Fish Communities of the Nepean River in the Vicinity of Pheasants Nest Weir

NSW Department of Primary Industries – Fisheries Research Report Series: 15

Fish communities of the Nepean River in the vicinity of Pheasants Nest Weir

by Lee Baumgartner and Nathan Reynoldson

September 2007

ISSN 1449-9959

NSW Department of Primary Industries – Fisheries Research Report Series

This series presents scientific and technical information on general fisheries research and the documents in the series are intended to be progress reports on ongoing investigations. Titles in this series may be cited as publications, with the correct citation on the front cover.

Fisheries Research in New South Wales

Fisheries research activities in the NSW Department of Primary Industries are based at various centres throughout the state. The studies conducted cover commercial and recreational fisheries and aquaculture, and conservation issues in coastal and riverine areas. The major role of the research is to provide information upon which relevant fisheries management policies and strategies are developed, monitored and assessed in terms of the Department’s obligations under the NSW Fisheries Management Act, 1994.

Title: Fish communities of the Nepean River in the vicinity of Pheasants Nest Weir Authors: Lee Baumgartner and Nathan Reynoldson Published By: NSW Department of Primary Industries (now incorporating NSW Fisheries) Postal Address: PO Box 21, Cronulla, NSW, 2230 Internet: www.dpi.nsw.gov.au

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ISSN 1449-9959 [Note: Prior to July 2004, this report series was published as the ‘NSW Fisheries Resource Assessment Report Series’ with ISSN number 1440-057X]

Fish of the Nepean River, Baumgartner and Reynoldson

TABLE OF CONTENTS

TABLE OF CONTENTS...... I LIST OF FIGURES & TABLES...... I ACKNOWLEDGEMENTS...... II NON-TECHNICAL SUMMARY ...... III 1. INTRODUCTION...... 5 2. METHODS...... 6 Study Sites...... 6 Fish collection methods...... 6 Data analysis ...... 6 3. RESULTS ...... 9 Fish community differences...... 9 Fish catches from sites further downstream...... 9 4. DISCUSSION ...... 15 Impacts of Pheasants Nest Weir...... 15 Potential Impacts of weirs lower in the Nepean system...... 16 5. REFERENCES...... 17

LIST OF FIGURES & TABLES

Figure 1. A map of the Nepean River catchment highlighting storages and weirs located on the main stem of the river systems...... 8 Figure 2. Mean abundance of total fish catches from all sites sampled upstream and downstream of Pheasants Nest Weir...... 11 Figure 3. Multidimensional scaling ordination of fish communities sampled upstream and downstream of Pheasants Nest Weir...... 11 Figure 4. Length-frequency distributions of gambusia sampled upstream and downstream of Pheasants Nest Weir...... 12 Figure 5. Length-frequency distributions of Australian smelt sampled upstream and downstream of Pheasants Nest Weir...... 12

Table 1. Summary of fish caught at six monitoring sites upstream and downstream of Pheasants Nest Weir on the Nepean River...... 10 Table 2. Summary of fish sampled by boat electrofishing at downstream sites...... 13

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ACKNOWLEDGEMENTS

This project was funded by the Sydney Catchment Authority as part of its environmental flows program. Tony Paull and Stephen Blockwell provided comments on experimental design and provided overall guidance for the project; particularly with OH&S requirements. Fieldwork was undertaken by Michael Rodgers, Leo Cameron, Justin Stanger, Cameron McGregor, Adam Baumgartner and Peter Heath. Leo Cameron, Bob Creese and Craig Boys provided helpful comments on the manuscript. This work was performed under NSW DPI research permit P01/0059(A) and the Aquatic Ecosystems ACEC permit Fish 98/14.

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NON-TECHNICAL SUMMARY

Fish communities of the Nepean River in the vicinity of Pheasants Nest Weir

PRINCIPAL INVESTIGATORS: Lee Baumgartner and Nathan Reynoldson

ADDRESS: Narrandera Fisheries Centre PO Box 182 Narrandera NSW 2700 Telephone: 02 6959 9021; Fax: 02 6959 2935 OBJECTIVE: 1. To obtain information about current fish species in the river to inform the design of the fishway. 2. To provide information about current fish populations in the river, and any species of concern, that may be affected by the proposed works at the weir. 3. To provide information about species and abundances and establish a baseline for ongoing monitoring of the effectiveness of the fishway.

NON TECHNICAL SUMMARY:

The Hawkesbury-Nepean system is the second-largest river system in New South Wales. Dams in the upper reaches of the major tributaries supply 97% of Sydney’s drinking water, whilst much of the sewage from western Sydney and the Blue mountains is discharged into the middle and lower reaches of the system. The NSW government has announced the release of new environmental flows from the dams on the Hawkesbury-Nepean River (and investigations of measures to improve fish passage at weirs).The Sydney Catchment Authority has subsequently undertaken new works at Avon Dam and is currently investigating further improvements to Cataract Dam, Cordeaux Dam, Nepean Dam, Pheasants Nest Weir and Broughtons Pass Weir. Once completed, enhanced environmental releases should commence in 2009.

At Pheasants Nest and Broughtons Pass Weir, the Sydney Catchment Authority is also investigating measures required to improve fish passage. The Department of Water and Energy is leading a review of the downstream weirs to develop the best procedures to facilitate the new environmental releases down the river and to improve fish passage. It is likely that the proposed works will require substantial modifications to several weirs in the lower Nepean region. The aim of this brief study was to obtain baseline data on fish communities in the Nepean River system prior to the reinstatement of environmental flows from upper dams. It also enabled a preliminary assessment of current impacts of Pheasants Nest Weir on fish communities of the Nepean River.

Fish were collected from four sites upstream and two sites downstream of Pheasants Nest Weir. The two downstream sites were 0 km and 6 km downstream of the weir and sought to determine the structure of fish assemblages potentially obstructed by the barrier. Upstream sites provided additional information about any species that cannot migrate past the structure. An additional six sites were sampled much further downstream of Pheasants Nest Weir. At these sites migratory fish were tagged to document any potential large distance migrations within the system. Information on movements of these tagged fish was used to identify migratory bottlenecks that might require the construction of fish passage facilities to improve fish communities within the system.

Although sampling was conducted over one year, and at a small number of sites, this study provided some preliminary evidence to suggest dams and weirs were impacting on fish

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communities of the Nepean River system. Few migratory species were sampled upstream of Pheasants Nest Weir and some potential migrants accumulated downstream. Downstream weirs also restricted the passage of catadromous species such as bullrout, bully mullet and freshwater mullet. Movements of tagged fish suggested that downstream migration is also important, as some species moved long distances to estuarine areas, despite the study only being undertaken over a six month period. It is recommended that additional research work be done in the Nepean River system to further assess the impact of migration obstructions and to inform the development of appropriate fish passage facilities.

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1. INTRODUCTION

The Hawkesbury-Nepean system is the second-largest river system in New South Wales (Gehrke and Harris, 1996). Dams in the upper reaches of the major tributaries supply 97% of Sydney’s drinking water, whilst much of the sewage from western Sydney and the Blue mountains is discharged into the middle and lower reaches of the system. For some time, the system has demonstrated signs of environmental stress which is typified by the prevalence of algal blooms and the proliferation of pest species (Cullen, 1995). Increased population growth throughout the catchment continually inflates the demand for consumptive water use, reduces the frequency of floods and freshets and places additional stress on the riverine ecosystem as a whole.

In recent times, the fish community of the system has demonstrated signs of decline. Species such as common galaxias, bully mullet, freshwater mullet and freshwater herring are reduced in abundance, and grayling are now rarely recorded (Gehrke and Harris, 1996). Whilst numerous factors have contributed to these declines, increasing flow regulation and the proliferation of dams and weirs is likely to have had a profound effect on fish within the system. Starting at Penrith and moving upstream, there are 13 barriers to fish passage on the main channel of the Nepean River. In addition, there are three headwater storages (Avon Dam, Nepean Dam and Cordeaux Dam) that are operated to regulate flow within the system.

The presence of barriers can disrupt fish passage, and also fragment populations (Ward and Stanford, 1979). Catadromous fish species are unable to make recolonising migrations from estuarine areas following the construction of a dam or weir (Gehrke et al, 2000). In addition, regulated flow from upland storages had a profound impact on macroinvertebrate communities within the system (Growns and Growns, 2001). The delivery of environmental flows may partly ameliorate these impacts but a holistic rehabilitation effort is required to improve the overall ecological health of the system (Bishop et al, 2002).

The NSW government has announced the release of new environmental flows from the dams on the Hawkesbury-Nepean River (and investigations of measures to improve fish passage at weirs). The Sydney Catchment Authority has subsequently undertaken new works at Avon Dam and is currently investigating further improvements to Cataract Dam, Cordeaux Dam, Nepean Dam, Pheasants Nest Weir and Broughtons Pass Weir. Once completed, enhanced environmental releases should commence in 2009. The project will involve the modification of several downstream weirs to minimise the reliance of upstream dams for environmental releases. It is likely that the proposed works will require substantial modifications to several weirs in the lower Nepean region. The aim of this brief study was to obtain baseline data on fish communities in the Nepean River system prior to the reinstatement of environmental flows from upper dams. It also enabled an assessment of the impact of Pheasants Nest Weir on fish communities of the Nepean River.

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2. METHODS

Study Sites

Fish were collected from four sites upstream and two sites downstream of Pheasants Nest Weir. The two downstream sites were 0 km and 6 km downstream of the weir and sought to determine the structure of fish assemblages potentially obstructed by the barrier. Upstream sites provided additional information about any species that cannot migrate past the structure. Each site was sampled four times between September 2006 and April 2007. Fish were also collected from six additional sites much further downstream of Pheasants Nest Weir to allow a proportion of fish to be implanted with passive integrated transponder (PIT) and dart tags.

Fish were tagged to determine the extent of longitudinal migrations undertaken along the river system. For instance, if fish were tagged downstream of the weir and were subsequently caught upstream, conditions favouring migration must have occurred. This could have biased samples from upstream because it was assumed that Pheasants Nest Weir was a complete barrier to migration. These extra sites were sampled once in February 2007.

Fish collection methods

Fish were collected from all six sites upstream and downstream of Pheasants Nest Weir using a Smith-Root Model 12 backpack electrofishing unit. A total of 8 electrofishing ‘shots’ (defined as 150 seconds of electrofishing time) were undertaken at each site. Sites were fished in a downstream direction and sampling activities were undertaken by two staff. One staff member operated the backpack unit whilst the second dip-netted any fish narcotised by the unit. Fish collections from the six downstream sites were done using a Smith-Root 7.5 kW electrofishing boat. Twelve electrofishing ‘shots’, of 90 second duration, were done by three staff at each site. One staff controlled the electrofishing controls and two staff dip-netted fish from the bow. Where the river channel was less than 10 m wide, banks were fished in a ‘zig-zag’ pattern. In areas where the river channel was wider, shots were alternated between banks. At the completion of each electrofishing operation, all fish were identified, counted and measured (maximum of 50 per species per shot). In addition, fish over 200 mm in size were fitted with both a passive integrated transponder (PIT) tag and a yellow external dart tag.

PIT tagging was undertaken to determine the length of migrations undertaken in the Nepean system. A PIT tag is a small glass capsule (Texas instruments; 23 mm; half duplex) that is individually coded with a 12-digit identification number. The identity of individual fish can be determined by scanning the fish with a hand-held tag detector or by placing remote antennas at strategic locations in a river system. These are planned for future installation at fishways in the Nepean River. PIT tags are internal and may not be obvious if re-captured by an angler. Therefore, all PIT tagged fish were also fitted with 65 mm external dart tags into the dorsal musculature. Dart tags contained contact information allowing details of fish caught by anglers to be sent to researchers.

Data analysis

Data were analysed using the S-Plus (Insightful Corporation, 2001) and Plymouth Routines in Multivariate Ecological Research (PRIMER) (Clarke and Warwick, 1994) statistical packages. Analyses were performed on fourth-root transformed data to determine spatial differences in fish communities. Multidimensional scaling ordinations of Bray-Curtis similarity measures were used to plot fish community data, in two dimensions, after pooling replicate shots at each site. For the purposes of this study, fish communities were defined by the relative abundance of species sampled during the course of routine electrofishing. A one-way analysis of similarities (ANOSIM), as

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described in Clarke and Warwick (1994), was used to determine if any differences in fish community composition existed between regions upstream and downstream of Pheasants Nest Weir. Where possible, each test was conducted using 20,000 Monte Carlo randomisations to calculate probabilities. A similarity percentages (SIMPER) test was subsequently performed to identify species contributing most to average dissimilarities within and between regions.

Two tailed Kolmogorov-Smirnov tests (KS: Sokal and Rohlf, 2001) were performed on the most common species from each site to assess differences in length frequency distributions between upstream and downstream regions. For the purpose of the present study, length frequency analysis revealed whether populations upstream and downstream of weirs were dominated by particular size classes.

All data were standardised to electrofishing time (fish per minute) and all statistical tests were considered significant at p < 0.05.

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Figure 1. A map of the Nepean River catchment highlighting storages and weirs (circles) located on the main stem of the river systems. Fish collection sites are shown as squares. Downstream tagging sites are highlighted by a triangle.

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3. RESULTS

Fish community differences

Backpack electrofishing yielded a total of 11 species and 1,472 individuals over the study period. Catches were dominated by the non-native gambusia (Gambusia holbrooki; 60.3%) and Australian smelt (Retropinna semoni; 21.9%). More species of fish were sampled downstream of Pheasants Nest Weir (Table 1), with mountain galaxias (Galaxias olidus), Australian bass (Macquaria novemeculeata), firetail gudgeon (Hypseleotris galii) and freshwater catfish (Tandanus tandanus) not sampled upstream. Additionally, when the catch of all species was pooled, it was evident that more fish were collected downstream of the weir (Figure 2). In particular, substantially higher numbers of Australian smelt were collected immediately downstream of Pheasants Nest Weir, suggesting an accumulation of potential upstream migrants.

Only seven species were sampled upstream of Pheasants Nest Weir. The threatened Macquarie perch (Macquaria australasica) was only collected 12 km downstream of Avon Dam and no Australian bass were caught upstream of Pheasants Nest Weir. Gambusia was sampled in higher relative abundances upstream of Pheasants Nest Weir, particularly from the Avon River.

Multidimensional scaling demonstrated a separation between fish community assemblages upstream and downstream of Pheasants Nest Weir, which was confirmed by ANOSIM (Global R = 0.15; p < 0.05; Figure 3). These differences were characterised by higher downstream abundances of Australian smelt, firetail gudgeon and long-finned eel (Anguilla reinhardtii) and higher upstream catches of gambusia, flatheaded gudgeon (Phylipnodon grandiceps) and cox’s gudgeon (Gobiomorphus coxii).

Two species, gambusia and Australian smelt, exhibited significant differences in length-frequency distributions upstream and downstream of Pheasants Nest Weir. Significantly smaller gambusia were sampled upstream of the weir, where a greater proportion of fish less than 30 mm were collected (KS = 0.443, p < 0.05; Figure 4). Significantly more Australian smelt greater than 40 mm were collected upstream of Pheasants Nest Weir (KS = 0.332, p < 0.05; Figure 5).

Sample sizes of other species were too small for statistical comparisons but some trends in length distribution were observed. For instance, no individuals of the catadromous long-finned eel less than 100 mm were sampled upstream of the weir. Although few individuals were sampled, the mean size of Cox’s gudgeon was also substantially greater from the two sites sampled immediately below Avon and Nepean Dams suggesting that smaller individuals may either accumulate downstream of the weir or have difficulty in gaining passage past the structure.

Fish catches from sites further downstream

Sampling at the six downstream Nepean River sites (Figure 1) yielded 516 fish from 16 species. Catches were dominated by Australian bass (21.7%), long-finned eel (16%) and bully mullet (13%). Catadromous species such as bully mullet and freshwater mullet were not sampled upstream of Theresa Park Weir. Similarly, bullrout (Notesthes robusta) and empire gudgeon (Hypseleotris compressa) were not sampled further upstream than Penrith Weirpool. Freshwater herring (Potamalosa richmondia) were not caught upstream of Wallacia. Australian bass and long-finned eel were the only catadromous species collected from all six sites. Gambusia were absent from lower regions and only collected from upper sites at Douglas Park and Menangle.

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Table 1. Summary of fish caught at six monitoring sites upstream and downstream of Pheasants Nest Weir on the Nepean River.

Species n Fish length Min Max (mm) Fish length Fish length (mean ± SD) (mm) (mm) 0 km d/s Avon Dam Long-finned eel 2 450 ± 212 300 600 Gambusia 333 26 ± 7 12 47 Cox’s gudgeon 2 112 ± 66 65 159 Flat-headed gudgeon 6 38 ± 7 30 47 Australian Smelt 3 35 ± 16 22 52 6-12 km d/s Avon Dam Gambusia 61 26 ± 8 12 44 Cox’s gudgeon 21 97 ± 30 46 144 Macquarie Perch 1 222 222 222 0 km d/s Nepean Dam Long-finned eel 6 692 ± 161 480 920 Cox’s gudgeon 4 168 ± 11 156 182 Flat-headed gudgeon 50 59 ± 12 24 88 Dwarf flat-headed gudgeon 1 50 50 50 Australian smelt 5 42 ± 13 36 66 6-12 km d/s Nepean Dam Long-finned eel 1 600 600 600 Cox’s gudgeon 18 80 ± 26 51 136 Flat-headed gudgeon 7 58 ± 7 49 68 Dwarf flat-headed gudgeon 1 57 57 57 Australian smelt 23 43 ± 12 29 71 0 km d/s Pheasants Nest Long-finned eel 4 330 ± 190 161 600 Gambusia 10 29 ± 6 16 37 Cox’s gudgeon 10 78 ± 24 44 117 Macquarie perch 6 163 ± 18 112 223 Australian smelt 111 44 ± 10 24 75 Mountain galaxias 1 63 63 63 6-12 km d/s Pheasants Nest Long-finned eel 15 438 ± 135 100 606 Gambusia 42 27 ± 8 12 53 Cox’s gudgeon 3 109 ± 49 56 153 Firetail gudgeon 59 37 ± 5 25 52 Australian bass 4 271 ± 29 240 309 Flat-headed gudgeon 18 36 ± 9 19 60 Dwarf flat-headed gudgeon 2 43 ± 6 38 47 Australian smelt 98 43 ± 5 28 40 Catfish 3 200 ± 110 73 265

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Mean Abundance Abundance Mean 4

0 Nepean River (D/S) Nepean River (U/S) Avon River (U/S)

Figure 2. Mean abundance (± one standard error) of total fish catches from all sites sampled upstream (white) and downstream (grey) of Pheasants Nest Weir. Each bar represents a site sampled within each river reach.

Stress = 0.17

Figure 3. Multidimensional scaling ordination of fish communities sampled upstream (white) and downstream (black) of Pheasants Nest Weir. Each dot represents a single sampling occasion.

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48 Gambusia n=103 n=343 40

16 Percentage frequency frequency Percentage 8

0 0 6 12 18 24 30 36 42 48 54 60

Length (mm)

Figure 4. Length-frequency distributions of gambusia sampled upstream (black) and downstream (white) of Pheasants Nest Weir.

60 Australian Smelt n=121

50 n=119

Percentage frequency frequency Percentage 10

0 0 8 16 24 32 40 48 56 64 72 80

Length (mm)

Figure 5. Length-frequency distributions of Australian smelt sampled upstream (black) and downstream (white) of Pheasants Nest Weir.

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A total of 150 fish were implanted with PIT tags as part of this project. These comprised larger- bodied species such as Australian bass, long-finned eel, bully mullet, carp, freshwater mullet, catfish and goldfish (Table 2). Since tagging, catches of Australian bass (n = 4) and Bully mullet (n = 6) have been reported by anglers. Australian bass did not disperse far from the tagging site. Two fish tagged at Wallacia were recaptured from the same site. Similarly, two fish tagged at Menangle, were also recaptured at that location. In contrast, bully mullet were highly mobile. Three fish tagged at Blaxlands Crossing (downstream of Warragamba Dam) in November 2006 were subsequently re-captured by anglers at the junction of the Hawkesbury and Nepean River in April 2007. Three other fish tagged within the Penrith weirpool were recaptured in May 2007 at Patonga, near the Hawkesbury River mouth.

Table 2. Summary of fish sampled by boat electrofishing at downstream sites (listed from downstream to upstream). nC is the number of fish caught and nT is the number of fish tagged. (Continued on next page).

Species nC nT Fish length Min Max (mm) Fish length Fish length (mean ± SD) (mm) (mm) Penrith Weirpool Long-finned eel 4 4 701 ± 197 422 880 Goldfish 18 12 274 ± 21 211 301 Carp 10 10 514 ± 182 252 745 Striped Gudgeon 3 0 129 ± 10 123 140 Firetail Gudgeon 1 0 36 36 36 Australian Bass 20 7 209 ± 81 96 362 Bully Mullet 19 19 417 ± 60 354 602 Freshwater Mullet 3 3 380 ± 27 363 411 Bullrout 2 0 209 ± 38 182 235 Flat-headed Gudgeon 9 0 49 ± 16 19 67 Freshwater Herring 2 0 179 ± 13 169 188 Australian Smelt 1 0 56 56 56 Catfish 3 3 413 ± 20 397 435 Wallacia Long-finned eel 2 2 494 ± 151 387 601 Carp 4 4 595 ± 15 581 615 Firetail Gudgeon 23 0 33 ± 4 26 43 Australian Bass 12 10 241 ± 34 192 297 Bully Mullet 4 4 343 ± 65 283 407 Freshwater Mullet 4 4 442 ± 4 437 446 Flat-headed Gudgeon 1 0 56 56 56 Freshwater Herring 2 0 233 ± 24 216 250 Downstream Warragamba Dam Long-finned eel 2 2 518 ± 47 485 551 Carp 3 3 548 ± 89 461 639 Firetail Gudgeon 5 0 39 ± 13 28 61 Australian Bass 13 7 215 ± 46 150 284 Bully Mullet 17 17 395 ± 19 358 443 Freshwater Mullet 5 5 418 ± 31 376 448 Flat-headed Gudgeon 1 0 60 60 60 Catfish 2 2 392 ± 41 363 421

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Table 2 (continued)

Species nC nT Fish length Min Max (mm) Fish length Fish length (mean ± SD) (mm) (mm) Theresa Park Weir Pool Long-finned eel 5 5 521 ± 91 374 612 Carp 1 1 735 735 735 Firetail Gudgeon 1 0 34 34 34 Australian Bass 6 3 255 ± 69 191 338 Catfish 2 2 504 ± 1 503 505 Douglas Park Long-finned eel 12 10 671 ± 254 322 1255 Goldfish 5 5 302 ± 29 250 316 Carp 1 1 547 547 547 Gambusia 41 0 29 ± 4 24 43 Cox’s Gudgeon 6 0 174 ± 27 144 224 Firetail Gudgeon 1 0 39 39 39 Australian Bass 1 1 384 384 384 Flat-headed Gudgeon 1 0 26 26 26 Menangle Long-finned eel 5 5 725 ± 111 556 827 Goldfish 14 13 250 ± 76 28 348 Carp 1 1 693 693 693 Gambusia 7 0 30 ± 4 25 37 Cox’s Gudgeon 1 0 129 129 129 Firetail Gudgeon 35 0 33 ± 4 25 45 Australian Bass 20 19 295 ± 47 189 378 Flat-headed Gudgeon 2 0 47 ± 19 33 60 Catfish 3 0 426 ± 18 408 443

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4. DISCUSSION

Impacts of Pheasants Nest Weir

This study has provided several indications that Pheasants Nest Weir is acting as a migration barrier to native fish in the Nepean River. Several species of catadromous fishes were not present upstream of the weir. Australian bass and firetail gudgeon both require access to estuarine areas for reproduction. Subsequently, these species often become extinct upstream of migration barriers that do not possess fish passage facilities (Gehrke et al, 2002). In such situations, opportunities for re- establishing the species in these areas are limited to the provision of fish passage facilities for upstream migrants or the development of a restocking program.

The presence of Cox’s gudgeon and long-finned eel at sites upstream of Pheasants Nest can be attributed to one of two processes. First, both of these species are known to climb (Buemer, 1996; Larson and Hoese, 1996) and can negotiate large barriers such as Tallowa Dam on the Shoalhaven River (Gehrke et al, 2000). Although low flows have persisted in the Nepean River for a number of years, these fish may have also colonised upstream reaches during previous periods of weir inundation during high flow events. Second, only very large individuals of climbing species were collected from upstream sites suggesting that even if inundation was providing passage opportunities, small individuals may not be able to ascend.

The abundances of some species were substantially greater from downstream sites, providing further evidence that some migrants are unable to pass the weir. Australian smelt accumulated downstream of the weir in substantial numbers suggesting that the weir was acting as a migration barrier. This species is known to form self-sustaining populations without the need to migrate (Baumgartner 2006). The provision of passage, especially for smaller-bodied species, can promote genetic mixing among fragmented population and also reduce potential for predation from piscivorous birds and therefore fish (Baumgartner, 2007). Enhancing opportunities for fish passage at Pheasants Nest Weir, therefore, would have important-long term benefits and should be considered for this site.

Previous studies have determined that the two main piscivores in the Nepean system are Australian bass and long-finned eels (Gehrke and Harris, 1996). In some systems, piscivorous fish can effectively regulate the abundances of smaller species if predation rates are substantial (Werner et al, 1983). No Australian bass were sampled upstream of Pheasants Nest Weir and long-finned eels were present in relatively low numbers, possibly creating a situation favouring the proliferation of small-bodied species. Maldon Weir is a substantial barrier on the Nepean River and known to be the upstream limit of migration for Australian bass and other catadromous species (Sammut and Erskine, 1995). The high abundances of Australian smelt downstream of Pheasants Nest may also be attributed to the relatively low abundances of Australian bass in this reach.

Macquarie perch were collected upstream and downstream of Pheasants Nest Weir but in low numbers. This species is threatened in coastal catchments and the population in the Nepean River requires conservation consideration. The population could potentially be threatened by loss of habitat, water quality changes or the introduction of predatory species (Bishop et al, 2002). More resources should be invested into determining the dynamics of the Nepean River Macquarie perch populations to identify any riverine processes requiring mitigation. Fisheries managers could then develop the necessary legislative requirements to help protect this important species.

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Potential Impacts of weirs lower in the Nepean system

Despite the short-term nature of this study, some evidence was obtained to suggest weirs downstream of Pheasants Nest were also acting as migration barriers to some species. Species richness was far greater at Penrith than at sites further upstream. Penrith Weir is fitted with a fishway that can provide upstream passage for migratory species (NSW DPI, 2006). Wallacia Weir, the next upstream in the system, does not contain any operational fish passage facilities and represents a substantial barrier to fish migration during low-flow periods (NSW DPI, 2006).

Bullrout, Bully mullet or freshwater mullet were not collected upstream of Wallacia during the present study. These species are catadromous (McDowall, 1996) and require access to estuarine areas to spawn. The present study was undertaken during a period of extreme low flows in the Nepean system, and any fish which had previously negotiated the structure may have returned to estuarine reaches since the last inundation event. Although not sampled in the present study, previous surveys have noted these species in the vicinity of Theresa Park and Mt Hunter (NSW DPI, Unpublished Data; Gehrke and Harris, 1996). These observations suggest that these species can colonise further upstream into the system when conditions are suitable.

The assertion that some species are able to return to estuarine reaches during low-flow periods was supported by observations of tagged bully mullet undertaking seaward migrations during the study period. A study on mullet movement in the Shoalhaven River determined that some individuals can re-enter saltwater and migrate northwards for hundreds of kilometres (Gehrke et al, 2001). Similarly, individuals tagged upstream of Penrith Weir were subsequently re-captured by anglers in saltwater reaches, near the mouth of the Hawkesbury River. Nineteen additional fish species known from the Nepean River also require access to estuarine reaches for reproduction purposes (Gehrke and Harris, 1996). Any fish passage facilities constructed in the Nepean River should therefore also provide downstream passage to ensure catadromous species, such as bully mullet, can return to spawning areas.

Previous studies have identified two major structures, Wallacia and Maldon Weirs as major migration barriers in the system (Gehrke and Harris, 1996; Mallen-Cooper and Smit, 2005; Baumgartner, in prep). These barriers obstruct fish movements under most flow conditions and substantially limit upstream migrations of catadromous fish. Provision of fishways at these structures would provide opportunities for upstream fish movements throughout the Nepean River. Other barriers within the system also provide fish passage, either through existing fishways, or during flooding events (Mallen-Cooper and Smit, 2005). However, the ecological mechanisms required for effective passage for the entire migratory community are still poorly understood. This lack of knowledge persists because fish passage through existing fishways has not been assessed. In addition, previous studies have been too limited in scope to provide an understanding of fish passage issues at a basin-wide scale. Further research is required to determine (i) the effectiveness of existing fish passage infrastructure for providing opportunities for upstream migrations and (ii) the ecology of different species and size classes requiring passage. Addressing both of these issues would enable informed decisions on the degree of infrastructure required to adequately re-instate passage at Nepean River weirs.

The present study was extremely limited in its ability to determine the overall impact of downstream weirs because fish were collected from few sites, and only one temporal sample was taken. Obtaining accurate assessments of rare or cryptic species is difficult when undertaking studies that sample only once. Furthermore, inter-annual variability in fish community composition cannot be accounted for and this makes it more difficult to ascertain the exact cause of any ecological patterns observed. Therefore, determining the full extent of fish passage obstruction in the Nepean River would require a much larger study, with additional sites that are temporally replicated to account for seasonal changes in expected fish migration rates.

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5. REFERENCES

Almodovar, A. and Nicola, G.G. (1999). Effects of a small hydropower station upon brown trout Salmo trutta in the River Hoz Seca (Tagus Basin, Spain) one year after regulation. Regulated Rivers: Research and Management, 15: 477–484. Baumgartner, L.J. (2006). The effects of dams and weirs in the Murray-Darling Basin. PhD Thesis, University of Canberra. Baumgartner, L.J. (2007). Diet and feeding habits of predatory fishes upstream and downstream of a low-level weir. Journal of Fish Biology, 70: 879–894. Bishop, K., Growns, I., Church, T., Warner, R. and Taylor-Wood, E. (2002). The health of the Hawkesbury-Nepean River. Report to the Independent Expert Panel on Environmental Flows, Sydney. Buemer, J.P. (1996). ‘Family Anguillidae; Freshwater Eels’ in McDowall, R.M. (ed.) The freshwater Fishes of South – Eastern Australia. Reed Publishing, Sydney. pp. 33-49. Clarke KR and Warwick RM. (1994). Change in Marine Communities: An approach to statistical analysis and interpretation. Bourne Press Limited, Plymouth. Cullen, P. (1995). Algal blooms in the Hawkesbury River. Technical Report 1, Hawkesbury- Nepean Catchment Management Trust, Windsor. Gehrke, P.C. and Harris, J.H. (1996). Fish and Fisheries of the Hawkesbury-Nepean River system. Final Report to Sydney Water Corporation, NSW Fisheries, Cronulla. 266pp. Gehkre, P.C., Gilligan, D.M. and Barwick, M. (2000). Changes in fish communities of the Shoalhaven River 20. years after construction of Tallowa Dam, Australia. Regulated Rivers: Research and Management, 18; 265–286. Growns, I. and Growns, J. (2001). Ecological effects of flow regulation of the macroinvertebrate and periphytic diatom assemblages in the Hawkesbury-Nepean River. Regulated Rivers Research and Management, 17: 275–293. Insightful Corporation. (2001). S-PLUS 6 for Windows users guide. Insightful Corporation, Seattle. Larson, H.K. and Hoese, D.F. (1996). Family Gobiidae, subfamilies Eleotridinae and Butinae: Gudgeons in McDowall, R.M. (ed.) The freshwater Fishes of South – Eastern Australia. Reed Publishing, Sydney. pp. 33–49. Mallen-Cooper, M and Smit K. (2005). A review of fishways in the Nepean River. Final Report to the Sydney Catchment Authority. McDowall, R. (1996). Freshwater Fishes of South-Eastern Australia. Reed Books, Sydney. NSW DPI. (2006). Reducing the Impact of Weirs on Aquatic Habitat – New South Wales Detailed Weir Review. Hawkesbury – Nepean CMA region. Report to the NSW Environmental Trust. NSW Department of Primary Industries, Flemington, NSW. Sammut, J and Erskine, W.D. (1995). Hydrological impacts of flow regulation associated with the upper Nepean Water Supply Scheme, NSW. Australian Geographer, 26: 71–86. Ward, J. and Standford, J. (1979). The ecology of regulated streams. Plenum Press: New York. Werner, E.E., Gilliam, J.F., Hall, D.J and Mittelbach, G.G. (1983). An experimental test of the effects of predation risk on habitat use in fish. Ecology, 64 (6): 1540–1548.

NSW DPI – Fisheries Research Report Series: No. 15 17 Fish of the Nepean River, Baumgartner and Reynoldson

Other titles in this series:

ISSN 1442-0147 (NSW Fisheries Research Report Series)

No. 1 Otway, N.M. and Parker, P.C., 1999. A review of the biology and ecology of the grey nurse shark (Carcharias taurus) Rafinesque 1810. 36pp. No. 2 Graham, K.J., 1999. Trawl fish length-weight relationships from data collected during FRV Kapala surveys. 105pp. No. 3 Steffe, A.S., Chapman, D.J. and Murphy, J.J., 1999. A description of the charter fishing boat industry operating in the coastal and estuarine waters of New South Wales during 1997-98. 33pp. No. 4 Reid, D.D. and Smith, I.R., 1998. The 1998 Pacific oyster survey. 14pp. No. 5 Walford, T. and Pease, B., 2000. Strategies and techniques for sampling adult anguillid eels. Proceedings of a workshop held at FRI, Cronulla, Australia, August 1999. 176pp. No. 6 Heasman, M. and Lyall, I., 2000. Proceedings of the workshop held on 3 March 2000 at the Sydney Fish Markets: Problems of producing and marketing the flat oyster Ostrea angasi in NSW. 57pp. No. 7 Heasman, M., 2003. Proceedings of the Sydney Rock Oyster Hatchery Workshop held on 8 and 9 August 2002 at Port Stephens, NSW. 164pp. No. 8 Allan, G.A., 2003. Proceedings of the Aquafin CRC Snapper Workshop held on 26 September 2002 at the Convention Centre, Melbourne (Aquafin CRC 2001/208). 107pp. No. 9 Faragher, R.A., 2004. Hooking mortality of trout: a summary of scientific services. 9pp. No. 10 Daly, T., 2004. Summary of Proceedings from the Perkinsus Workshop held at the Cronulla Fisheries Centre on 3 September 2003. 32pp.

ISSN 1449-9959 (NSW Department of Primary Industries - Fisheries Research Report Series)

No. 11 Baumgartner, L., 2005. Fish in Irrigation Supply Offtakes: A literature review. 22pp. No. 12 Ganassin, C. and Gibbs, P., 2005. Descriptions of the wildlife species that commonly occur in the marine and estuarine waters of NSW. 88pp. No. 13 Nell, J., 2006. Manual for mass selection of Sydney rock oysters for fast growth and disease resistance. 57pp + 110pp attachments. No. 14 Gilligan, D. and Rayner, T., 2007. The distribution, spread, ecological impacts and potential control of carp in the upper Murray River. 25pp. No. 15 Baumgartner, L., 2007. Fish communities of the Nepean River in the vicinity of Pheasants Nest Weir. 18pp.

18 NSW DPI – Fisheries Research Report Series: No. 15