where saline water is impounded and evaporates.

A figure of 800 EC (480 mg/L) is considered the upper salinity limit for drinking water desirability (MDBC 1999a). MDBC (1999) uses 1500 EC (900 mg/L) as a measure where consumptive use and irrigation use of water is restricted and where direct adverse biological effects are likely to occur in river, stream and wetland ecosystems. Once levels reach above 5000 EC (3000 mg/L) water changes from fresh to saline, with ecosystems changing in species composition.

Research indicates that the physical salinity tolerance of adult freshwater fish is greater than that of freshwater macroinvertebrates, algae and macrophytes (Hart et al. 1991, Ryan and Davies 1996). Hart et al. (1991) note that Australian fish species are quite tolerant to moderate salinities up to 10 000 mg/L. Ryan and Davies (1996) indicate that most adult freshwater fish can tolerate salinities of up to 7000 to 13 000 mg/L. Anderson (1991) suggested a maximum recommended level of 7 500 mg/L since eggs, larvae and juveniles can be less tolerant to salt. It is important to test the salinity tolerance of the entire life cycle of a species and results on adult fish only can be misleading (Williams and Williams 1991). Williams and Williams (1991) tested several Murray Darling fish species which were quite tolerant to salinity. They suggested this was either due to inheriting this tolerance from their marine or estuarine ancestors, or the species or their recent ancestors having experienced high salinities in their environments in the recent past. Prior to European settlement and the significant changes, the Murray Darling Basin probably experienced greater fluctuations in salinity; during periods of very low flow pools with higher salinity levels formed.

There may be considerable variation in the salinity tolerance within a taxon depending on environmental conditions and local genetic differences. The results of toxicity tests (e.g. lethal concentrations required to kill 50% of individuals (LC50) and no observable effect limit (NOEL)) may be difficult to compare directly to the situation in the field. In some cases, it may a species’ tolerance to salinity as well as numerous other environmental

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Silver perch – A Resource Document conditions which influence its presence in an area. Fish can be affected both directly and indirectly by salinity. For example, increased salinity can affect their food source of invertebrates, algae and macrophytes, as well as habitat complexity and quality of instream habitat (e.g. shelter, breeding sites). Degradation of riparian habitat due to salinity could also affect instream habitat, by contributing to erosion by decline in soil structure and instability of stream banks.

The tolerance of fish species to water quality conditions may vary between areas and so tolerances may not be able to be universally transferable (Ryan et al. 1999). This may be the case for silver perch. In general early life history stages tend to be the most susceptible; Ryan and Davies (1996) indicated that fry are the most sensitive stage for fish.

Investigating the entire salinity profile can be valuable, particularly in deep pools. McGuckin (1991) notes that when saline water enters rivers, it may result in the development and persistence of saline pools. Characteristics such as stream morphology, hydrology and flow patterns can ultimately influence the extent and duration of stratification and deoxygenation (McGuckin et al. 1991). Stable saline pools have been recorded in a number of river systems, including the Wimmera River. The stratification and associated low levels of dissolved oxygen that can develop in saline pools can be a more significant threat to the instream environment that the actual salinity. The susceptibility of fish species to low dissolved oxygen levels can vary significantly. For example, some salmonid species (eg. trout) can have a relatively high tolerance to salinity, but low tolerance to low dissolved oxygen levels (S. Saddlier, DNRE, pers. comm. 1998). Where increases in salinity are associated with changes to other water quality parameters the effects can be quite dramatic even at small increases in salinity (Kefford and Robley 1996).

Impact of increased salinity levels on silver perch A reasonable amount of research has been undertaken on the salinity of silver perch, including eggs, larvae and juveniles. Guo et al. (1993) investigated the effect of increasing salinity and time of first exposure to salinity on silver perch eggs and larvae. The time of transfer after fertilisation influenced the hatching rate of eggs at different salinities. A high mortality was recorded early in development; this declined greatly 16 hours after fertilisation. Egg hatching rate could be determined by observing embryos; since embryos which exhibited tail twitching movements all hatched successfully (Guo et al. 1993). However, Rowland (NSW Fisheries, pers. comm. 1998) notes that this is not always the case. When eggs were transferred to saline water in early stages of development they showed low hatching rates above 3 000 mg/L (3 ppt)1. At salinities higher than 6 000 mg/L eggs exhibited abnormal development and died within 8 hours; above 9 000 mg/L no eggs hatched. However, if eggs were transferred to saline water at a later stage of development (after cleavage) hatching rates improved. High hatching rates occurred at a salinity of less than 9 000 mg/L, a few between 9 000-12 000 mg/L, very few up to 15 000 mg/L, none at 18 000 mg/L. If larvae were hatched in saline water

1 Guo et al. (1993, 1995) use ppt. For consistency, these values have been converted to mg/L (TDS) Freshwater Ecology, NRE & Murray Darling Basin Commission 80

Silver perch – A Resource Document above 9 000 mg/L, they died within 4 days after hatching. Eggs transferred to highly saline water (9 000-15 000 mg/L) developed at a slower rate and took longer to hatch. O’Brien (1996) has also tested the salinity tolerance of silver perch eggs. Post hardened eggs had an EC50 (EC50 data-concentration resulting in a 50% reduction of hatching) of 11 160 mg/L (18 600 uS/cm). The eggs were placed into the saline test conditions 1 hour after fertilisation.

Guo et al. (1993) found that larvae which hatched in 6 000 mg/L saline water survived better compared to those which hatched in freshwater, and suggested that saline water may inhibit diseases, a factor which can often cause mortality of larvae. This corresponds to the use of saline water as a treatment for diseases in silver perch (Rowland and Ingram 1991, Selosse and Rowland 1990). The higher survival of eggs when they were transferred five hours or more after fertilisation compared to before may be because eggs become water hardened. Guo et al. (1993) recommended further study on the mechanisms and function of water hardening of eggs.

Guo et al. (1995) investigated the salinity tolerance and osmoregulation of juvenile silver perch. Fish were transferred directly from freshwater to a range of salinities. A mortality of 40% was recorded at a salinity of 15 000 mg/L, while at higher levels, all juvenile fish died within 18 hours. When fish were acclimatised to a salinity of 12 000 mg/L for seven days, a mortality of 100% occurred at a salinity of 18 000 mg/L within 24 hours. Stress was observed at salinities above 15 000 mg/L. Plasma osmotic concentrations in fish increased with water salinity. Silver perch have chloride cells and accessory cells in the gill epithelium which are involved in hydromineral regulation. Guo et al. (1995) concluded that in comparison with other native fish species of the Murray Darling River system, silver perch has a poor tolerance to salinities above 15 000 mg/L. They note that the species has not been recorded in natural environments of a higher salinity than 3 000 mg/L (<3 is freshwater).

Direct transfer and slow acclimation tests have been carried out on silver perch by B. Pierce (SARDI). Direct transfer tests involve transferring fish to a range of specific salinity concentrations and recording mortalities; the LC50 is recorded for a solution in which 50% of the fish are killed. Silver perch had an LC50 direct transfer tolerance of 13 700 mg/L. Slow acclimation tests involve a gradually increasing salinity, a situation which is more likely to be experienced in the field. Ryan et al (1999) noted that there are difficulties in interpreting such test results. It can be difficult to determine the specific salinity when death occurred since rates of increase used in the laboratory tests are usually higher than those experienced in the field. Also a time range in which 50% of the fish died is used since the specific time taken to kill the fish cannot be determined. Silver perch had an LC50 slow acclimation tolerance of 16 000 mg/L. Ryan et al. (1999) indicated that there is a general belief that salinity alone is not particularly hazardous to adult fish. Pierce (SARDI, pers. comm. in Ryan et al. 1999) notes that salinity rarely reaches levels directly toxic to most Victorian native fish which have evolved from marine ancestors.

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Silver perch – A Resource Document Ryan et al. (1999) carried out in situ salinity tolerance research on a number of fish species within the Murray Darling Basin. They observed that tolerances recorded in laboratory trials are usually higher than those observed in the field. Trials on silver perch were carried out in Victoria in the Wimmera River and the Campaspse River using juvenile fish (40-60 mm). Stratified saline pools were selected and cages of fish were submerged beneath the halocline for intervals of time varying from 5 to 120 minutes. Fish were checked at intervals with recooperation periods of 15 minutes in fresh, well oxygenated water. Tolerance periods were determined based on whether 50% of the fish or more failed to recover. In the Wimmera River, juvenile silver perch were able to tolerate conductivities of 23 400-26 400 mg/L (39 000-44 000 uS/cm) and dissolved oxygen of 0-2% saturation for only about 10 minutes. In the Campaspse River, juvenile silver perch tolerated conductivities of 3000-6960mg/L (5000-11 600 uS/cm) and dissolved oxygen of 5-13% saturation for a similar period of time. The principle feature affecting silver perch in the Campaspse River was likely to be the low dissolved oxygen content which juvenile fish may be sensitive to. Conditions in the Campaspse do not reach the extremes beneath the halocline experienced in the Wimmera River. In terms of dissolved oxygen, the levels in the Campaspse may have been stressful to fish while they would have been toxic in the Wimmera. Since adult fish can tolerate at least 10 000 mg/L, conditions would not have been toxic in the Campaspse River and would have been toxic in the Wimmera River. Other factors such as temperature may have influenced the results. Ryan et al.(1999) noted overall trends with the range of fish species investigated. In general, juvenile fish tended to be significantly less tolerant to saline conditions than adults, probably because of their underdeveloped osmoregulatory capabilities and lack of acclimatisation over their life history. Native species, which would be better adapted to saline conditions having evolved in the naturally saline Murray Darling Basin, were more tolerant than introduced species. Introduced species tended to have a greater dissolved oxygen tolerance.

Juvenile silver perch were the most sensitive of all species tested (Ryan et al. 1999). Since silver perch eggs and larvae can tolerate salinities of 3000 to 6000 mg/L (5000- 10 000 uS/cm) (Guo et al. 1993), Ryan et al.(1999) suggested the concentration of dissolved oxygen was the limiting factor in the Wimmera and Campaspse Rivers. Ryan et al.(1999) indicated that the silver perch may have been stressed within the cages, being a very high energy fish with no ability to escape. Such a high energy fish species would be expected to require a higher oxygen consumption; therefore in an low dissolved oxygen environment, it may be the first native species to be detrimentally affected by these conditions.

In rearing ponds, low dissolved oxygen concentrations can occur at the bottom of deep sections of the ponds. Thurstan and Rowland (1995) note that silver perch can tolerate dissolved oxygen concentrations of 2.0 mg/L in rearing ponds, although this may be close to their limit. Rowland (NSW Fisheries, pers. comm. 1998) notes that silver perch will still feed, although less aggressively, at 2.2 mg/L and that lethal levels must be somewhat lower. Silver perch can tolerate pH of 6.0 to 10.2 (Rowland 1995) and concentrations of unionised ammonia as high as 0.65 mg/L (S. Rowland, NSW Fisheries, pers. comm.

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Silver perch – A Resource Document 1998). Silver perch is considered a hardy species in relation to water quality (S. Rowland, NSW Fisheries, pers. comm. 1998).

Whether silver perch experiences sub lethal response to salinity is also unknown. Ryan and Davies (1996) indicated that the Yabbie Cherax destructor shows a high salinity tolerance but at high salinities shows stress and its response to threat decreases becoming slower and more vulnerable to predation. Ryan and Davies (1996) suggest that freshwater fish probably have a preference for salinities of less than 9 000 mg/L, and that levels above this probably cause stress.

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8.3 Algal blooms Algal blooms are a natural occurrence within inland rivers although their frequency and intensity is likely to have increased as a result of environmental degradation. While algal blooms are now a common occurrence, it is not known whether they have played a significant role in the decline of silver perch. There do not appear to be any documented cases of large numbers of silver perch dying after significant algal blooms. Whether algal blooms and associated water quality problems have had less obvious, sublethal effects on silver perch is not known. They may well cause occasional fish kills in certain areas, particularly those experiencing slow flowing or stagnant water, low winds, high concentrations of nutrients, sunlight and warm water temperatures.

Algae has been recorded as a significant component of the diet of silver perch, increasing as fish grow. Whether the algae is digested either partially or completely is not known and fish may possibly only digest fauna which live on the algae (S. Rowland, NSW Fisheries, pers. comm. 1998). Research is required to identify the range of divisions of algae consumed by silver perch, including whether the species feeds on blue green algae which can form algal blooms.

No research appears to have been undertaken on the toxicity of particular blue green algae (or the other associated water quality problems associated with blooms) to silver perch. It is not known whether different life history stages of silver perch may vary in their ability to tolerate toxins or whether silver perch are able to avoid algal blooms. Possible changes to the species' food resource during algal blooms is also not known.

8.3.1 Background Algae can provide food and shelter for aquatic organisms and can play an important role in the ability of aquatic ecosystems to absorb nutrients and heavy metals (Entwisle et al. 1997). They can also however cause water quality problems. Blue green algae are cyanobacteria, microscopic organisms which photosynthesise. The most common types within the Murray Darling Basin are Anabaena, Microcystis and Nodularia. Blooms, which are a natural occurrence, typically give water a green appearance with a thick surface scum and have a distinctive odour and taste. The frequency and intensity of blooms prior to European settlement is unknown (MDBC 1994) and it is likely that many have gone unreported and undetected in the past (Government of Victoria 1995). The earliest record in the Murray Darling Basin was at Lake Alexandrina at the mouth of the Murray River in South Australia in 1878. Blooms have now become a common occurrence in surface waters including in reservoirs, rivers and dams (MDBC 1993) and are considered a major water quality problem in Australia (Bowling and Baker 1996). There is no clear evidence that the incidence of blooms has increased, particularly since the 1970s and the increasing number of reports may be a reflection of increased awareness (http:///www.mdbc.gov.au). In recent years there have been major algal blooms along parts of the Murray River in South Australia and New South Wales, parts of Barwon-Darling and Namoi rivers in New South Wales as well as numerous impoundments (MDBC 1994). During the summer of 1989-90 there were exceptional Freshwater Ecology, NRE & Murray Darling Basin Commission 84

Silver perch – A Resource Document outbreaks of algal blooms in water storages across Murray Darling Basin. In 1992/3, 140 blooms were recorded in Queensland, New South Wales, Victoria and South Australia (Commonwealth of Australia 1993). The most dramatic example was a bloom which occurred over about 1000 km of the Barwon-Darling River in New South Wales in 1991, resulting in a state of emergency being declared.

Blooms are often considered a symptom of environmental degradation associated with river regulation, loss of riparian vegetation, excessive nutrient discharges and toxin discharges (ANZECC and ARMCANZ, draft 1999). Instream factors which affect algal growth include nutrients, light, temperature, turbidity, conductivity, salinity, carbon availability, river flow and macrophytes (MDBC 1993). A range of factors interact in the development of blooms and it is not possible to attribute occurrence to a single factor. The conditions which produce algal blooms now tend to occur more frequently; these include high concentrations of nutrients such as phosphorous and nitrogen, slow flowing or stagnant water, sunlight and warm water temperatures (MDBC 1994). Nutrients sources include sewage and industrial effluents, irrigation return flows, intensive keeping, fish farming and aquaculture activities, urban stormwater and runoff from agriculture and forestry (Harvey 1993). Phosphorous seems to be a key nutrient in causing blooms, while nitrogen levels tend to influence the quality rather than the quantity of the algal crop (MDBC 1993). Recent research indicated that in dry years, nutrients primarily came from point sources while in wet years they came primarily from diffuse sources including agricultural land, forests and as a result of erosion (MDBC 1994). Sewage was considered the most significant point source of nutrients followed by irrigation drains (MDBC 1993). Nutrients which are stored in sediments and are subsequently released into water may be an important source after external nutrient levels decline (Government of Victoria 1996). While there is a perception that carp play a significant role in algal bloom development, this requires investigation (Commonwealth of Australia 1993). High nutrient levels can also cause abundant growth the exotic water plants including Water Hyacinth Eichhornia crassipes and Alligator weed Alternanthera philoxeroides.

The 1991 bloom in the Barwon-Darling River occurred spontaneously in most parts of the river and was triggered by a long period of low flow between August and December resulting from drought conditions and water abstraction. High water temperatures, high dissolved oxygen and pH and very high phosphorous levels were also recorded (Bowling and Baker 1996). Low flowing water can allow settling out of suspended material and the subsequent improved light penetration can result in increased photosynthesis. Increased flows enable flushing of algal blooms and increased water turbulence and mixing can reduce their occurrence. Waterbodies with good growths of aquatic vegetation may play a role in limiting algal growth by using up nutrients (MDBC 1993). Riparian vegetation can also reduce the amounts of nutrients entering water from surrounding land (MDBC 1993).

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Silver perch – A Resource Document 8.3.2 Management The management of algal blooms has received much attention in recent years. In 1993, a report was prepared by the senate standing committee on environment, recreation and the arts which looked at the impact of toxic algae upon Australian waterways (Commonwealth of Australia 1993). The Murray Darling Basin developed an Algal Management Strategy (MDBC 1994) after first identifying sources of nutrient pollution in the Murray Darling Basin (MDBC 1993) and compiling background papers concerning the issue (MDBC 1993a). The strategy aims to reduce the frequency and intensity of blooms and other water quality problems related to nutrient pollution in streams and storages within the Murray Darling Basin and establishes a framework to coordinate planning and management actions. Management issues which are being addressed by the range of authorities responsible include improved flow management which is an action for the shorter term, and reducing nutrient inputs which is a more medium to long term action. MDBC (1994) indicates that the environmental streamflow requirements to minimise the risk of algal blooms are not fully understood.

State committees have been established to coordinate appropriate actions. The range of activities which are relevant to managing algal blooms include the development and implementation of catchment and nutrient management strategies, development of legislation and guidelines, provision of advice and extension services and improvement of sewage plants, improved stormwater management, protection of remnant vegetation and establishment of vegetation corridors to intercept nutrients, water quality monitoring, provision of minimum flows and short flushes, reserving water for environmental contingencies, creation of riffle zones to increase water turbulence, installation of mixers in storages to prevent thermal stratification, installation of variable offtakes and limiting drawdown in reservoirs to enhance macrophyte growth. There are a range of other strategies which have associated goals related to improving water quality (Decade of Landcare Plan, National Water Quality Management Strategy, National River Health Program National Waterwatch Program and relevant state programs).

8.3.3 Assessment of Significance of the Threat Algal blooms may or may not be toxic. Little research has been conducted into the environmental implications of algal blooms other than the obvious fish kills and wildlife losses (Commonwealth of Australia 1993). Toxic algal blooms can cause liver damage, stomach upsets, skin irritations and disorders of nervous system in humans and stock deaths have been recorded (MDBC 1994).

Little is known about the toxicity of blooms to fish species and it is not known whether phytoplanktivorous, zooplanktivorous or piscivorous fish shown different responses to blooms (Codd 1995). Codd (1995) questioned whether cyanobacterial toxins affect fish populations directly or indirectly via the food web. There is some evidence of fish avoiding particular areas and feeding in non toxic areas of a bloom (Codd 1995). Intraperitoneal injection of toxins to goldfish, rainbow trout and carp have caused death at similar doses to those recorded in rodents (Codd 1995).

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Silver perch – A Resource Document In Australia, no deaths of birds or fish have been unequivocally attributed to cyanobacterial poisoning (MDBC 1993). Bowmer et al. (1996) provides a summary of documented fish kills in New South Wales. There are a small number of cases where fish deaths were attributed to poor water quality, including suspected deoxygenation due to algal blooms and no flow. Silver perch were not amongst the species identified. Fish kills in eutrophic waterbodies at times of blooms have usually been linked to oxygen depletion due to bloom respiration at night or high microbial oxygen demand during the decay process, physical gill blockage by cyanobacterial cells and colonies, high ph and high ammonia levels during bloom senescence. These characteristics occur together and are typical of blooms (Codd 1996).

Algae has been recorded as a significant component of the diet of silver perch, increasing as fish grow. Whether the algae is digested either partially or completely is not known and fish may possibly only digest fauna which live on the algae (S. Rowland, NSW Fisheries, pers. comm. 1998). Research is required to identify the range of divisions of algae consumed by silver perch, including whether the species feeds on blue green algaes which can form algal blooms.

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8.4 Agricultural Chemicals Concern over the impact of pesticides on the aquatic environment has focussed on the use of DDT and subsequently endosulphan in the cotton industry. The cotton industry has expanded rapidly since the early 1960s. DDT was primarily used from the 1960s to about 1980, mainly in the Namoi area with endosulphan now being the most commonly used pesticide in the industry. Most cotton (70-80%) is now grown in inland New South Wales with the remainder in Queensland.

The specific role these pesticides may have played in the decline of silver perch is unknown. Since silver perch has declined over a far broader area than where cotton is grown, pesticide usage is unlikely to be the principle reason for the species' demise. However it is possible pesticide usage has contributed to the species' decline in areas where silver perch encounter contaminated water.

Water quality monitoring since the 1970s indicates that DDT and endosulphan residues have been recorded both in river water as well as in fish flesh. While there was limited monitoring of pesticide levels in silver perch, it seems reasonable to suggest that DDT residues would have been present in wild samples of this species since residues were recorded in a range of other species. Research, primarily overseas, has indicated that DDT can affect reproductive success and cause physiological and behavioural effects on aquatic organisms. However it is not known whether DDT residues caused lethal or sublethal effect on silver perch in the past. Documentation of fish kills from the past is poor and no toxicity trials for DDT have been carried out on any life history stages for silver perch.

Endosulphan dissipates quite rapidly in water, although its metabolites are more persistent when associated with soils and sediments. Some trials have been undertaken on the toxicity of endosulphan to silver perch. Of six native and introduced fish species trialed, silver perch was one of the least sensitive to the pesticide (Sunderam et al. 1992). In further trials, the toxicity of silver perch to endosulphan was found to be affected by temperature (Patra et al. 1995a). Further research is required to determine both the sublethal and lethal effects of endosulphan on all life history stages of silver perch.

There has been a concerted effort in recent years to minimise the impacts of pesticides on the riverine environment. Implementation of best management practices on farms means that it is likely that levels of pesticide residues reaching rivers will in general not be as high as in the past. However, aquatic organisms in rivers including silver perch will probably occasionally be subjected to low level concentrations of pesticides with pulses of higher levels following events such as storms. Whether agricultural chemicals other than DDT and endosulphan represent a threat to silver perch, either in the past or currently, has not been documented.

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Silver perch – A Resource Document 8.4.1 Background Agriculture, both dryland and irrigation, is a dominant land use in the Murray Darling Basin. A wide range of products are grown including crops such as wheat, barley, rice, oilseeds, cotton as well as horticultural crops, sown pastures and livestock production including sheep, cattle, pigs and poultry. While there are numerous issues associated with agriculture which may affect the aquatic environment (e.g. water usage), one of the main concerns relates to the long term effects of intensive use of pesticides. Pesticide use in the cotton industry particularly has been the focus of much attention.

In the cotton industry, chemicals are applied at all stages of cultivation, and those used are constantly changing (Whyte and Conlon 1990). Chemicals may spread from a treated area by spray drift, volatilisation, deep percolation, and runoff of tailwater or stormwater (Barrett et al. 1991). Pesticides may reach waterways in soluble form or attached to soil particles and may ultimately remain in suspension or settle out. Fairweather (1999) notes that all irrigated crops use pesticides and chemical compounds and some contaminated water tends to reach waterbodies even under the best management regimes. These chemical compounds tend to occur as mixtures and their toxicity will depend on the chemicals used and details of their exposure, the aquatic biota exposed, and the physiochemical nature of the water.

The cotton industry Cotton growing first began in the early 1960s in the Namoi valley, followed by the Gwydir and Macintyre areas in the 1970s, and the Macquarie area mainly in the 1980s. It is now expanding west along the Darling River and the Hillston area of the Lachlan river (Whyte and Conlon 1990). The area harvested in Australia has increased substantially from 35 000 ha in 1970/71 to 245 000 ha in 1989/90 (Whyte and Conlon 1990) to 289 605 ha by 1999 (http:/www.mdbc.gov.au). Between 70 and 80% of cotton is now grown in New South Wales with the remainder in Queensland.

About 40 different chemicals are currently used in the cotton industry with a heavy reliance on insecticides, herbicides, conditioners, defoliants, fertilisers and wetting agents. Most concern has focussed on the pesticides DDT and more recently endosulphan. These two pesticides are discussed below in relation to their impact on the aquatic environment.

8.4.2 Management There have been a number of very detailed reports on the cotton industry, the fate and impact of pesticides in the environment and the state of rivers in cotton growing areas (Whyte and Conlon 1990, Barrett et al. 1991, Petersen and Batley 1991, Arthington 1995, Bowmer et al. 1996).

In 1993, the Land and Water Resources Research and Development Corporation (LWRRDC) and the Cotton Research and Development Corporation (CRDC) and MDBC established a national research and development program, which aimed to minimise the impact of pesticides on the riverine environment using the cotton industry as a model. Freshwater Ecology, NRE & Murray Darling Basin Commission 89

Silver perch – A Resource Document The research program included a range of projects including on farm studies on pesticide transport, examining the effect of different farming practices and the role of transport methods in river contamination and ecotoxicology (Kennedy et al. 1998). The project aimed to assess the importance of different methods of transport, develop means of retaining the residues on farms, measure impacts on riverine systems and identify best practices.

There has been large scale monitoring of water quality by DLWC in New South Wales since 1991 in the northern inland cotton growing areas of the Namoi, Gwydir, Barwon- Darling, Macintyre and Macquarie rivers. This work has found that pesticides (particularly endosulphan) are recorded in rivers often in concentrations which exceed recommended guidelines. The lower reaches of the Macintyre, Gwydir and Namoi catchments are contaminated by pesticides to a greater degree than the upper reaches (Muschal and Cooper 1999). Fairweather (1999) noted that there has been a declining trend in concentrations of endosulphan including in the Namoi and Gwydir areas but increasing trends in the Border and Darling rivers. Fairweather (1999) notes that this program has been successful and improvements have been achieved by raising awareness, improving farm practices and because of strong public scrutiny. A Best Management Practices Manual was developed for farmers in 1997. A conference was held in 1998 by LWRRDC about minimising the impact of pesticides on the riverine environment (LWRRDC 1999); this summarised key findings of recent research. Chemical application work and the control of irrigation tailwater are considered key components in minimising the environmental effects of endosulphan (Anthony 1999).

Fairweather (1999) noted that monitoring of billabongs found that toxicity of water varies greatly over time, some pesticides were always recorded in drainage water during the irrigation season, pesticides were always picked up in mixtures and intensive and regular sampling was required to detect all pesticides.

Significant pesticide contamination in irrigation runoff water can occur after first application, with Kennedy et al. (1999) recording residues of 1-30 ug/L initially which then declined to about 2 ug/L after a month. Kennedy et al. (1999) emphasised that this water was recirculated within the farm boundary. There is always the possibility of storm events and heavy rains causing runoff into rivers which could potentially cause significant environmental contamination. Traces of insecticides, most commonly atrazine and endosulphan, are detected usually at low levels in rivers (Anthony 1999). Kennedy et al. (1999) noted the whether impacts on biota would be lessened by dilution was impossible to assess. Kennedy et al. (1999) notes that most of endosulphan applied is rapidly dissipated. However some residues remain which require careful management to prevent significant contamination away from the farm. Residues may move to nearby wetlands and rivers in large storm events. Techniques such as band spraying can reduce the extent of soil contamination.

Water quality monitoring of the Macquarie Marshes has recorded minimal levels of endosulphan (0.01-0.1 g/L) on isolated occasions in water and sediments. Water quality

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Silver perch – A Resource Document guidelines suggest levels should not exceed 0.01 g/L (DLWC 1996). The Macquarie Marshes Land and Water Management Plan (Macquarie Marshes Catchment Committee 1997) indicates that pesticide levels lower than those in Namoi, Macintyre and Gwydir valleys. Endosulphan has been the main pesticide recorded, with occasional records of atrazine and prometryn.

8.4.3 Assessment of the Significance of the Threat DDT Organochlorine pesticides have been used in the Australian agriculture industry since the 1940s. DDT was once a principle pesticide, with its use peaking in the late 1960s/early 1970s. Over half the national usage of DDT was in the Namoi area in the 1970s (State Pollution Control Commission 1980). This pesticide began to be phased out of the cotton industry in 1981. DDT use ceased following the banning of Australian meat because of contamination in 1987 (Olsen et al. 1993).

DDT is stable, has a low water solubility, high solubility in organic solvents and lipids, can attach to sediment particles and become concentrated in bottom sediments. Extensive research, primarily overseas, indicated that DDT can persist in the environment and have lethal and sublethal effects on aquatic organisms such as phytoplankton, invertebrates and fish, including affecting reproductive success (including survival and development of embryos, inducement of abortion) and physiological and behavioural effects (USEPA 1975). USEPA (1975) noted that fish kills due to DDT had been recorded and that the pesticide could cause sublethal effects in fish that meant they were less able to compete successfully in the aquatic environment. Toxicity to DDT can increase with increasing water temperature.

In Australia, a number of investigations were undertaken of pesticide contamination in cotton growing areas of New South Wales including the Namoi and Gwydir areas (State Pollution Control Commission 1980, Gilbert et al. 1990, a, Gilbert et al. 1992), as well as research on the effects of DDT on non target fish (Mowbray 1978). Symptoms of organochlorine poisoning include hyperactivity, loss of balance and uncoordinated movement. Mowbray (1978) found that carp displayed symptoms at much lower doses of DDTR and endrin compared to gambusia, golden perch and Australian smelt. DDT residues were recorded in freshwater catfish and silver perch, although the numbers investigated were small (Mowbray 1978). For fish species in general, Mowbray (1978) indicated that deaths in the field have been recorded when whole body concentrations were 1-16 ug/L, most commonly averaging between 4-7 ug/L, although sometimes healthy fish with concentrations of >5 ug/L were recorded. Fish species tested for DDT contamination in the Namoi valley were golden perch, freshwater catfish, bony bream and carp (State Pollution Control Commission 1980, Gilbert et al. 1990a). Fish within this cotton growing area had significantly higher levels of DDT compared to those in other areas with 20% of fish tested exceeding the recommended Australian NHMRC maximum residue limit of 1 ug/g (0.001 ug/L) (Gilbert et al. 1990a). A study of pesticide contamination in the Gwydir and Namoi areas between 1981-3 after DDT had been phased out found that 18% fish tested (golden perch, freshwater catfish, bony bream,

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Silver perch – A Resource Document spangled perch, redfin and carp) exceeded the Australian NHMRC maximum residue recommended limit (Gilbert et al. 1992). Gilbert et al. (1990a) found indications that fry survival of fish could be affected because of high fish bodyfat levels. Such levels are similar to those in reproductive organs and egg yolks; concentrated levels of residues in yolks will subsequently be used by fry. Most aquatic organisms concentrate DDT residues in tissues at far higher levels than those occurring in surrounding water (USEPA 1975).

Mowbray (1978) indicated that changes in mortality of fish may not be correlated with levels of residue accumulation and so residue cannot be used as a criterion for determining whether death rates of fish increase with pesticides. He considered it most likely that fish would experience chronic doses of pesticides over time rather than acute doses. Mowbray (1978) found that while there was a high variability between fish, at most doses of pesticide gambusia produced more than 20% dead fry. Mowbray (1978) also suggested gambusia may have developed a degree of tolerance to DDT.

A study in the Namoi valley recorded the highest residues in water and sediment in tailwater drains; these usually ranged between 1-5 ug/L but occasionally reached 20- 40 ug/L (State Pollution Control Commission 1980). This water would not usually drain into rivers and streams. Maximum sediment residues during spraying were 0.113 ug/L. It was found that residue levels of DDT and its metabolites in both the water and sediments of the Namoi River did not increase between seasons even after prolonged usage. During heavy rain and floods some residues flushed into rivers (State Pollution Control Commission 1980). Gilbert et al. (1992) investigated pesticide contamination in the Gwydir and Namoi areas between 1981-3. Although DDT had been phased out of use in 1981 it was still detected. While endosulphan had begun to be used as a pesticide, it was not detected. Less contamination was found in the Gwydir area as was expected, since DDT had been used for a longer period in the Namoi area.

Endosulphan Since the early 1970s, the use of other insecticides such as endosulphan has increased. Endosulphan is now the most commonly used insecticide in the cotton industry and in New South Wales it is usually applied between November and February (Nowak and Julli 1992). As it is used in large amounts, this pesticide has been well studied and has been found to be highly toxic to fish (Barrett et al. 1991). Endosulphan has been used in recent research as an indicator of pathways for fate and transport for other chemicals (Anthony 1999).

Endosulphan dissipates through volatilisation and degradation in foliage and runoff water (Kennedy et al. 1998). It consists of alpha and beta isomers; while they have different chemical and physical properties, they only have a half life of a few days in water. The toxic metabolite endosulphan sulfate however, has a half life in water of several weeks. Endosulphan and its metabolites are more persistent when associated with soils and sediments (Hyne et al. 1999). Kennedy et al. (1999) indicated that endosulphan sulphate in soil has a half life of about three months. Suspended or bottom sediments may reduce

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Silver perch – A Resource Document toxicity of endosulphan. It may leach from soils during rain and floods (Nowak and Julli 1992) which could increase endosulphan sources entering rivers and contributing to fish kills (Hyne et al. 1999).

Hyne et al. (1999) investigated the toxicity of endosulphan for invertebrates in field and laboratory trials. They found endosulphan concentrations correlated with reductions in population densities of macroinvertebrate taxa tested. LC50 values for several species were within the range of concentrations measured in river water during land runoff following storm events (Hyne et al. 1999). Monitoring indicated changes in macroinvertebrate communities in irrigated agriculture sites and no changes at reference sites in some growing seasons (Brooks 1999). In a study of pesticide impacts on billabongs also recorded declines in abundance and species richness of aquatic macroinvertebrates and small vertebrates (Fairweather 1999).

Work undertaken on the toxicity of endosulphan to a number of native and introduced fish species in Australia has found they are sensitive to endosulphan at low concentrations. Laboratory trials indicated carp were the most sensitive species (LC50 of 0.1 ug/L) followed by bony bream (0.2 ug/L), golden perch (0.5 ug/L), rainbow trout (1.6 ug/L), with rainbowfish and silver perch the least sensitive (2.4 ug/L) (Sunderam et al. 1992). Research has shown that fish eggs are not sensitive to endosulphan at environmental concentrations (R. Hyne, NSW EPA, pers. comm. 1997).

Laboratory and field experiments provide varying results on the toxicity of pesticides. Extrapolating laboratory findings may be inappropriate for deciding impacts in rivers (Anthony 1999). Laboratory trials tend to produce lower LC50s, while field trials may produce higher levels since they are influenced by factors such as vegetation and sediments. Barrett et al. (1991) note the example of healthy populations of carp occurring at sites with concentrations of >1 ppb. In laboratory studies, turbidity did not significantly improve endosulphan toxicity. The short term acute toxicity of endosulphan to fish was approximately doubled by high temperatures. Plant material and to a lesser extent higher pH increases degradation of endosulphan (Chapman 1999). Chapman (1999) also suggested that chronic toxicity of endosulphan to fish occurs at concentrations which are only slightly lower than those for acute toxicity. The main water quality parameters likely to affect toxicity would be turbidity and temperature.

Chapman (1999) notes that water temperature and suspended solids are two of the most significant water quality parameters which may affect pesticide toxicity. Trials with silver perch found that increases in water temperature caused little change in endosulphan toxicity (at 1.5 ug/L) to fish over 96 hours. There was however, a two-fold increase in the short term (24 hours) toxicity as temperature increased from 15oC to 35oC, with most of the increase in toxicity occurring between 25 and 30oC (Patra et al. 1995). Patra et al. (1995) suggested that endosulphan was adding to the stress to fish at extreme high and low temperatures. Critical thermal maximum (CTM) tests found that after exposure to sublethal concentrations of endosulphan (0.3-1.0 ug/L), the CMT temperatures were significantly reduced by about 3oC (Patra et al. 1995a). Patra et al. (1995a) observed that

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Silver perch – A Resource Document under field conditions, fish which were predisposed to sub-lethal levels of endosulphan had a reduced ability to survive high natural temperatures.

The general guideline value for the safety of aquatic life is 0.01 ug/L (Bowmer et al. 1996). Barrett et al. (1991) indicates that fish kills are possibly expected at concentrations of >0.3 ppb. Fish kills attributed to endosulphan have been recorded in cotton growing areas. Bowmer et al. (1996) observe that fish kills are quite rare, and question whether this may be because many are not recorded or involve small fish which are not seen. Toxic fish kills are very transient and often hard to detect. Bowmer et al. (1996) list recorded fish kills in New South Wales and Queensland; there are no specific reports of silver perch kills. Reasons for fish kills of this species included possible pesticides pollution, poor water quality, deoxygenation due to an algal bloom and stress resulting from a snow melt. An analysis of fish kills found that a higher proportion of kills were associated with irrigated cotton growing than would be expected and that pesticides suspected in over half of kills particularly in Queensland (Bowmer et al. 1996). Bowmer et al. (1996) indicated that an endosulphan residue in the gills at a concentration of >0.5 mg/kg probably indicates that a fish died from endosulphan poisoning.

Other chemicals used in the cotton industry Bowmer et al. (1996) noted that many of the pesticides other than endosulphan break down rapidly or are strongly absorbed by soil, and few are detected in river water. However, compounds of concern include some organophosphates and pyrethroid insecticides, insect growth regulators and herbicides such as atrazine (Bowmer et al. 1996). Whyte and Conlon (1990) note that other frequently used insecticides such as phyrethroids, organophosphates and carbamates have the potential to be cause environmental damage but their pattern of usage means they are less of a threat compared to endosulphan. Pyrethroids are toxic when active but degrade rapidly, most organophosphates have a moderate to low toxicity to fish and carbamates are of a lesser threat (Whyte and Conlon 1990). Levels of profenofos in some areas during the spraying season can affect enzyme activity in fish and may be toxic to invertebrates (Chapman 1999). Whyte and Conlon (1990) conclude that herbicides generally represent a minor threat to the environment; Trifluralin and diuron have the greatest potential to be detrimental. The environmental impact of defoliants and conditioners is not well known and requires research.

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9 Riparian Vegetation The character and composition of riparian zones within the Murray Darling Basin has changed significantly in many areas since European settlement. Riparian vegetation has been damaged or cleared as a result of agriculture and forestry, stock grazing, cropping, salinity, roading, altered fire and flow regimes, weed invasions and recreational activities. It represents a broad ranging threat to the aquatic environment which has been occurring for well over a century and its role in the decline of silver perch is difficult to determine.

The specific impacts of degradation and destruction of riparian vegetation on silver perch have not been determined and no research has been undertaken to compare the distribution and abundance of fish in relation to health of riparian zones. Degradation and loss of riparian vegetation may adversely affect instream habitat in terms of loss of shading, loss of organic inputs, increased runoff, increased erosion and streambank slumping and sedimentation. Such changes may have affected silver perch in relation to food sources, water quality and breeding success. Increased erosion could cause smothering of eggs and larvae and reduce recruitment success, primarily in still water habitats where eggs and larvae may settle out. The loss of riparian vegetation may have resulted in a decrease in diversity of habitats such as slower flowing areas, although whether these are an important habitat to silver perch is not known. Whether woody debris originating from riparian zones is an important habitat component is also unknown. However some research suggests that silver perch is not wood dependent, as other species such as Murray cod, trout cod, golden perch and carp appear to be to varying degrees (S. Nicol, DNRE, pers. comm.).

9.1 Background Riparian vegetation generally refers to an arbitrarily defined zone immediately adjacent to or verging watercourses. LWRRDC (1999a) describe this zone as 'any land which adjoins, directly influences, or is influenced by a '. In large river systems this zone may include flood prone habitats like wetlands as well as forests and understorey vegetation (Walker 1993). This zone provides a link between the terrestrial and aquatic environment and is often highly productive with vegetation which reflect superior soils and availability of water. Riparian vegetation can include overstorey, understorey and aquatic macrophytes.

Riparian vegetation provides a key source of organic material for rivers and streams; many aquatic invertebrates feed on decaying leaves and wood, and these in turn provide a food source for predatory invertebrates and fish (Reich 1998). Logs and debris which fall into streams provide fish with shelter and form a substrate for food and also breeding sites. Shade provided by riparian vegetation decreases the amount of direct and diffuse sunlight reaching the water and reduces daily and seasonal extremes in water temperature (LWRRDC 1999a). It influences rates of photosynthesis and primary productivity. Riparian vegetation stabilises riverbanks, intercepts and slows surface runoff and reduces flow velocities instream and can act as a filter to pollutants and nutrients. Riparian vegetation provides a diversity of habitats including woody debris, rock ledges, Freshwater Ecology, NRE & Murray Darling Basin Commission 95

Silver perch – A Resource Document overhanging vegetation and undercut banks. It increases the variance in channel form thereby providing a greater diversity of instream habitats (LWRRDC 1999a).

The character of riparian areas has changed significantly since European settlement. Riparian vegetation has suffered destruction as well as modification as a result of land clearance for agriculture and forestry, stock grazing, cropping, salinity, roading, altered fire regimes, changes to flow regimes, weed invasions and recreational activities. A wide range of flora and fauna may be directly or indirectly dependent on riparian zones, and may occur permanently in riparian zones or utilise them at some time during their life history. Degradation or loss of riparian vegetation may adversely affect instream habitat in terms of loss of shading, loss of organic inputs, increasing runoff, increased erosion and streambank slumping and sedimentation. Increased light transmittance and primary production as a result of clearance of riparian vegetation may lead to algal blooms. Removal of riparian vegetation may cause increases in water temperature which may affect invertebrates and fish. Campbell (1993) noted that in Australia riparian zones often have open canopies so that the amount of light reaching streams with undisturbed vegetation is naturally high. Destruction of riparian vegetation may cause fragmentation of populations thereby threatening habitat and genetic diversity.

There has been no comprehensive survey of the condition of riparian zones throughout the Murray Darling Basin. A survey of the Murray River vegetation indicated that there has been extensive clearing on floodplains between the Edwards and Murray rivers and that riparian vegetation was generally in poor condition (Margules et al. 1990). A literature review of riparian communities in the New South Wales part of the Murray Darling Basin by Parsons (1991) indicated that degradation may be significant and that our understanding of the ecological responses to changes was limited.

The decline of riverine and riparian species along the Murray River is related to flow regulation which began in the 1920s and greatly expanded after about 1950 (Walker and Thoms 1993). The establishment and survival of plants can be strongly influenced by water regimes. For example, weir pools provide a favourable habitat for those plant species associated with stable water conditions. Sharley (1993) noted that flow regulation, impoundments and water diversions have been identified as the major cause of riparian zone degradation in the Murray River. Flooding regimes can affect the recruitment and growth of vegetation. Blanch et al. (1999) identified four categories for littoral and floodplain plants: infrequently flooded, floodplain (flooded moderate frequently), widespread species generally tolerant of flooding and exposure and permanently flooded (hydrologically stable).

The introduction of exotic plant species has also changed riparian zones significantly. Willows were first planted in parts of Australia about 150 years ago and are now a significant component of many rivers in southern parts of the basin. Willows dominate the banks of the Murray River in areas where water levels are stabilised by weirs (Walker 1993). In South Australia, willows dominate over about one third of the Murray River (Schultze and Walker 1997). While planting willows is an effective method to control

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Silver perch – A Resource Document erosion and prevent streambank collapse, their vigorous growth and invasive habitat has caused major channel diversions and habitat loss (Reich 1998). Willows may spread by seed, detached branches or attached branches and roots (Cremer 1999) and can form a dense canopy that limits the establishment and growth of other vegetation. These deciduous trees provide all organic input to water once compared to natives which provide organic input through the year. Further research is required to fully understand the impact of willows on the aquatic environment. Schultze and Walker (1997) note that it has been argued that foliage and roots may reduce water velocity and increasing sedimentation, that shading may decrease primary productivity, reduce summer food supplies and discourage species attracted to light. Native vegetation provides fallen timber and the open nature of redgums provides for a diverse understorey and macrophytes. Alternately it has been argued that willows may increase diversity by providing readily processed litter or by stimulating production through nutrients from leaf leachates. Schultze and Walker (1997) did not identify differences in aquatic invertebrate assemblages in areas with willows and areas with Red Gums, although there were differences in the way they were used as a food resource. Schultze and Walker (1997) argue that the effect of willows may be so general that invertebrate assemblages may have changed even in areas where redgums are predominant. Read and Barmuta (1999) suggested that willows potentially affect instream fauna in a range of direct and indirect ways which are likely to be most significant in summer low flow periods. They suggested the greatest effects would probably occur in smaller sized streams. Growns et al. (1998) argue that the presence of willows is preferable to aquatic assemblages than complete removal of riparian vegetation.

Since riparian zones provide a source of food for fish species as well as cover for shelter and breeding, it would be expected that changes in riparian zones would be detrimental to fish. Growns et al. (1998) compared fish assemblages in reaches of the Hawkesbury- Nepean River and found that the number of fish species and total fish abundance were significantly greater in habitats adjacent to vegetated banks. Vegetated banks provided a greater complexity of instream habitats and greater cover than degraded banks.

9.2 Management In 1993, the Land and Water Resources Research and Development Corporation (LWRRDC) considered rehabilitation and management of riparian systems a high priority area and established a national research and development program. The program aimed to establish a better understanding of the processes operating in riparian lands and the interactions with surrounding ecosystems. A series of issue papers on riparian management and technical guidelines for land managers and government staff to assist non technical people to implement best management of riparian land (LWRRDC 1999a,b). The guidelines address components of riparian zones and how they should be managed and outline legislation in each state. LWRRDC (1999a) also summarises current research projects underway to address riparian issues.

The importance of protecting and managing riparian zones is increasingly being acknowledged with remedial actions being undertaken at a range of levels from national

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Silver perch – A Resource Document to local. There is a wide range of people and groups involved in the management and protection of riparian zones including state government departments, river management authorities, landcare groups and individual landholders. There is a general trend towards integrated catchment management with rehabilitation of riparian zones a key aspect. Buffer strips along rivers are traditionally recommended as a means of maintaining and improving water quality in forestry and agricultural areas (Campbell 1993). Whether buffer strips are effective in protecting streams from water quality degradation depends on the width of strip and the type of vegetation. Rehabilitation programs can include fencing fencing off areas to prevent stock access, engineering techniques to control erosion and removing weed species (both riparian and instream) and reestablishing native species. Well meaning actions such as willow removal may in fact make the situation worse by causing increased erosion. Thus management actions need to be planned and prioritised taking into account all factors which can affect their success and practicality.

9.3 Assessment of the Significance of the Threat The character and composition of riparian zones within the Murray Darling Basin has changed significantly in many areas since European settlement. Riparian vegetation has been damaged or cleared as a result of agriculture and forestry, stock grazing, cropping, salinity, roading, altered fire and flow regimes, weed invasions and recreational activities. It represents a broad ranging threat to the aquatic environment which has been occurring for well over a century and its role in the decline of silver perch is difficult to determine.

The specific impacts of degradation and destruction of riparian vegetation on silver perch have not been determined and no research has been undertaken to compare the distribution and abundance of fish in relation to health of riparian zones. Degradation and loss of riparian vegetation may adversely affect instream habitat in terms of loss of shading, loss of organic inputs, increased runoff, increased erosion and streambank slumping and sedimentation. Such changes may have affected silver perch in relation to food sources, water quality and breeding success. Increased erosion could cause smothering of eggs and larvae and reduce recruitment success, primarily in still water habitats where eggs and larvae may settle out. The loss of riparian vegetation may have resulted in a decrease in diversity of habitats such as slower flowing areas, although whether these are an important habitat to silver perch is not known. Whether woody debris originating from riparian zones is an important habitat component is also unknown. However some research suggests that silver perch is not wood dependent, as other species such as Murray cod, trout cod, golden perch and carp appear to be to varying degrees (S. Nicol, DNRE, pers. comm.).

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10 Removal of Woody Debris Desnagging has occurred for well over a century although there appeared to be an increase in removal of snags corresponding to increases in irrigation in the 1950s and 1960s. While desnagging has been recognised as being detrimental to native fish species, some operations still continue and further education is required to limit the unnecessary loss of woody debris from rivers in the Murray Darling Basin.

The habitat requirements of silver perch are poorly understood and so the significance of woody debris as a habitat component is unknown. Silver perch have been recorded in a range of habitats including open water as well as amongst snags. There have been some general observations that the species prefers faster flowing waters and open areas compared to those that are heavily snagged. While species such as Murray cod, trout cod, golden perch and carp appear to be wood dependent to varying degrees, this trend has not been identified for silver perch. They do not appear to preferentially use snags compared to other habitat components (S. Nicol, DNRE, pers. comm. 2000). Since fish are often observed in faster flowing water woody debris may not be a significant habitat component in terms of offering a refuge from high water velocities. Whether silver perch use woody debris as a habitat marker, as a refuge from high water velocity and protection from predators or as a nursery for larvae and juveniles is unknown. Silver perch feed on a wide range of invertebrates as well as filamentous algae. Some of these food items occur within woody debris such as rotifers, chironomid larvae and small crustaceans. Whether woody debris provides a significant component of the species’ food supply is not known. Trials with larvae have suggested they rarely use cover and do not seek shelter in shade (Gehrke 1990, Thurstan 1991), which may suggest woody debris is not a significant habitat component. It seems unlikely that woody debris is an important habitat component in relation to the species’ breeding strategy, since eggs are semi pelagic and have been recorded floating downstream.

10.1 Background In the past, Large Woody Debris (LWD) and smaller debris has been deposited in rivers and streams from falling riparian trees or washed in from floodplains. It has been recognised for some time that LWD is important for river ecology, often being a key structural habitat component, especially in lowland rivers. By removing this debris, habitat availability and the complexity of the aquatic environment is reduced. For example, LWD can help in the creation of a range of water types by assisting in the formation of pools, source holes, gutters and general undulations in substrate. LWD falls into a river, smaller pieces get caught against it, leaves and litter accumulate, and pools form where sand, silt and other fine sediments settle out (Bilby and Likens 1980). Pools can be an important refuge for fish at times of low flows. Snags may be very important in deeper lowland streams, where substrates are generally made of finer particles, and are more uniform providing less available habitat diversity compared to upland streams

LWD can provide spawning sites for some fish species which lay adhesive eggs in or on logs, provide refuge sites from high water velocity, sunlight and predators, provide Freshwater Ecology, NRE & Murray Darling Basin Commission 99

Silver perch – A Resource Document nursery areas for larvae and juveniles, a source of invertebrate food as well as act as territory markers. Woody debris may be consumed by invertebrates, or used as an attachment site for feeding or pupation, for oviposition, a nursery site for early instars, for nesting, molting or emergence (Triska and Cromack 1980). Litter and organic debris can be retained amongst woody debris, thus functioning as a long term nutrient source to instream organisms and providing retention time for organic processors.

Recent research has indicated that large woody debris is a primary habitat determinant for Murray cod, trout cod and golden perch (Koehn and Nicol 1998). Species such as Murray cod use snags as markers, showing strong site fidelity and returning to the same snags over time. A population of two-spined blackfish Gadopsis bispinosus increased following an increase in woody debris (Koehn 1987). There are at least 34 species which use LWD, and suggests this may be an underestimate since little is known about the habitat and spawning requirements of many species (Koehn, unpublished data).

It has more recently been recognised that woody debris can significantly influence geomorphology of alluvial rivers (Brooks 1999a). Brooks (1999a) indicates that there is evidence that LWD actually plays a role in maximising channel stability rather than causing instability. In most cases, snags have little effect on the frequency and duration of large floods or on bank erosion. A modelling exercise indicated that full desnagging of a channel can cause a 10 fold increase in bedload transport and that reorienting logs to 30 degrees from the bank is equivalent to removing 50% of the debris (Brooks 1999a).

10.2 Management For over a century, the amount of LWD in rivers has declined significantly because of deliberate removal and the loss of riparian vegetation. Snags have also been realigned within rivers to be more parallel to flow. Desnagging has occurred to improve boat navigation and in the belief that it assists in controlling bank erosion and flooding, and improves irrigation supply. Many lowland rivers in Australia once appeared to have had very high amounts of woody debris. For example, 22 000 items of debris were removed from a 233 km stretch of the Murray River as late as 1976-1986 (Gippel et al. 1996).

Clearing of snags from lowland rivers occurred early in European settlement. Lloyd (1988) described desnagging of the Murray River for 30 years up until the late 1880s when river trade began to wane. More recently, desnagging became a standard practice of many River Improvement Trusts (with their associated legislation such as the River Improvement Act 1958 in Victoria). The increase in irrigation in the 1960s brought increased focus on desnagging, with the belief that removing debris allowed greater flows down river. In New South Wales, guidelines were developed in the early 1980s and site inspections are now undertaken to determine appropriate snag management. The practice of desnagging has declined greatly in this state with more emphasis placed on realignment of snags (P. O’Connor, NSW Fisheries, pers. comm. 1999). The same may be said of Victoria. Desnagging still occurs to some degree, although realignment is a more prevalent practice. In Queensland, desnagging has occurred primarily in the upper Condamine, since the 1950s (D. Moffatt, DNR, pers. comm. 1999). This practice is still

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Silver perch – A Resource Document underway, with possible plans to increase removal of timber in some areas (D. Moffatt, DNR, pers. comm. 1999). Desnagging rivers can be an expensive management exercise.

A greater understanding of the importance of snags in the aquatic environment has led to consideration of reintroducing snags into rivers. Brooks (1999) suggests that providing healthy riparian zones and trapping logs by in channel structures may be the best long term approach to resnagging rivers. Resnagging programs would need to consider issues such as where to source the debris, how many to introduce, what species are appropriate and how they should be orientated within a river. Research is currently being undertaken to reintroduce snags into a section of the Murray River. This project includes the determination of design criteria for resnagging operations and cost/benefit analyses (J. Lieschke, DNRE, pers. comm. 1999). There are also several other resnagging trials being undertaken in the Murrumbidgee River and in East Gippsland.

10.3 Assessment of Significance of the Threat While desnagging has occurred for well over a century, there appeared to be an increase in removal of debris corresponding to the increase in irrigation and the creation of River Improvement Trusts in the 1950s and 60s. In terms of trends in commercial catch of silver perch, there was a relatively consistent pattern of decline in New South Wales from the early 1960s onwards and in South Australia from the early 1980s onwards. Whether there is any link between desnagging and the species decline is not clear, particularly since the significance of both large and small woody debris to silver perch as a habitat component is unknown.

There has been no comprehensive investigation of the significance of habitat components for silver perch. While the species occurs in a wide range of habitats within the Murray Darling Basin, there have been some general observations that the species prefers faster flowing waters and open areas compared to those that are heavily snagged. Further research is required however to accurately assess the species’ habitat requirements. Silver perch have been recorded in a range of habitats including open water as well as amongst snags. While species such as Murray cod, trout cod, golden perch and carp appear to be wood dependent to varying degrees, this trend has not been identified for silver perch. They do not appear to preferentially use snags compared to other habitat components (S. Nicol, DNRE, pers. comm. 2000). Whether silver perch use woody debris as a habitat marker, as a refuge from high water velocity and predators or as a nursery for larvae and juveniles is entirely unknown. Silver perch feed on a wide range of invertebrates as well as filamentous algae, some of which may occur within woody debris such as rotifers, chironomid larvae and small crustaceans. Whether woody debris provides a significant component of the species’ food supply is not known. Trials with larvae have suggested they rarely use cover and do not seek shelter in shade (Gehrke 1990, Thurstan 1991), which may suggest woody debris is not a significant habitat component. Since silver perch have been observed in faster flowing waters, woody debris may not be a significant habitat component in terms of offering a refuge from high water velocities. It seems unlikely that woody debris is an important habitat component in relation to the species’ breeding strategy, since eggs are pelagic and have been recorded floating downstream.

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11 Aquatic Vegetation While there is a lack of historical scientific evidence, it is generally believed that there has been a severe decline in abundance of aquatic vegetation in many inland rivers in the last 50 years. The significance of aquatic vegetation as a habitat component for silver perch is unknown. Silver perch occur within a wide range of habitat within the Murray Darling Basin including sites with and without aquatic vegetation. A strong preference of silver perch to areas with abundant aquatic vegetation has not been demonstrated. Research is required to determine whether aquatic vegetation provides a significant habitat component for silver perch, either for juveniles and/or adults. It is not known whether silver perch use aquatic vegetation as a refuge to evade predators or as cover from prey, although no observations of such behaviour have been described. Since silver perch has often been recorded in faster flowing and open water habitats, aquatic vegetation may not be a significant habitat component in terms of providing shelter from high water velocities. The habitat preferences of juveniles are also not well known. It is possible that aquatic vegetation could provide nursery habitat for juveniles. Moffatt (NRE, pers. comm. 1998) however recorded juveniles in the Warrego River in side channels where there was a lack of shade, shelter and plant life. Since eggs are semi buoyant and pelagic it is unlikely that aquatic vegetation is used as a spawning site. Larvae appear to rarely use cover and do not seek shelter in shade (Gehrke 1990, Thurstan 1991) which suggests aquatic vegetation may also not be a significant habitat component for larvae. Aquatic vegetation may provide a food source for silver perch, both directly and indirectly, although the significance of this is also not known.

11.1 Background Aquatic vegetation can include growth forms such as free floating, surface floating and attached, submerged, emergent or both. Historically, very little has been written about the composition, distribution or abundance of this vegetation within the Murray Darling Basin. Roberts and Sainty (1996) suggest this may be partly due to there being no economic interest in aquatic vegetation. Past perceptions of the value of aquatic vegetation also probably partly explain the lack of documented information. Aquatic vegetation is still commonly referred to as "weed". Criticisms of aquatic vegetation have included that it is an impediment to navigation blocking boat passage along rivers, clogging boat propellers and water values for pumping, it restricts swimming, can cause skin irritations and that the decay of rotting vegetation smells unpleasant (Roberts and Sainty 1996).

There has been no comprehensive survey of waterplants in inland rivers in Australia, nor specifically within the Murray Darling Basin. There have been some surveys of riverine areas which focus primarily on floodplain rather than riverbank habitats. Surveys of aquatic habitats often still fail to include detail of aquatic vegetation and submerges and floating-leafed species remain poorly documented. Submerged vegetation is often out of sight, which makes survey more difficult. Information concerning the past distribution of aquatic vegetation comes largely from incidental observations of explorers and those interested in natural history, as well as potentially by investigating sediment cores. The Freshwater Ecology, NRE & Murray Darling Basin Commission 103

Silver perch – A Resource Document oral history of the Lachlan River by Roberts and Sainty (1996) clearly demonstrates the value of such information.

Despite the lack of historical scientific evidence, Roberts (1999) believes that there has been a severe decline in the abundance of waterplants in many inland rivers in the last 50 years. Bowling and Maker (1996) note that there is anecdotal evidence to suggest a major decline in macrophyte presence in the Barwon-Darling River in recent years. The oral history of the Lachlan River indicates that aquatic vegetation was previously abundant in areas along the river. Ribbonweed and Curly Pondweed Potamogeton cripsus are believed to have declined along inland rivers, but are still abundant in parts of the Murray River such as weir pools in the lower Murray and Darling rivers. Submerged waterplants are considered generally missing from the middle reaches of the Murray River, the middle and lower reaches of the Murrumbidgee River and from the Darling River downstream of Mungindi (Roberts 1999). Thoms et al. (1998) indicated that the declines are likely to be due to a combination of disturbance by carp, growth limiting conditions due to high turbidity and deep water conditions during most of the growing season. Roberts and Sainty (1996) note that Phragmites is now much less abundant in the Murray Darling Basin. Although this species once lined stretches of inland rivers and was found in the understorey of some floodplain forests, it is now restricted to small, fragmented patches or has disappeared in some areas. Roberts (CSIRO, pers. comm. 2000) note there are still large areas of this emergent macrophyte in several floodplain wetlands on the westward flowing rivers which are tributaries of the Murray Darling system. Frankenberg (1997) suggested the species has declined because of grazing and drainage programs. Emergent species such as Cumbungi and Phragmites tend to occur in the uppermost 0.5 m while rooted species such as milfoil Myriophyllum sp. occur to a depth of 2 m (Walker et al. 1995).

Ogden (1996) noted the widespread decline of submerged aquatic waterplants in large, deep billabongs in the south east of the Murray Darling Basin, having been replaced by phytoplankton-dominated billabongs. He indicated that the historical contraction probably related to submerged rather than emergent vegetation and that it occurred prior to the introduction of carp. There is still a range of other growth forms which need to be investigated in relation to their past and present distribution. A survey of sites along the Murrumbidgee River found that in general riparian and aquatic vegetation upstream of weirs and near lakes had the best structural diversity, with species richness greatest upstream of weirs (DLWC 1995). Walker et al. (1995) note that the decline of plants in riparian wetlands in the lower Murray River are compensated to a degree by the proliferation of littoral species since the establishment of weirs and impoundments.

Role of aquatic vegetation Aquatic vegetation can play a key role in the structure and functioning of aquatic ecosystems (Bornette et al. 1994). It can be important for a range of aquatic fauna including invertebrates and fish by: - Providing habitat (structural complexity, surface area, can increase available habitat) - Providing food (as a direct and indirect food source)

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Silver perch – A Resource Document - Providing refuge (for prey to evade predators, and cover for predators, shelter from high water velocities) - Providing spawning sites (for adult fish, can trap invertebrate eggs) - Providing nursery habitat (for juvenile fish) - Stabilising sediments (enabling finer particles to settle out, preventing scouring by strong water currents, creating habitat for the establishment of other species of aquatic vegetation) - Acting as a physical filter - Influencing physical and chemical components of water (nutrient release of decaying plants, growing plants can act as a nutrient sink, aerating water, can prevent wind mixing of water column etc.).

Aquatic vegetation is habitat for invertebrate fauna. Humphries (1996) notes that different species of macrophytes tend to support different assemblages, abundances and numbers of macroinvertebrates. In general those with dissected leaves support greater abundances of invertebrates per unit biomass, although this is not always the case. Boulton and Lloyd (1992) indicated that there have been observations of a relationship between invertebrate species diversity and aquatic plant structural complexity. Suren and Lake (1989) note that while direct consumption of living macrophytes is rare for invertebrates, many graze epiphytic periphyton. Invertebrates such as dragonfly larvae are often sit-and-wait predators and macrophytes provide them with camouflage. Grazers may feed on epiphytic algae, shredders on decaying plants and detritivores on fine organic matter that accumulates within plant beds (Kaenel et al. 1998). Structural complexity can interfere with foraging predators and decrease risks for small and juvenile fish (Dibble et al. 1996). Different ages of fish may use different aquatic vegetation. For example, Northern Pike Esox lucius fry prefer emergent vegetation, young use emergent, floating and submergent vegetation, while adults used submergent vegetation (Casselman and Lewis 1996).

Factors affecting the presence of aquatic vegetation Rivers and streams of the Murray Darling Basin are likely to have varied in the composition and abundance of aquatic vegetation in the past, with species having evolved according to local environmental conditions. Their ability to grow and reproduce is influenced by a wide range of factors including hydrological regimes (flooding frequency, depth, current etc), light and nutrient levels, substrate type and texture. There may be zone partitioning, e.g. occasionally flooded, seasonally flooded, marginal/emergent and aquatic plant zones. Walker et al. (1992) described upper, middle and lower littoral zones.

Changes to inland rivers since European settlement are likely to have resulted in changes to the composition and abundance of aquatic vegetation. It is difficult however to determine reasons for the decline of aquatic vegetation because of the existence and interaction of numerous factors (Rybicki and Carter 1986). The factors responsible for the establishment and abundance of macrophytes in Australian inland waters is also not well known. Walker et al. (1995) note that some species appear to be sensitive to local

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Silver perch – A Resource Document environmental variables while others are not, and discuss the concept of plant assemblages being ranked according to disturbance such as fluctuations in discharge which can initiate change and succession.

Briggs (1981) describes the freshwater wetlands in Australia, including within the Murray Darling Basin, according to structure, floristics as well as a combination of both. Many aquatic plants are specifically adapted to particular hydrological cycles for regeneration, establishment and survival. Blanch et al. (1999) note that species from permanent backwaters and wetlands on floodplains have colonised the main river channels since river regulation. While some species may be tolerant of a broad range of water regimes, others may be restricted to areas with stable water levels or infrequently flooded areas (Blanch et al. 1999). Roberts and Ludwig (1991) note that most aquatic plants require lowered water levels for recruitment and that stabilising water levels may result in lower species richness, less structural diversity and less emergent macrophytes. Highly regulated impoundments can sometimes have no aquatic vegetation, while those with more moderate changes in water level can have a greater species richness. For example, along the Cudgegong River Burrendong Dam experiences significant fluctuations in water levels and has very little aquatic vegetation, while the smaller Windamere Dam experiences much smaller fluctuations and has abundant aquatic vegetation. Walker et al. (1995) found that Cyperus sp. and Red Water Milfoil were present downstream of several weirs where high variation in water levels occur, while Cumbungi preferred areas above weirs where water levels were quite stable. Walker et al. (1992) note that some plants such as Red Water Milfoil favour lower pool environments where the dominant regime is erosional. Others such as Cumbungi are most frequent in depositional upper-pool environments, but that such sites were restricted because of willows. Significantly, Walker et al. (1992) note that many others are too rare or patchily distributed to be able to describe their typical environments. Blanch et al. (1999) state that the timing of aquatic plant surveys can be important since plants can respond rapidly to changing water levels.

Rivers within the Murray Darling Basin vary dramatically in their characteristics, ranging from very clear to highly turbid waters. Habitat degradation has increased turbidity in many areas and river regulation means that low flows which are times when rivers flow slowly enough to allow some particle settlement, are now less common. Soil characteristics in some areas, however, mean they may always have experienced times of high turbidity. Increased turbidity can cause a shift in dominance from submergent species to emergent species probably because of decreased light penetration (Niemeier and Humbert 1986, Fisher and Claflin 1995). Submerged plants can have specific light requirements which may be affected by depth, turbidity and planktonic algal growth. A number of authors have discussed the theory of ponds and shallow waterbodies having phases, ranging from clear water with abundant aquatic vegetation to turbid water with little or no aquatic vegetation and dominated by phytoplankton (Smart et al. 1996, Casanova et al. 1997). It is not known whether these can be applicable to Australian ponds and lentic habitats. However, Casanova et al. (1997) found a strong relationship between low species richness of aquatic vegetation in dams and poor water quality.

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Silver perch – A Resource Document Species composition can be influenced by the nutrient composition of water (Simpson and Eaton 1986). While rooted plants may obtain nutrients from substrates, periphytic algae obtain them from the water. Sedimentation can also influence survival (Rybicki and Carter 1986). The loss of submerged species does not necessarily mean that they will be replaced by emergent species. Aquatic vegetation may vary in its ability to tolerate physical disturbance and how rapidly recolonisation can occur. Submerged vegetation can also decline with high levels of boat disturbance (Vermaat and De Bruyne 1993). Higher flow velocities resulting from modification to flow regimes may have affected the distribution and abundance of aquatic vegetation.

11.2 Management Little attention has been given to management of aquatic vegetation in inland rivers of Australia and it has primarily focussed on control of weed species or those considered to be causing problems. The issue of restoration of aquatic vegetation has only recently been considered. Much research is required to understand past species composition, population dynamics, effective methods of propagation and survival, sources of stocks for reestablishment etc. It is important to understand why particular species have declined and what management is required for successful reestablishment.

11.3 Assessment of Significance of the Threat The significance of aquatic vegetation as a habitat component for silver perch is unknown. Silver perch occur within a wide range of habitat within the Murray Darling Basin including sites with and without aquatic vegetation. A strong preference of silver perch to areas with abundant aquatic vegetation has not been demonstrated. Research is required to determine whether aquatic vegetation provides a significant habitat component for silver perch, either for juveniles and/or adults. It is not known whether silver perch use aquatic vegetation as a refuge to evade predators or as cover from prey, although no observations of such behaviour have been described. Since silver perch has often been recorded in faster flowing and open water habitats, aquatic vegetation may not be a significant habitat component in terms of providing shelter from high water velocities. The habitat preferences of juveniles are also not well known. It is possible that aquatic vegetation could provide nursery habitat for juveniles. Moffatt (NRE, pers. comm. 1998) however recorded juveniles in the Warrego River in side channels where there was a lack of shade, shelter and plant life. Since eggs are semi buoyant and pelagic it is unlikely that aquatic vegetation is used as a spawning site. Larvae appear to rarely use cover and do not seek shelter in shade (Gehrke 1990, Thurstan 1991) which suggests aquatic vegetation may also not be a significant habitat component for larvae. Aquatic vegetation may provide a food source for silver perch, both directly and indirectly, although the significance of this is also not known.

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12 Diseases Diseases have the potential to pose serious threats to native species such as silver perch both in hatcheries and wild populations. Diseases may cause mortalities, producing obvious fish kills or have sublethal effects which could affect long term survival and reproduction which are less likely to be observed. Fish kills of species including silver perch have occasionally been reported, although the cause has usually not been identified. No mass mortalities of the species have been recorded in the wild.

The past and present distribution of diseases in native fish within the Murray Darling Basin is not well understood. Three diseases and one parasite are discussed below. While they have been recorded in the last 30 years in introduced species and hatchery fish, their occurrence prior to this time is unknown. • Epizootic Haematopoietic Necrosis Virus (EHNV) - The virus has first reported in the mid 1980s and its occurrence in the wild prior to this date is unknown. Fish kills have only been recorded for introduced species (primarily redfin). Langdon and Humphrey (1987) suggested the disease was probably widespread in Victoria and New South Wales. It has been recorded on specific occasions in a number of lakes. Silver perch has been found to be highly susceptible to EHNV and the virus may well have played some role in the species’ decline as suggested by Langdon (1989). • Viral Encephalopathy and Retinopathy (VER) (formerly known as Barramundi Picorna-like Virus BPLV)- The virus was first reported in the late 1980s and has largely been restricted to farmed barramundi in northern Australia. Thus it is very unlikely to have played a role in the decline of silver perch. Barramundi farming is permitted in New South Wales and Victoria; while there are conditions to prevent escapes of fish and disease, it must be assumed that there is some degree of risk, however minor, of release of the virus into waterways. Experimental trials found that VER caused mortalities of silver perch (Glazebrook 1995) and additional trials have been recommended (Whittington, NSW Agriculture, pers. comm. 1998). • Goldfish Ulcer Disease (GUD) - The disease was first reported in the mid 1970s and its current prevalence is unknown. It appears to primarily be of concern for hatcheries and predominantly affects salmonids. Rowland and Ingram (1991) noted that fingerlings of silver perch appeared to be resistant to GUD. More recently Whittington et al. (1995) recorded Aeronomas salmonicida in silver perch from a farm where goldfish with GUD had occurred seven years previously. • Asian Fish Tapeworm - The tapeworm has first recorded in the late 1990s. Its distribution appears compatible with carp. The parasite commonly affects young fish and shows a low host specificity. Thus it is possible that it could become established in native species such as silver perch.

The occurrence of diseases and parasites in hatchery-reared silver perch are quite well documented since this species is a significant aquaculture species. While the species has been found to be susceptible to several diseases, further research is required to understand the prevalence of all potential diseases in fish in the wild as well as lethal and Freshwater Ecology, NRE & Murray Darling Basin Commission 108

Silver perch – A Resource Document sublethal effects on fish including eggs, larvae and juveniles. Since some of the diseases of concern occur in a number of introduced species which have a widespread distribution within the Murray Darling Basin, they represent a clear potential threat to silver perch. Since there have also been extensive stocking programs of silver perch this may be quite a significant issue.

12.1 Background Little is known about the prevalence of diseases in freshwater fish in Australia, or their impact on native fish species including silver perch. Rowland and Ingram (1991) note that bacterial and viral diseases have the potential to pose serious threats to native fish in hatcheries and in the wild. Outbreaks of disease can occur if fish are susceptible, when pathogens are present and when environmental conditions are conducive to an outbreak. The release of hatchery-reared fish carrying a disease into the wild could pose significant problems to fish populations in rivers and impoundments. Rowland and Ingram (1991) outline a range of diseases known from hatcheries, primarily concerning Murray cod, golden perch and silver perch.

Rowland and Ingram (1991) note that fish kills are occasionally recorded from New South Wales waters, but the cause is usually not identified because of delays in investigations. The same is no doubt true in other states. Fish kills may be due to a range of reasons (e.g. low dissolved oxygen, toxic pollution) as well as infectious diseases.

Three diseases and one parasite of concern are discussed below in detail; epizootic haematopoietic necrosis virus (EHNV), Viral encephalopathy and retinopathy (VER), Golden Ulcer Disease (GUD) and the Asian Fish Tapeworm Bothriocephalus acheilognathi. Other diseases and parasites are discussed briefly.

Epizootic Haematopoietic Necrosis Virus (EHNV) EHNV, a new iridovirus, was first reported from dead redfin at Lake Nillahcootie, near Benalla, in 1984 (Langdon et al. 1986). This disease, which can cause sudden high fish mortalities following necrosis of tissues and organs (Langdon 1989), is only recorded in Australia and is one of four diseases in the world which must be reported to international disease surveillance authorities if an outbreak occurs (Callinan and Rowland 1995).

Outbreaks of the disease have been recorded at Lake Nillahcootie and in north east Victoria (Langdon and Humphrey 1987). Mass mortalities of young redfin have also been recorded from other lakes in New South Wales and Victoria including , Lake Dog, Lake Green, Lake Learmonth, Lake Carcoar, Lake Burrendong, Waranga Basin (Langdon and Humphrey 1987) and Lake Eildon (G. Strongman, DNRE, pers. comm. 1999). Fish kills appear to affect specific age classes and it is rare to see a variety of age classes in one kill (A. Baxter, DNRE, pers. comm. 1998). Langdon and Humphrey (1987) suggested the disease was probably widespread in Victoria and New South Wales. They note two outbreaks have been recorded in cultured rainbow trout in southern New South Wales, in one case possibly spread to the trout from redfin, and the source of the other outbreak was unknown (Langdon et al. 1988). The first

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Silver perch – A Resource Document confirmed reports of EHNV in South Australia was in 1991, where it was recorded in dead redfin in the lower Murray River system (Lake Alexandrina and Lake Albert) and in the Mount Bold Reservoir, and more recently as far up as Renmark and metropolitan reservoirs (Safish 1991).

Langdon et al. (1986) indicated the implications of the presence of this virus on wild and farmed fish had not yet been established. While the origin and natural host of the virus are apparently unknown, Langdon (1989) notes that various species may be carriers. Trials were carried out to investigate transmission, host range, reservoirs of infection and resistance of the virus in redfin and 11 other introduced and native teleosts. Redfin appeared to be the most susceptible teleost species tested. The native species silver perch, macquarie perch, and mountain galaxias Galaxias olidus and the introduced gambusia were found to be highly susceptible to the disease. Murray cod were considered to be susceptible to infection and a possible carrier species. They considered golden perch and Australian bass Macquaria novemaculeata unlikely to develop the disease in the wild. Goldfish, carp, barramundi and Australian smelt appeared unresponsive to the disease. Langdon (1989) suggested that the decline of silver perch could be partly due to the virus. EHNV has the ability to survive prolonged periods both in the aquatic environment and on materials such as fishing rods and nets. Rowland and Ingram (1991) note that as yet no outbreaks have been recorded in native fish.

More recently, work has focused on developing appropriate diagnostic tests for EHNV (J. Humphrey, DNRE, pers. comm. 1998). Humphrey (DNRE, pers. comm. 1998) is not aware of recent outbreaks of EHNV which he considers surprising since it used to occur each year at the same time at a number of locations. Populations may have become immune to the virus, or refractory to infection.

Viral Encephalopathy and Retinopathy (VER) Viral Encephalopathy and Retinopathy(VER) was formerly known as Barramundi Picorna-like Virus (BPLV). It attacks the central nervous system, causing loss of orientation, vacuolation of the brain, and mortality (Glazebrook 1995). The virus was first discovered in 1989, although mass mortalities of farmed barramundi Lates calcarifer had occurred repeatedly between 1986 and 1989 in northern Australia. Whittington (1996) noted the virus may spread (a) directly from infected fish (horizontally) or via ova (vertically) or (b) indirectly via (i) discharges of hatchery water, (ii) movements of piscivorous birds, or (iii) mechanical means including on fishing gear. He indicated there is some suggestions that the virus has been transmitted horizontally from fingerlings to larvae at the start of feeding. While vertical transmission is a possibility because of observations of other similar viruses, Munday et al. (1992) suggest this may not be common.

Some preliminary tests have been carried out on the effect of VER on other fish species. Glazebrook (1995) found that all macquarie perch and silver perch tested died within four days of exposure to the virus, while about half the Murray cod died after 10 days. Golden perch, goldfish, snapper Pagrus auratus and rainbow trout were not susceptible to the

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Silver perch – A Resource Document virus, while brown trout and mulloway Argyrosomus hololepidotus could carry the virus asymptomatically (Glazebrook 1995). Whittington (1996) concludes that fish species other than barramundi may be highly susceptible to the virus. Further research is required to clarify the susceptibility of a range of fish species, including additional testing of the species listed above (Whittington, NSW Agriculture, pers. comm. 1998).

Glazebrook (1995) expressed his concern over the movement of eggs and larvae from northern Queensland to near Narrandera in New South Wales, via South Australia for testing. He noted the virus has now been recorded in South Australia on three occasions.

Mass mortalities have apparently ceased once disinfection and quarantine measures are put in place. Whittington (1996) observed that prevention of infection could be based on buying disease/infection free stocks, and prevention of disease based on management to minimise environmental risk aspects such as poor nutrition or water quality. Carrier fish should be identified and removed, the segregation of age classes could prevent horizontal transmission and mechanical vectors should be disinfected. NSW Fisheries (1997) notes that economic losses due to the virus can now be eliminated through effective control techniques in hatcheries.

Glazebrook (1995) cautions that the accidental release of VER from contaminated farm effluent into rivers, creeks and billabongs of Murray Darling River system could have serious consequences. It is likely that most fish kills would be restricted to young fish. Glazebrook (1995) indicated that histopathological examination is not sufficient as the only screening method and recommended additional tests. Reliable diagnostic techniques such as immuno-assay, need to be developed for this virus.

In Victoria, a moratorium on the importation of live barramundi for aquaculture, has been removed. In guidelines for farming barramundi in Victoria, NRE (1997) notes that “In the event of barramundi escaping to waterways in Victoria, the likely hood (sic) of establishing self sustaining populations is considered remote as water temperatures are considerably lower than in their natural range.” Current permit conditions specify import restrictions, criteria for hatcheries to follow, the need for fish health accreditation reports, site selection and farm management conditions. J. Humphrey (DNRE, pers. comm. 1998) believes current regulations are quite adequate, if systems are enclosed, inspected regularly and predation is prevented. C. Balinger (DNRE, Fisheries, pers. comm. 1998) indicated that several commercial operators have shown interest in farming barramundi in Victoria, and farming on a small scale has commenced in Melbourne. He indicated that farming barramundi has reasonable potential in Victoria.

In New South Wales, a policy paper on barramundi farming has been prepared concerning where fingerlings may be purchased, mode of farm operation, management of waste water etc. The principle issues considered in assessing permit applications relate to prevention of fish escapes, prevention of disease, control and disposal of effluent and a sound business plan (NSW Fisheries 1997). Several aquaculture permits for the culture of

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Silver perch – A Resource Document barramundi have been authorised in New South Wales. In South Australia, Primary Industries Fisheries have developed a permit with stringent conditions prohibiting the movement of fish unless they are disease free according to testing protocol.

Goldfish Ulcer Disease (GUD) GUD is a bacterial disease affecting goldfish caused by an atypical strain of the bacterial fish pathogen Aeromonas salmonicida. Humphrey and Ashburner (1993) note that it is considered a major fish pathogen internationally. GUD was initially recorded on a commercial goldfish hatchery in Victoria in 1974, but has spread because of unsuccessful attempts to control the disease in hatcheries, the lack of regulations concerning hatcheries and the movement of goldfish as live bait (Whittington and Cullis 1988). Distribution of goldfish from the first hatchery resulted in the occurrence of GUD in aquarium goldfish in five states by 1977 (Humphrey and Ashburner 1993). In 1985, it had been recorded in two fish hatcheries in New South Wales and there were confirmed records from wild populations of goldfish (Humphrey and Ashburner 1993) including at Lake Burrumbeet, Lake Hume and Lake Cargelligo. Rowland and Ingram (1991) note that fingerlings of Murray cod, golden perch and silver perch appear to be resistant to GUD, but salmonids are highly susceptible. This has caused significant concern in the salmonid aquaculture industry. Carson and Handlinger (1988) note that the presence of GUD in New South Wales and Victoria represents a significant threat to maintaining salmonid stocks free from the furunculosis which GUD can cause. More recently, Aeromonas salmonicida was identified in ulcerating dermal lesions in carp, roach Rutilus rutilus and silver perch (Humphrey and Ashburner 1993). Whittington et al. (1995) recorded Aeromonas salmonicida in silver perch from a farm on which goldfish with GUD had occurred seven years previously; it was not known if the infection had occurred on the farm in the intervening years.

Asian Fish Tapeworm The Asian Fish Tapeworm Bothriocephalus acheilognathi is a cestode originally recorded in cyprinid fish in China. Previously known host species include the following introduced species; carp, tench, roach and gambusia all of which occur in Australia and have widespread distributions (Dove et al. 1997). The parasite was recently recorded in carp, gambusia and western carp gudgeon Hypseleotris klunzingeri in the ACT and NSW, representing the first known occurrences in Australia (Dove et al. 1997). More recently, the parasite has been also been recorded from Hypseleotris sp. 4, and Hypseleotris sp. 5, flat-headed gudgeon Philypnodon grandiceps, Australian smelt, and the introduced goldfish (A. Dove, University of Queensland, pers. comm. 1998). Dove also observes that the parasite has a distribution in Australia compatible with the Boolara strain carp and that it does not appear to have spread beyond that range using other hosts. The parasite does not appear to mature properly in native hosts. Further work is required to determine whether other species act as hosts and the impact of the parasite on populations of native species. The annual mass mortality of western carp gudgeon in the Australian Capital Territory appears to be associated with the presence of the parasite (A. Dove, University of Queensland, pers. comm. 1998).

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Silver perch – A Resource Document 12.2 Management Management of diseases and parasites in hatchery-reared fish are addressed within government policies and protocols for hatchery operations such as quarantine and disinfection measures.

12.3 Assessment of the Significance of the Threat EHNV Redfin is an exotic species first introduced into Australia in the 1860s now has a wide distribution throughout south eastern Australia. While it is not known when EHNV occurred prior to its first report in 1984, it is considered to be unlikely to have been much earlier than this date. Since experiments have indicated silver perch is highly susceptible to this virus, it may well have played a role in the species’ decline as suggested by Langdon (1989).

VER Limited experiments have been undertaken on the susceptibility of native fish species to VER. These trials have indicated that the virus can kill silver perch (Glazebrook 1995), although further testing is warranted (Whittington, NSW Agriculture, pers. comm. 1998). Since this virus was only discovered in Australia in the late 1980s, and has largely been restricted to farmed barramundi in northern Australia, it is very unlikely it is a major reason for the previous decline of silver perch. However, the development of a barramundi industry within the Murray Darling Basin means there is some potential risk of the disease escaping into natural waterways.

GUD Goldfish was introduced into Australia in the 1860s and is now widespread throughout New South Wales, Victoria and South Australia (Brumley 1996). It is found in still and sluggish waters, habitats which silver perch can occur in. The spread of GUD since its first report in the mid 1970s and its current prevalence in the wild is not well understood and so its possible role in the decline of silver perch is unknown. While Rowland and Ingram (1991) observed fingerlings of silver perch to be apparently resistant to GUD, Whittington et al. (1995) recorded Aeromonas salmonicida in the species on a farm many years after goldfish with GUD had occurred.

Asian Fish Tapeworm While the Asian Fish Tapeworm was first recorded in Australia in 1997, its previous presence in the country and its current prevalence within the Murray Darling Basin is not known. Dove et al. (1997) note that since species which are known to be hosts overseas are widespread in Australia, it is possible that the parasite has already become established in a range of native Australian fish species. The potential pathogenicity of the parasite is of concern, causing histopathological changes. The parasite apparently commonly affects young fish and shows a low host specificity. It is currently not known whether silver perch are a host to the Asian Fish Tapeworm, so the impact of this parasite is also unknown. Dove (University of Queensland, pers. comm. 1998) considers it highly likely that young silver perch are hosts to the tapeworm where their distribution with carp Freshwater Ecology, NRE & Murray Darling Basin Commission 113

Silver perch – A Resource Document overlap significantly. Preliminary investigations on a small number of silver perch have failed to record the tapeworm.

Other diseases recorded in silver perch Rowland and Ingram (1991) outlined the known diseases recorded in fish hatcheries and farms, particularly those affecting silver perch, golden perch and Murray cod. Methods of control and appropriate management practices to limit their presence were also discussed. Little is known about diseases in freshwater fish in Australia, although bacterial and viral diseases have the potential to pose serious threats to native fish in hatcheries and in wild (Rowland and Ingram 1991). Outbreaks of disease can occur if fish are susceptible, when pathogens are present and when environmental conditions are conducive to an outbreak.

The following parasites and diseases have been recorded in silver perch: the protozoans Ichthyophthirius multifiliis, Ichthyobodo necator, Chilodonella hexasticha, Trichodina and Tetrahymena sp., fungi Aphanomyces piscicida and Saprolegnia sp., flatworms, 4 different nematodes, externally attached copepods of the Lernaea genus, monogenetic trematodes, columnaris disease (Flexibacter columnaris) and tail rot (Merrick and Schmida 1984, Robinson 1981, Rowland and Ingram 1991, Thurstan and Rowland 1995, Callinan and Rowland 1995). Silver perch are particularly susceptible to the protozoan Ichthyophthirius multifiliis, which is one of the most virulent ectoparasites known to freshwater fish (Selosse and Rowland 1990). Most infestations occur in late spring, summer and early autumn at temps of 18 to 27oC, and infested fish can die; however infestations can occur at temperatures around 15oC (S. Rowland, NSW Fisheries, pers. comm. 1998). The protozoan Chilodonella can also cause high mortalities. Mass mortalities of silver perch fry have occurred following infestations of Trichodina, which Thurstan and Rowland (1995) consider the most common ectoparasite, and Tetrahymena. Thurstan and Rowland (1995) note Ichthyobodo necator can cause high losses. Goussia and Eimeria, coccidian protozoans, have been observed to cause chronic wasting in silver perch (Rowland and Ingram 1991). Beumer et al. (1983) and Rowland and Ingram (1991) list other parasites recorded from silver perch. Dove (University of Queensland, pers. comm. 1998) recorded diplectanids (gill parasitic flatworms) in two silver perch, from Narrandera Inland Fisheries Research Station and Leslie Dam. He recorded Dermocystidium from fingerlings at a hatchery in Queensland; the life cycle of this parasitic fungus is unknown, although the infected fish apparently had large spleens, indicating some kind of immune response. Dove also found Spirocamallanus from several silver perch in Leslie Dam; it is an intestinal parasite often found in freshwater fish. Salt water can be used to control diseases in silver perch eggs as well as to kill parasites in larvae and fry (Rowland and Ingram 1991).

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13 Aquaculture Industry, Translocations and Genetic Implications There is a very significant and substantial aquaculture industry for silver perch in Australia and extensive research has been undertaken to determine efficient methods of production. The potential to produce large numbers of silver perch means that large numbers of fingerlings have been released in many areas, both within and outside of the Murray Darling Basin. In particular, many impoundments in New South Wales and Queensland have been stocked with silver perch, in many cases for put-and-take fisheries.

Silver perch may be bred in hatcheries for a range of reasons, including aquaculture, for put-and-take fisheries in impoundments and for stocking open systems as a restoration activity for conservation. Such reasons for fish breeding can in some cases be potentially conflicting. Responsible management of genetic resources must be seen as a key issue for the future if stocking programs of silver perch continue, particularly of open systems.

Stocking programs cannot be seen as an answer to the decline of silver perch in the wild and emphasis must be placed on habitat protection and restoration and the amelioration of threats which caused the species' initial and ongoing decline. However, stocking programs may be able to play a role in the restoration of riverine populations. There is a crucial need to ensure that breeding programs in hatcheries maximise the genetic composition of stocks. Inappropriate breeding and stocking programs, where limited numbers of broodstock are used, may in fact cause more harm than good in the long term survival of silver perch. There is already a large scale aquaculture industry for silver perch which is expanding rapidly. There are already indications that there is limited genetic variation within some hatchery stocks and signs of genetic problems in hatchery stocks such as poor quality fish and abnormalities are beginning to become apparent. Responsible management of hatchery stocks which are destined for release in the wild is a crucial issue for silver perch in the future.

13.1 Background The potential of silver perch as an aquaculture species has long been recognised (Lake 1967c). A workshop on the aquaculture of silver perch held in New South Wales discussed culture in farm dams, production in earthen ponds, design of aquaculture systems, water quality, diseases, artificial diets and predators, as well as marketing (Rowland and Bryant 1995). Rowland et al. (1995) note that improved performance of silver perch could be achieved with further research to develop cost effective diets, optimum stocking densities and feeding regimes, fish husbandry techniques, production strategies and implementation of selective breeding programs. Production of silver perch has the potential to develop into a very large industry in Australia, based on high volume, relatively low cost production (Rowland et al. 1995).

Silver perch are considered quite a good fish both for sport and eating (Merrick 1996), having an attractive appearance and colour (Rowland and Barlow 1990). Due to the Freshwater Ecology, NRE & Murray Darling Basin Commission 115

Silver perch – A Resource Document characteristics of silver perch outlined below, Rowland and Barlow (1990) considered this species to have the highest biological potential for aquaculture in Australia. Rowland (1984) developed techniques to produce large numbers of fry by hormone induced spawning. Rowland et al. (1983) indicated the high public demand to stock fry in farm dams, lakes and impoundments. In New South Wales, almost 520 000 silver perch were stocked in public waters between 1976 and 1983. Hatcheries now produce more than one million fry each year in New South Wales, Queensland and Victoria (Rowland and Barlow 1990). Hatcheries raising silver perch are primarily in New South Wales, followed by Victoria and Queensland (Kibria et al. 1996). Kibria et al. (1996) indicated there is interest in overseas countries, such as China and Taiwan, in cultivating silver perch.

Silver perch is hardy with good survival rates. Its behaviour of congregating in schools and spending time close to the water’s surface also facilitates management. Growth can be uniform and rapid, with fish reaching 500 mm in 15 to 18 months without fertilisation of ponds or additional feeding. Silver perch are omnivorous and will eat a range of food including artificial food which varies in texture and flavour. While Rowland and Barlow (1990) note food conversion ratios were unknown, they suggested additional protein needs would not be significant and relatively cheap diets may be suitable. Silver perch will not eat each other, and have been raised in ponds with densities of up to 1000 fish per hectare. Major diseases which affect silver perch in hatcheries are known, and Rowland and Ingram (1991) outline appropriate treatment. The species has quite a high meat recovery, has a white flesh and few bones (Rowland and Barlow 1990). Silver perch can also easily be raised in earthen ponds (Rowland et al. 1995, Thurstan and Rowland 1995).

Rowland (1984) developed successful techniques of inducing breeding in silver perch using hormones. Adults were injected with human chorionic gonadotrophin (HCG) or preparations of the pituitary gland of (CPG). Injection of females with 200IU/kg HCG produced the best results; all females spawned and the mean hatch rate of eggs was almost 89%. For species such as silver perch, which require specific environmental conditions to spawn in the wild, manipulation of reproduction with hormones is a valuable technique (Rowland et al. 1983). Such techniques enable great control over aspects such as timing of aspects of reproduction.

Thurstan (1991) emphasised the importance of providing sufficient food within rearing ponds. If adequate amounts of zooplankton are not available, larvae may not survive. The high variability in survival of larvae recorded in rearing ponds may be due to the difficulty in guaranteeing a sufficient supply of prey (Thurstan 1991). Plankton blooms can be initiated in rearing ponds; these may represent similar conditions to an inundated floodplain (Thurstan 1991). Once the food supply within ponds has declined to the point where fish growth cannot be supported, the fish are harvested.

Rowland and Barlow (1990) emphasised the importance of carrying out extensive research and development prior to setting up large scale aquaculture ventures. They

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Silver perch – A Resource Document believed the initial research phase to establish an aquaculture industry for silver perch would take between 5 and 10 years. A major research project commenced at the NSW Fisheries Eastern Freshwater Fish Research Hatchery in 1990. This project investigated the feasibility of, and techniques required in, growing silver perch intensively (Allan and Rowland 1992). An appropriate diet needs to be nutritionally adequate as well as cost effective. SP35 has been developed as a suitable feed for the pond production of fingerlings as well as market size fish (400-500 g). Three commercial feed companies are developing silver perch feeds, and there was a recent shipment of fry and fingerlings to China (Kibria et al. 1996). Anderson and Arthington (1989) found that silver perch accumulated more body fat when fed high fat diets compared to those on fat free diets.

Marking fish in some way can enable future identification between wild or hatchery raised stock. Tagging programs can assist in identifying and estimating population sizes, patterns of distribution and migration, and determining the success of releasing hatchery raised young (Ingram 1993). Ingram (1993) evaluated tagging of coded wire tags in both silver perch and golden perch; fish were of two sizes, 500-710 mm and 210-390 mm. After 30 days, all silver perch had retained cheek implanted tags. Cheek implanted tags proved much more effective than snout implanted tags, possibly because of the larger size of the contact area in the cheek. Factors such as operator experience and using a head mould to hold fish can influence the success of tagging. Larger fish are easier to handle and therefore less likely to be injured by handling. Survival of tagged fish was very high and feeding did not appear to be affected by the presence of a tag.

Studying scale growth patterns can be used as a method of aging fish; this method assumes that seasonal changes in growth rates create annual marks in circuli patterns (Willett 1993). Willett (1994) noted that discrimination between different fish stocks can be achieved by analysing scale circulus patterns. This method of identification of fish is advantageous; large numbers of fish do not need to be manually tagged, survival, behaviour and growth are unaffected, and the markers are permanent (Willett 1996). Methods of physically marking fish, such as internal and external tags and chemical markers, can be time consuming to apply, be expensive to recover and may vary in their retention rates (Willett 1993).

Patterns of circulus growth are influenced by genetic as well as environmental characteristics such as pond size, water temperature, fish density and food availability (Willett 1993, Willett 1994). By changing water temperatures by 5oC, Willett (1994) was subsequently able to identify different groups of silver perch. Fish raised in water temperatures of 30oC were significantly larger; while there was a degree of overlap in growth of fish raised in water temperatures of between 20 and 25oC. This is not unexpected, since 30oC would be moving towards the species’ limit, while 20 to 25oC fall within its preferred temperature range (Willett 1994). Willett (1996) compared stocking success of stocking silver perch measuring 35 mm and 50 mm in two impoundments in Queensland. The manipulation of the growth rates of the two size groups of silver perch enabled Willett (1996) to distinguish between them using scale circulus pattern analysis.

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Silver perch – A Resource Document Results indicated that relative survival of both sizes was similar, suggesting the release of 35 mm fish would be the most cost effective approach.

Kibria et al. (1997) discuss the potential problem if effluents from aquaculture industry cause water pollution; the release of nutrients (such as nitrogen and phosphorus) from feeds can cause algal blooms and eutrophication. They found silver perch produced less solid and nutrient load at 25oC compared to less than 30oC and less than 20oC. Better growth rate and good food conversion ratio at 25oC could be linked to quite a low pollution load at this temperature. The New South Wales Silver Perch Aquaculture Policy indicates that no effluent water is to leave any farm and enter natural waterways. It must be retained in an effluent/settlement dam and either used for irrigation, reused for fish culture or allowed to evaporate (S. Rowland, NSW Fisheries, pers. comm. 1998).

In a study in New South Wales, over an eight month period, 90% of 19 400 silver perch stocked in four ponds were taken by cormorants (Barlow 1995). Fingerlings (3-75 g) were taken by Little Black Cormorants Phalacrocorax sulcirostris and Little Pied Cormorants P. melanoleucos, while larger fish (55-373 g) were taken by Black Cormorants P. carbo. Flocks are attracted to fish farms because of the high fish density, and the ease at finding and catching fish. Enclosing ponds with netting was found to be the best method of excluding predatory birds (Barlow 1995).

The Victorian Marine and Freshwater Resources Institute (MAFRI) is currently evaluating the integration of aquaculture with irrigated farming systems, to enhance productivity, water use efficiency and overall sustainability (McKinnon et al. 1996). The project aims to adapt and develop suitable husbandry and general production techniques to allow viable, cost effective, commercial fish production of silver perch in these systems. Trials are also being undertaken to determine the potential for inland mariculture in Victorian saline groundwater evaporation basins, using fish species such as silver perch, as well as shellfish species (Ingram et al. 1996).

Reasons for breeding fish Fish may be bred in hatcheries for a range of reasons, including aquaculture, development of put and take fisheries in impoundments, stocking of open systems and for endangered species (Keenan 1995). The methods of breeding fish can in some cases be potentially conflicting. For example, the aquaculture industry may manipulate genetic composition to produce fish with specific beneficial traits such as higher growth rates. However, the breeding of fish for stocking and conservation reasons should aim to maximise levels of natural genetic diversity (Keenan et al. 1996). Keenan (1995) notes that the significance of genetic manipulation in artificial breeding programs is often ignored. The impact of poorly planned stocking and breeding programs is likely to increase for species which continue to decline in the wild (Keenan et al. 1996).

Breeding programs for silver perch which aim to restore populations in rivers must aim to maximise genetic variation to ultimately maximise the chance of success of such stockings.

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Reason for stocking programs Clearly it would be preferable to protect sufficient natural habitat and conditions to enable successful reproduction of wild populations, rather than undertake stocking programs. A sufficiently large area needs to be protected to enable successful natural reproduction under the influence of natural selection.

However, stocking programs may be able to play a role in the restoration of riverine populations of silver perch. Releases may enhance natural reproduction, and may enable establishment or maintenance of fisheries where natural reproduction is not feasible (Willett 1996). Stocking programs may be undertaken to restore populations in areas with suitable habitat where the species has declined due to high fishing pressure as well as to reestablish populations in degraded habitats which have been rehabilitated. There seems little value in stocking sites which are degraded. It would be preferable to understand the reasons for such a decline and if possible reverse them before undertaking a stocking program. However since the reasons for the decline of silver perch are not fully understood, restocking programs of silver perch of suitable genetic composition in particular rivers may be a worthwhile management action. Stocking program should aim to rebuild populations to acceptable abundance levels so that they will then be able to maintain themselves by natural reproduction.

Genetic considerations for breeding programs There is a growing recognition that genetic issues need to be addressed in the breeding and release of native fish as a conservation tool. A Fish Genetics Workshop held in New South Wales in 1985 addressed concerns over the low number of broodstocks used in breeding programs, the mixing of wild and stocked populations and the need to develop suitable breeding and stocking programs for threatened fish species.

Genetic variability Natural populations of animals often show high levels of genetic variability which is closely linked to fitness-related characters concerning reproduction, growth and development (Smith and Chesser 1981). Loss of genetic variation has been recorded in hatchery stocks of fish, probably largely as a result of using limited numbers of broodstock through the founder effect, random genetic drift and inbreeding (Crozier 1994). Traits which can often show the influence of inbreeding depression relate to reproductive capacity (e.g. fecundity, egg size, hatchability) and physiological efficiency (e.g. fry deformities, growth rate, feed conversion efficiency, survival) (Kincaid 1983). The effects of inbreeding can vary between species (Gjerde et al. 1983). A small breeding population will increase inbreeding and decrease genetic variation leading to a reduced overall fitness, viability and productivity. This is because the population cannot adjust to environmental changes because of the loss of much of its genetic potential (Keenan 1995). While a finite population used for breeding will always exhibit some level of inbreeding over time, the larger the population, the slower this will occur and the normal influences of selection may avoid a serious problem (Kincaid 1983). Genetic

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Silver perch – A Resource Document variability should be maximised in breeding programs so that following stocking, the natural environment can be relied upon for selection of the fittest (Krueger et al. 1980).

Population genetics of silver perch Ryman (1981) emphasises the importance of understanding the level of genetic diversity still existing in wild populations. Keenan et al. (1996) investigated the extent of genetic subdivision and inbreeding in populations of silver perch in rivers and impoundments within the Murray Darling Basin, using electrophoretic and morphometric techniques. Keenan et al. (1996) observed reduced levels of genetic diversity in stocked populations of silver perch. Despite extensive sampling, only three polymorphic loci were identified. Another four loci showed unresolvable variability (C. Keenan, DPI, pers. comm. 1998). Three of the five stocked populations showed a reduced genetic variability compared to two wild populations and two other stocked samples. This limited genetic variation within some stocked populations was probably the result of hatcheries using small numbers of broodstock. I recent years, signs of genetic problems in hatchery stocks, such as poor quality fish and abnormalities, have begun to be apparent (S. Rowland, NSW Fisheries, pers. comm. 2000).

Mixing of wild and hatchery-reared fish Stocking programs in rivers often fail to consider the potentially harmful effects on wild populations (Keenan et al. 1996). The release of hatchery reared fish with reduced genetic variation, or a different genetic composition, may result in genetically harmful interactions with wild populations (Ryman 1981). Reduced genetic variation may affect the productivity and stability of fish populations and reduce adaptive potential. Keenan et al. (1996) recommend that the stocking of species between drainage basins should not occur due to the threat of inter species hybrid inviability. In North America, Pierce (1989) noted that local wild stocks of rainbow trout disappeared after the introduction of hatchery produced fish, due to hybridisation; the hybrids were also not well adapted to the local conditions. Hatchery reared fish released into an impoundment for a put and take fishery can potentially escape into river systems (Keenan 1995). Rowland (1995) notes that to date, there has been little evidence of escapes from farm dams in the eastern drainage of New South Wales.

Survival of hatchery-reared fish Johansson (1981) raises the issue of fish in hatcheries being well adapted to rearing conditions and possibly less adapted to the wild. When animals are held in captivity for long periods of time they may perform less well in the wild (Brown 1985). High levels of mortality of hatchery-reared fish can occur, which may be partly due to these fish failing to recognise predators since they have not attained this experience in life before their release. Brown and Smith (1998) found that hatchery-reared trout could be conditioned to identify chemical cues of a predator and recommended this be carried out before fish were released into the wild.

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Silver perch – A Resource Document Frankham (1985) notes that fish should spend as little time as possible in captivity so that there is no serious relaxation of natural selection; resampling broodstock from the wild each generation can avoid adaptation to captivity.

Effective population size It is recognised that breeding fish in hatcheries will always influence the genetics since a finite number of fish are involved. An acceptable balance must be reached where genetic variation is maximised and breeding programs are feasible. There is much debate over what may be the most appropriate numbers of broodfish to use in breeding programs. Whether broodfish are continually replaced or not influences the suggested numbers.

Both Frankham (1985) and Brown (1985) recommended having an effective population size of a few hundred fish. Blankenship and Leber (1995) suggest about 125 broodfish, with supplementation of these with 25% replacement each year from wild stocks. Kincaid (1983) considers the minimum number of breeding adults for maintaining a random mating broodstock should be at least 50 pairs or 50 of the least numerous sex with spawning adults taken throughout the spawning periods. This target considers the inability of most aquaculture broodstocks to be truly random mating and the need to keep the rate of inbreeding accumulation at a low level. Franklin (1985) notes that without continual introduction of new broodstock, an effective population size of at least 50 animals is needed. If new broodstock are introduced, a lower number of animals can be used. By using equal numbers of each sex in breeding programs, ensuring that each parent contributes equally to breeding generations, the effective population size of 50 can be achieved by 13 mating pairs (Brown 1985). Allendorf and Ryman (1987) consider 25 male and 25 females to be the absolute minimum number.

Source of broodstock Tave (1993) notes that obtaining broodstock is the most important step in population management, since this initial population determines the amount of initial genetic variation. It is important that broodstock are not taken from areas which are known to have been stocked before since this may affect the potential genetic variation available.

The loss of genetic variation in breeding numbers of fish in natural habitats is also of concern (Keenan 1995). Obtaining sufficient broodstock from the wild may also be a problem. Wohlfarth (1986) notes that when a fishery has declined significantly, the possibility that genetic changes have occurred within a population is generally not considered.

Thurstan and Rowland (1995) note that deciding where broodstock should come from depends on what is intended for the offspring. They recommend that if fish are being restocked in the wild, broodstock should come from the same area. If fish are to be stocked in an area with no existing population, then broodstock should be obtained from a range of sites. This will maximise the chances of a population adapting to its new environment (Frankham 1985).

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Silver perch – A Resource Document Gehrke et al. (1996) observed that the decline in abundance and distribution of silver perch in the wild may mean that obtaining wild brood fish for aquaculture may not be possible. New South Wales has recently introduced a moratorium on taking broodfish from the wild. In the past, Thurstan and Rowland (1995) indicated that in New South Wales, broodstock was taken from seven naturally reproducing populations in that state. Broodstock should be replaced every five years, since spawning success declines probably as a result of repeated hormone inducement treatment.

Age of release There is debate over what age and size of fish should be released in stocking programs to maximise their survival. There is a theory that larger sizes are likely to survive better due to less predator pressure, although the costs of rearing fish for longer within a hatchery may be significantly greater. Brown (1985) suggests that for stock rehabilitation programs, stock should be released at as early a stage as possible, to minimise the artificial selection that may occur in captivity.

There appear to be few studies on the effectiveness of stocking different sizes of native fish in Australia. Gehrke et al. (1996) stocked Lake Waldaira, a small deflation basin lake on the Murrumbidgee River floodplain, with approximately 250 000 silver perch larvae in 1993. No silver perch were subsequently recaptured. While this experiment only involved one lake, Gehrke et al. (1996) noted the results corresponded with other stocking programs and suggested that stocking floodplains with reared fish larvae may not be an effective method of enhancing natural recruitment.

Some work has been carried on stockings of the introduced trout. Moy (1974) observed a significant level of predation of yearling rainbow trout (7.5 cm) in a reservoir in Victoria, with 90% of redfin caught having trout in their stomachs. Following a subsequent release of larger rainbow trout (20.0 cm), no redfin sampled had trout in their stomachs. Baxter et al. (1985) also observed a significant level of predation of recently released rainbow trout fingerlings in Lake Burrumbeet by redfin in 1983. They suggested that establishing or maintaining a trout fishery in a redfin dominated water would need a number of stockings of large numbers of fingerlings. Trout stocking trials in three sites in Victoria recommended that when redfin are known to be present in a waterbody that releasing yearlings would be preferable to fingerlings, and where there are not large numbers of redfin and waters are plankton rich the release of fingerlings may be viable (Baxter 1987). A decline in the trout fishery in Brushy Lagoon in Tasmania was also attributed to redfin consuming the stocked young trout (Jarvis 1998).

Miller (1958) observed that survival for hatchery-reared trout released into lakes and streams where a resident population already exists is poor. Mortalities appear to occur quite soon after release, and may be due to competition with resident fish with introduced fish competing for niches and food.

Hesthagen et al. (1989) discuss the success of releasing takeable size (25-61 m) hatchery reared brown trout in a river in Norway. They recommended stocking just prior to when

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Silver perch – A Resource Document heavy angling pressure occurred for high harvest rates, and noted that survival of stocked fish was low between the first and second year after stocking, possibly due to malnutrition and/or decreased catchability.

13.2 Management Keenan (1995) notes that most hatcheries usually have less than 20 broodfish either taken from a wild population or including siblings from a single generation reared in the hatchery.

NSW Fisheries (1998) has a set of aquaculture permit guidelines. These relate to construction details, release of effluent, disease prevention etc. It notes that “Broodstock may not be accessed from the wild without a collection permit other than by purchasing fish from licenced commercial fishers”. Permits are required for liberation of fish into waters, and the sale of live fish interstate. NSW Fisheries recommends that five locally caught pairs being used as broodstock (I. Lyall, NSW Fisheries, pers. comm. 1998).

The Department of Natural Resources and Environment, Victoria (DNRE) is committed to developing native finfish aquaculture enterprises in line with economically sustainable development protocols. As part of an industry wide review of the role of the department in aquaculture, DNRE staff are reviewing and overseeing aspects of relevant licensing and policies issues including genetic issues. Current practice requires a general permit to be held by hatchery operators, the requirements of which are currently under review. Currently, broodstock collection is largely done on an ad hoc basis. There are nine permits for hatcheries to hold silver perch although it is thought that there are only two hatcheries currently breeding silver perch (A. Bearlin, DNRE, pers. comm. 2000). The number of broodfish held by these operations is not known.

In Queensland, hatchery production of native finfish has expanded considerably in the last 15 years. The major increase in hatchery production has occurred with golden perch and silver perch. A significant part of this expansion is probably attributable to the initiation of the Recreational Fishing Enhancement Program in 1986-87 to create or enhance freshwater recreational fisheries in impoundments and to encourage community support for inland fisheries. Local management groups assist with the stocking of these dams with native fish species and raise additional funds for the purchase of fingerlings for supplementary stockings. The main species involved in the restocking progam have been golden perch, silver perch and Australian bass. While there is an awareness of genetic issues, in part due to the development of programs by the Bribie Island Aquaculture Centre, there is no industry code governing the management issues. The recommendations of Rowland and Bryant (1995) are generally utilised.

In South Australia, there is currently high interest in developing a native finfish aquaculture industry. Primary Industry Research South Australia bears the responsibility for granting aquaculture licences. There are currently no guidelines or policy other than to follow accepted practice and experience of other growers due to the infancy of the industry.

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Translocation policy There have been substantial introductions and translocations of hatchery reared silver perch in eastern Australia (Musyl and Keenan 1996) as well as translocations of wild adults. Silver perch have been translocated to various river systems, as well as impoundments and private dams. They have been released into many eastern coastal river systems of New South Wales and south eastern Queensland and south western Western Australia (Merrick 1996). A National Policy for the Translocation of Live Aquatic Organisms (Ministerial Council on Forestry, Fisheries and Aquaculture 1999) has been prepared which discusses issues surrounding translocations, provides national policy principles and guidelines for development of policies in each states where policies do not exist.

Queensland Silver perch have been stocked both between catchments within the Murray Darling Basin as well as outside the basin. A translocation policy is currently being developed in Queensland. It is proposed that translocations will be considered only where there is a clear potential economic, social or conservation benefit and where no alternative native species in a drainage basin have similar potential. It is proposed that it will not be permitted in catchments where the integrity of the native fish community remains substantially intact, where there are one or more threatened fish species (conservation priority catchments) and where there are several native fish species of value (translocation unnecessary catchments (QFMA 1996). The proposed policy also supports the translocation of threatened species with an emphasis on establishing breeding populations and releasing fish into environments where they can reproduce. This is seen as a last resort management tool, and an assessment of disease risk must be made. Stocking of public waters in Quensland requires a permit.

New South Wales In New South Wales, silver perch have been stocked between catchments within the Murray Darling Basin, as well as west of the divide. A permit is requried from NSW Fisheries for the release of fish into rivers. An Introduction and Translocation Policy (NSW Fisheries 1994) states proposals to stock fish into rivers within their natural range will be evaluated on a case by case basis, and that normally broodstock would be required to be taken from the same populations into which the progeny are to be stocked. NSW Fisheries (1994) notes that hatchery fingerlings of silver perch are stocked into farm dams in the eastern drainages and in large impoundments in the Hunter River drainage. The species is now thought to be widespread in farm dams east of the Great Dividing Range. Some have escaped into the wild, although it was noted that there is no evidence that they are affecting local fish populations in the east. NSW Fisheries (1994) also noted that there was no evidence that silver perch have bred or established viable populations in the eastern drainages and suggested that this was unlikely. A permit is required from NSW Fisheries to stock silver perch into farm dams in eastern drainages.

Victoria

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Silver perch – A Resource Document There have not been extensive stocking programs for silver perch in Victoria. The Victorian Fisheries policy indicates that native fish will be stocked for conservation purposes in waters within the known former range of the species, except where species management or research needs arise or exist, including maintenance of existing populations (DCE 1988).

South Australia Pierce (SARDI, pers. comm. 1998) is not aware of any stocking of silver perch in river systems in South Australia, although there have been stockings in dams for put-and-take fishing.

13.3 Assessment of the Significance of the Threat The release of hatchery-reared silver perch both within and outside its former range is not likely to have played a significant role in the species’ decline of silver perch. It is however, a very important management issue for the future, primarily in relation to genetic implications of poor breeding programs. Stocking programs in river systems often do not take into account the potentially detrimental effects on wild stocks (Keenan et al. 1996). As the population size of species such as silver perch continue to decline in natural populations and hatchery production increases, the potential impacts of poorly planned stocking and breeding programs will increase. The decline of silver perch throughout most of its range means that retaining genetic resources is crucial for the future.

There is a significant aquaculture industry for silver perch in Australia. While many fish are bred as table food, a certain component are purchased and released for recreational fishing and conservation purposes. Large scale stocking programs of silver perch have been undertaken in the past in parts of the Murray Darling Basin. It is likely that the genetic composition of many fish bred in hatcheries is restricted which may have implications if these fish survive and breed in the wild. Silver perch are stocked in numerous farm dams in eastern Australia, and there is the potential for fish to escape from farm dams into river systems (Wager 1994). For example, while silver perch have been recorded in the north of Brisbane, they have never officially been stocked into that river. Wager (1994) observed silver perch to move over a farm dam spillway into a tributary of the Caboolture River. In New South Wales, Rowland (1995d) notes that to date there is little evidence that many fish have escaped from farm dams into the eastern drainages, even though common gully dams regularly overflow. However, he notes that silver perch have been recorded in the Clarence and systems, and that the presence of this species may be detrimental to endemic species such as the threatened eastern freshwater cod and Australian bass. Rowland (1995d) notes that it is unlikely that self-sustaining populations of silver perch would occur in these short coastal rivers, since they have fast and strong floods.

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14 Commercial Fishing The role the commercial fishery for silver perch may have played in the decline of this species is difficult to determine. In particular, the lack of historical catch per unit effort data means it is not possible to accurately determine how trends are influenced by changes in fishing effort or changes in actual fish stocks.

There was once a significant commercial catch of silver perch within the Murray Darling Basin: primarily in New South Wales and South Australia with a smaller catch in Victoria. There has never been a commercial fishery for this species in Queensland. There was a relatively consistent pattern of decline of commercial catches in New South Wales from the early 1960s onwards and in South Australia from the early 1980s onwards. There is now no longer a commercial fishery for silver perch within the Murray Darling Basin.

There have been general comments made by many authors that the primary reasons for declines in fish populations are related to changes in the environment. Once populations reach low numbers, the influence of pressures such as commercial fishing may increase. Thus, regardless of the possible impact of commercial fishing on silver perch in the past, the ban on commercial fishing should continue to be supported while the species remains in such a precarious position throughout the majority of the Murray Darling Basin. The introduction of a voluntary ban by commercial fishers in New South Wales and South Australia within the last ten years indicates the general recognition of the need to cease commercial taking of the species while it remains at such low population levels.

14.1 Background Historically, a commercial fishery existed for silver perch within the Murray Darling Basin. A review of the trends in this fishery provides some insight into the previous and current status of this species. Harris (1995) observed that both commercial and recreational fisheries can provide important information about the condition of the aquatic environment over time and space. There is a potential for commercial fisheries to provide useful monitoring data on status of fish stocks (Reid et al. 1997). Fisheries are affected by the health of the environment, fish ecology and exploitation, as well as social and technological change, such as improvements in fishing gear and changing economic values (Harris 1995). These aspects need to be recognised and quantified. The amount of fishing effort is very important in interpreting trends in catch rates. Fish numbers may decline due to a degraded ecosystem, with it being unlikely that consistent high yields of fish can be produced in a degraded environment (Harris 1995). It is likely that fish numbers, particularly in a variable system such as the Murray Darling Basin, will fluctuate naturally over time, being influenced by environmental conditions. For example, Harris (1995) indicated that strong population recruitment in silver perch occurs following flooding. Pierce and Doonan (1999) note that a key goal for fisheries managers is to match the amount of fishing with natural fluctuations in fish populations to avoid management-induced population crashes. Freshwater Ecology, NRE & Murray Darling Basin Commission 126

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There has been a general decline in the freshwater fishery within the Murray Darling Basin and Cadwallader (1985) suggested it was unlikely that the commercial inland fisheries would increase significantly in the future. Silver perch were once the fourth most important commercially exploited inland freshwater species (Pollard et al. 1980). A breakdown of commercial catch of silver perch between states using data from 1961 to 1975 indicated that New South Wales and South Australia dominated the industry: New South Wales 68%, South Australia 28% and Victoria 4% (Pollard et al. 1980). Pollard et al. (1980) noted the slow decline of catches in the Murray River from 23 200 kg in 1958/59 to 2300 kg in 1975/76. While these figures have been revised since this time, the same trend of decline is apparent in the catch data. The level of catch compared to fishing effort needs to be considered when interpreting such figures but unfortunately detailed historical records are not available. A more detailed assessment of the history of the fishery in each state is provided below.

South Australia The commercial fishery in South Australia is separated into the River Murray fishery ('reaches') and Lakes/Coorong fishery. The management of the fishery on a reach basis where commercial fishermen have sole access to certain stretches of river is unique in Australia. Poole (1984) provides a useful summary of the history of this inland fishery. In 1936, there were 310 licensed fishermen: 115 in the Lakes/Coorong area, 195 allocated to reaches (Poole 1984). In 1976, Reynolds (1976) indicated that there were 45 full time and 32 part time fishermen in the Lakes/Coorong fishery and 28 full time and 45 part time fishermen in the reach fishery. In the Murray River, Rohan (1988) noted the clear decline in the number of reaches being fished over time. He indicated that while the number of licence holders has declined, since 1976 the amount of area available to be fished had not been reduced and licensed fishers had been given access to backwaters. In 1988, Rohan (1988) suggested that the fishing effort had probably increased, noting that there had also been an expansion in the area available to commercial fishermen and that there were no restrictions on the number of drum nets which could be used. Currently, there are 39 owner-operators in the Lakes/Coorong fishery and 30 owner-operators in the river reaches (Pierce and Doonan 1999). An inquiry into the fish stocks of inland waters in South Australia by the Parliament of South Australia observed that while the number of commercial operators had declined in recent years, the rights of those remaining fishermen were increased in some ways (Parliament of South Australia 1999).

Commercial catch information is available from 1968 onwards with information from the SAFIC and SAFISH magazines, Cadwallader (1985) and data provided by Pierce (SARDI, pers. comm. 1998). Catch records prior to 1968 are patchy (Walker 1982) and individual species were not identified (Pollard et al. 1980). Pillar (1981) indicated that catch per unit effort data had only recently been monitored in South Australia. Pierce (SARDI, pers. comm. 1998) notes that catch per unit effort is confounded by climatically dominated catchability and thus should not be interpreted in isolation.

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Silver perch – A Resource Document The amount of native fish taken commercially from the Murray River has declined significantly since the 1950s. There has also been a shift away from the Murray River, with larger catches taken from the Lakes fishery. Rohan (1988) indicated that compared to the importance of River Murray water for drinking and irrigation, the commercial fishery is insignificant. In recent years, the commercial catch in South Australia has focused on four species; golden perch, Murray cod, carp and bony bream (Rohan 1988).

Figure 14.1 Commercial catch of silver perch in South Australia between 1968/69 and 1996/97

12000 10000 8000 6000 4000

Catch (kg) 2000 0 1976/77 1978/79 1980/81 1982/83 1984/85 1986/87 1988/89 1990/91 1992/93 1994/95 1996/97 Date

In relation to silver perch, there has been a clear decline in the South Australian fishery. Commercial fishermen have been voluntarily releasing silver perch for almost a decade, and so catch statistics will inaccurately illustrate the apparent decline (B. Pierce, SARDI, pers. comm. 1998). When commercial South Australian fishermen were surveyed in 1992, they indicated that 77% of fishermen returned silver perch. As indicated in Figure 14.1 the catches of silver perch have varied significantly between 1976/77 and 1996/97. There was a peak of 10 665 kg in 1983 followed by a rapid decline.

A number of reviews of the management of the commercial fishery in South Australia have been undertaken in recent years. Revised management arrangements for the river fishery were prepared in 1989, while a draft plan for structural adjustment in the South Australian river fishery was undertaken in 1996. This draft plan included recommendations allowing the transfer of individual licences, a reduction of licences to between 25 and 30, opportunity to relocate and restructure fishing areas, a cap on amount of fishing gear and access to backwaters. In 1999, the Parliamentary Environment, Resources and Development Committee prepared a report on the environmental impact of both commercial and recreational fishing on native fish stocks in inland waters. The Parliament of South Australia (1999) recommended: • the immediate investigation into a fair and equitable way of phasing out commercial fishers from Murray River over a period of no more than 10 years. All those who

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Silver perch – A Resource Document have a vested interest in the future sustainability of the River Murray should be considered to share whatever cost is associated with the phase out • the investigation of the introduction of a docket system • that commercial fishers should not have access to harvesting native fish in backwaters (which was in conflict to the recommendations of the 1996 draft plan) • that commercial fishers be actively encouraged and supported to take up fish farming of native species outside the riverine environment • further investigation into the appropriateness of use of gill nets. It was noted that the current number was grossly excessive

In response to these recommendations, the South Australian government has rejected the inquiry’s call for a phase out commercial fishing in the Murray River within 10 years. Commercial fishermen were not allowed access to native fish in backwaters until the Inland Fisheries Management Committee had considered the issue. The maximum number of gill nets that could be used at any one time was reduced from 50 to 30. Other recommendations supported included the increase in funding for research into inland fish stocks and the publication of annual native fish stock assessments.

Pierce and Doonan (1999) note that it would be ideal if accurate estimates of fish stock abundances and dynamics were available for use in determining sustainable harvest limits for commercial and recreational fisheries. Since such information is not available, stocks are currently assessed in South Australia according to information on habitat health, abundance of juveniles and adults and non target prey species, as well as commercial and recreational catch data.

Victoria The commercial fishery of silver perch in Victoria does not appear to have ever been substantial. However, it seems impossible to quantify the exact take since there is little accurate data available regarding commercial catch data for freshwater fish in Victoria (D. Tippet, MAFRI, pers. comm. 1997). A government inquiry in 1960 indicated that the number of professional fishing licences in 'inland waters' in Victoria between 1946 and 1959 ranged from 29 to 57 (State Development Committee 1960). Pollard et al. (1980) indicated that there was no catch data from Victoria from 1972/73 onwards. Cadwallader (1985) provides some figures on commercial catch of silver perch in Victoria between 1976 and 1984 (provided by the former Victorian Commercial Fisheries Branch, Department of Agriculture and Rural Affairs). Accurate data is available for only about the last four years. Prior to this date, returns apparently contain numerous errors. Many fishermen filled out forms incorrectly, and can no longer be contacted to review the data. Much of the information has not been checked or collated (D. Tippet, MAFRI, pers. comm. 1999). Currently, there are six licensed fishers and several other permit holders in Victoria who predominantly exploit carp. Available information on commercial catch of silver perch is provided in Figure 14.2.

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Silver perch – A Resource Document Figure 14.2 Commercial catch of silver perch in Victoria between 1976/77 and 1983/84 and 1993/94 and 1996/97

1800 1600 1400 1200 1000 800 600

Catch (kg) 400 200 0 1976/77 1978/79 1980/81 1982/83 1984/85 1986/87 1988/89 1991/92 1993/94 1995/96 Date

New South Wales The number of inland commercial fishing licences has declined significantly from 1300 in the 1920s, to a peak of 280 in 1971, 214 in 1981, 76 in 1986/87, 56 in 1992 and 40 in 1996/97 (O’Connor 1988, Brown 1994, Reid et al. 1997). Recent declines are due to a freeze in new licences in 1983 and stronger licensing provisions. Reid et al. (1997) note that the fishery currently only operates on about 5% of the available length of the waterways. The inland commercial fishery is small and opportunistic depending on seasonal conditions. In 1994, Richardson (1994) indicated that the commercial fishing only occurred in the lower section of the Murray River in the waters to the west of the Darling River and several specified lakes. In 1997, Reid et al. (1997) stated that the commercial fishery was now centred mainly in the lower Murray River, the Murray/Wakool/Edwards rivers complex and the Menindee Lakes, with those rivers in the north only being fished infrequently due to the long periods of low to no flows. Commercial fishing for native finfish is being phased out with the closure taking effect on 1 September 2001 (A. Sanger, NSW Fisheries, pers. comm. 1999). The number of participants in the fishery will decline, as some are willing to surrender their licences in exchange for an ex gratia payment. The commercial inland fishery for finfish is to be phased out in September 2001.

New South Wales has the most detailed commercial catch data for the Murray Darling Basin system. Accurate data is available since 1947; before this the annual reports did not separate catch data by individual species, and so commercial catch of silver perch before this date cannot be derived. Pease and Scribner (1993) note that the annual report of New South Wales published details of commercial catch data between 1883 and 1981. A database system to process fisheries catch statistics has since been developed and all licensed fishermen are required to submit monthly catch returns. There has been a closed season from September through November in the inland fishery since 1940 (Peace and Grinberg 1995). Freshwater Ecology, NRE & Murray Darling Basin Commission 130

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Peace and Grinberg (1995) note that catch per unit effort cannot be calculated for most of the historical data since sufficient information is not available prior to 1990. It is therefore impossible to determine whether variations in catch are due to differences in fishing effort or changes in actual fish stocks. However, Peace and Grinberg (1995) note that there were peaks in fishing effort in 1948/49 and 1952/53. The peaks in catches which occurred in the mid 1950s and late 1970s probably corresponded to times of high rainfall and increased river flows (Peace and Grinberg 1995).

In terms of silver perch, catch peaked in 1958/59 with 43 940 kg harvested followed by a relatively consistent pattern of decline until the fishery collapsed in the mid 1980s (Figure 14.3). Catches of silver perch have been concentrated in the Murray and Murray Riverina, Lower Murray, Darling, Murrumbidgee and Lachlan regions (Peace and Grinberg 1995).

Figure 14.3 Commercial catch of silver perch in New South Wales between 1947/48 and 1995/96

50000 40000 30000 20000

Catch (kg) 10000 0 1947/48 1951/52 1955/56 1959/60 1963/64 1967/68 1971/72 1975/76 1979/80 1983/84 1987/88 1991/92 1995/96 Date

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14.2 Management In South Australia, many commercial fishermen have been voluntarily releasing silver perch for about 10 years. In 1997, the species was declared a protected fish in this state by regulation under section 42 of the Fisheries Act 1982. In New South Wales, commercial fishermen introduced a voluntary ban on commercial handing of silver perch in 1993. In Victoria, the commercial catch of silver perch has been minimal for many years. Since the species was listed under the Flora and Fauna Guarantee Act 1988 in 1998, the taking of silver perch now requires a permit. In Queensland, there has never been a commercial fishery for silver perch.

14.3 Assessment of Significance of the Threat It is difficult to determine what role commercial fishing of silver perch may have played in this species' decline. It is likely that in the past, prior to extensive changes within the Murray Darling Basin, population levels for species including silver perch were influenced by the natural variability of the environment, particularly by events such as floods and droughts. The species was once a significant component of the commercial catch in South Australia and New South Wales.

Whether a commercial fishery for silver perch was ever sustainable is impossible to determine. Baker and Pierce (1997) observed that decisions concerning management policies are dominated by user groups such as commercial fishers. The lack of historical information in relation to catch per unit effort makes the interpretation of trends in the fishery difficult. However, it is clear that there is now no longer a commercial fishery for this species, whatever the reason. In South Australia, a report by the Parliament of South Australia (1999) on fish stocks of inland waters questioned whether the River Murray fishery is being managed sustainably. The report noted the lack of published scientific data on status of native fish stocks means that the fishery is being managed without the required information. Pierce (SARDI, pers. comm. in Parliament of South Australia 1999) argues commercial catch information provides an index of abundance. Pierce notes that SARDI uses an index of relative abundances to manage the fishery, using netting at specific habitat points set up along the river which provide a measure each year of fish produced.

Reid et al. (1997) indicate that a number of authors have suggested that environmental impacts on the river system has been the most pervasive influence on decline in fish numbers. Cadwallader and Lawrence (1990) argue that the impact of overfishing is likely to be hidden by natural population fluctuations until late in the population decline. They suggested that in comparison to habitat degradation, overfishing is likely to be insignificant, at least initially, in a markedly declining population. Once a species had declined significantly then its vulnerability to overfishing is likely to increase.

Walker (1982) states that speculation on the role of overfishing on the decline of native fish species has largely been rejected or ignored. Rowland (1989) suggested that the

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Silver perch – A Resource Document decline of Murray cod at least until the 1930s was primarily caused by overfishing. Walker (1982) indicated that it is likely that native fish are likely to be susceptible to heavy fishing pressure during periods when their recruitment has been reduced. Once populations are at a low level it is more difficult for them to recover. Lloyd et al. (1991) argued that overfishing can occur when the target species has a low fecundity or breeding success or is relatively sedentary or very large so that relatively few individuals can be supported by the environment. Roughley (1951) noted a general, heavy decline in the fish population in the Murray River and suggested possible reasons for this included a "great intensification of fishing, both legal and illegal.." and "the destruction of great quantities of immature fish by unlicensed fishermen".

In the Parliament of South Australia (1999) report of fish stocks in South Australian inland waters, Walker (1999) stated that the three main reasons for the change in fish stocks have been "… changing environment, particularly effects of flow regulation, interactions with introduced species…and the third and much more contentious and much less understood factor is the effect of fishing practices." This report included discussion of work by Stapleton (1996) for the Bookmark Biosphere Trust which stated that there are two major hypotheses, not mutually exclusive, regarding the low populations of native fish [found in the study]: - lack of recruitment due to lack of fish food which in turn might be attributed to poor water quality and/or effects of management of the river system - overpredation (fishing), fish mortality attributed to other factors Stapleton (1996, cited in Parliament of South Australia 1999) argued that overpredation may be significant because: - Both native and exotic species which are not attractive to the market are present in gill nets in significantly higher numbers than are those which are attractive to the market - Size distributions are weighted towards small fish - Seasonality does not change trends in data - Water quality is sufficient to support macroinvertebrates (fish food) - which are common in these waters.

In South Australia, Poole (1984) observed that even as early as 1936 there was rising concern over the lack of fish in the river. Commercial fishermen apparently blamed recreational fishermen who at the time were not subject to size regulations and were accused of taking large numbers of undersized fish. Poole (1984) questions whether this was true, although he noted that there was a decline in numbers of cod being caught.

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15 Recreational Fishery The lack of comprehensive data on the recreational fishing catch and effort in the past means it is difficult to determine the impact this may have had on silver perch. In general, overfishing has not been considered a key reason for declines of freshwater fisheries but rather one of numerous threats which may contribute to declines. However, once a species such as silver perch has experienced a significant decline in abundance and distribution, it may become more vulnerable to fishing pressure.

Throughout most of its range, silver perch now has a patchy distribution and is frequently caught in only small numbers in rivers. Large scale stockings of silver perch have taken place in impoundments in New South Wales, primarily for put-and-take fisheries. It is appropriate that there are fishing regulations controlling the take of silver perch within the Murray Darling Basin. There has been an increase in the controls over taking in recent years which reflects the recognition of the species' significant decline. These controls vary, with the strongest protection in South Australia where the species is fully protected. Silver perch is also protected in all rivers in New South Wales. While silver perch remains in such a vulnerable state, fishing regulations should continue and it may be appropriate to increase the level of protection.

Fisheries have generally been managed in a reactionary way with regulations only being implemented once populations have declined significantly. A more cautious approach to managing fisheries is appropriate where regulations are put in place that can be relaxed in the future if the status of silver perch populations improves.

15.1 Background Recreational fishing, including marine and freshwater, is Australia's largest outdoor participation sport and recreational activity, with more than 4.5 million people fishing each year (NRFWG 1994). Participation in recreational freshwater fishing is clearly high. In Victoria, of the 841 000 people who fished in 1996, 38% fished only in freshwater while 14% fished in both freshwater and marine areas (http://www.nre.vic.gov.au). In New South Wales in 1996, there were about 265 000 recreational freshwater anglers fishing in New South Wales (Pepperell 1996). In South Australia, there are about 160 000 recreational fishers for the entire inland resource, where the River Murray component is the principle area targeted (Pierce and Doonan 1999). In Queensland, there are between 700 000 and 1 million recreational anglers although their efforts are concentrated in marine areas and freshwater impoundments (Gwynne 1995). Only a small proportion fish exclusively in freshwater. These figures indicate the popularity of freshwater fishing and emphasise the potential 'take' by recreational anglers.

There have traditionally been conflicts concerning resource sharing between commercial and recreational fishermen. Kearney (1995) indicated that some recreational fishers blame declines in the resource on commercial fishers. Recreational take is significant for many fisheries, and in the case of most freshwater fish species may be larger than the commercial component (NRFWG 1994). While the commercial fishery in Murray Freshwater Ecology, NRE & Murray Darling Basin Commission 134

Silver perch – A Resource Document Darling Basin has declined since a peak after World War II, the recreational fishery has boomed (Harris 1995). The increase in recreational fishing may be due to increased leisure time, improved access and equipment. Numerous studies indicate that there is a very significant expenditure on recreational fishing each year (equipment, boats, vehicles, accomodation, bait, tackle etc).

Illegal take has also been a long standing concern which has never been quantified accurately and is very difficult to monitor. This includes those who do not follow existing regulations as well as those recreational fishers who sell their catch. In South Australia, Pierce (SARDI, pers. comm. Parliament of South Australia 1999) noted "… that the harvest capable out of illegal wire nets is similar to ... a commercial drum net." and that "... poaching takes a harvest the equivalent of the entire commercial sector harvest."

15.2 Management A National Policy for Recreational Fishing was developed in 1994. It included goals relating to maintaining and enhancing stocks and their habitats, developing partnerships between relevant groups, allocating the resource in a fair and reasonable way and establishing information and funding bases (NRFWG 1994). Key principles identified for recreational fishing included that fisheries management decisions should be based on sound information including fish biology, fishing activity, catches and economic and social values of recreational fishing and that adequate funding was required to manage recreational fisheries.

Additional funding to improve the recreational fishery is likely to become available since a recreational fishing license has recently been introduced in New South Wales and an all water licence in Victoria in recent years. There have also been proposals for recreational angling licenses in South Australia. In Queensland, no licence is required for recreational fishing although a permit is required for particular stocked impoundments.

Regulating fishing activities, including implementing bag limits, size limits, closed seasons and areas and gear restrictions are all techniques for managing recreational fisheries. Regulations should not be perceived as permanent and non negotiable (Diggles and Simpson 1998) and regular assessments and reviews of regulations are appropriate. The current regulations relating to silver perch are provided in Table 15.1.

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Table 15.1 Recreational fishing regulations for silver perch

State Size Limit (cm) Bag Limit Queensland 30 Combined total of 10 of silver perch, Welch's grunter and Barcoo grunter New South Wales 25 • Catch and release (rivers) • Five fish (impoundments) South Australia Fully protected Fully protected ACT Fully protected Fully protected Victoria 25 • Bag limit of 5 in lakes and impoundments north of the Great Dividing Range • Silver perch must not be taken from all other waters north of the Great Dividing Range (excluding the Wimmera Basin) • Bag limit of 5 in all waters south of the Great Dividing Range (including the Wimmera Basin)

Fishing regulations for silver perch vary between states. Since the species has declined significantly throughout most of its range it is clearly appropriate that regulations are in place. Silver perch was declared fully protected in South Australia in 1997 and the taking of this species will be prohibited until "… such time as there is evidence that their stocks have increased to levels which can sustain fishing" (Diggles and Simpson 1998). Such an approach is similar to that taken previously for Murray cod in South Australia. In the late 1980s a moratorium was implemented because poor stocks were observed with recruitment failure following a succession of drought years. Good recruitment subsequently occurred following a series of annual floods. Once stocks had increased a regulated fishery was reopened in 1994 (Diggles and Simpson 1998).

The recreational fishing regulations for silver perch have changed in recent years. A review of fishing regulations and native fish in 1992 recommended measures to protect fish resources should be more conservative (Richardson 1994). New South Wales authorities recommended changes to angling laws to provide total protection of silver perch from commercial and recreational fishing in rivers, indicating angling should only be permitted in dams populated with stocked fish (Gehrke et al. 1996). In 1995, Mallen- Cooper et al. (1995) noted that in South Australia there was a bag limit of six fish and a size limit of 33 cm, that there were no restrictions in Victoria and that there was a bag limit of 10 fish in New South Wales in rivers. Thus, stronger regulations have become

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Silver perch – A Resource Document established in recent years which reflects the recent recognition of the significant decline of silver perch.

Minimum size limits aim to protect fish until they have reached a size by which they will have bred at least once. An issue of concern regarding this approach is whether a fishery will selectively harvest the faster growing members of each year class leaving slower growing ones and reducing average growth rate and size of maturity of whole population (Diggles and Simpson 1998). Mallen-Cooper et al. (1995) emphasised the variation in growth rates and size at maturity of silver perch. While Mallen-Cooper et al. (1995) indicated the mean minimum size of fish at maturity was 263 mm (215 mm for males, 312 mm for females), Merrick and Schmida (1984) stated females matured at 340 mm and males at 233 mm standard length. Fisheries managers need to take such regional variations into account. Existing size limits may not adequately protect silver perch in reaching maturity before being susceptible to recreational fishing. Mallen-Cooper et al. (1995) also indicated that since females tend to be larger, using minimum size limits may cause more female fish to be taken. They recommended having a bag limit of two, with a maximum size of 35 cm to be exceeded by one fish. In Queensland, the minimum size of 30 cm was selected on the basis that fish of this size were likely to have spawned at least once (although this is not relevant in impoundments where they never spawn) (QFMA 1996). QFMA (1996) noted that there has been some suggestion that the size limit be reduced to 25 cm in impoundments, to enable anglers to catch them more readily. However, it was acknowledged that having different minimum size limits for natural and impounded populations may be confusing, and potentially detrimental to silver perch in the wild.

Establishment of regulations can assist in promoting the concept of recreational fishers only taking fish for their immediate needs. Kearney (1995) indicated changes are occurring in the approach of many recreational fishers. For example, fishing competitions in the past were often based on how many fish someone could kill, while catch and release fishing competitions are now increasingly common. Further, exceeding bag limits is now considered an anti social behaviour by many.

A recreational fishery needs to be managed sustainably so that there are enough fish to reproduce and replace those taken by anglers. Baker and Pierce (1997) note that in the past decision-making has been based largely on commercial fishing. Management has often been reactive with regulations implemented once data indicates that declines have occurred. A more precautionary approach would provide for future increases in fishing effort and pressure and would mean that regulations may not need to be changed as often.

There has been a lack of comprehensive data in the past in terms of recreational catch, effort and catch per unit effort data which makes it difficult to adequately regulate the recreational fishery. Relevant biological information needed includes stock structures, size of populations, growth rates, recruitment rates, and population trends over time. While several states undertake creel surveys and monitor angler information for

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Silver perch – A Resource Document particular impoundments, there is as yet no comprehensive database for recreational catch and effort throughout the Murray Darling Basin.

The recreational fishery for silver perch is based on wild fish populations as well as hatchery-reared stocked fish. Large scale stocking programs of silver perch has occurred in New South Wales and Queensland, primarily in impoundments for a put-and-take fishery. Some anglers have also translocated fish to establish new populations for recreational angling.

15.3 Assessment of Significance of the Threat The lack of adequate data makes it hard to quantify the level of recreational fishing for silver perch. Silver perch may be particularly vulnerble to recreational catch if they become aggregated below barriers. The presence of algae as a dietary component of adults may however render them difficult to capture. While there were fewer restrictions on take in the past, the level of fishing effort has probably increased. Pollard et al. (1980) suggested that anglers were not attracted to silver perch despite them being an edible fish that was caught in large numbers and provided good sport. In recent years, silver perch has generally not been a principle species targeted by recreational fishers. This may partly be due to the widespread decline of the species. A recreational fishing survey in Queensland in 1996 found that 4.3% of those interviewed targeted silver perch in the last year. In 1988, O’Connor (1988) indicated that silver perch was one of main species sought by recreational fishermen in New South Wales. In Victoria in 1984, Barnham (1984) indicated that of those who targeted inland native species, silver perch was targeted by only 1.4% of those surveyed. A more recent recreational fishing survey in Victoria in 1996 found that silver perch was not sought by anglers (Victorian Fisheries 1997). In South Australia, Pillar (1981) acknowledged collecting recreational fishing data was only in its early stages. Pillar (1979) recorded details of recreational fishing activity on one section of the Murray River from the New South Wales border to Purnong in South Australia. Between 1973 and 1978, he interviewed numbers ranging from less than 50 to over 1 000 people, who caught less than 50 to almost 600 silver perch. Rohan (1988) subsequently indicated that catches of recreational fishers are not monitored in this state. He considered the Murray River region to be the second most popular recreational fishing area in South Australia. Walker (1982) indicated that although recreational fishing in the Reach and Lakes fishery is intense, policing of the limited regulations is low. He noted competition and interference with fishing gear of commercial operators is a problem of an unknown level.

In Australia, there is limited information on the impact of recreational fishing on native freshwater aquatic species. Fishing pressure is often included in the list of threats and reasons for decline but there is little direct evidence of its specific impact. While declines and the destruction of many key recreational fisheries have occurred, it is difficult to separate the effects of a range of impacts. For example, in Tasmania, overfishing and habitat degradation have had a damaging impact on populations of giant freshwater crayfish (Cullen and Lake 1995). Geddes (1990) discussed declines in some populations

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Silver perch – A Resource Document of yabby and River Murray Crayfish which were at least partly attributed to fishing pressure.

Overfishing is not thought to be a principle cause of declines in freshwater fisheries (Kearney et al. 1999). However, Kearney et al. (1999) argue fishing can have direct effects on target species as well as indirect effects on the composition of aquatic communities by loss of individuals and biomass which can direct fishing pressure to other species. Cadwallader and Lawrence (1990) argued that compared to threats such as habitat degradation, overfishing is likely to be initially insignificant even for a fish population which is declining significantly. The effect of overfishing may also be hidden by natural fluctuations in population size until a species is in the late stages of decline when small numbers and patchy distribution render species highly vulnerable to overfishing (Cadwallader and Lawrence 1990). Thus once a species is in a vulnerable state, fishing pressure may become significant when combined with a range of other threats. Fishing pressure may vary between areas with intense pressure at popular fishing spots which could contribute to local declines.

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16 Threat Assessment by Recovery Team

During the initial stage of this project, the recovery team undertook an exercise to rank the significance of threats to silver perch. This involved comparing each threat to all others and marking which threat was more important. At the end of this comparison, a total score was calculated for each threat. The ranking of threats was made based on the existing knowledge of recovery team members. It must be recognised, as has become apparent in the preparation of this resource document, that there are many gaps in our knowledge about the impact of many threats to silver perch.

Table 16.1 summarises the results of this threat ranking exercise. It indicates that there were differences in rankings between states, which would be expected due to the different pattern and distribution of particular threats between states, as well as differences in people's perceptions of the importance of threats. It was difficult to score some threats because many are interrelated and complex. However some trends were apparent. Alteration to flow regimes and barriers were ranked as very significant threats in all states. This probably demonstrates the general awareness that changes to flow regimes may affect the spawning success and movement of silver perch and that barriers restrict the movement of this species which is known to undergo migrations. Introduced species, which referred primarily to carp, was ranked highly in most states which may reflect general habitat degradation often associated with carp. Loss of floodplains was ranked quite highly in Queensland, New South Wales and Victoria which reflects the belief that floodplains may be important to recruitment; the exact significance of floodplains however still requires further research. The loss of aquatic plants was also ranked reasonably highly by all states, although little is known of how important this habitat component is for the species. Diseases and parasites were ranked quite highly which may reflect the awareness that silver perch is suceptible to EHN and VER and possibly also reflects concerns over the large scale stockings of hatchery-reared silver perch within the Murray Darling Basin. Interestingly, snag removal was ranked as the most important threat in Queensland, although the significance of snags as a habitat for silver perch is unknown. Food chain changes and sedimentation were also ranked highly in Queensland which varied from the ranking within other states. Fishing, both commercial and recreational, was not considered a significant threat throughout the Murray Darling Basin and salinity was also ranked low in significance.

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Table 16.1 Ranking of risks to silver perch by recovery team members

Threat SA Vic NSW Qld Overall Ranking Alteration of flow regimes 1 2 1 2 1 Barriers 2 1 3 2 2 Introduced species 3 4 2 6 3 Loss of floodplains 8 3 4 2 4 Loss of aquatic plants 7 6 6 4 5 Alteration of temperature 10 5 3 6 5 regimes Diseases and parasites 4 9 3 8 6 Water quality issues 6 12 4 5 7 Sedimentation 10 8 5 3 8 Community changes 9 7 7 4 9 Loss of riparian vegetation 11 12 5 4 10 Food chain changes 8 4 12 3 11 Snag removal 11 7 10 1 12 Fish stocking (genetic 5 13 11 7 13 issues) Salinity 12 11 8 10 14 Recreational fishing 13 10 11 9 15 Commercial fishing 13 14 9 11 16

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17 References

ACT Parks and Conservation Service (1995) A review of recreational fishing in the ACT. Public discussion paper prepared by the ACT Parks and Conservation Service.

Allan, G. L. (1995) Development of artificial diets for silver perch. In: Proceedings of silver perch aquaculture workshops, Grafton and Narrandera, April 1994. NSW Fisheries. p. 77-88.

Allan, G. and Rowland, S. J. (1992) Development of an experimental diet for silver perch (Bidyanus bidyanus). Austasia Aquaculture 6(3): 39-40.

Allan, J. and Lovett, S. (1997) Impediments to managing environmental water provisions. Bureau of Resource Sciences, Canberra.

Allen, G (1989) Freshwater fishes of Australia. TFH Publications, Australia.

Anderson, J. R. (1990) Age determination for native freshwater fish in the Murray Darling Basin. In: Australian Society for Fish Biology Proceedings No. 12. The Measurement of Age and Growth of Fish and Shellfish. Lorne 22-23 August 1990. Bureau of Rural Resources. [Ed. Hancock, D. A.]. p. 45-60.

Anderson, J. R. (1991) The implications of salinity and salinity management initiatives, on fish and fish habitat in the Kerang Lakes Management Area. Arthur Rylah Institute for Environmental Research Technical Report Series No. 103. Department of Conservation and Environment, Victoria.

Anderson, A. J. and Arthington, A. H. (1989) Effect of dietary lipid on the fatty acid composition of silver perch (Leipotherapon bidyanus) lipids. Comparative Biochemistry and Physiology 93B(3): 715-720.

Anderson, J. R. and Morison, A. K. (1989) Environmental flow studies for the Wimmera River. Part D. Fish populations - Past, present and future; Conclusions and recommendations. Arthur Rylah Institute for Environmental Research Technical Report Series No. 76. Department of Conservation, Forests and Lands, Victoria.

Anthony, D. (1999) Outcomes of the program for the Australian cotton industry and directions for the future. In: Minimising the impact of pesticides on the riverine environment: Key findings from research with the cotton industry - 1998 conference. LWRRDC Occasional Paper 23/98. Land and Water Resources Research and Development Corporation. p. 13-15.

ANZECC and ARMCANZ (draft 1999) National Water Quality Management Strategy. Australian and New Zealand guidelines for fresh and marine water quality, Volume 2:

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Silver perch – A Resource Document Aquatic ecosystems - rationale and background information. Prepared under the auspices of Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand.

Arthington, A. (1995) State of the rivers in cotton growing areas - Northern New South Wales and the Border rivers within Queensland. LWRRDC Occasional Paper Series No. 02/95. Land and Water Resources Research and Development Corporation, Environmental Protection Agency, NSW.

Baker, D. L. and Pierce, B. E. (1997) Does fisheries management reflect societal values? Contingent valuation evidence for the River Murray. Fisheries Management and Ecology 4: 1-15.

Barlow, C. G. (1995) Bird predation of silver perch in ponds. In: Proceedings of Silver Perch aquaculture workshops, Grafton and Narrandera, April 1994. NSW Fisheries. p. 89-96.

Barlow, C. G., McLouglin, R. and Bock, K. (1986) Complementary feeding habits of golden perch and silver perch in farm dams. Proceedings of Linnean Society of New South Wales 109(3): 143-152.

Barrett, J. W. H., Petersen, S. M. and Batley, G. E. (1991) The impact of pesticides on the riverine environment with special reference to cotton growing. A report for the Cotton Research and Development Corporation, and the Land and Water Resources Research and Development Corporation. New South Wales.

Baxter, A. F. (1987) The trout fingerling stocking trials in Birch Creek, and Lake Modewarre, 1982-84. Fisheries Management Report No. 15. Department of Conservation, Forests and Lands, Victoria.

Baxter, A. M. (1985) Genetic aspects of the propagation and distribution of fish. In: Proceedings: Fish Genetics Workshop, Cronulla New South Wales 31 July-1 August 1985. [Ed. Rowland, S. J. and Barlow, R.]. p. 38-45

Baxter, A. F., Vallis, S. L. and Hume, D. J. (1985) The predation of recently released rainbow trout fingerlings Salmo gairdneri by redfin Perca fluviatilis in Lake Burrumbeet, October-December 1983. Arthur Rylah Institute for Environmental Research Technical Report Series No. 16. Department of Conservation, Forests and Lands, Victoria.

Beumer, J. P., Ashburner, L. D., Burbury, M. E., Jette, E. and Latham, D. J. (1983) A checklist of the parasites of fishes from Australia and its adjacent Antarctic territories. Commonwealth Agricultural Bureaux, Canberra.

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Silver perch – A Resource Document Bilby, R. E. and Likens, G. E. (1980) Importance of organic debris in the structure and function of stream ecosystems. Ecology 61: 1107-1113.

Blanch, S., Ganf, G. G. and Walker, K. F. (1999) Tolerance of riverine plants to flooding and exposure indicated by water regime. Regulated Rivers: Research and Management 15: 43-62.

Blandowski, W. (1857) Recent discoveries in natural history on the Lower Murray. Philosophical Institute of Victoria 2(1): 124-137.

Blyth, J. D., Doeg, T. J. and St Clair, R. M. (1984) Response of the macroinvertebrate fauna of the Mitta Mitta River, Victoria, to the construction and operation of Dartmouth Dam. 1. Construction and initial filling period. Occasional Papers from the Museum of Victoria 1: 83-100.

Bornette, G., Amoros, C. and Lamouroux, N. (1994) Theoretical habitat templets, species traits and species richness: aquatic macrophytes in the Upper Rhone River and its floodplain. Freshwater Biology 31: 487-505.

Boulton, A. J. and Lloyd, L. N. (1992) Flooding frequency and invertebrate emergence from dry floodplain sediments of the River Murray, Australia. Regulated Rivers: Research and Management 7: 137-151.

Bowling, L. C. and Baker, P. D. (1996) Major cyanobacterial bloom in the Barwon- Darling River, Australia, in 1991, and underlying limnological conditions. Marine and Freshwater Research 47: 643-57.

Bowmer, K. H., Bales, M. and Roberts, J. (1994) Potential use of irrigated drains as wetlands. Water Science and Technology 29(4): 151-158.

Bowmer, K. H., Fairweather, P. G., Napier, G. M. and Scott, A. C. (1996) Biological impacts of cotton pesticides. LWRRDC Occasional Paper No. 03/96. Occasional Paper Series. Land and Water Resources Research and Development Corporation, Cotton Research and Development Corporation, and Murray Darling Basin Commission.

Briggs, S. V. (1981) Freshwater wetlands. Chapter 15. In: Australian vegetation. [Ed. Groves, R. H.]. Cambridge University Press, Melbourne. p. 335-360.

Brooks, A. (1999) Biological monitoring: Central and north-west regions water quality program. Minimising the impact of pesticides on the riverine environment: Key findings from research with the cotton industry - 1998 conference. LWRRDC Occasional Paper 23/98. Land and Water Resources Research and Development Corporation. p. 73-78.

Freshwater Ecology, NRE & Murray Darling Basin Commission 144

Silver perch – A Resource Document Brooks, A. (1999a) Large woody debris and the geomorphology of a perennial river in southeast Australia. In: Second Australian Stream Management Conference 8-11 February 1999, Adelaide, South Australia. p. 129-136.

Brown, G. E. and Smith, R. J. F. (1998) Acquired predator recognition in juvenile rainbow trout Oncorhynchus mykiss: conditioning hatchery-reared fish to recognise chemical cues of a predator. Canadian Journal of Fisheries and Aquatic Science 55: 611-617.

Brown, P. (1994) The Murrumbidgee River fishery: An historical perspective. In: The Murrumbidgee - past and present. [Ed. Roberts, J. and Oliver, R.]. CSIRO Division of Water Resources, Griffith, 1994. p. 20-26.

Brumley, A. (1996) Chapter 15 - Family Cyprinidae. In: Freshwater Fishes of South- Eastern Australia. [Ed. McDowall, R.]. Reed Books, NSW. p.99-106.

Brumley, A. R., Morison, A. K. and Anderson, J. R. (1987) Revisions of the conservation status of several species of warmwater native fish after surveys of selected sites in northern Victoria (1982-1984). Arthur Rylah Institute for Environmental Research Technical Report Series No. 33. Department of Conservation, Forests and Lands, Victoria.

Cadwallader, P. (1977) J. O. Langtry’s 1949-1950 Murray River investigations. Victorian Fisheries and Wildlife Department Paper No. 13.

Cadwallader, P. (1978) Some causes of the decline in range and abundance of native fish in the Murray Darling River system. Proceedings of the Royal Society of Victoria 90: 211-223.

Cadwallader, P. (1979) Distribution of native and introduced fish in the Sevens Creek river system, Victoria. Australian Journal of Ecology 4: 361-385.

Cadwallader, P. (1983) A review of fish stockings in the larger reservoirs of Australia and New Zealand. FAO Fisheries Circular No. 757. Food and Agriculture Organisation of United Nations, Rome 1983.

Cadwallader, P. (1985) Freshwater fisheries production in Australia. Australian Fisheries 44(9): 12-15.

Cadwallader, P. (1986) Fish of the Murray-Darling system. Chapter 13B. In: Ecology of River Systems. Dr. W. Junk Publishers. The Netherlands. [Ed. Davies, B., R. and Walker, K. F.]. p. 679-694.

Cadwallader, P. and Backhouse, G. N. (1983) A guide to the freshwater fish of Victoria. Government Printer, Melbourne.

Freshwater Ecology, NRE & Murray Darling Basin Commission 145

Silver perch – A Resource Document

Cadwallader, P. and Lawrence, B. (1990) Fish. Chapter 22. In: The Murray. Murray Darling Basin Commission. Canberra. [Ed. Mackay, N. and Eastburn, D.] p. 317-335.

Callanan, M. D. (1985) Survey of the fish resources of the Darling River. Division of Fisheries, Department of Agriculture, New South Wales.

Callinan, R. B. and Rowland, S. J. (1995) Diseases of silver perch. In: Proceedings of silver perch aquaculture workshops, Grafton and Narrandera, April 1994. NSW Fisheries. p. 67-76.

Campbell, I. C. (1993) Riparian stream linkages: An Australian perspective on instream issues. Ecology and Management of Riparian Zones in Australia, Occasional Paper Series No. 05/93. [Ed. Bunn, S. E., Pusey, B. J., Price, P.]. Land and Water Resource Development Corporation, Canberra. p. 21-30.

Campbell, I. C. and Doeg, T.J. (1989) Impact of timber harvesting and production on streams: A review. Australian Journal of Marine and Freshwater Research 40: 519-39.

Cantlon, B. and Blanch, S. (1999) Cold water pollution from dams: The problem and some options for remediation. Inland Rivers Networks News 4(2): 5-7.

Carson, J. and Handlinger, J. (1988) Virulence of the aetiological agent of goldfish ulcer disease in Atlantic salmon Salmo salar L. Journal of Fish Diseases 11: 471-479.

Casanova, M. T., Douglas-Hill, A., Brock, M. A., Muschal, M. and Bales, M. (1997) Farm ponds in New South Wales, Australia: Relationship between macrophyte and phytoplankton abundances. Marine and Freshwater Research 48: 353-360.

Casselman, J. M. and Lewis, C. A. (1996) Habitat requirements of northern Pike Esox lucius. Canadian Journal of Fisheries and Aquatic Science. Volume 53: Supplement 1: 161-174.

Chapman, J. (1999) Laboratory ecotoxicology studies and implications for key pesticides. Minimising the impact of pesticides on the riverine environment: Key findings from research with the cotton industry - 1998 conference. LWRRDC Occasional Paper 23/98. Land and Water Resources Research and Development Corporation. p. 62-67.

Childs, M. R. and Clarkson, R. W. (1996) Temperature effects on swimming performance of larval and juvenile Colorado squawfish: Implications for survival and species recovery. Transactions of the American Fisheries Society 125 (6): 940-947.

Close, A. (1990) The impact of man on the natural flow regime. In: The Murray. Murray Darling Basin Commission. Inprint Ltd. Brisbane, Queensland. p.61-74.

Freshwater Ecology, NRE & Murray Darling Basin Commission 146

Silver perch – A Resource Document Codd, G. A. (1995) Cyanobacterial toxins: Occurrence, properties and biological significance. Water Science and Technology 32(4): 149-156.

Commonwealth of Australia (1993) Water Resources - Toxic Algae. A report from the Senate Standing Committee on Environment, Recreation and the Arts.

Cremer, K. (1999) Willow management for Australian rivers. Natural Resource Management Special Issue December 1999. Australian Association of Natural Resource Management.

Crivelli, A. J. (1983) The destruction of aquatic vegetation by carp. Hydrobiologia 106: 37-41.

Crozier, W. W. (1994) Maintenance of genetic variation in hatchery stocks of altantic salmon Salmo salar L.: experiences from the River Bush, Northern Ireland. Aquaculture and Fisheries Management 25: 382-392.

CNR (1995) Threatened fauna in Victoria - in 1995. Department of Conservation and Natural Resources, Victoria.

Davey, G. W., Doeg, T. J. and Blyth, J. D. (1987) Changes in benthic sediment in the Thomson River, southeastern Australia, during construction of the . Regulated Rivers: Research and Management 1: 71-84.

DCE (1988) Policy statement - Native fish stocking in public waters. Infosheet No. 7. Fisheries Management Notes. Department of Conservation and Environment, Victoria.

Dibble, E. D., Killgore, K. J. and Dick, G. O. (1996) Assessment of fish-plant interactions. American Fisheries Society Symposium 16: 357-372.

Dick, A. (1997) Carp - can they be controlled. Rural Research 175: 17-21.

Diggles, B. and Simpson, D. (1998) A discussion paper on the regulation of recreational fishing in South Australia. South Australian Fisheries Management Series. Primary Industries and Resources South Australia Fisheries Policy Unit.

Doeg. T. J. and Koehn, J. D. (1994) Effects of draining and desilting a small weir on downstream fish and macroinvertebrates. Regulated Rivers: Research and Management 9: 263-277.

Doeg, T. J. and Milledge, G. A. (1991) Effect of experimentally increasing concentrations of suspended sediment on macroinvertebrate drift. Australian Journal of Marine and Freshwater Research 42: 519-26.

Freshwater Ecology, NRE & Murray Darling Basin Commission 147

Silver perch – A Resource Document Dove, A., Cribb, T. H., Mockler, S. P. and Lintermans, M. (1997) The Asian Fish Tapeworm Bothriocelphalus acheilognathi in Australian freshwater fishes. Marine and Freshwater Research 48: 181-183.

DLWC (1995) State of the rivers report: Murrumbidgee catchment 1994-5. Volume 1. Department of Land and Water Conservation, New South Wales.

DLWC (1996) Macquarie Marshes water management plan 1996. Department of Land and Water Conservation and National Parks and Wildlife Service.

DLWC (1996a) New South Wales wetlands management policy. Department of Land and Water Conservation, New South Wales.

Driver, P., Harris, J. H., Norris, R. H. and Closs, G. P. (1997) The role of the natural environment and human impacts in determining biomass densities of common carp in New South Wales rivers. In: Fish and Rivers in Stress: The New South Wales Rivers Survey. NSW Fisheries, Cooperative Research Centre for Freshwater Ecology, Resource and Conservation Assessment Council. p. 225-245.

Elliot, J. M. (1981) Some aspects of thermal stress on freshwater teleosts. In: Stress in fish. [Ed. A.D. Pickering]. Academic Press, London and New York. p. 209-245.

Entwisle, T. J., Sonneman, J. A. and Lewis, S. H. (1997) Freshwater algae in Australia: A guide to conspicuous genera. Sainty and Associates, Pty. Ltd., New South Wales.

Environment ACT (2000) Fish stocking plan for the ACT: 2001-2005. Environment ACT.

Fairweather, P. G. (1999) Pesticide contamination and irrigation schemes: What have we learnt so far? Chapter 14. In: A Free-flowing River: The Ecology of the Paroo River. National Parks and Wildlife Service, Sydney. [Ed. Kingsford, R. T.]. p. 223-32

Faragher, R. A. and Lintermans, M. (1997) Alien fish species from the New South Wales Rivers Survey. Chapter 8. In: Fish and Rivers in Stress - The New South Wales Rivers Survey. NSW Fisheries, Cooperative Research Centre for Freshwater Ecology, Resource and Conservation Assessment Council. p. 201-224.

Fisher, J. R. and Claflin, T. O. (1995) Declines in aquatic vegetation in navigation pool No. 8, Upper Mississippi River between 1975 and 1991. Regulated Rivers: Research and Management 11: 157-165

Fletcher, A. R., Morison, A. K. and Hume, D. J. (1985) Effects of carp Cyprinus carpio L. on communities of aquatic vegetation and turbidity of waterbodies in the lower Goulburn River Basin. Australian Journal of Marine and Freshwater Research 36: 311-327.

Freshwater Ecology, NRE & Murray Darling Basin Commission 148

Silver perch – A Resource Document

Frankenberg, J. (1997) Guidelines for growing phragmites for erosion control. Cooperative Research Centre for Freshwater Ecology, Murray-Darling Freshwater Research Centre. Albury, New South Wales.

Frankham, R. (1985) Selection of captive populations. In: Proceedings: Fish genetics workshop, Cronulla New South Wales, 31 July-1 August 1985. [Ed. Rowland, S. J. and Barlow, R.]. p.30-34.

Franklin, I. R. (1985). Conservation Genetics. In: Proceedings: Fish genetics workshop, Cronulla New South Wales, 31 July-1 August 1985. [Ed. Rowland, S. J. and Barlow, R.]. p.35-37.

Geddes, M. C. and Puckridge, J. T. (1988) Survival and growth of larval and juvenile native fish - The importance of the floodplain. In: Proceedings of the workshop on native fish management. Murray Darling Basin Commission, Canberra 16-17 June 1988.

Gehrke, P. C. (1990) Clinotactic responses of larval silver perch (Bidyanus bidyanus) and golden perch (Macquaria ambigua) to simulated environmental gradients. Australian Journal of Marine and Freshwater Research 41: 523-528.

Gehrke, P. C. (1991) Enhancing recruitment of native fish in inland environments by accessing alienated floodplain habitat. Australian Society for Fish Biology Proceedings No. 16 Recruitment Processes. Hobart August 1991. [Ed. Hancock, D. A.]. p. 205-209.

Gehrke, P. C. (1994) Influence of light intensity and wavelength on phototactic behaviour of larval silver perch Bidyanus bidyanus and golden perch Macquaria ambigua and the effectiveness of light traps. Journal of Fish Biology 44: 741-751.

Gehrke, P. and Harris, J. H. (1994) The role of fish in cyanobacterial blooms in Australia. Australian Journal of Marine and Freshwater Research 45: 905-15.

Gehrke, P. C., Brown, P., Schilller, C. B., Moffatt, D. B. and Bruce, A. M. (1995) River regulation and fish communities in the Murray Darling River systems, Australia. Regulated Rivers Research and Management 11: 363-375.

Gehrke, P. Brown, P. and Schiller, C. (1996) Recruitment ecology of native fish larvae and juveniles. In: Riverine Environment Research Forum. Proceedings of the inaugural River Environment Research Forum of Murray Darling Basin Commission Natural Resource Management Strategy. 4-6 October, Attwood, Victoria. [Ed. Banens, B. and Lehane, B.] p. 9-16.

Freshwater Ecology, NRE & Murray Darling Basin Commission 149

Silver perch – A Resource Document Gilbert, W. S., Singh, G. and Ahmad, N. (1990) Pesticide contamination of the agricultural environment in the Namoi Valley 1976-77. New South Wales Agriculture and Fisheries, Technical Bulletin 39.

Gilbert, W. S., Singh, G. and Ahmad, N. (1990a) Pesticide contamination of birds and fish in the Namoi Valley 1977. New South Wales Agriculture and Fisheries, Technical Bulletin 40.

Gilbert, W. S., Singh, G. and Ahmad, N. (1992) Pesticide residue studies in the Gwydir and Namoi cotton growing environments 1981 and 1983. NSW Agriculture, Technical Bulletin 45.

Gippel, C. J. (1989) The use of turbidimeters in suspended sediment research. Hydrobiologia 176/177: 465-480.

Gippel, C., O'Neill, I. C., Finlayson, B. L. and Schnatz, I. (1996) Hydraulic guidelines for the reintroduction and management of large woody debris in lowland rivers. Regulated Rivers: Research and Management 12: 223-236.

Gjerde, B., Gunnes, K. and Gjerdrem, T. (1983) Effect of inbreeding on survival and growth in rainbow trout. Auqaculture 34: 327-332.

Glazebrook, J. S. (1995) Disease risk associated with the translocation of a virus lethal for Barramundi Lates calcarifer Bloch. Master of Environmental Management report, Griffith University, Queensland.

Government of Victoria (1996) Blue-green algae and nutrients in Victoria: A resource handbook.

Grant, E. M. (1978) Guide to Fishes. Department of Harbours and Marine, Brisbane.

Grant, E. (1987) Fishes of Australia. E.M. Grant Pty. Ltd. Publ. Queensland.

Growns, I. O., Pollard, D. A. and Gehrek, P. C. (1998) Changes in river fish assemblages associated with vegetated and degraded banks, upstream and within nutrient-enriched zones. Fisheries Management and Ecology 5: 55-69.

Guo, R., Mather, P. and Capra, M. F. (1993) Effect of salinity on the development of silver perch Bidyanus bidyanus eggs and larvae. Comparative Biochemistry and Physiology 104A(3): 531-535.

Guo, R., Mather, P. and Capra, M. F. (1995) Salinity tolerance and osmoregulation in silver perch Bidyanus bidyanus Mitchell (Teraponidae) an endemic Australian freshwater teleost. Marine and Freshwater Research 46: 947-952.

Freshwater Ecology, NRE & Murray Darling Basin Commission 150

Silver perch – A Resource Document Gwynne, H. L. (1995) Management of recreational fishing in Queensland. In: Recreational fishing: What's the catch? Proceedings of Australian Society for Fish Biology Workshop Canberra 30-31 August 1994. [Ed. Hancock, D. A.]. p. 220-223.

Hamlyn, A. and Thomas, M. (1995) A brief history of fish stocking in southern Queensland - Where are we at? In: Proceedings of a symposium held in Townsville, Queensland 11 November 1995. Fish stocking in Queensland- Getting it right. Queensland Fisheries Management Authority. [Ed. Cadwallader, P. and Kerby, B.] p. 25-33

Hamlyn, A., Thomas, M. and Brooks, S. (1997) Freshwater monitoring and stocking review - period 1 July 1995 to 30 June 1996. Report No. 14. Queensland Department of Primary Industry, Fisheries Division.

Harris, J. H. (1995) The use of fish in ecological assessments. Australian Journal of Ecology 20: 65-80.

Harris, J. (1996) Environmental rehabilitation and carp control. Chapter 3. In: Controlling carp - Exploring the options for Australia.. [Eds. Roberts, J. and Tilzey, R.] p. 21-36.

Harris, J. (1997) Fish bypass technology. In: Proceedings of the second national fishway technical workshop. [Eds. Berghuis, A., Long, P. and Stuart, I.]. p. 19-26.

Harris, J. H. and Battaglene, G. C. (1989) The introduction and translocation of native freshwater fish in south eastern Australia. In: The Introduction and translocation of fish and their ecological effects. Australian Society of Fish Biology Workshop, Magnetic Island 24-25 August 1989. Proceedings No. 8. Bureau of Rural Resources. [Ed. Pollard, D. A.]. p. 136-142.

Harris, J. H., Edwards, E. D. and Curran, S. J. (1992) Bourke weir fish passage study. Fisheries Research Institute Internal Report. 18pp.

Harris, J. H. and Gehrke, P. C. (1997) Fish and rivers in stress: The New South Wales Rivers Survey. NSW Fisheries, Cooperative Research Centre for Freshwater Ecology, Resource and Conservation Assessment Council. New South Wales.

Hesthagen, T. et al. (1989) Survival, exploitation and movement of takeable size brown trout Salmo trutta L. in a Norwegian river. Aquaculture and Fisheries Management 20: 475-484.

Hogan, A. (1995) A history of fish stocking in northern Queensland - Where are we at? In: Proceedings of a symposium held in Townsville, Queensland 11 November 1995. Fish stocking in Queensland- Getting it right. Queensland Fisheries Management Authority. [Ed. Cadwallader, P. and Kerby, B.]. p. 8-24.

Freshwater Ecology, NRE & Murray Darling Basin Commission 151

Silver perch – A Resource Document Hume, D. J., Fletcher, A. R. and Morison, A. K. (1983) Final Report Carp Program. Arthur Rylah Institute for Environmental Research. Fisheries and Wildlife Division, Ministry for Conservation.

Humphrey, J. D. and Ashburner, L. D. (1993) Spread of the bacterial fish pathogen Aeromonas salmonicida after importation of infected goldfish Carassius auratus into Australia. Australian Veterinary Journal 70 (12): 453-454.

Humphries, P. (1996) Aquatic macrophytes, macroinvertebrate associations and water levels in a lowland Tasmanian river. Hydrobiologia 321: 219-233.

Humphries, P., King, A. J. and Koehn, J. D. (1999) Fish, flows and flood plains: Links between freshwater fishes and their environment in the Murray Darling River system. Environmental Biology of Fishes 56: 129-151.

Hutchison, M. J. (1991) Distribution patterns of redfin perch Perca fluviatilis Linnaeus and western pygmy perch Edelia vittata Castelnau in the Murray River System, Western Australia. Records of the Western Australia Museum 15(1): 295-301.

Hynes, J. D., Brown Jr, E. H., Helle, J. H., Ryman, N. and Webster, D. A. (1980) Guidelines for culture of fish stocks for resource management. Canadian Journal of Fisheries and Aquatic Science 38: 1867-1876.

Hyne, R. V., Lim, R. P. and Leonard, A. W. (1999) Relationship between endosulphan concentrations and macroinvertebrate densities in the Namoi River over two cotton growing seasons. Minimising the impact of pesticides on the riverine environment: Key findings from research with the cotton industry - 1998 conference. LWRRDC Occasional Paper 23/98. Land and Water Resources Research and Development Corporation. p. 68-72.

Ingram, B. A. (1993) Evaluation of coded wire tags for marking fingerling golden perch Macquaria ambigua (Percichthyidae) and silver perch Bidyanus bidyanus (Teraponidae). Australian Journal of Marine and Freshwater Research 44: 817-824.

Ingram, B. et al. (1996) Potential for inland mariculture in Victorian saline groundwater evaporation basins. Austasia Aquaculture 10(2): 61-63.

Jackson, P. D. (1997) Strategy for fish passage in Queensland. In: Proceedings of the second national fishway technical workshop. [Ed. Berghuis, A., Long, P. and Stuart, I.] p. 1-8.

Jackson, P. D. (1993) Australian Threatened Fishes - 1993 Supplement. Australian Society of Fish Biology Newsletter 23(2): 22-25.

Freshwater Ecology, NRE & Murray Darling Basin Commission 152

Silver perch – A Resource Document Jackson, P. D. (1995) Australian Threatened Fishes - 1995 Supplement. Australian Society of Fish Biology Newsletter 25(2): 30-34.

Jackson, P. D. (1996) Australian Threatened Fishes - 1996 Supplement. Australian Society of Fish Biology Newsletter 26(1).

Jarvis, D. (1998) New life for Brushy Lagoon. On the Rise - Inland Fisheries Commission Newsletter 27(1): 4-5.

Jensen, A. (1996) Identifying and redressing the ecological consequences of river regulation in the lower River Murray. First National Conference on Stream Management in Australia. Merrijig 19-23 February 1996. p. 163-169.

Johansson, N. (1981) General problems in Atlantic Salmon rearing in Sweden. Fish Gene Pools. Ecological Bulletin 34. Proceedings of an International Symposium arranged by the Commission for Research on Natural Resources of the Swedish Council for Planning and Coordination for Research in Stockholm 23-25 January 1980. [Ed. Ryman, N.]. p. 75-83.

Junk, W. J., Bayley, P. B. and Sparks, R. E. (1989) The flood pulse concept in river- floodplain systems. In: proceedings of the International Large River Symposium. Canadian Special Publication of Fisheries and Aquatic Sciences. p. 110-127.

Kaenel, B. R., Matthaei, C. D. and Uehlinger, U. R. S. (1998) Disturbance by aquatic plant management in streams: Effects on benthic invertebrates. Regulated Rivers: Research and Management 14: 341-356.

Kearney, R. E. (1995) Recreational fishing: what's the catch? In: Recreational fishing: what's the catch? Australian Society for fish biology workshop proceedings. Canberra 30-31 August 1994. [Ed. Hancock, D. A.]. p.10-23.

Keenan, C. (1995) Genetic implications of fish stocking programs. In: Proceedings of a symposium held in Townsville, Queensland 11 November 1995. Fish stocking in Queensland- Getting it right. Queensland Fisheries Management Authority. [Ed. Cadwallader, P. and Kerby, B.] p. 64-71

Keenan, C., Watts R. and Serafini, L. (1996) Population genetics of golden perch, silver perch and eel-tailed catfish within the Murray-Darling Basin. In: Riverine Environment Research Forum. Proceedings of the inaugural River Environment Research Forum of Murray Darling Basin Commission Natural Resource Management Strategy. 4-6 October, Attwood, Victoria. [Ed. Banens, B. and Lehane, B.] Murray Darling Basin Commission. p.17-26.

Keenan, C., Watts R. and Serafini, L. (1998) Population genetics of golden perch Macquaria ambigua, silver perch Bidyanus bidyanus and eel-tailed catfish Tandanus

Freshwater Ecology, NRE & Murray Darling Basin Commission 153

Silver perch – A Resource Document tandanus within the Murray Darling Basin. Final Report of NRMS Project Number M262.

Kennedy, I. R., Sanchez-Bayo, F., Kimber, S. W. L., Beasley, H. and Ahmad, N. (1999) Movement and fate of endosulphan on-farm (New South Wales). In: Minimising the impact of pesticides on the riverine environment: Key findings from research with the cotton industry - 1998 conference. LWRRDC Occasional Paper 23/98. Land and Water Resources Research and Development Corporation. p. 33-37.

Kibria, G., Nugegoda, D., Fairclough, R. and Lam, P. (1996) Australian native species in aquaculture. Victorian Naturalist 113(5): 264-267.

Kibria, G., Nugegoda, D., Fairclough, R. and Lam, P. (1997) Pollution from aquaculture. Chemistry in Australia, January/February 1997: 19-20.

Kincaid, H. L. (1983) Inbreeding in fish populations used for aquaculture. Aquaculture 33: 215-227.

King, A. (1995) The effects of carp on aquatic ecosystems - a literature review. A Report prepared for the Environmental Protection Authority New South Wales, Murray Region.

King, A. Robertson, A. I. and Healey, M. R. (1997) Experimental manipulations of the biomass of introduced carp (Cyprinus carpio) in billabongs. I. Impacts on water- column properties. Marine and Freshwater Research 48: 435-43.

Kinghorn, B. P. (1983) A review of quantitative genetics in fish breeding. Aquaculture 31: 283-304.

Koehn, J. (1987) Artificial habitat increases abundance of two-spined blackfish Gadopsis bispinosus in Ovens River, Victoria. Arthur Rylah Institute for Environmental Research Technical Report Series No. 56. Department of Conservation, Forests and Lands, Victoria.

Koehn, J. (1996) Habitats and movements of freshwater fish in the Murray Darling Basin. In: Proceedings of the Murray Darling Basin Commission Riverine Environment Forum, October 1995, Atwood, Victoria Victoria. Murray Darling Basin Commission, Canberra. p. 27-32.

Koehn, J. D. and Morison, A. K. (1990) A review of the conservation status of freshwater fish in Victoria. Victorian Naturalist 107(1): 13-25.

Koehn, J., Doeg, T., Harrington, D. and Milledge, G. (1997) Dartmouth Dam: Effects on the downstream aquatic fauna. In: Proceedings of inaugural Riverine Environment Research Forum Natural Resources Management Strategy held at 4-6 October

Freshwater Ecology, NRE & Murray Darling Basin Commission 154

Silver perch – A Resource Document Attwood, Victoria. Murray Darling Basin Commission, Canberra, Australia. [Ed. Banens, R. J. and Lehane, R.]. p. 49-56.

Koehn, J. D. and Nichol, S. (1998) Habitat and movement requirements of fish. In: Proceedings of inaugural Riverine Environment Research Forum Natural Resources Management Strategy held at 4-6 October Attwood, Victoria. Murray Darling Basin Commission, Canberra, Australia. [Ed. Banens, R. J. and Lehane, R.]. p. 1-6.

Koehn, J. D., Gehrke, P. C. and Brumley, A. R. (in press) Managing the impacts of carp. Bureau of Rural Science.

Koehn, J. D. and Nichol, S. (in press) Native Fish Managment Strategy for the Murray- Darling Basin. Priorities and recommended actions. Murray Darling Basin Commission, Canberra.

Krueger, C. C., Charrett, A. J., Dehring, J. R. and Allendorf, F. W. (1980) Genetic aspects of fisheries rehabilitation programs. Canadian Journal of Fisheries and Aquatic Science 38: 1877-1881.

Lake, J. S. (1959) The freshwater fishes of New South Wales - Silver perch. State Fisheries Research Bulletin No. 5. p.7.

Lake., J. S. (1967a) Rearing experiments with five species of Australian freshwater fishes. I Inducement to spawn Australian Journal of Marine and Freshwater Research 18: 137-153.

Lake., J. S. (1967b) Rearing experiments with five species of Australian freshwater fishes. II Morphogenesis and ontogeny. Australian Journal of Marine and Freshwater Research 18: 155-173.

Lake, J. S. (1967c) Silver perch. In: Freshwater fish of the Murray Darling River system. State Fisheries Bulletin No. 7. New South Wales.

Lake, J. S. (1967d) Principal fishes of the Murray Darling River system. Chapter 8. In: Australian Inland waters and their Fauna. Australian National University Press, Canberra. [Ed. Weatherley, A. H.]. p. 192-213.

Lake. J. S. (1971) Freshwater Fishes of Rivers of Australia. Thomas Nelson, Sydney.

Langdon, J. S. (1989) Experimental transmission and pathology of epizootic haematopoetic necrosis virus EHNV in redfin perch Perca fluviatilis L., and 11 other teleosts. Journal of Fish Diseases 12: 295-310.

Freshwater Ecology, NRE & Murray Darling Basin Commission 155

Silver perch – A Resource Document Langdon, J. S., Humphrey, J. D., Williams, L. M., Hyatt, A. D. and Westbury, H. A. (1986) First virus isolation from Australian fish: An iridovirus-like pathogen from redfin perch Perca fluviatilis L. Journal of Fish Diseases 9: 263-268.

Langdon, J. S. and Humphrey, J. D. (1987) Epizootic haematopoietic necrosis, a new viral disease in redfin perch Perca fluviatilis L. in Australia. Journal of Fish Diseases 10: 289-297.

Langdon, J. S., Humphrey, J. D. and Williams, L. M. (1988) Outbreak of an EHNV-like iridovirus in cultured rainbow trout Salmo gairdneria Richardson, in Australia. Journal of Fish Diseases 11: 93-96.

Leggett, R. (1992) A report of freshwater fish and water quality at Eulo and other sites in . Queensland Naturalist 31 (5-6): 119-122.

Lintermans, M. (2000) The status of fish in the Australian Capital Territory: A review of current knowledge and management requirements. Environment ACT Technical Report No. 15.

Lintermans, M. and Burchmore, J. (1996) Family Cobitidae. Chapter 18 In: Freshwater Fishes of South-Eastern Australia. Reed Books. p. 114-115.

Llewellyn, L. C. (1983) Distribution of fish in New South Wales. Australian Society for Limnology Special Publication No. 7.

Lloyd, L. (1988) Either drought or plenty: Water development in New South Wales. Department of Water Resources, New South Wales. Kangaroo Press, Sydney.

Lloyd, L., Puckridge, J. and Walker, K. (1991) The significance of fish populations in the Murray-Darling system and their requirements for survival. In: Proceedings of the third Fenner Conference on the Environment. [Ed. Dendy, T. and Coombe, M.]. South Australian Department of Environment and Planning. p. 86-99.

Lugg, A. (1999) Eternal winter in our rivers: addressing the issue of cold water pollution. Internal Report for NSW Fisheries.

LWRRDC (1999) Minimising the impact of pesticides on the riverine environment: key findings form research with the cotton industry - 1998 conference. LWRRDC Occasional Paper 23/98. Land and Water Resources Research and Development Corporation.

LWRRDC (1999a) Riparian land management technical guidelines, Volume 1. Land and Water Resources Research and Development Corporation, Canberra.

Freshwater Ecology, NRE & Murray Darling Basin Commission 156

Silver perch – A Resource Document LWRRDC (1999b) Riparian land management technical guidelines, Volume 2. Land and Water Resources Research and Development Corporation, Canberra.

Mackay, N. J. (1973) Histological changes in the ovaries of the golden perch Plectroplites ambiguus associated with the reproductive cycle. Australian Journal of Marine and Freshwater Research 24: 95-101.

Mackay, N. J. and Shafron, M. (1988) Water quality. In: Proceedings of the workshop on native fish management. Murray Darling Basin Commission, Canberra.

Maheshwari, B. L., Walker, K. F. and McMahon, J. A. (1995) Effects of regulation on the flow regime of the River Murray, Australia. Regulated Rivers: Research and Management 10: 15-38.

Mallen-Cooper, M. (1993) Habitat changes and declines of freshwater fish in Australia: What is the evidence and do we need more? In: Sustainable Fisheries Through Sustaining Fish Habitat. [Ed. Hancock, D. A.] Australian Society for Fish Biology Workshop Proceedings. Victor Harbour, South Australia. AGSP. Canberra. p. 118- 123.

Mallen-Cooper, M. (1994) Swimming ability of adult golden perch Macquaria ambigua (Percichthyidae) and adult silver perch Bidyanus bidyanus (Teraponidae) in an experimental vertical-slot fishway. Australian Journal of Marine and Freshwater Research 45: 191-198.

Mallen-Cooper, M. (1997) Priorities for fishways in semi-arid and tropical streams. In: Proceedings of the second national fishway technical workshop. [Ed. Berghuis, A. Long, P. and Stuart, I.]. p. 27-34.

Mallen-Cooper, M. (in press) Developing fishways for nonsalmonid fishes: A case study from the Murray River in Australia. Proceedings of the Symposium on New Fish Passage Technologies, Monterey, August 25-29, 1997 [Ed. Odeh, M.]. American Fisheries Society.

Mallen-Cooper, M. and Edwards, E. (1991) Fish passage through the Main Weir at Menindee Lakes during the flood of September 1990. Fisheries Research Institute Internal Report. 17pp.

Mallen-Cooper, M. and Brand, D. (1992) Assessment of two fishways on the River Murray and historical changes in fish movement. NSW Fisheries Research Institute, report to the Murray Darling Basin Commission.

Mallen-Cooper, M. and Thorncroft, G. A. (1992) Fish passage and fish abundance at Brewarrina Weir following a bloom of blue-green algae. NSW Fisheries Research Institute Internal Report. 12pp.

Freshwater Ecology, NRE & Murray Darling Basin Commission 157

Silver perch – A Resource Document

Mallen-Cooper, M.and Stuart, I. G. (1995) Recruitment patterns of golden perch and silver perch in the Murray River: The importance of small floods. Abstract: Proceedings of Annual Conference of Australian Society of Fish Biology Newsletter. p.79.

Mallen-Cooper, M., Stuart, I. G., Hides-Pearson, F. and Harris, J. (1995) Fish migration in the Murray River and assessment of the Torrumbarry fishway. Final report for Natural Resource Management Strategy Project N002. NSW Fisheries Research Institute and the Cooperative Research Centre for Freshwater Ecology.

Mallen-Cooper, M. and Stuart, I. G. (in prep) Age, growth and some non-flood recruitment of two potamodromous fishes in a large semi-arid/temperate river system.

Margules and Partners Pty Ltd, P. and J. Smith Ecological Consultants, Department of Conservation, Forests and Lands (1990) Riparian vegetation of the River Murray. Murray Darling Basin Commission, Canberra.

McDowall, R. (1996) Freshwater Fishes of south-eastern Australia. Chapter 30 - Freshwater Perches. Reed Books, New South Wales. p. 183-185.

McGuckin, J., Anderson, J. R. and Gasior, R. J. (1991) Salt affected rivers in Victoria. Arthur Rylah Institute for Environmental Research Technical Report Series No. 118. Department of Conservation and Environment Victoria.

McKinnon, L., Gooley, G. and Gasior, R. (1996) Integrated silver perch aquaculture trials in the Goulburn-Murray Irrigation district of Victoria. Austasia Aquaculture 10(1): 45- 47.

MDBC (1987) Murray Darling Basin Environmental Resources Study. Murray Darling Basin Ministerial Council, Canberra.

MDBC (1993) Algal management strategy - background papers. Murray Darling Basin Commission, Canberra.

MDBC (1993a) Algal management strategy -Technical advisory group report. Murray Darling Basin Commission, Canberra.

MDBC (1994) The algal management strategy for the Murray Darling Basin. Murray Darling Basin Commission, Canberra.

MDBC (1995) An audit of water use in the Murray Darling Basin. Murray Darling Basin Commission.

Freshwater Ecology, NRE & Murray Darling Basin Commission 158

Silver perch – A Resource Document MDBC (1996) Setting the cap: Report of the independent audit group. Murray Darling Basin Commission.

MDBC (1997) Review of cap implementation 1996/97. Murray Darling Basin Commission.

MDBC (1998) Water audit monitoring report 1996/97. Report on the final year of the interim cap in the Murray Darling Basin. Murray Darling Basin Commission.

MDBC (1999) The salinity audit of the Murray Darling Basin: A 100 year perspective, 1999. Murray Darling Basin Ministerial Council, Canberra.

MDBC (2000) National management strategy for carp control. Prepared by Carp Control Coordination Group. Murray Darling Basin Commission, Canberra.

MDBC (in prep.) Barriers to fish migration: Inventory of potential barriers and priorities for action. Murray Darling Basin Commission, Canberra.

Merrick, J. R. and Schmida, G. E. (1984) Australian Freshwater Fishes - Biology and Management. Griffin Press Ltd. South Australia.

Merrick, J. R. (1996) Freshwater grunters or perches, Family Terapontidae. Chapter 26. Silver perch. In: Freshwater fishes in south eastern Australia. [Ed. McDowall, R.] Reed Books. p.164-166.

Metzeling, L., Doeg, T. and O'Connor, W. (1995) The impact of salinization and sedimentation on aquatic biota. Conserving biodiversity: Threats and solutions. [Ed. Bradstock, R. A. et al.]. Surrey Beatty and Sons. p. 126-136.

Midgely, S. H. (1989) Some river systems of south west Queensland. Unpublished report by Midgely and Midgely Consultants for the Minister for Primary Industries, Queensland.

Miller, R. B. (1958) The role of competition in the mortality of hatchery Trout. J. Fish Res. Bd. Canada 15(1): 27-45.

MMCC (1997) Macquarie marshes land and water management plan. Macquarie Marshes Catchment Committee.

Mottell (1995) Flood plain resources study lower Balonne flood plain in New South Wales. Mottell Pty. Ltd., Land and Water Management Consultants, Swan Hill.

Mowbray, D. L. (1978) The ecological effects of pesticides on non-target organisms: A study of the environmental impact of pesticides on wildlife in the Namoi River Valley

Freshwater Ecology, NRE & Murray Darling Basin Commission 159

Silver perch – A Resource Document cotton growing area, 1972-1976. PhD, School of Biological Sciences, University of Sydney.

Moy, D. (1974) Survival of Trout liberated into Barker’s Creek Reservoir, Harcourt. Freshwater Fisheries Newsletter 6: 19-20.

Muschal, M. and Cooper, B. (1999) Regional level monitoring of pesticides and their behaviour in rivers. Minimising the impact of pesticides on the riverine environment: Key findings from research with the cotton industry - 1998 conference. LWRRDC Occasional Paper 23/98. Land and Water Resources Research and Development Corporation. p. 44-50.

Niemeier, P. E. and Hubert, W. A. (1986) The 85 year history of the aquatic macrophyte species composition in a eutrophic prairie lake (United States). Aquatic Botany 25: 83-89.

NRE (1997) Guidelines for farming Barramundi in Victoria. Department of Natural Resources and Environment, Victoria. Fisheries Branch.

NRE (in press) Condamine-Balonne water allocation management plan. Environmental flows technical report. Department of Natural Resources and Environment, Queensland.

NRFWG (1994) Recreational fishing in Australia: A national policy. National Recreational Fisheries Working Group. Department of Primary Industries and Energy.

NSW Fisheries (1915) Report on the fisheries of New South Wales for the year 1914. Legislative Assembly, New South Wales, Fisheries.

NSW Fisheries (1994) Introduction and translocation policy. New South Wales Fisheris Policy Paper R94/1.

NSW Fisheries (1997) Barramundi farming policy A97/1. NSW Fisheries.

NSW Fisheries (1997a) NSW Fisheries - Status of fisheries resources 1996/7.

O’Brien, T. (1996) Assessment of the impact of saline drainage on key fish species. In: Riverine environment research forum. Proceedings of the inaugural river environment research forum of Murray Darling Basin Commission Natural Resource Management Strategy. 4-6 October, Attwood, Victoria. [Ed. Banens, B. and Lehane, B.] Murray Darling Basin Commission. p.43-48.

O’Connor, P. F. (1988) Fisheries Management in inland New South Wales. In: Proceedings of the workshop on native fish management. Murray Darling Basin Commission, Canberra 16-17 June 1988.

Freshwater Ecology, NRE & Murray Darling Basin Commission 160

Silver perch – A Resource Document

Ogden, R. (1996) Potential for the restoration of aquatic macrophytes in billabongs. In: First national conference on stream management in Australia, Merrijig 19-23 February 1996. p. 99-104.

Ogilby, J. D. and McCulloch, A. R. (1916) A revision of the Australian Terapons with notes on some Papuan species. Memoirs of the Queensland Museum 5: 99-126.

Olsen, P., Fuller, P. and Marples, T. C. (1993) Pesticide-related eggshell thinning in Australian raptors. Emu 93: 1-11.

Parliament of South Australia (1999) Fish stocks of inland waters. Parliament of South Australia, Environment, Resources and Development Committee. Thirty-first report of the committee.

Patra, R. W. C., Chapman, J. C., Lim, R. P. and Gehrke, P. C. (1995) Effects of temperature on the acute toxicity of endosulphan to silver perch Bidyanus bidyanus. Poster abstract PW129. In: Proceedings of the second SETAC world congress 16th annual meeting. Vancouver, B. C., Canada.

Patra, R. W. C., Chapman, J. C., Lim, R. P. and Gehrke, P. C. (1995a) Effects of sub- lethal concentrations of endosulphan on the critical thermal maxima of freshwater fish. Poster abstract PW113. In: Proceedings of the second SETAC world congress (16th annual meeting. Vancouver, B. C., Canada.

Peace, B. C. and Grinberg, A. (1995) New South Wales commercial fisheries statistics 1940 to 1992. Fisheries Research Institute, NSW Fisheries.

Peace, B. C. and Scribner, E. A. (1993) New South Wales commercial fisheries statistics 1990/1991. NSW Fisheries, Fisheries Research Institute.

Peace, B. C. and Scribner, E. A. (1994) New South Wales commercial fisheries statistics 1991/1992. NSW Fisheries, Fisheries Research Institute.

Peace, B. C. and Kathuria, A. (1996) New South Wales commercial fisheries statistics 1992/1993. NSW Fisheries, Fisheries Research Institute.

Pepperell. J. G. (1996) Recreational fishing in New South Wales April 1995 to April 1996. Report prepared for NSW Fisheries.

Peterie, C and Blanch, S. (1999) Fishways: The answer to weirs? Inland Rivers Network News 4(2): 8-10.

Freshwater Ecology, NRE & Murray Darling Basin Commission 161

Silver perch – A Resource Document Peterson, S. M. and Batley, G. E. (1991) Fate and transport of endosulphan and diuron in aquatic ecosystems. Prepared for the Australian Water Research Advisory Council, AWRAC Project Number 88/20.

Pen, L. J. and Potter, I. C. (1992) Seasonal and size-related changes in the diet of perch, Perca fluviatilis L., in the shallows of an Australian river, and their implications for the conservation of indigenous teleosts. Aquatic Conservation: Marine and Freshwater Ecosystems 2: 243-253.

Pierce, B. E. (1988) Improving the status of our River Murray fishes - A discussion paper on the potential of cooperative management. In: Proceedings of the workshop on native fish management. Murray Darling Basin Commission, Canberra. 16-17 June 1988. p. 7-18.

Pierce, B. E. (1989) Biological impacts of translocations on North American salmonids, or how to use the latest United States fisheries management techniques to decimate otherwise useful fisheries. In: The Introduction and translocation of fish and their ecological effects. Australian Society of Fish Biology Workshop, Magnetic Island 24- 25 August 1989. Proceedings No. 8. Bureau of Rural Resources. [Ed. Pollard, D. A.] p. 127-135.

Pierce, B. E. and Doonan, A. M. (1999) A summary report on the status of selected species in the River Murray and Lakes and Coorong Fisheries. South Australian Fisheries Assessment Series 99/09. South Australian Research and Development Institute.

Pillar, J. (1979) Summary of recreational fisheries in the River Murray 1973-78. SAFIC 3(3): 10-14

Pillar, J. (1981) A new age for fish in an old river. SAFIC 5: 15-18

Pollard, D. A., Llewellyn, L. C. and Tilzey, R. D. J. (1980) Management of freshwater fishes. Chapter 22. In: An ecological basis for water resources management. [Ed. Williams, W. D.] ANU Press Canberra, Australia. p. 225-267.

Poole, D. E. (1984) Inland fishery on the River Murray - Some historical facts. SAFIC, February 1984.

Pressey, R. L. (1986) Wetlands of the River Murray below Lake Hume. River Murray Commission.

Pribble, J. (1979) A proposal to assess the biological and environmental impact of carp Cyprinus carpio on Victorian waters. Fisheries and Wildlife Division, Ministry for Conservation.

Freshwater Ecology, NRE & Murray Darling Basin Commission 162

Silver perch – A Resource Document Puckridge, J. T., Sheldon, F., Walker, K. F. and Boulton, A. J. (1998) Flow variability and the ecology of large rivers. Marine and Freshwater Research 49: 55-72.

Queensland Fisheries Management Authority (1996) Queensland Freshwater Fisheries. Discussion paper No. 4. Freshwater Fisheries Management Advisory Committee, Queensland Fisheries Management Authority.

Queensland Fisheries Management Authority (1998) Queensland Freshwater Fisheries. Draft management plan and regulatory impact statement. Freshwater Fisheries Management Advisory Committee, Queensland Fisheries Management Authority.

Reich, P. (1998) Riparian vegetation. Trees and Natural Resources June 1998: 14-15.

Reid, D. D., Harris, J. D. and Chapman, D. J. (1997) New South Wales inland commercial fishery data analysis. FRDC Project No. 94/027.

Reynolds, L. F. (1976) Decline of the native fish species in the River Murray. SAFIC 1(8): 19-24.

Reynolds, L. F. (1983) Migration patterns of five fish species in the Murray-Darling River system. Australian Journal of Marine and Freshwater Research 34: 857-871.

Richardson, B. A. (1994) The human impacts on the ecology of freshwater fish in western New South Wales. Chapter 16. In: Future of the Fauna of Western New South Wales. [Ed. Lunney, D., et al.]. 169-173.

Richardson, W. B., Wickham, S. A. and Threlkeld, S. T. (1990) Foodweb response to the experimental manipulation of a benthivore Cyprinus carpio, zooplanktivore Menidia beryllina and benthic insects. Archiv fur Hydrobiologie 119(2): 143-165.

Roberts, J. (1999) Taking the pulse. In: A free-flowing river: the ecology of the Paroo River. [Ed. R. T. Kingsford]. New South Wales National Parks and Wildlife Service. p. 233-242.

Roberts, J. and Ludwig, J. A. (1991) Riparian vegetation along current-exposure gradients in floodplain wetlands of the River Murray, Australia. Journal of Ecology 79: 117-127.

Roberts, J., Chick, A., Oswald, L. and Thompson, P. (1995) Effects of carp Cyprinus carpio L. an exotic benthivorous fish on aquatic plants and water quality in experimental ponds. Marine and Freshwater Research 46: 1171-80.

Roberts, J. and Sainty, G. (1996) Listening to the Lachlan. Murray Darling Basin Commission, Sainty and Associates, New South Wales.

Freshwater Ecology, NRE & Murray Darling Basin Commission 163

Silver perch – A Resource Document Roberts, J. and Tilzey, R. (1996) Controlling carp - exploring the options for Australia. Proceedings of a workshop 22-24 October 1996, Albury. CSIRO Land and Water, Griffith. [Editors].

Robertson, A. I., King, A. J., Healey, M .R., Robertson, D. J. and Helliwell, S. (1995) The impact of carp on billabongs. A report prepared for the Environmental Protection Authority, New South Wales, Murray Region.

Robertson, A. I., Healey, M. R. and King, A. J. (1997) Experimental manipulations of the biomass of introduced carp Cyprinus carpio in billabongs. II. Impacts on benthic properties and processes. Marine and Freshwater Research 48: 445-54.

Robinson, S. E. (1981) The effect of the parasitic copepod Lernae cyprinaceae on the survival and condition of fish stocked in an urban impoundment. Australian Society for Fish Biology Newsletter 11(2): 22.

Rohan, G. (1988) River fishery (South Australia) - Review of management arrangements. In: Proceedings of the workshop on native fish management. Murray Darling Basin Commission, Canberra 16-17 June 1988. p. 37-54

Roughley, T. C. (1951) Fish and Fisheries of Australia. Angus and Robertson, Sydney.

Rowland, S. J. (1984) The hormone-induced spawning of silver perch Bidyanus bidyanus (Mitchell (Teraponidae). Aquaculture 42: 83-86.

Rowland, S. J. (1989) Aspects of the history and fishery of the Murray cod Maccullochella peeli (Mitchell) (Percichthyidae). Proceedings of the Linnean Society of New South Wales 111(3): 201-213

Rowland, S. J. (1995a) Water quality in the intensive pond culture of silver perch. In: Proceedings of Silver Perch aquaculture workshops, Grafton and Narrandera, April 1994. NSW Fisheries. p. 51-66.

Rowland, S. J. (1995b) The silver perch and its potential for aquaculture. In: Proceedings of Silver Perch aquaculture workshops, Grafton and Narrandera, April 1994. NSW Fisheries. p. 9-11.

Rowland, S. J. (1995c) Aspects of the reproductive biology and hatchery production of Murray cod, golden perch and silver perch from the Murray Darling River system. In: Proceedings of a seminar and workshop held on 26-27 September 1994. Translocation issues in Western Australia. Fisheries Management Paper No. 83. Fisheries Department of Western Australia. p. 38-49.

Rowland, S. J. (1995d) Stocking of freshwater fishes and policy in New South Wales. In: Proceedings of a seminar and workshop held on 26-27 September 1994. Translocation

Freshwater Ecology, NRE & Murray Darling Basin Commission 164

Silver perch – A Resource Document issues in Western Australia. Fisheries Management Paper No. 83. Fisheries Department of Western Australia. p. 50-61.

Rowland, S. J., Dirou, J. F. and Selosse, P,. M. (1983) Production and stocking of golden perch and silver perch in New South Wales. Australian Fisheries 42(9): 24-28.

Rowland, S. J. and Barlow, C. G. (1990) Fish Biology - The right prerequisites. A case study with freshwater silver perch (Bidyanus bidyanus). Austasia Aquaculture 5(5): 27-30.

Rowland, S. J. and Ingram, B. A. (1991) Diseases of Australian native freshwater fishes, with particular emphasis on the ectoparasitic and fungal diseases of Murray cod (Maccullochella peeli), golden perch (Macquaria ambigua) and silver perch (Bidyanus bidyanus). Fisheries Bulletin No. B. New South Wales Agriculture and Fisheries, Sydney.

Rowland, S. J., Allan, G., Hollis, M. and Pontifex, T. (1995) Production of Australian freshwater silver perch Bidyanus bidyanus (Mitchell 1838) at two densities in earthen ponds. Aquaculture 36: 317-328.

Rowland, S. J. and Bryant, C. (1995) Silver perch culture. Proceedings of silver perch aquaculture workshops, Grafton and Narrandera, April 1994. NSW Fisheries. [Ed.].

Ryan, T. and Davies, P. (1996) Environmental effects of salinity and nutrients from salt disposal: approaches to the development of management criteria. Flora and Fauna Technical Report No. 137. Department of Natural Resources and Environment.

Ryan, T., Gasior and Steegstra, D. (1999) Habitat degradation associated with saline stratification. A report for the Murray Darling Basin Commission, Natural Resource Management Strategy Project V238. Department of Natural Resources and Environment, Victoria.

Rybicki, N. B. and Carter, V. (1986) Effect of sediment depth and sediment type on the survival of Vallisneria americana Michx grown from tubers. Aquatic Botany 24: 233- 240.

Ryman, N. (1981) Conservation of gnetic resources: experiences from the brown trout Salmo trutta. In: Fish Gene Pools. Ecological Bulletin 34. Proceedings of an International Symposium arranged by the Commission for Research on Natural Resources of the Swedish Council for Planning and Coordination for Research in Stockholm 23-25 January 1980. [Ed. Ryman, N.]. p. 61-74.

Schaugaard, C. J. and Crowl, T. A. (1994) The effect of temperature regime and food availability on growth rates of Colorado squawfish Ptychocheilus lucius in the Green

Freshwater Ecology, NRE & Murray Darling Basin Commission 165

Silver perch – A Resource Document River, Utah. 1994 Annual Symposium of the Desert Fishes Council, Furnace Creek, CA (USA), 17-20 Nov. 1994.

Schultze, D. J. and Walker, K. F. (1997) Riparian eucalypts and willows and their significance for aquatic invertebrates in the River Murray, South Australia. Regulated Rivers: Research and Management 13: 557-577.

Scott, T., D., Glover, G. J. M. and Southcott, R. V. (1980) The marine and freshwater fishes of South Australia. Second Edition. D. J. Woolman, Government Printer, South Australia.

Selosse, P. M. and Rowland, S. J. (1990) Use of common salt in treating Ichthyophthiriasis in Australian warmwater fish. The Progressive Fish Culturalist 52: 124-127.

Shafron, M., Croome, R. and Rolls, J. (1990) Water Quality. In: The Murray. Murray Darling Basin Commission, Canberra. p. 147-166.

Sharley, T. (1993) Management issues in the Murray Darling Basin. In: Ecology and management of riparian zones in Australia. LWRRDC Occasional Paper Series No. 05/93. [Ed. Bunn, S., Pusey, B. J., Price, P.]. Land and Water Resources Research and Development Corporation. p. 99-110.

Simpson, P. S. and Eaton, J. W. (1986) Comparative studies of the photosynthesis of the submerged macrophyte Elodea canadensis and the filamentous algae Caldophor glomerata and Spirogyra sp. Aquatic Botany 24: 1-24.

Smart, R. M., Doyle, R. D. and Madsen, J. D. (1996) Establishing native submerged aquatic plant communities for fish habitat. American Fisheries Society Symposium 16: 347-356.

Smith, D. I. (1999) Water in Australia: Resources and Management. Oxford University Press, South Melbourne.

Smith, M. H. and Chesser, R. K. (1981) Rationale for conserving genetic variation for fish gene pools. In: Fish Gene Pools. Ecological Bulletin 34. Proceedings of an international symposium arranged by the Commission for Research on Natural Resources of the Swedish Council for Planning and Coordination for Research in Stockholm 23-25 January 1980. [Ed. Ryman, N.]. p. 13-20.

State Development Committee (1961/62) Report of the State Development Committee on the fishing industry in Victoria.

State Pollution Control Commission (1980) Namoi environmental study. State Pollution Control Commission, New South Wales.

Freshwater Ecology, NRE & Murray Darling Basin Commission 166

Silver perch – A Resource Document

Stanford, J. A. and Hauer, F. R. (1992) Mitigating the impacts of stream and lake regulation in the Flathead River catchment, Montana,, USA: An ecosystem perspective. Aquatic Conservation and Marine and Freshwater Ecosystems 2(1): 35- 63.

Sunderam, R. I. M., Cheng, D. M. H. and Thompson, G. B. (1992) Toxicity of endosulphan to native and introduced fish in Australia. Environmental Toxicology and Chemistry 11: 1469-1476.

Suren, A. M. and Lake, P. S. (1989) The fauna of macrophytes and unvegetated substratum in two southern Victorian streams. Bulletin Australian Society of Limnology 12: 1-14

Tave, D. (1993) Genetics for fish hatchery managers. Second Edition. Van Nostrand Reinhold, New York.

Tenison-Woods (1883) Fish and Fisheries of New South Wales. Government Printer, Sydney.

Thoms, M. and Walker, K. F. (1993) Channel changes associated with two adjacent weirs on a regulated lowland alluvial river. Regulated Rivers: Research and Management 8: 271-284.

Thoms, M. C., Sheldon, F., Roberts, J., Harris, J. and Hillman, T. J. (1996) Scientific panel assessment of environmental flows for the Barwon-Darling River. Department of Land and Water Conservation, Cooperative Research Centre for Freshwater Ecology.

Thoms, M., Suter, P., Roberts, J., Koehn, J., Jones, G., Hillman, T. and Close, G. (1998) Report of the River Murray Scientific Panel on Environmental Flows. River Murray- Dartmouth to Wellington and the Lower Darling River.

Thorncraft, G. A. and Harris, J. H. (1996) Assessment of rock-ramp fishways. NSW Fisheries Research Institute and the Cooperative Research Centre for Freshwater Ecology. Report for Environmental Trusts, NSW Environmental Protection Authority, Border Rivers Commission, Department of Land And Water Conservation, and Wyong Shire Council.

Thorncraft, G. A. and Harris J. H. (1997) Rock-ramp and lock fishways as tools for solving fish passage problems. In: Proceedings of the second national fishway technical workshop. [Ed. Berghuis, A. , Long, P. and Stuart, I.]. p. 203-215.

Freshwater Ecology, NRE & Murray Darling Basin Commission 167

Silver perch – A Resource Document Thorne, T. and Brayford, H. (1997) The aquaculture of non endemic species in Western Australia: silver perch Bidyanus bidyanus. Fisheries Management Paper No. 107. Fisheries Department of Western Australia.

Thresher, R. E. (1996) Physical removal as an option for the control of feral carp populations. In: Controlling carp: Exploring the options for Australia. [Eds. Roberts, J. and Tilzey, R.]. p 58-73.

Thurstan, S. J. (1991) Commercial extensive larval rearing of Australian freshwater native fish. In: Australian Society for Fish Biology Workshop Proceedings No. 15 Larval Biology. Hobart August 1991. [Ed. Hancock, D. A.]. p. 71-75.

Thurstan, S. and Rowland, S. (1995) Techniques for the hatchery production of silver perch. Proceedings of silver perch aquaculture workshop. Grafton and Narrandera, April 1994. Austasia Aquaculture/NSW Fisheries. p. 29-39.

Triska, F. J. and Cromack, K. Jr. (1980) The role of wood debris in forests and streams. In: Forests: Fresh Perspectives from Ecosystem Analysis. Proceedings of the 40th Annual Biology Colloquium. Oregon State University Press, Oregon. [Ed. Waring, R. H.].

Trueman, W. and Lucker, C. (1983) Recollections of Mr R. D. Mckenzie of the fishery of north east Victoria since 1908 with special reference to the endangered species trout cod or blue nose cod Macullochella macquariensis. Native Fish Australia.

Tunbridge, B. R. and Glenane, G. (1982) Fisheries value and classification of freshwater and estuarine waters in Victoria. Fisheries and Wildlife, Victoria.

Tunbridge, B. R. and Rogan, P. L. (1976) A guide to the inland angling waters of Victoria. Fisheries and Wildlife, Victoria.

Tunbridge, B. R. and Rogan, P. L. (1981) A guide to the inland angling waters of Victoria. Fisheries and Wildlife, Victoria.

Tunbridge, B. R., Rogan, P. L. and Barnham, C. A. (1991) A guide to the inland angling waters of Victoria. Department of Conservation and Environment, Victoria.

Unmack, P. J. (1994) Desert fishes down under. In: Proceedings of the Desert Fishes 1994 Symposium 26: 71-95.

USEPA (1975) DDT - A review of scientific and economic aspects of the decision to ban its use as a pesticide. United States Environmental Protection Agency, Washington, July 1975.

Freshwater Ecology, NRE & Murray Darling Basin Commission 168

Silver perch – A Resource Document Vari, R. P. (1978) The Terapon Perches (Peroidei, Teraponidae). A cladistic analysis and taxonomic revision. Bulletin of the American Museum of Natural History 159. Article 5.

Vermaat, J. E. and De Bruyne, R. J. (1993) Factors limiting the distribution of submerged waterplants in the lowland River Vecht (The Netherlands). Freshwater Biology 30: 147-157

Victorian Fisheries (1997) Tracking survey on recreational fishing in Victoria - 1996. Victorian Fisheries, Department of Natural Resources and Environment, Victoria.

Voelz, N. J. , Poff, N. L. and Ward, J. V. (1994) Differential effects of a brief thermal disturbance on caddisflies (Trichoptera) in a regulated river. American Midland Naturalist 132(1): 173-182.

Wager, R. (1993) The distribution and conservation status of Queensland freshwater fish. Department of Primary Industries, Fisheries Division, Brisbane.

Wager, R. (1994) Fish translocation and biodiversity of Queensland freshwater fishes. Australian Biologist 7(1): 23-32.

Walker, K. F. (1981) Effects of weirs on the environment of the Lower River Murray. SAFIC 5(6): 26-29.

Walker, K. F. (1982) Impact of Murray-Darling Basin Development of Fish and Fisheries. Chapter 4.4. Indo-Pacific Fisheries Commission Workshop on inland fisheries for planners. FAO Fisheries Report No. 28.

Walker, K. F. (1993) The River Murray, Australia: A semiarid lowland river. In: The Rivers Handbook, hydrology and ecological principles. [Ed. Calow, P. and Petts, G. E.]. Blackwell Scientific Publications. p. 472-492.

Walker, K. F. and Hillman, T. J. (1977) Limnological survey of the River Murray in relation to Albury-Wodonga 1973-1976. Albury-Wodonga Development Corporation and Gutteridge Haskins and Davey.

Walker, K. F., Hillman, T. J. and Williams, W. D. (1978) Effects of impoundments on rivers: an Australia case study. Verh. Internat. Verein. Limnol. 20: 1695-1701.

Walker, K. F., Thoms, M. C. and Sheldon, F. (1992) Effects of weirs on the littoral environment of the River Murray, South Australia. In: River Conservation and Management. [Eds. Boon, P. J. , Calow, P. and Petts, G. E.]. p. 271-292.

Freshwater Ecology, NRE & Murray Darling Basin Commission 169

Silver perch – A Resource Document Walker, K. F. and Thoms, M. C. (1993) Environmental effects of flow regulation on the lower River Murray, Australia. Regulated Rivers: Research and Management 8: 103- 119.

Walker, K. F., Boulton, A. J., Thoms, M. C. and Sheldon, F. (1995) Effects of water-level changes induced by weirs on the distribution of littoral plants along the River Murray, South Australia. Australian Journal of Marine and Freshwater Research 45: 1421-38.

Warburton, K., Retif, S. and Hume, D. (1998) Generalists as sequential specialists: Diets and prey switching in juvenile silver perch. Environmental Biology of Fishes 51: 445- 454.

Weatherley, A. H. (1963) Zoogeography of Perca fluviatilis (Linnaeus) and Perca flavescens (Mitchell) with special reference to the effects of high temperature. Proceedings of the Zoolological Society London 141: 557-576.

Whittington, R. J. (1996) Strategy meeting on Barramundi encaphalitis virus disease - implications for New South Wales. Unpublished report, New South Wales Department of Agriculture.

Whittington, R. J. and Cullis, B. (1988) The susceptibility of salmonid fish to an atypical strain of Aeromonas salmonicida that infects goldfish, Carassius auratus (L.) in Australia. Journal of Fish Diseases 11: 461-470.

Whittington, R. J., Djordjevic, S. P., Carson, J. and Callinan, R. B. (1995) Restriction endonuclease analysis of atypical Aeronomas salmonicida isolates from Goldfish Carassius auratus, silver perch Bidyanus bidynaus and Greenback Flounder Rhombosolea tapirina in Australia. Diseases of Aquatic Organisms 22: 185-191.

Whyte, R. J. and Conlon, M. L. (1990) The New South wales Cotton Industry and the Environment. State Pollution Control Commission.

Williams, W. D. (1980) Australian freshwater life. Macmillan Comp. Australia Pty. Ltd. Melbourne.

Williams, W. D. (1987) Salinization of rivers and streams: An important environmental hazard. Ambio 16(4): 180-185.

Williams, M. D. and Williams, W. D. (1991) Salinity tolerances of four species of fish from the Murray Darling River system. Hydrobiologia 210: 145-160.

Willett, D. J. (1993) Discrimination between hatchery stocks of silver perch Bidyanus bidyanus (Mitchell) using scale growth patterns. Aquaculture and Fisheries Management 24: 347-354

Freshwater Ecology, NRE & Murray Darling Basin Commission 170

Silver perch – A Resource Document Willett, D. J. (1994) Use of temperature to manipulate circulus patterns on scales of silver perch Bidyanus bidyanus (Mitchell) for the purpose of stock discrimination. Fisheries Management and Ecology 1: 157-163.

Willett, D. J. (1996) Use of scale patterns to evaluate stocking success of Silver Perch Bidyanus bidyanus (Mitchell) released at two different sites. Marine and Freshwater Research. 47: 757-761.

Wohlfarth, G. W. (1986) Decline in natural fisheries - A genetic analysis and suggestion for recovery. Canadian Journal of Fisheries and Aquatic Science 43: 1298-1306.

Wolf, A. E. , Willis, D. W. and Power, G. J. (1996) Larval fish community in the Missouri River below Garrison Dam, North Dakota. Journal of Freshwater Ecology 11(1): 11-19.

Wood, P. J. and Armitage, P. D. (1997) Biological effects of fine sediment in the lotic environment. Environmental Management 21(2): 203-217.

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18 Personal Communications

Baxter, A., Victorian Fisheries, Department of Natural Resources and Environment, Victoria Dove, A., formerly PhD student, Department of Parasitology, University of Queensland Harrington, D., (formerly) Department of Natural Resources and Environment, Victoria Hogan, A., Department of Primary Industries, Hutchison, M., Department of Primary Industries, Deception Bay, Queensland Humphrey, J., formerly Department of Natural Resources and Environment, Victoria Hyne, R., New South Wales Environment Protection Authority, Gore Hill, NSW Keenan, C., Department of Primary Industries, Bribie Island Aquaculture, Queensland Lay, C., Fishways Development Officer, NSW Fisheries Lieschke, J., Department of Natural Resources and Environment, Victoria Love, D., Department of Land and Water Conservation, Dubbo, NSW Lugg, A., NSW Fisheries, Nowra Mallen-Cooper, M., fish ecology consultant Moffatt, D., Department of Natural Resources, Toowoomba Nicol, S., Department of Natural Resources and Environment, Victoria O'Brien, T., Department of Natural Resources and Environment, Victoria O'Connor, B., Department of Natural Resources and Environment, Victoria O'Connor, J., Department of Natural Resources and Environment, Victoria O'Connor, P. NSW Fisheries Pierce, B., South Australian Research and Development Institute, West Beach Raadik, T., Department of Natural Resources and Environment, Victoria Rowland, S., NSW Fisheries, Grafton Sanger, A., NSW Fisheries, Albury Saddlier, S., Department of Natural Resources and Environment, Victoria Strongman, R., Department of Natural Resources and Environment, Victoria Stuart, I., Department of Natural Resources and Environment, Victoria Tippet, D., Marine and Freshwater Resources, Queenscliff, Victoria Whittington, R., NSW Agriculture, New South Wales Winwood, J., formerly executive officer, SA Recreational Fishing Council

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ISBN 978-1-74208-128-1 (Print) ISBN 978-1-74208-129-8 (Online)