Spencer Gulf Prawn ( latisulcatus) Fishery

Fishery Assessment Report to PIRSA Fisheries

2007

Dixon, C.D., Roberts, S.D. and Hooper, G.E.

SARDI Aquatic Sciences Publication No. RD03/0079-4 SARDI Research Report Series No. 161

Fishery Assessment Report to PIRSA Fisheries

Spencer Gulf Prawn (Melicertus latisulcatus) Fishery

Dixon, C.D., Roberts, S.D. and Hooper, G.E.

2007

SARDI Aquatic Sciences

SARDI Aquatic Sciences Publication No. RD03/0079-4 SARDI Research Report Series No. 161

This Fishery Assessment Report updates the 2005 report for the Spencer Gulf Prawn Fishery and is part of SARDI Aquatic Sciences ongoing assessment program for ’s Prawn Fisheries. The aims of the report are to synthesise information for the Spencer Gulf Prawn Fishery, to assess the current status of the resource and consider the uncertainty associated with the assessment, to comment on the current biological Performance Indicators and Reference Points, and to identify future research needs for the fishery.

Title Spencer Gulf Prawn (Melicertus latisulcatus) Fishery Sub-title Fishery Assessment Report to PIRSA Fisheries Author(s) Dixon, C.D., Roberts, S.D. and Hooper, G.E.

South Australian Research and Development Institute SARDI Aquatic Sciences 2 Hamra Avenue West Beach SA 5024

Telephone: (08) 8207 5400 Facsimile: (08) 8207 5406 http://www.sardi.sa.gov.au

DISCLAIMER

The authors warrant that they have taken all reasonable care in producing this report. This report has been through SARDI Aquatic Sciences internal review process, and was formally approved for release by the Chief Scientist. Although all reasonable efforts have been made to ensure quality, SARDI Aquatic Sciences does not warrant that the information in this report is free from errors or omissions. SARDI Aquatic Sciences does not accept any liability for the contents of this report or for any consequences arising from its use or any reliance placed upon it.

© 2007 SARDI Aquatic Sciences This work is copyright. Apart from any use as permitted under the Copyright Act 1968, no part may be reproduced by any process without prior written permission from the author.

Printed in Adelaide: February 2007

SARDI Aquatic Sciences Publication No. RD03/0079-4 SARDI Research Report Series No. 161

Reviewers: Dr Stephen Mayfield (SARDI), Mr Paul Rogers (SARDI) and Mr Martin Smallridge (PIRSA). Approved by: Dr Tim Ward

Signed: Date: 26 February 2007 Distribution: PIRSA Fisheries, Spencer Gulf and West Coast Prawn Fisherman’s Association, SARDI Aquatic Sciences Library, Spencer Gulf Prawn Fishery licence holders Circulation: Public Domain

TABLE OF CONTENTS

LIST OF TABLES ...... v LIST OF FIGURES...... vi ACKNOWLEDGEMENTS ...... 1 EXECUTIVE SUMMARY...... 2 1. GENERAL INTRODUCTION ...... 4 1.1 Overview ...... 4 1.2 Description of the Fishery...... 5 1.2.1 Fishery location...... 5 1.2.2 The Spencer Gulf environment...... 7 1.2.3 Nursery areas...... 9 1.2.4 Commercial fishery ...... 12 1.2.5 By-catch ...... 14 1.2.6 By-product...... 15 1.2.7 Recreational, indigenous and illegal catch...... 16 1.3 Management of the Fishery...... 17 1.3.1 Management milestones ...... 17 1.3.2 Current fishery management...... 17 1.3.3 Prawn Fishery Management Plan...... 18 1.3.4 Performance Indicators and Reference Points ...... 19 1.3.5 Real Time Management...... 20 1.4 Biology of the Western King Prawn ...... 21 1.4.1 Distribution and taxonomy...... 21 1.4.2 Reproductive biology...... 21 1.4.3 Larval and juvenile phase ...... 24 1.4.4 Stock structure ...... 27 1.4.5 Growth...... 27 1.4.6 Length weight relationship...... 30 1.4.7 Movement determined from tagging studies...... 30 1.4.8 Natural mortality...... 32 1.4.9 Prawn health...... 32 1.5 Stock Assessment...... 33 1.6 Current Research and Monitoring Program...... 33 1.6.1 Catch and effort research logbook...... 33 1.6.2 Stock assessment surveys ...... 33 2. FISHERY STATISTICS ...... 34 2.1 Introduction...... 34 2.2 Catch and Effort ...... 34 2.2.1 Inter-annual trends...... 34 2.2.2 Trends among regions ...... 35 2.2.3 Trends within years ...... 37 2.2.4 Catches during the spawning season ...... 38 2.3 Catch-Per-Unit-Effort (CPUE)...... 39 2.3.1 Inter-annual trends...... 39 2.3.2 Trends among regions ...... 40 2.3.3 Trends within years ...... 41 2.3.4 Trends within fishing periods...... 42 2.4 Prawn Size...... 43

iii 2.4.1 Inter-annual trends...... 43 2.4.2 Trends within years ...... 44 2.4.3 Daily prawn grades...... 46 2.5 By-product ...... 48 2.5.1 Inter-annual trends...... 48 2.5.2 Trends within years ...... 49 3. STOCK ASSESSMENT SURVEYS...... 50 3.1 Introduction...... 50 3.2 Annual Trends...... 52 3.2.1 Mean catch rate...... 52 3.2.2 Prawn size...... 53 3.2.3 Sex ratio ...... 54 3.3 November Surveys...... 54 3.3.1 Catch rates and prawn size...... 54 3.3.2 Correlation between November surveys and prawn catch...... 55 3.4 February Surveys...... 57 3.4.1 Catch rates and prawn size...... 57 3.4.2 Recruitment...... 57 3.5 April Surveys...... 58 3.6 Comparison of catch rates and prawn size between months ...... 59 3.7 Estimates of egg production from survey catch...... 60 3.8 Spot Surveys...... 63 4. SURVEYS, CATCH AND PRAWN SIZE FOR 2004/05 HARVEST PERIODS... 65 4.1 Introduction...... 65 4.2 Survey and Catch Data by Harvest Period...... 66 4.2.1 Harvest Period 1...... 66 4.2.2 Harvest Period 2...... 66 4.2.3 Harvest Period 3...... 66 4.2.4 Harvest Period 4...... 67 4.2.5 Harvest Period 5...... 67 4.2.6 Harvest Period 6...... 67 5. PERFORMANCE INDICATORS...... 80 5.1 Fishing Effort ...... 80 5.2 Size at Capture...... 80 5.3 Recruitment Indices...... 80 5.4 Proportion of Virgin Spawning Biomass...... 81 6. DISCUSSION...... 82 6.1 Assessment of the Spencer Gulf Prawn Fishery ...... 82 6.2 Information Available for Assessment of the Fishery...... 82 6.3 Current Status of the Spencer Gulf Prawn Fishery...... 84 6.3.1 Annual stock assessment...... 84 6.3.2 Assessment of harvest strategy development and management for 2004/05. 85 6.4 Performance Indicators...... 85 6.5 Future Research...... 87 7. REFERENCES ...... 89 8. APPENDIX ...... 96

iv LIST OF TABLES Table 1.1 The number of Fishery Habitat Area’s and the estimated proportion and distance of coastline of tidal flat only and mangrove forest for each of South Australia’s three prawn fisheries...... 10 Table 1.2 Production figures and species harvested in major Australian prawn fisheries.12 Table 1.3 Major management milestones for the Spencer Gulf Prawn Fishery...... 17 Table 1.4 Current management arrangements...... 18 Table 1.5 Biological and ecological objectives and strategies of the Management Plan for the Spencer Gulf and West Coast prawn fisheries...... 19 Table 1.6 Summary of the biological and economic Performance Indicators and Reference Points for the Spencer Gulf Fishery...... 19 Table 1.7 Fecundity relationships for M. latisulcatus in Gulf St. Vincent, Western Australia and Queensland...... 23 Table 1.8 Sex-specific growth parameters for M. latisulcatus estimated from tag-recapture and cohort analysis in the West Coast and from tag-recapture in Spencer Gulf and ...... 28 Table 2.1 Analytical categories assigned to reported prawn grades from the commercial logbook data...... 43 Table 2.2 The number of prawn per kg estimated for reported prawn grades from the commercial logbook data...... 46 Table 2.3 Statistics associated with mean daily prawn size estimated from prawn grade data provided in commercial logbooks...... 47 Table 3.1 Number of stock assessment survey shots done in fishing regions of Spencer Gulf from February 1982 to April 2005...... 51 Table 3.2 Spearman’s Rank Correlations between commercial fishing catches (November/December and annual), and the percent change in mean catch rate during November Fishery-independent Surveys (FIS) for the fishing years between 1994/95 and 2004/05...... 56 Table 3.3 Egg production from female prawns captured from surveys conducted during November 2004...... 61 Table 3.4 Mean (SE) catch rates and prawn size from spot surveys, and regional total catch and CPUE in the remaining of the fishing month immediately subsequent to spot surveys...... 63 Table 5.1 Summary of Performance Indicators for the 2002/03 and 2003/04 fishing years of the Spencer Gulf Prawn Fishery...... 80

v LIST OF FIGURES

Figure 1.1 Location of South Australia’s three commercial prawn fisheries...... 5 Figure 1.2 Fishing blocks and regions of the Spencer Gulf Prawn Fishery...... 6 Figure 1.3 The bathymetry of Spencer Gulf ...... 7 Figure 1.4 Comparison of mean monthly sea surface temperature (SST, ºC) for the Australian prawn fisheries that target M. latisulcatus...... 8 Figure 1.5 Sea-surface temperatures over the continental shelf of South Australia during late summer/early autumn, 1995...... 8 Figure 1.6 Important juvenile nursery habitat, mangrove forest and tidal flats, around coastal Spencer Gulf...... 11 Figure 1.7 Double rig trawl gear and location of hopper sorting and prawn grading systems used in the Spencer Gulf Prawn Fishery...... 13 Figure 1.8 Trawl net configuration showing trawl boards, head rope, ground chain and cod end with crab bag...... 13 Figure 1.9 Slipper lobster, Ibacus peronii, caught by South Australian prawn fishers...... 15 Figure 1.10 Female prawn reproductive maturation trends based on the percentage of ripe (Stage 3 & 4) prawns...... 22 Figure 1.11 The relationship between fecundity (ovary weight) and carapace length (CL) for M. latisulcatus in GSV, Western Australia and Queensland...... 23 Figure 1.12 Life cycle of M. latisulcatus...... 25 Figure 1.13 Mean larval density in Spencer Gulf during 1993 and 1994...... 26 Figure 1.14 Sex-specific growth curves for M. latisulcatus estimated from tag-recapture and cohort analysis in the West Coast and from tag-recapture in Spencer Gulf and Gulf St Vincent ...... 29 Figure 1.15 Length weight relationship for juvenile M. latisulcatus in GSV ...... 30 Figure 1.16 Generalised movement patterns of tagged M. latisulcatus in Spencer Gulf.... 31 Figure 2.1 Total catch (t) and effort (hrs) for Spencer Gulf from 1968 to 2004/05...... 35 Figure 2.2 Average annual catches from regions of Spencer Gulf from 1988/89 to 2004/05...... 36 Figure 2.3 Average monthly catches from Spencer Gulf for 5-year periods from 1974/75 to 1998/99 and the six years between 1999/2000 and 2004/05...... 37 Figure 2.4 Catches from November and December relative to the total annual catch from 1973/74 to 2003/04 in Spencer Gulf...... 38 Figure 2.5 Annual catch and catch-per-unit-effort (CPUE) for Spencer Gulf from 1968 to 2004/05...... 39 Figure 2.6 Mean (+ SE) annual catch-per-unit-effort for the 10 fishing regions within Spencer Gulf from 2000/2001 to 2004/05...... 40 Figure 2.7 Mean monthly catch-per-unit-effort (CPUE) in Spencer Gulf for 5-year periods between 1974/75 and 1998/99 and the six years between 1999/2000 and 2004/05...... 41

vi Figure 2.8 Mean (+ SE) daily catch-per-unit-effort (CPUE) during 2000/01–2004/05 in Spencer Gulf with respect to the lunar phase...... 42 Figure 2.9 Size composition of prawns in the commercial catch in 1978/79, 1998/99, 2002/03, 2003/04 and 2004/05...... 44 Figure 2.10 Size composition of prawns during each month fished in 2002/03, 2003/04 and 2004/05...... 45 Figure 2.11 Mean daily prawn size estimated from prawn grade data provided in commercial logbooks during 2002/03, 2003/04 and 2004/05...... 47 Figure 2.12 Reported catch of by-product (slipper lobster and calamary) from the Spencer Gulf Prawn Fishery between 2002/03 and 2004/05...... 48 Figure 2.13 Average daily reported catch of by-product (slipper lobster and calamary) from the Spencer Gulf Prawn Fishery during fished months between 2002/03 and 2004/05...... 49 Figure 3.1 Mean annual catch rate obtained during surveys and throughout the fishing year in Spencer Gulf from 1981/82 to 2004/05...... 52 Figure 3.2 Correlation between survey catch rates calculated as kg/h and kg/km between 1981/82 to 2004/05...... 53 Figure 3.3 Mean sex-specific prawn size (mm, CL) from surveys conducted in Spencer Gulf between 1981/82 and 2004/05...... 53 Figure 3.4 Mean (+ SE) catch rate (kg/h) and mean (± SE) prawn size (mm, CL) from November surveys conducted in Spencer Gulf between 1981/82 and 2004/05..... 54 Figure 3.5 Mean November fishery-independent survey (FIS) catch rate (kg/h), pre- Christmas catches and total catches from 1980/81 to 2004/05...... 55 Figure 3.7 Mean (+ SE) recruitment index for up to 39 stations surveyed in February in the northern region of Spencer Gulf from 1982 to 2005. The line represents the target reference point (40/nm)...... 58 Figure 3.8 Mean (+ SE) catch rate (kg/h) and mean (± SE) prawn size (mm, CL) from April surveys conducted in Spencer Gulf between 1981/82 and 2004/05...... 58 Figure 3.9 Mean (+ SE) catch rate (kg/h) from November, February and April surveys conducted in Spencer Gulf between 1993/94 and 2004/05...... 59 Figure 3.10 Mean (+ SE) prawn size (mm, CL) during November, February and April surveys from 1994/95 to 2004/05...... 60 Figure 3.11 Mean (SE) graded catch (kg) per shot (males and females) and the estimated number of fertilized eggs (eggs % 107) produced by females captured from survey shots during November 2004...... 62 Figure 3.12 Correlation between regional spot survey CPUE and subsequent regional fishing CPUE in the same month...... 64 Figure 4.1 Catch rate and mean size during the November 2004 Stock Assessment survey, prior to harvest period 1...... 68 Figure 4.2 Commercial catch and mean size from blocks fished during harvest period 1...... 69

vii Figure 4.3 Catch rate and mean size during the December 2004 Spot survey, prior to harvest period 2...... 70 Figure 4.4 Commercial catch and mean size from blocks fished during harvest period 2...... 71 Figure 4.5 Catch rate and mean size during the February 2005 Stock Assessment survey, prior to harvest period 3...... 72 Figure 4.6 Commercial catch and mean size from blocks fished during harvest period 3...... 73 Figure 4.7 Catch rate and mean size during the April 2005 Stock Assessment survey, prior to harvest period 4...... 74 Figure 4.8 Commercial catch and mean size from blocks fished during harvest period 4...... 75 Figure 4.9 Catch rate and mean size during the May 2005 Spot survey, prior to harvest period 5...... 76 Figure 4.10 Commercial catch and mean size from blocks fished during harvest period 5...... 77 Figure 4.11 Catch rate and mean size during the June 2005 Spot survey, prior to harvest period 6...... 78 Figure 4.12 Commercial catch and mean size from blocks fished during harvest period 6...... 79

viii ACKNOWLEDGEMENTS

Funds for this research were provided by PIRSA Fisheries, obtained through licence fees. SARDI Aquatic Sciences provided substantial in-kind support. We are grateful to the numerous scientific observers, SARDI staff and volunteers that have assisted with Spencer Gulf prawn surveys. Considerable thanks go to the Spencer Gulf and West Coast Prawn Fishing Association, in particular Greg Palmer, Samara Miller, Barry Evans, David Craig and Jenny Kranz. The catch and effort data were provided by Angelo Tsolos of the Fisheries Statistics Unit (FSU) at SARDI Aquatic Sciences. Annette Doonan (FSU) developed some GIS maps. The report was reviewed by Dr Stephen Mayfield (SARDI), Mr Paul Rogers (SARDI) and Mr Martin Smallridge (PIRSA). Dr Tim Ward formally approved the report for release.

1 EXECUTIVE SUMMARY

1. This report updates the 2005 fishery assessment report and is part of SARDI Aquatic Sciences ongoing assessment program for the Spencer Gulf Prawn Fishery (SGPF). It aims to (1) synthesise and assess the information available for the SGPF, (2) assess the status of the resource and consider uncertainty associated with that assessment, (3) comment on current biological Performance Indicators for the fishery, and (4) identify future research priorities.

2. The SGPF was established in 1968 and catches increased rapidly to reach 2,287 t in 1973/74. Annual catches have since fluctuated between 1,048 and 2,522 t. Nominal fishing effort peaked at 45,786 hrs in 1978/79 and fell to 18,905 hrs in 2002/03.

3. During 2004/05, total catch (1,939 t), catches during the early spawning period (430 t) and CPUE (91 kg/hr) were all slightly lower than the previous 5-year average (2,006 t, 469 t and 97 kg/hr, respectively). The distribution of catches during 2004/05 was similar to previous years, with 48% of the catch harvested from the Wallaroo region.

4. The size of prawns harvested in 2004/05 reflected the size of prawns observed during surveys on most occasions. However, high catches of small and medium-sized prawns were harvested from adjacent fishing blocks in the Cowell, Main Gutter and South Gutter regions during harvest period 3 (March). The difference in the size of prawns harvested and observed during surveys was likely due to the lag time between survey and fishing.

5. During 2002/03, 2003/04 and 2004/05 mean harvested prawn size was 206, 221 and 214 prawns/7kg, respectively, and the limit Reference Point for prawn size (280 prawns/7kg) was exceeded on 24, 66 and 24 vessel nights, respectively.

6. Reporting rates of retained by-product increased from 2002/03 to 2004/05. The retained catch of calamary decreased from 30.9 t in 2003/04 to 26.7 t in 2004/05. The retained catch of slipper lobster increased from 3.1 t to 3.2 t during the same period.

7. Survey catch rates during November, February and April 2004/05 were similar to survey catch rates obtained during 2003/04 and the previous 5-year average.

8. The recruitment index during 2004/05 (45.6 prawns/nautical mile) was higher than the previous year (41.2), the 5-year average (41.5) and the target reference point (40).

2 9. With the exception of the recruitment index for 2002/03, the Performance Indicators of “fishing effort”, “prawn size”, and “recruitment index” in 2002/03 and 2003/04 were within the reference range. The Performance Indicator “percent virgin spawning biomass” could not be calculated. The current suite of Performance Indicators needs to be reviewed.

10. Historical reductions in effort, relatively stable catches, increases in prawn size over time, and stable long-term trends in survey data suggest that the SGPF is currently being fished within sustainable limits.

11. For rigorous assessment of the Spencer Gulf Prawn Fishery it is suggested that: appropriate Performance Indicators are developed; independent annual assessment of stock status is maintained, and; transparent decision-rules that underpin harvest strategy development and management are documented, applied and audited.

3 1. GENERAL INTRODUCTION

1.1 Overview

This Fishery Assessment Report for the Spencer Gulf Prawn Fishery is a “living” document that is part of SARDI Aquatic Sciences ongoing assessment programs for South Australian Prawn Fisheries. It updates the previous stock assessment report for this fishery (Dixon et al. 2005a). The aims of the report are: (1) to synthesise information for the Spencer Gulf Prawn Fishery; (2) to assess the current status of the resource and consider the uncertainty associated with the assessment; (3) to comment on the current biological Performance Indicators and Reference Points; and (4) to identify future research needs for the fishery.

The report is divided into six sections. The first section is the General Introduction that: (1) outlines the aims and structure of the report; (2) describes the fishery including the Spencer Gulf environment and the fishery’s history; (3) outlines current management arrangements; (4) identifies the Biological Performance Indicators (PI) and Target and Limit Reference Points; (5) summarises the biological knowledge of Spencer Gulf prawns; (6) provides a synopsis of previous stock assessment reports for the fishery; and (7) outlines the current research program.

Section two presents the analyses and interpretation of fishery-dependent logbook data from 1968–2005, documenting trends in catch, effort, CPUE, prawn size and by-product. Analyses of fishery-independent data including catch rates, prawn sizes and sex ratios observed during stock assessment surveys between 1982 and 2005 are provided in section three. Section four spatially compares fishery-independent survey data with commercial logbook data for catch rate and prawn size to enable an assessment of harvest strategy decisions throughout the fishing season.

Section five provides assessment of the fishery against the biological Performance Indicators outlined in the Management Plan (MacDonald 1998). The final section, section six, (1) summarises the information available for stock assessment of the Spencer Gulf Prawn Fishery, (2) assesses the status of the resource and comments on the uncertainty associated with that assessment, (3) identifies the limitations of the current PI and suggests an alternative approach and (4) outlines future research needs to improve stock assessment for this fishery.

4 1.2 Description of the Fishery

1.2.1 Fishery location

There are three commercial prawn (Melicertus latisulcatus) fisheries in South Australia: Spencer Gulf, Gulf St. Vincent (GSV) and the West Coast (Figure 1.1). The Spencer Gulf Prawn Fishery is the largest of these in terms of total area, production, and number of licence holders.

Fishing is permitted in all waters north of the geodesic joining Cape Catastrophe (Latitude 34º 35.4’S, Longitude 136º 36.0’E) on and Cape Spencer (Latitude 34º 9.6’S, Longitude 135º 31.2’E) on , with the exception of several permanently closed areas. Spencer Gulf is divided into 125 prawn fishing blocks aggregated into regions reflective of the main trawl grounds of the fishery (Figure 1.2).

r

# Ced una

301 30 2 303 30 4 305 30 6 30 7 308309 31031131 2 31 9 31 8 31 7 31 6315 31 4 313 ! Smoky Bay 320 321 322 323324 32 5 32632 7 ! 337 33 6 33533433 3 33 2 33 1 33 0 329 32 8 34 7 338 33 9 340 34 1 34 2 34 3 34 4 345 34 6 34 8 357 356 355 35 4 353352 351 350 34 9 Chinaman Creek 35 8 35 9 36 0 36 1 362 363 36436 5 366 36 7 !Streaky Bay 36 836 9 37 037 1372 3 7 3 374 37 5 3 76 377 387 386 385 38 438338 2 381 380 379 378

Whyalla! 2 3 4 1 10 5 11 6 101 9 ! ! 8 12 13 7 Venus Bay 1415 Port Pirie 20 1918 17 16 10 2 11411 0 105 24 26 11310 9 106 21 22 23 30 11 7 25 27 103 11 6 11210710 4 29 28 33 32 31 11 5 111108 Port Broughton !Elliston ! 34 36 Co we ll ! 35 37 120 121125 39 38 West Coast 11 9 122 40 42 115 43 114 124 44 11611 8 123 47 Arno Bay! 109 11 3 46 117 52 45 !Wallaroo 103 110 112 51 11 1 10 4 102 53 50 49 48 10 5 101 56 54 Port Neill! 57 67 10 7 10 6 60 66 20 3 20 2 20 1 61 59 58 55 68 108 20 4 20 5 20 6 65 76 62 63 77 64 69 20 9 20 8 20 7 70 75 Ardrossan! 21 0 211 21 2 74 73 72 71 79 81 82 1 83 84 85 ! 80 2 5 90 89 4 6 88 3 10 0 87 86 7 16 8 15 91 10 17 92 9 14 ! 94 18 32 95 !Ha rd wi cke Bay 13 19 93 12 31 33 Port Ade 20 30 98 11 21 34 29 35 22 28 49 99 23 36 48 97 96 ! 27 37 50 26 47 51 38 46 ! 120 112 25 52 Mari on B ay 11 1 39 45 64 ! 119 113 24 40 53 63 Port Noar 11 8 110 10 0 44 54 11 4 109 41 43 62 65 117 11 5 101 99 55 61 Spencer Gulf 10 8 102 42 56 66 116 10 7 98 89 60 67 75 10 3 97 57 59 106 10 4 90 88 68 74 96 91 58 69 73 105 95 87 82 92 86 70 72 Gulf St Vincent 9493 83 81 71 ! 85 84 Victor Harbor 80 76 Kingscote ! 79 77 78

020 40 80 120 160 Kilometres

Figure 1.1 Location of South Australia’s three commercial prawn fisheries.

5 Northern region

Middlebank & Shoalwater region

Main Gutter region Cowell region West Gutter Wallaroo region region

South Gutter region Wardang region

Thistle Corny Point Island region region

Figure 1.2 Fishing blocks and regions of the Spencer Gulf Prawn Fishery.

6 1.2.2 The Spencer Gulf environment

Spencer Gulf is a shallow embayment <40 metres depth in northern areas and up to 60 metres depth in southern areas (Figure 1.3). Sediments are predominately sand and mud, and seagrass habitats are common at depths <10 metres. Due to minimal freshwater input and high summer evaporation rates, it is an inverse estuary, with salinity increasing towards the head of the gulf (Nunes & Lennon 1986).

Figure 1.3 The bathymetry of Spencer Gulf

7 Sea Surface Temperatures (SSTs) in South Australia are lower and more variable than in other northern fisheries that target M. latisulcatus (eg. Broome and Shark Bay, Figure 1.4). Figure 1.5 illustrates the warmer SSTs in the north of both gulfs in South Australia, the cooler surface waters in the south of Spencer Gulf, and the considerable decrease in the surrounding open ocean.

30 o

Broome 25 Shark Bay

West Coast (SA) 20 GSV

15 Spencer Gulf

Sea surface temperature C 10 Jan Mar May Jul Sep Nov Feb Apr Jun Aug Oct Dec Month

Figure 1.4 Comparison of mean monthly sea surface temperature (SST, ºC) for the Australian prawn fisheries that target M. latisulcatus. Figure reproduced from Carrick 2003.

Figure 1.5 Sea-surface temperatures over the continental shelf of South Australia during late summer/early autumn, 1995. A colour-coded key in degree Celsius is situated at the top of the map. Figure from Linnane et al (2005), sourced from CSIRO.

8 1.2.3 Nursery areas

In South Australia, juvenile M. latisulcatus occur predominately on intertidal sand- and mud-flats, generally located between shallow subtidal / intertidal seagrass beds and mangroves higher on the shoreline (Kangas and Jackson 1998; Tanner and Deakin 2001). In Spencer Gulf, juvenile prawn abundances were significantly greater in the mid intertidal zone compared to the lower and upper zones (Roberts et al. 2005), while in GSV abundances were similar within the intertidal zone (Kangas and Jackson 1998).

Bryars (2003) documented a detailed inventory of important coastal fisheries habitats in South Australia. The entire South Australian coastline was divided into a number of Fisheries Habitat Areas (FHA) that corresponded to each of the 3 South Australian prawn fisheries as follows:

1) West Coast Prawn Fishery: FHA 1–6, 8–12, 14–17, 19 (Nullabor to ) 2) Spencer Gulf Prawn Fishery: FHA 20, 23, 25–37 (Thorny Passage to Formby Bay) 3) Gulf St Vincent Prawn Fishery: FHA 38–45, 55, 54, 62 (Foul Bay to Yankalilla Bay, and north- eastern ).

Each FHA has a comprehensive description, including colour-coded maps of up to 12 habitat types. Of these, the habitat types ‘tidal flats’ and ‘mangrove forests’ were determined as appropriate juvenile prawn habitat. ‘Tidal flats’ included mud flats, sand flats and intertidal unvegetated soft bottoms. It was noted that these tidal flats were often associated with adjacent mangrove forests. However, ‘tidal flat’ habitat also included intertidal seagrass meadows and intertidal macroalgal environments, which are unsuitable habitat for juvenile prawn settlement (and therefore these are a potential source of error in calculations). ‘Mangrove forest’ were characterised by a soft sediment substrate in the upper intertidal zone dominated by grey mangrove (Avicennia marina). Mangrove forest always overlapped with tidal flat habitat (see Figure 1.6) and was thus labelled mangrove forest (+ tidal flat) for this report. The proportion of the coastline for each FHA containing tidal flat only and mangrove forest (+ tidal flat) was estimated to the nearest 10% from the maps (Bryars 2003). This enabled estimation of the percent length of coast for each habitat type, which were summed across each fishery. The total length of coastline for each fishery was calculated from satellite imagery (http://earth.google.com). Table 1.1 provides summary estimates for each South Australian Prawn Fishery.

9 Table 1.1 The number of Fishery Habitat Area’s (Bryars 2003) and the estimated proportion and distance of coastline of tidal flat (TF) only and mangrove forest (+ TF) for each of South Australia’s three prawn fisheries. Coastline Tidal flat (TF) only Mangrove (+ TF) Fishery # FHA’s (km) % km % Km Spencer Gulf 15 992 51 508 25 245 GSV 11 551 41 225 14 79 West Coast 16 1310 24 310 3 45

The Spencer Gulf coastline was approximated at 992 km, where 753 km (76%) was tidal flat only and 245 (25%) was mangrove forest (+ tidal flat) (Table 1.1). Areas with the greatest juvenile prawn nursery habitat were the Far Northern Spencer Gulf (~201 km of tidal flat only and 67 km of mangrove forests (+ tidal flat)), (~95 km of tidal flat only and 57 km of mangrove forests (+ tidal flat)) and False Bay (~63 km of tidal flat only and 49 km of mangrove forests (+ tidal flat)) (Figure 1.6). These areas of identified nursery habitat correspond well with sites in Spencer Gulf previously found to contain the greatest abundances of juvenile prawns. Juvenile abundances were significantly greater in the north, with False Bay (the northern most site consistently sampled) found to have the greatest abundance (Roberts et al. 2005).

The extent of available juvenile habitat appears to correlate well with production from each fishery (Table 1.2), particularly with respect to mangrove habitat (Table 1.1). Of note, the importance of mangrove habitat for prawn recruitment has long been debated (see Lee 2004).

10

Figure 1.6 Important juvenile nursery habitat, mangrove forest and tidal flats, around coastal Spencer Gulf. Reproduced from Bryars (2003).

11 1.2.4 Commercial fishery

The Spencer Gulf Prawn Fishery is a single species fishery that targets the western king prawn M. latisulcatus. A smaller penaeid, Metapenaeopsis crassima, occurs in Spencer Gulf but is of no commercial value. M. latisulcatus were first trawled in Spencer Gulf in 1909 by the FIS Endeavour. The first commercial prawn trawling attempts occurred in 1948 but the first commercial quantity of prawns was not harvested until October 1968 (Carrick 2003).

Prawns are harvested at night using demersal, otter-trawl, double-rig gear (Figure 1.7). Considerable technological advancements have been made in the fishery including the use of “crab bags” to exclude mega-fauna by-catch (Figure 1.8), “hoppers” for efficient sorting of the catch and rapid return of by-catch (Figure 1.7), and “graders” to sort the prawns into marketable size categories (Figure 1.7). Many vessels in the prawn fishery fleet are “factory vessels” that process the catch on-board.

The Spencer Gulf Prawn Fishery is the fourth most valuable prawn fishery in Australia (Table 1.2). South Australian prawn fisheries (Spencer Gulf, GSV and West Coast) are the only single species prawn fisheries in Australia. M. latisulcatus is also a target species of the Shark Bay (78% of total catch) and Broome fisheries (44% of total catch).

Table 1.2 Production figures and species harvested in major Australian prawn fisheries. * includes by- product

Fishery Year Production (t) Value Vessels Prawn sp. harvested (% W. king) $000,000 Northern 2004–05 5,035 (<1.5%) 65 95 Banana, tiger, endeavour, king North Qld 2003 9,348* 110* 630 Banana, tiger, endeavour, king, bay NSW 2000–01 3,411* 32* 238 Eastern king, school Shark Bay 2001 1,696 (78%) 25.2 27 Western king, tiger, endeavour, coral Broome 2001 142 (44%) 1 5 Western king, coral Spencer Gulf 2004–05 1,939 (100%) 31.8 39 Western king GSV 2004–05 213 (100%) 3.8 10 Western king West Coast 2004–05 21 (100%) 0.3 3 Western king

12

Figure 1.7 Double rig trawl gear and location of hopper sorting and prawn grading systems used in the Spencer Gulf Prawn Fishery. Figure from Carrick (2003).

Figure 1.8 Trawl net configuration showing trawl boards, head rope, ground chain and cod end with crab bag. Figure from Carrick (2003).

13 1.2.5 By-catch

The predominant by-catch of the Spencer Gulf Prawn Fishery consists of small (<10 grams) non-commercial teleost fish (mainly Degens leatherjacket, Thamnaconus degeni and sand trevally, Pseudocaranx wrighti) and blue-swimmer crabs (Portunus pelagicus) (Carrick 1997; Svane 1998). The ratio of by-catch to prawn catch by weight is 1:2 for the Spencer Gulf Prawn Fishery (Carrick 1997), which is low by Australian standards (Poiner et al. 1998). Survival of by-catch species has been improved over time with the introduction of crab bags and the hopper system. The non- surviving component of discarded by-catch is rapidly consumed by dolphins, seagulls, blue swimmer crabs, leatherjackets and sealice (Svane 1998; Roberts 2000; Svane 2005).

Incidental capture of turtles in the Spencer Gulf fishery is rare. The mean annual turtle catch in the Spencer Gulf fishery was 0.1 year-1 between 1978 and 1998 (Carrick 1999).

Considerable research has been done on by-catch reduction devices. Initially, pilot studies were conducted to determine the differences in catch rates of both prawns and by-catch species for standard diamond mesh cod-ends (57.2 mm) compared to square mesh cod-ends of various sizes (Kangas and Jackson 1995).

Subsequent to these studies, a FRDC funded project evaluated the effectiveness of various by- catch reduction devices (BRD’s) in both GSV and Spencer Gulf (Broadhurst et al. 1999; McShane et al. 1998). Results showed that substantial losses of prawn catch were observed when using BRD’s compared with conventional rigging, particularly for solid grid BRD’s. Results were more favourable for tests of composite panels of square and conventional mesh, with substantial reductions in the capture of by-catch species without detection of significant differences in the weight of prawn catches. However, there were often substantial reductions in the number of prawns harvested, suggesting that fewer smaller prawns were captured in the composite mesh panels. Also, tests were conducted on the effects of both a reduction in the twine diameter and an increased mesh size in the body of the trawl net (Broadhurst et al. 2000). Again, both modifications were successful at significantly reducing by-catch without significantly reducing the weight of prawns caught.

14 1.2.6 By-product

Spencer Gulf prawn fishers are permitted to take two “by-product” species: slipper lobster (Ibacus spp) and Southern calamary (Sepioteuthis australis). The life history and population status of both these species in Spencer Gulf is poorly understood. In comparison, the GSV prawn fishery recently agreed to develop and conduct the first comprehensive monitoring program for by- product species, I. peronii and S. australis (Dixon et al. 2006) to address the Department of Environment and Heritage (DEH) management recommendations (Anon, 2004). The GSV by- product monitoring program is now an established component of fishery-independent surveys. Such a monitoring program is yet to be established in the SGPF.

1.2.6.1 Slipper lobster

The species of slipper lobster (Family: Scyllaridae) frequently captured in South Australia was identified as I. peronii, commonly known as the “Balmain Bug” (pers. comm. Dr Wolfgang Zeidler and Mr Thierry Laperousaz, South Australian Museum, Figure 1.9). I. peronii inhabits depths of 4– 288 m (Brown and Holthuis, 1998), and is a long-lived , with relatively low fecundity (5,000–50,000) compared to other Scyllarid lobsters (Stewart & Kennelly, 1997, 2000). NSW populations exhibited limited movement, with average displacements of 0.35 km over a 248-day period (Stewart & Kennelly, 1998). Another species of slipper lobster, I. alticrenatus (common name: Wollongong Bug), also resides in South Australian waters although is rarely caught by prawn trawlers as it inhabits water depths of 82–696 m (Brown and Holthuis, 1998).

There are no management restrictions on the capture of I. peronii in South Australia. In Queensland and New South Wales management strategies imposed on the trawl fisheries include bans on the taking of egg-bearing females, and a minimum legal size of 100 mm carapace width (CW).

Figure 1.9 Slipper lobster, Ibacus peronii, caught by South Australian prawn fishers.

15 1.2.6.2 Southern calamary

Southern calamary (Sepioteuthis australis) is the most common squid species in southern Australian inshore waters and a key component of the marine ecosystem as a primary consumer of and fishes, and as a food source for a variety of predators (Coleman 1984; Gales et al. 1993). In southern Australia, it ranges from Dampier in Western Australia to Moreton Bay in Queensland, including Tasmania. It also occurs in northern New Zealand waters. For most of its distribution, S. australis inhabits coastal waters and bays, usually in depths of <70 m (Winstanley et al. 1983). Like many other inshore squid species, S. australis is of increasing commercial significance, contributing to multi-species, multi-gear, marine fisheries in all southern Australian states, particularly South Australia and Tasmania.

The South Australian calamary resource is shared by three sectors: the recreational sector; the commercial sector of the Marine Scalefish Fishery (MSF), and; South Australia’s prawn fisheries. Whilst catches from the MSF are regularly documented (see Steer et al. 2006), poor information on the catches from the recreational fishery and commercial prawn fisheries lead to considerable uncertainty in assessing the status of the calamary resource in South Australia (see Steer et al. 2006).

1.2.7 Recreational, indigenous and illegal catch

Significant recreational catches of M. latisulcatus are precluded by current fisheries regulations that require recreational prawn catches to be taken from waters >10 m in depth using hand held nets. Levels of indigenous and illegal fishing are considered negligible (Anon 2003).

16 1.3 Management of the Fishery

The Spencer Gulf Prawn Fishery is managed by Primary Industries and Resources South Australia (PIRSA) under the framework provided by the Fisheries Act 1982. General regulations for South Australia’s prawn fisheries (commercial and recreational) are described in the Fisheries (General) regulations 2000, with specific regulations located in the Scheme of Management (Prawn fisheries) Regulations 1991. These three documents provide the statutory framework for management of the Spencer Gulf Prawn Fishery.

1.3.1 Management milestones

Management arrangements have evolved in the Spencer Gulf Prawn Fishery since its inception in the late 1960’s, with the milestones presented in Table 1.3.

Table 1.3 Major management milestones for the Spencer Gulf Prawn Fishery

Date Management milestone 1968 Licence limitation Trawling prohibited in waters of <10 metres Commercial recording of catch and effort introduced 1969 Prawn Resources Regulations established Spencer Gulf divided into two zones 1971 Spencer Gulf zones removed 1974 Spatial closure north of implemented 1976 Licences capped at 39 1981 Spatial closure adjacent to Port Broughton implemented 1991 Scheme of Management (Prawn Fisheries) Regulations introduced 1998 Management Plan implemented

1.3.2 Current fishery management

The Spencer Gulf Prawn Fishery is a limited entry fishery with 39 licensed operators. Trawling activities are banned during daylight hours and must be conducted in waters >10m depth. Effort is restricted both spatially and temporally throughout the fishing year by spatial closures. Effective effort (fishing power) is restricted by gear restrictions including vessel size and power, type and number of trawl nets towed, maximum headline length and minimum mesh sizes (Table 1.4).

17

Table 1.4 Current management arrangements

Prawn fishery management strategy Specification Permitted prawn species harvested Melicertus latisulcatus Permitted by-product species harvested Ibacus spp., Sepioteuthis australis Limited entry Yes Number of licences 39 Corporate ownership of licences Yes Licence transferability Yes Maximum number of days fishing 85 Minimum depth trawled 10 metres Method of capture Demersal Otter Trawl Trawl net configuration Single or double Maximum total headline length 29.26 metres Minimum mesh size 4.5 cm Maximum length of vessel 22 metres Maximum engine capacity 272 kW Catch and effort data Daily logbook submitted monthly Catch and disposal records Daily CDR records Recreational fishery Depth> 10 metres, hand nets only Recreational licence Not required

There are generally 6 fishing periods within each fishing year. Each fishing period lasts a maximum of 18 nights from the last to first quarter of the moon in November, December, March, April, May and June. Harvest strategies for each period are determined on the basis of data collected during fishery-independent and fishery-dependent surveys.

1.3.3 Prawn Fishery Management Plan

The Management Plan for the Spencer Gulf, GSV and West Coast prawn fisheries (MacDonald 1998) was developed by the Prawn Fishery Management Committee (PFMC). This plan provides guideline policies, objectives and strategies for the sustainable management of these fisheries. The biological and ecological objectives and strategies are relevant to this report and are summarised in Table 1.5. The Management Plan is currently under review.

18

Table 1.5 Biological and ecological objectives and strategies of the Management Plan (MacDonald 1998) for the Spencer Gulf and West Coast prawn fisheries

Objective Strategy Biological Maintain 50% virgin spawning • adopt a precautionary approach biomass • collect catch, effort and fishery-independent data to assess against the performance indicators • maintain sustainable exploitation rates • monitor effort levels Prevent growth overfishing and • produce annual stock assessment reports to determine capture of small prawns subsequent harvest levels • annual review of research • improve harvest practices and strategies Ecological Minimise the impact of trawling • monitor and assess trawl by-catch on the environment • improve technology to minimise environmental impacts • minimise impacts on other fisheries Minimise trawl by-catch • communicate with other user groups of Spencer Gulf waters • promote conservation of seagrass and juvenile prawn Monitor marine environment habitats

1.3.4 Performance Indicators and Reference Points

The Management Plan specifies Performance Indicators (PI) and Reference Points that form the basis upon which performance of the fishery is assessed (Table 1.6). “Target Reference Points” define the desirable management target. “Limit Reference Points” define the level where intervention is required to prevent long-term stock decline. Caddy and McMahon (1995) provide detailed background into the conceptual and applied aspects of Reference Points for fisheries management.

Table 1.6 Summary of the biological and economic Performance Indicators and Reference Points for the Spencer Gulf Fishery (MacDonald 1998).

Performance Indicator Target Reference Point Limit Reference Point Effort (days) 70-80 85 % Virgin Spawning Biomass 50% 40% Recruitment Index (juv./shot) 40 35 Size at capture <40 per kg ≥ 40 per kg

19 The target reference point for effort is based on “effective days”, which are calculated as the number of nominal days multiplied by a coefficient estimating the annual fishing power of the fleet. Percent virgin spawning biomass is calculated as the difference in the abundance of prawns between April and the following November surveys, accounting for the seasonal effects of catchability and natural mortality. The recruitment index in the Management Plan (MacDonald 1998) was initially defined as the number of prawns from a standard trawl shot that were <30 mm carapace length (CL). Carrick (2003) suggested a more appropriate measure as the square root transformation of the numbers of prawns (males <33 and females <35 mm carapace length) per nautical mile trawled from 39 stations in the north of the Gulf during February surveys. Size at capture is calculated as the mean number of prawns per kg from the annual catch.

1.3.5 Real Time Management

Real Time Management (RTM) is an ongoing process of determining sustainable harvest strategies based on information gathered during fishery-independent and fishery-dependent processes. During fishing, the “committee at sea” provides information to PIRSA Fisheries on an ongoing basis, upon which the prescribed harvest strategy may be amended in “real time”.

The committee at sea acquires information from two sources. Firstly, fishers report prawn catches that violate the size (>40 prawns per kg) criteria prescribed in the Management Plan (MacDonald 1998). Secondly, fishers conduct “spot” surveys in areas closed to fishing. The spot survey locations are determined through collaboration among industry, PIRSA Fisheries and SARDI Aquatic Sciences, and are coupled with the most recent stock assessment survey results and historical information.

20 1.4 Biology of the Western King Prawn

1.4.1 Distribution and taxonomy

M. latisulcatus, is distributed throughout the Indo-west Pacific (Grey et al. 1983). Its distribution in South Australia is unique, as it is at its lowest temperature range, restricted to waters of Spencer Gulf and GSV and along the west coast including the commercially fished areas of Ceduna, Venus Bay and Coffin Bay. King (1977), Sluczanowski (1980) and Carrick (1982, 1996) provide detailed accounts of the distribution of western king prawn in Spencer Gulf.

The western king prawn is a benthic species that prefers sandy areas to seagrass or vegetated habitats (Tanner & Deakin 2001). Both juvenile and adult prawns show a strong diel behavioural pattern of daytime burial and nocturnal activity (Rasheed & Bull 1992; Primavera & Lebata 2000). Strong lunar and seasonal differences in activity are also exhibited, where prawn activity (and catchability) is greater during the dark phase of the lunar cycle and during warmer months.

The distribution and abundance of M. latisulcatus within gulfs and estuaries is affected by salinity and the presence of sandy substrate (Potter et al. 1991). Higher abundances are associated with salinities above 30 ‰ (Potter et al. 1991). In physiological studies on M. latisulcatus, optimal salinity ranged from 22 to 34 ‰, and 100% mortality occurred at salinities below 10 ‰ (Sang & Fotedar 2004). Juvenile M. latisulcatus are more efficient osmoregulators than adults, tolerating greater variation in salinity. Important nursery areas in Western Australia and South Australia are characterised as being hyper-saline (35–55 ‰) (Carrick 1982; Penn et al. 1988).

1.4.2 Reproductive biology

In the Spencer Gulf Prawn Fishery adult prawns aggregate, mature, mate and spawn in deep water (>10 metres) between October and April, with the main spawning period between November and February (Figure 1.10; Carrick, 1996). During mating the male transfers a sperm capsule (spermatophore) to the female reproductive organ (thelycum). The success of this insemination depends on the female prawn having recently moulted.

During the peak spawning period, the sex ratio of M. latisulcatus caught in West Australia (WA) was shown to significantly change to that of a female biased catch. This was attributed to higher catchability of females due to increased foraging-feeding activity necessitated by food requirements during ovary development (Penn 1976; Penn 1980). Similarly during November and December, female biased populations of M. latisulcatus were documented in GSV (Svane 2003; Svane & Roberts 2005).

21

Figure 1.10 Female prawn reproductive maturation trends based on the percentage of ripe (Stage 3 & 4) prawns. Figure from Carrick (2003).

Spawning and fecundity are affected by water temperature, with the minimum for spawning being 17°C for M. latisulcatus in WA (Penn 1980). The peak reproductive period in Queensland (QLD) populations of M. latisulcatus was between June and July when water temperature dropped below 25°C (Courtney & Dredge 1988). Thus, the ideal temperature range for spawning in M. latisulcatus appears to be 17–25°C.

The proportion of female M. latisulcatus with fertilized eggs also varies with size. The smallest ripe female recorded in WA populations was 29 mm Carapace Length (CL) (Penn 1980). In Spencer Gulf, the smallest ripe female was 24 mm CL (February 2005 fishery-independent survey data, SARDI). While females can mature at a small size, insemination rate increases with size. Courtney & Dredge (1988) showed that ~50% of females were inseminated at 34 mm CL, while ~95% were inseminated at 42 mm CL in QLD populations of M. latisulcatus. In Spencer Gulf, Carrick (1996) defined the relationship between maturity and size with the logistic equation:

⎡ 1 ⎤ Proportion fertilized=× 8.3 10−6 + ⎢ −−(0.277(CL 36.45)) ⎥ ⎣⎢1+ e ⎦⎥

22 There are no data on the fecundity of M. latisulcatus in Spencer Gulf. Table 1.7 and Figure 1.11 presents the results of fecundity studies for M. latisulcatus in GSV (Kangas unpublished, cited in Carrick 2003), Shark Bay (Penn 1980) and the North East Coast of Queensland (Courtney and Dredge 1988). In all three fisheries, fecundity increases exponentially with carapace length, however this is more pronounced in the cooler waters of GSV (see Figure 1.11). Thus larger prawns make a greater contribution to total egg production due to both greater insemination rates, as well as greater fecundity (Penn 1980; Courtney & Dredge 1988; Carrick 1996).

Table 1.7 Fecundity relationships for M. latisulcatus in Gulf St. Vincent, Western Australia and Queensland. Fecundity = a × carapace length^ b

Location a b Gulf St. Vincent, SA 7.94 ×10-6 3.462 Shark Bay, WA 6.95 ×10-5 2.916 Nth East Coast, QLD 4.8 ×10-6 3.52

In the Eastern King prawn (P. plebejus) females greater than 50 mm CL contribute little to egg production, while the bulk of the eggs are produced by middle to upper size ranged prawns of 35–48 mm CL (Courtney et al. 1995). Such ovarian senescence in old female M. latisulcatus has not been documented.

16 GSV WA 14 QLD

12

10

8

6 Ovary weight (g)

4

2

0 25 30 35 40 45 50 55 60 65 Carapace length (mm)

Figure 1.11 The relationship between fecundity (ovary weight) and carapace length (CL) for M. latisulcatus in GSV, Western Australia and Queensland.

23 Spawning frequency for M. latisulcatus was related to moulting frequency in several previous studies. This was concluded because: no recently moulted female was found with well-developed (stage 3 or 4) ovaries (Penn 1980; Courtney & Dredge 1988); females lost the spermatophores with the exuvae at moult (Penn 1980) and; the average interval for both moulting and spawning was the same in tagging experiments (Penn 1980). The average moult interval, and hence spawning interval, for mature untagged females in WA populations during the spawning season was estimated at 30–40 days (Penn 1980).

Multiple spawning events can occur in M. latisulcatus as spawning frequency is related to moulting frequency. There are three lines of evidence supporting the concept of multiple spawning: (1) spent ovaries are difficult to identify since immediate ovary development meant they were often classified as stage 2 (Penn 1980; Courtney & Dredge 1988); (2) in an experiment where ripe females were tagged and released, 15 re-captured individuals were found to have spawned and moulted, and had ovaries at an early stage of development during the same season (Penn 1980) and; (3) artificial spawning of P. orientalis in aquaria, using eyestalk ablation, provided direct evidence for the multiple spawning capacity of Penaeids (Arnstein & Beard 1975). In addition to multiple spawning within a season, females may repeat the process in subsequent years. This was determined by the large proportion of females in different size cohorts being reproductively active during the spawning season (Penn 1980).

Prawn reproduction can also be affected by parasite load and disease status. Courtney et. al. (1995) showed that parasitisation by bopyrid isopods affected the reproductive output of P. plebejus. Bopyrid isopods have been observed to parasitise South Australian population of M. latisulcatus (Dr. Shane Roberts, personal observation). In F. indicus, it was shown that viral infections affected moulting and reproduction in Penaeid shrimp (Vijayan et. al. 2003). In addition, environmental pollution from coastal industries can increase the susceptibility of prawns to disease and reduce reproductive output (Nash et. al. 1988). These issues are poorly understood for M. latisulcatus in South Australia.

1.4.3 Larval and juvenile phase

M. latisulcatus has an offshore adult life and an inshore juvenile phase (Figure 1.12). Prawn larvae undergo metamorphosis through four main larval stages: nauplii, zoea, mysis and post-larvae. The length of the larval stage depends on water temperature, with faster development in warmer water (Hudinaga 1942). It has previously been suggested that the larval period of M. latisulcatus in Spencer Gulf could exceed 40 days, where water temperatures over the main spawning and larval period range from 19–25° C (after Shokita 1984, cited in Carrick 2003).

24 Prawn larvae are generally dispersed by wind-driven and tidal currents (Carrick 2003). Larval sampling in Spencer Gulf has shown that larvae are broadly distributed (Figure 1.13), but highest densities are found north of Cowell (Carrick 1996). Latitude, water temperature and salinity were all shown to influence the distribution and abundance of larvae (Carrick 2003). Larval densities varied significantly among years, probably due to differences in environmental conditions and spawning stock status.

Figure 1.12 Life cycle of M. latisulcatus (Carrick 2003).

Post-larvae settle in inshore nursery areas when 2-3 mm CL and can remain there for up to 10 months, depending on the time of settlement (Carrick et al. 1996). The post-larvae produced from early spawning events settle in nursery areas during December or January where they grow rapidly before emigrating to deeper water in May or June. Alternatively, post-larvae produced from spawning after January settle in nurseries from March and then grow slowly. They “over- winter” in the nursery areas before recruiting to the trawl grounds in February of the following year (Carrick 2003). The effect of over-wintering on adult growth and survival are unquantified.

Over-wintering mortalities in nurseries ranged from 0.2–16.5% (mean = 7.9%) per week, with evidence of density dependent mortality (Kangas 1999). The mean natural mortality in Spencer Gulf nurseries during winter was estimated at 5% per week (Carrick 2003). These estimates of natural mortality for juvenile M. latisulcatus are considerably lower than for other prawn species (Carrick 1996).

25

Figure 1.13 Mean larval density (√no./100 m3) in Spencer Gulf during 1993 and 1994. Figure from Carrick (2003).

26 In Spencer Gulf, spatial and temporal differences in juvenile prawn abundances were evident (Roberts et al. 2005). Even so, inter-annual patterns were generally consistent across sites. Abundances were greatest between February and May, with key nursery sites identified as False Bay, Shoalwater Pt, Plank Pt, Mt Young, 5th Creek and Port Pirie, all in the north of the gulf (Carrick 1996; Roberts et al. 2005).

1.4.4 Stock structure

Analyses using r-DNA have shown significant genetic differences in haplotype distribution of M. latisulcatus between South Australia and Western Australia (South Australian Museum/SARDI cited in Carrick 2003). However, an analysis on the genetic structure of M. latisulcatus within South Australia using electrophoresis suggested a homogenous stock (Richardson, 1982 cited in Carrick 2003).

1.4.5 Growth

Prawns undergo a series of moults to increase their size incrementally. The shedding of hard body parts during moulting means that the age of individuals cannot be reliably determined as is possible for teleost and cartilaginous fishes, through the examination of otoliths and vertebrae. The inability to directly age prawns has increased the reliance on tag-recapture and cohort analysis for the determination of growth rate.

Uncertainties associated with each method of growth estimation include: • growth suppression by the tagging process (Penn 1975; Menz & Blake 1980), • short time at liberty for tag-recaptures influenced by seasonal growth, • bias in size at release and time at liberty during tag-recapture experiments, • inability to distinguish cohorts, effect of catchability, and net migration on cohort analysis, • measurement error (both methods).

Between 1984 and 1991 >150,000 prawns were individually tagged with streamer tags. The carapace length of each prawn was measured and the tag and location details recorded prior to release. Some 9,000 tagged prawns were recaptured between 1985 and 1992. Sex-specific growth parameters, derived using a modified Von Bertalanffy growth model (Carrick 2003), show that male prawns grow slower and attain a smaller maximum size than females (Table 1.8). Maximum growth rates occurred during late summer and autumn, and growth was negligible from July to December (Carrick 2003). Growth was strongly seasonal because winter water temperatures in Spencer Gulf are at the lower limits of their preferred temperature range (Wu 1990). The von

27 Bertalanffy limited growth model is dL/dt = k(L∞ - L), where k is a function of temperature. The

r formula for growth is usually re-written as L(t) = L∞ (1-e (t-t0)), where r is the specific growth rate, t is time, and k=r. The constant r is species (and gender) dependent and determines the rate of growth.

Growth estimates from the Spencer Gulf are compared to those estimated from the GSV and the West Coast Fishery in Table 1.8 and Figure 1.13. Kangas & Jackson (1997) estimated growth rates from 464 tag-recaptures in the GSV while in the West Coast Prawn Fishery growth was estimated from 510 tag-recaptures as well as from length-frequency cohort analyses (Wallner 1985).

Seasonal growth and differences between genders were evident in each fishery. Prawns in Spencer Gulf attained a similar size to GSV prawns, although a slower growth rate was evident for male prawns in GSV (Figure 1.14). Also, prawns in both gulfs attain a greater size and growth rate than their West Coast counterparts. Whilst this may be an artefact of the uncertainty associated with West Coast prawn growth estimates (see Dixon & Roberts 2006), growth may be slower due to the cooler summer water temperatures of the West Coast’s oceanic environment.

Table 1.8 Sex-specific growth parameters for M. latisulcatus estimated from tag-recapture and cohort analysis in the West Coast (Wallner 1985) and from tag-recapture in Spencer Gulf (Carrick 2003) and Gulf St Vincent (Kangas & Jackson 1997).

Growth parameters Fishery Method Sex -1 K (yr ) L∞ (mm) West Coast Cohort Male 0.73 44.1 Female 0.88 53.9 West Coast Tag Male 0.83 39.4 Female 0.36 60.4 Spencer Gulf Tag Male 0.86 46.1 Female 0.61 64.0 GSV Tag Male 0.62 47.2 Female 0.54 65.3

28 70 Females Spencer Gulf - tag 60 GSV - tag WC - tag 50 WC - cohort

40

30

Carapace length (mm) 20

10

0 012345 Age (years)

70 Males

60 Spencer Gulf - tag GSV - tag 50 WC - tag WC - cohort

40

30

Carapace length (mm) 20

10

0 012345 Age (years)

Figure 1.14 Sex-specific growth curves for M. latisulcatus estimated from tag-recapture and cohort analysis in the West Coast (Wallner 1985) and from tag-recapture in Spencer Gulf (Carrick 2003) and Gulf St Vincent (Kangas & Jackson 1997).

29 1.4.6 Length weight relationship

The relationship between prawn carapace length (CL, mm) and weight (g) was determined from a sample of over 2000 prawns from Spencer Gulf (Carrick 2003). The power relationship described by the equation “Weight = a × carapace length^ b” varies between males (a = 0.00124, b = 2.76) and females (a = 0.00175, b = 2.66). Kangas (1999) determined the length weight relationship for juvenile prawns in GSV (a = 0.00066, b = 2.91, N = 325) (Figure 1.15). The size range of individuals was 2.4–20.4 mm CL, where sexes could not be distinguished at such small sizes.

Figure 1.15 Length weight relationship for juvenile M. latisulcatus in GSV (Kangas 1999).

1.4.7 Movement determined from tagging studies

The tag-recapture data (see 1.4.5) were analysed to determine the movement patterns of prawns in Spencer Gulf (Carrick 2003). The generalised movement patterns were: (1) a net movement from the north to the south in northern Spencer Gulf, (2) a general east to north-east movement from northern Cowell and the top of the Gutter, (3) south-east movement from southern Cowell and the Gutter towards Corny Pt., and (4) negligible movement from Wallaroo (Figure 1.16, Carrick 2003).

30

Figure 1.16 Generalised movement patterns of tagged M. latisulcatus in Spencer Gulf. Figure from Carrick 2003.

While the use of external tags (as used for prawns in South Australia) has been associated with higher prawn mortality rates (Benzie et al. 1995) and suppressed growth rates (Penn 1975; Menz & Blake 1980), particularly for small individuals, it is unclear how these tags affect prawn movement. Potential effects on growth and mortality can be reduced with the use of antibiotic/antifungal ointment on the tag to reduce post-tag mortality from infection (Courtney et al. 2001) and selective tag colour to reduce prawn predation (Benzie et al. 1995).

31 1.4.8 Natural mortality

Daily instantaneous rate of natural mortality for M. latisulcatus in Spencer Gulf range between 0.003 and 0.005.day-1 (King 1977). This value was similar to that estimated for M. latisulcatus in GSV (0.003.day-1; Kangas & Jackson 1997, Xiao and McShane 2000) the West Coast Prawn Fishery (0.001 to 0.014.day-1; Wallner 1985) and Western Australia (0.002 to 0.005.day-1; Penn 1976).

1.4.9 Prawn health

The health of South Australian populations of M. latisulcatus and the potential effects of coastal pollutants, parasites and disease on growth, survival and reproduction is poorly understood. In Spencer Gulf juvenile habitats appear to have been influenced by oil spills (Roberts et al. 2005) and industrial effluent Carrick (2003). In GSV, anecdotal evidence suggests that juvenile prawn abundances at have significantly declined since the early 1970’s, probably due to human induced factors including increased nutrient loading (Kangas 1999). The disturbance of acid sulfate soils as a result of coastal development were recently identified as a major cause of habitat degradation in GSV, including mangrove dieback at St Kilda and contaminated tidal flats in the Barker Inlet (SA Coast Protection Board, 2003). Common marine pollutants in South Australia include heavy metals, high nutrient loads from coastal industries and petroleum (hydrocarbon) discharges (Edyvane 1999). Although these sources of pollution are common, and potentially directly affect juvenile prawn nurseries, little research has been conducted to address these issues.

Parasite load and disease status are limiting factors in marine populations, although generally overlooked in fisheries management (Harvell et. al. 2004). These factors affect populations through mortality as well as suppression of growth and reproduction, and have yet to be determined for SA prawn populations.

32 1.5 Stock Assessment

The first stock assessment for the Spencer Gulf Prawn Fishery was completed in 1998 (Carrick and McShane 1998). Subsequent stock assessments in 2000 and 2001 were the first to consider the biological PI’s of the fishery (Carrick and Williams 2000, 2001). The 2003 stock assessment report was the first version of a “living” document (Carrick 2003) that constituted a considerable advance on previous stock assessments. This included a description of the life history and management of the fishery, detailed spatial and temporal analyses of fishery-dependent and fishery-independent data, assessment of the fishery against the performance indicators defined in the Management Plan, and a review of the biology of M. latisulcatus. The 2005 stock assessment considerably built on previous stock assessments and represents the current stock assessment template (Dixon et al. 2005a).

1.6 Current Research and Monitoring Program

The current research program conducted by SARDI Aquatic Sciences in support of the Spencer Gulf Prawn Fishery comprises five components. These are: (i) administer a daily logbook program; (ii) collate catch and effort information; (iii) conduct independent stock assessment surveys prior to, during and toward the end of the fishing year, to inform harvest strategies and assess the fishery against the PI’s; (iv) manage and analyse by-catch, juvenile sampling and tagging data; and (v) produce an annual report that assesses the status of the Spencer Gulf Prawn Fishery, including assessment of the fishery against the PI’s defined in the Management Plan.

1.6.1 Catch and effort research logbook

Licence holders are required to complete a daily and monthly logbook after the completion of fishing in each month. The logbook has undergone several modifications throughout time to improve the information available for assessment. During 1986 the catch and effort reporting blocks were modified to better reflect the fishing grounds and distribution of effort. More recent changes to the logbook include incorporation of the location (GPS position) of at least 3 trawl shots per night, size-grade data of the prawn catch, and reporting of retained by-product.

1.6.2 Stock assessment surveys

The first stock assessment surveys were done in Spencer Gulf in February 1982. Surveys are conducted using industry vessels with independent observers, to assess stock status and for the development of harvest strategies.

33 2. FISHERY STATISTICS

2.1 Introduction

This section of the report presents summaries and analyses of the catch and effort data for the Spencer Gulf Prawn Fishery. Data were obtained from two sources: annual data from 1968 to 1973 and monthly data from January 1973 to June 1988 were obtained from SAFIC annual reports (1973 to 1988); while data from 1988/89 to 2004/05 were obtained from daily logbooks. Logbook data prior to 1988/89 are currently being validated. Daily logbooks provide data for each trawl shot including start/finish times, estimated prawn catch, fishing block, and depth. Average trawl speed, prawn catch by size-grade, and by-product catch are provided for each night.

In this report, a fishing year is defined as the period from November to October. Fishing is mostly conducted between November and June, during six periods of varying length between the last and first quarter of the moon (maximum length 18 days). Estimated prawn catch for each shot was adjusted using validated post-harvest catches reported in monthly logbooks. Results are discussed in terms of the regions defined in Figure 1.2.

2.2 Catch and Effort

2.2.1 Inter-annual trends

Prawns were first harvested commercially from Spencer Gulf in 1968 (Figure 2.1). Catches increased to ~1,000 t over the next four years, and to >2,000 t in 1973/74. Catches over the last 30 years have ranged between 1,048 t in 1986/87 and 2,522 t in 2000/01. The average catch over the last 5 fishing years was 2,012 t, approximately 5% greater than the historical mean of 1,910 t. Catch during 2004/05 was 1939 t.

Commercial effort increased rapidly from 6,795 hr in 1968 to 45,786 hr during 1978/79 (Figure 2.1), but has declined steadily and significantly in the years since (Linear Regression (LR): r2 = 0.96, df = 24, P < 0.01). Effort during 2004/05 was 21,360 hrs, which was 47% of the peak in 1978/79.

34 3000 50

Catch Effort 2500 40

2000 30

1500 Catch (t) 20

1000 Effort (hours*1000)

10 500

0 0

8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 6 7 7 7 7 8 0 ' ' ' ' ' /7 /7 /7 /7 /7 / /7 /8 /8 /8 /8 / /8 /8 /8 /8 /8 /9 /9 /9 /9 /9 /9 /9 /9 /9 /9 /0 /0 /0 / /0 /0 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 7 7 7 7 7 7 7 7 8 8 8 8 8 8 8 8 8 8 9 9 9 9 9 9 9 9 9 9 0 0 0 0 0 Fishing-year

Figure 2.1 Total catch (t) and effort (hrs) for Spencer Gulf from 1968 to 2004/05. Data for 1968–1972 are reported as calendar year. Data for 1972/73 are from January to October 1973. From 1973/74 data are reported in fishing years.

2.2.2 Trends among regions

The spatial distribution of catches from Spencer Gulf has changed since 1988/89 (Figure 2.2). The annual catch from the Northern region peaked at 933 t in 1991/92, but has not exceeded 250 t over the last 7 fishing years. Peak catches in the Northern region coincided with the lowest annual catch from the Wallaroo region (206 t). During the past 8 fishing years, the Wallaroo, Middlebank/Shoalwater and Main Gutter regions have produced consistent annual catches, averaging 867, 479 and 182 t respectively. Other regions have produced variable annual catches. The reduction in fishing effort in the north of the Gulf in recent years reflects the management decision to target larger and more valuable prawns, which generally occur further south.

35 1200 800 North Middlebank/Shoalwater 1000 600 800

600 400 400 200 200

0 0 88/89 91/92 94/95 97/98 00/01 03/04 88/89 91/92 94/95 97/98 00/01 03/04

1200 300 Wallaroo Main Gutter 1000 225 800

600 150 400 75 200

0 0 88/89 91/92 94/95 97/98 00/01 03/04 88/89 91/92 94/95 97/98 00/01 03/04

200 60 Cowell West Gutter 150 45

100 30

50 15 Catch (t)

0 0 88/89 91/92 94/95 97/98 00/01 03/04 88/89 91/92 94/95 97/98 00/01 03/04

200 200 South Gutter Wardang

150 150

100 100

50 50

0 0 88/89 91/92 94/95 97/98 00/01 03/04 88/89 91/92 94/95 97/98 00/01 03/04 200 300 Thistle Island Corny Point

150 225

100 150

50 75

0 0 88/89 91/92 94/95 97/98 00/01 03/04 88/89 91/92 94/95 97/98 00/01 03/04

Fishing-year

Figure 2.2 Average annual catches from regions of Spencer Gulf from 1988/89 to 2004/05. Note change in Y-axis scales between graphs.

36 2.2.3 Trends within years

The distribution of monthly catches has changed over the last 30 years (Figure 2.3). Between 1974/75 and 1978/79, some prawns were captured in all months. Temporal closures were introduced in 1978/79. From 1979/80 to 1983/84, the average catch for April was ~600 t. Since 1984/85 most of the catch has been taken during March, April and May.

1974/75-1978/79 1979/80-1983/84 750 750

600 600

450 450

300 300

150 150

0 0 Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct

1984/85-1988/89 1989/90-1993/94 750 750

600 600

450 450

300 300

150 Catch (t) Catch 150

0 0 Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct

1999/2000-2004/05 750 1994/95-1998/99 750

600 600

450 450

300 300

150 150

0 0 Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct

Month

Figure 2.3 Average monthly catches from Spencer Gulf for 5-year periods from 1974/75 to 1998/99 and the six years between 1999/2000 and 2004/05.

37 2.2.4 Catches during the spawning season

The main spawning period for M. latisulcatus in Spencer Gulf extends from November to March. As catch levels during the early spawning period are important for the sustainability of the fishery, levels of catch harvested in November and December, related to total catch, are considered here. Figure 2.4 shows the pre-Christmas (November and December) catch and the total annual catch.

From 1981/82 to 1986/87, the total annual catch declined from 2,491 t to the record low for the established fishery of 1,048 t. This record low catch followed increases in the pre-Christmas catch from 297 in 1979/80 to 833 t in 1983/84. This is the only period in the history of fishery that pre-Christmas catch has exceeded 500 t in three consecutive years (1981/82, 1982/83, 1983/84). Since 1984/85 catches pre-Christmas have exceeded 500 t on only 4 occasions: 1991/92, 1995/96, 1998/99 and 2001/02. In each case, the total annual catch declined the following year.

3000 Other months Nov&Dec 2500

2000

1500 Catch (t) 1000

500

0 73/74 78/79 83/84 88/89 93/94 98/99 03/04 Fishing-year

Figure 2.4 Catches from November and December relative to the total annual catch from 1973/74 to 2003/04 in Spencer Gulf.

38 2.3 Catch-Per-Unit-Effort (CPUE)

2.3.1 Inter-annual trends

Annual (nominal) CPUE has varied substantially since the inception of the fishery (Figure 2.5). Up to 1985/86, CPUE generally fluctuated between 40 and 70 kg/hr, but a peak of 82.9 kg/hr was recorded in 1973/74. The lowest CPUE, 35.2 kg/hr, was recorded in 1986/87. CPUE increased during the late 80s and throughout the 90s and exceeded 100 kg/hr for the first time in 1997/98. CPUE has remained above 80 kg/hr thereafter, except during 2002/03 (78.3 kg/hr). Between 2000/01 (114.2 kg/hr) and 2004/05 (90.8 kg/hr) the trend in CPUE has generally decreased. The general historical increase in CPUE over time mainly reflects increases in the fishing power of the fleet. Decisions to target larger prawns also affected CPUE. Hence, CPUE does not accurately reflect prawn abundance over the entire history of the fishery. However, changes in CPUE over shorter time periods (eg between years) when variations in fishing power are smaller, more reliably reflects changes in prawn biomass (Figure 2.5).

3000 120 Catch Annual CPUE

2500 100

2000 80

1500 60 Catch (t)

1000 40 CPUE (kg/hour)

500 20

0 0

8 9 0 1 2 3 4 5 6 7 8 6 6 7 7 7 7 7 7 7 7 7 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 05 ' ' ' ' ' / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /

72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 Fishing-Year

Figure 2.5 Annual catch and catch-per-unit-effort (CPUE) for Spencer Gulf from 1968 to 2004/05. Data for 1968–1972 are reported as calendar year. Data for 1972/73 are from January to October 1973. From 1973/74 data are reported in fishing years.

39 2.3.2 Trends among regions

Over the past 5 fishing years (2000/2001–2004/05) CPUE has varied among regions (Figure 2.6). Generally, CPUE had declined with latitude, being higher in the Northern, Shoalwater/Middlebank, Wallaroo and Main Gutter regions than regions further south. Regional differences in CPUE have influenced the long-term CPUE trends for the fishery (Figure 2.5), as

the distribution of effort has changed over time.

140

120

100

80

60 CPUE (kg/hour)

40

20

0

k r r r g d t utte Cowell rdan Islan Northern Wallaroo st Gutte th Gutte Wa ater/M'ban Main G We ou Corny Poin w S Thistle S' Region

Figure 2.6 Mean (+ SE) annual catch-per-unit-effort (CPUE, total annual catch/total annual effort) for the 10 fishing regions within Spencer Gulf from 2000/2001 to 2004/05.

40 2.3.3 Trends within years

Within-year trends in CPUE have varied since 1974/75 (Figure 2.7). From 1974/75 to 1978/79, CPUE was highest between February and April, peaking at >70 kg/hour in March. During 1979/80 to 1983/84, when fishing was confined to eight months of the year, the highest CPUEs were recorded in February and April. Between 1984/85 and 1993/94, CPUE was relatively consistent across the months fished. In recent years (since 1999/00), the CPUE in April has been almost twice the average CPUE for the remainder of the fished months.

1974/75-1978/79 120 120 1979/80-1983/84

90 90

60 60

30 30

0 0 Nov Jan Mar May Jul Sep Nov Jan Mar May Jul Sep Dec Feb Apr Jun Aug Oct Dec Feb Apr Jun Aug Oct

1989/90-1993/94 120 1984/85-1988/89 120

90 90

60 60

30 30

0 0

CPUE (kg/hour)CPUE Nov Jan Mar May Jul Sep Nov Jan Mar May Jul Sep Dec Feb Apr Jun Aug Oct Dec Feb Apr Jun Aug Oct

1999/2000-2004/05 120 1994/95-1998/99 120

90 90

60 60

30 30

0 Nov Jan Mar May Jul Sep 0 Dec Feb Apr Jun Aug Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Month

Figure 2.7 Mean monthly catch-per-unit-effort (CPUE) in Spencer Gulf for 5-year periods between 1974/75 and 1998/99 and the six years between 1999/2000 and 2004/05. Months that contributed <2% of the catch for the period were omitted.

41 2.3.4 Trends within fishing periods

Between 2000/01 and 2004/05, the highest CPUE during each fishing period was on the night of the new moon (Figure 2.8). There was no obvious trend in CPUE prior to the new moon. After the new moon, CPUE declined steadily and significantly (LR: r2 = 0.98, df = 9, P<0.01).

160

120

80

40 Daily CPUE (kg/hour)

0 -8-7-6-5-4-3-2-10123456789 Days from new moon

Figure 2.8 Mean (+ SE) daily catch-per-unit-effort (CPUE) during 2000/01–2004/05 in Spencer Gulf with respect to the lunar phase.

42 2.4 Prawn Size

Information on prawn size was obtained from prawn-grade data. These data were only available from 1978/79, 1998/99, 2002/03, 2003/04 and 2004/05. The grade is determined from the number of prawns to the pound (i.e. U10 = under 10 prawns per pound, etc). In 1978/79 and 1998/99, data were reported as the proportion of the commercial catch that was comprised of four size categories (U10, 10/15, 16/20 and 20+, Carrick 2003). In 2002/03, 2003/04 and 2004/05, the data were reported as the proportion of the commercial catch occurring in each of 29 size classes (see Table 2.1). To facilitate interpretation of the prawn-grade data among all five fishing years, data from 2002/03, 2003/04 and 2004/05 were converted to four size categories based on the decision rules provided in Table 2.1. A fifth category, SB (Soft and Broken) was established for prawns that were unable to be graded. In this report, prawns in the U10, 10/15, 16/20, 20+ and Soft and Broken categories are referred to as XL, large, medium, small and SB respectively.

Table 2.1 Analytical categories assigned to reported prawn grades from the commercial logbook data.

Prawn grade Categories in logbook U10 (XL) U6, U8, U10, L, XL 10/15 (Large) 10/15, 9/12, U12, 13/15, LM, 10/20 (50%), 12/18 (50%) 16/20 (Medium) 16/20, M, 10/20 (50%), 12/18 (50%) 20+ (Small) 20+, 19/25, 21/25, 21/30, 26+, 30+, 31/40, S, SM SB (Soft & Broken) S/B, B&D, MIX, REJ, SMS, blank, ERR

2.4.1 Inter-annual trends

Small prawns comprised >40% of the catch in 1978/79, but <7% in the four recent fishing years (Figure 2.9). The proportion of medium prawns was similar in 1978/79 and the four recent fishing years. XL prawns comprised more than twice as much of the catch in 1998/99, 2002/03, 2003/04 and 2004/05 as they did in 1978/79. The distribution of the catch among size categories was similar in 1998/99 and 2002/03. However, between 2002/03 and 2003/04 there was a significant decrease in the proportion of XL prawns (Chi square test, χ2 = 260.0, d.f. = 1, P<0.01) and large prawns (χ2 = 11.5, d.f. = 1, P<0.01), and a significant increase in the proportion of medium prawns (χ2 = 146.5, d.f. = 1, P<0.01) and small prawns (χ2 = 95.2, d.f.= 1, P<0.01). During 2004/05 the proportion of large prawns increased, however large prawns still comprised less of the catch than during 1998/99 and 2002/03.

43 100%

75%

20+ 16-20 50% 10-15 U10

Proportion of catch 25%

0% 1978/79 1998/99 2002/03 2003/04 2004/05 Season/Year

Figure 2.9 Size composition of prawns in the commercial catch in 1978/79, 1998/99, 2002/03, 2003/04 and 2004/05.

2.4.2 Trends within years

The size composition of prawns varied between months within the 2002/03, 2003/04 and 2004/05 fishing years (Figure 2.10). The proportion of XL prawns in the catch was highest during March of 2002/03, 2003/04 and 2004/05 (35, 25 and 26% respectively) and lowest during May in 2002/03 (18%) and 2003/04 (12%), and during June 2004/05 (13%). The proportion of large prawns was substantially higher during May (45, 46 and 46%) and June (49, 45 and 47%) than all other months in 2002/03, 2003/04 and 2004/05 respectively. The proportion of medium prawns was highly variable, peaking during May in 2002/03 (27%), December 2003/04 (33%) and both April and June 2004/05 (31%). The proportion of small prawns in the catch was highest during December and April 2002/03 (6%), December 2003/04 (11%) and March 2004/05 (8%). SB were in highest proportion during December 2002/03 (12%). The proportion of SB was consistent during all other months and fishing years (5–7%).

44

2002/03 100% 90% 80% SB 70% 20+ 60% 50% 16/20 40% 10/15 30% 20% U10 Proportion of grades (%) 10% 0% Nov Dec Mar Apr May Jun

2003/04 100% 90%

80% SB 70% 20+ 60% 50% 16/20 40% 10/15 30% U10 20% Proportion of grades (%) 10% 0% Nov Dec Mar Apr May Jun

2004/05 100% 90%

80% SB 70% 20+ 60% 50% 16/20

40% 10/15 30% U10 20% Proportion of grades (%) 10% 0% Nov Dec Mar Apr May Jun

Figure 2.10 Size composition of prawns during each month fished in 2002/03, 2003/04 and 2004/05.

45 2.4.3 Daily prawn grades

The number of prawns per kilogram for each of the 23 prawn grades was estimated from the prawn grade name (i.e. prawn grade 10–15 was estimated as 12.5 prawns per pound equalling 27.5 prawns per kg) and are presented in Table 2.2.

Table 2.2 The number of prawn per kg estimated for reported prawn grades from the commercial logbook data.

Prawn grade Prawns per kg Prawn grade Prawns per kg Prawn grade Prawns per kg

U6 13.2 10/15 27.5 21/25 50.6 U8 15.4 13/15 30.8 S 56.1 XL 15.4 10/20 33.0 20+ 56.1 U10 19.8 12/18 33.0 21/30 56.1 L 19.8 M 39.6 26+ 61.6 9/12 23.1 16/20 39.6 30+ 78.1 U12 24.2 SM 48.4 31/40 78.1 LM 27.5 19/25 48.4

The average number of prawns per 7 kg (one bucket) for each vessel’s daily catch was calculated from the catch by grade data provided in commercial logbooks and the number of prawns per kg for each grade (Table 2.3; Figure 2.11): ()catch[] grade××() ppkg []7 grade ∑ ∑()catch[] grade where, catch is the total daily catch, ppkg is the number of prawns per kg, grade is the relevant prawn grade.

Mean annual prawn size (prawns per 7 kg) was determined as the weighted mean prawn size from each daily catch: ()catch××() catch pp7 kg ∑∑ ∑ ∑()catch where, catch is the total daily catch, pp7kg is the mean daily prawns per 7 kg. Prawns harvested in 2002/03 were larger than those harvested in either 2003/04 and 2004/05. In 2002/03, the modal size was 200 prawns per 7 kg and the mean size was 206 prawns per 7 kg. Prawns harvested in 2003/04 were the smallest in size, with a modal size class of 220 prawns per

46 7 kg and a mean size of 221 prawns per 7 kg. The modal size of prawns harvested in 2004/05 was 210 prawns per 7 kg, with a mean size of 214 prawns per kg (Table 2.3).

Similarly, the number of nights when harvested prawns were smaller than 220, 240 and 280 prawns per 7 kg was lowest in 2002/03 and highest in 2003/04. The number of nights when the mean harvest size exceeded the Limit Reference Point (280 prawns per 7 kg) was 24, 66 and 24 during 2002/03, 2003/04 and 2004/05, respectively (Figure 2.11).

Table 2.3 Statistics associated with mean daily prawn size estimated from prawn grade data provided in commercial logbooks.

Year Mode Mean Nights Nights >220 Nights >240 Nights >280 (pp7kg) (pp7kg) measured pp7kg pp7kg pp7kg

2002/03 200 206 1938 542 230 24 2003/04 220 221 2085 919 448 66 2004/05 210 214 2250 767 244 24

400

2002/03 2003/04 300 2004/05

200 Fishing nights

100

0 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 Daily prawn size (prawns per 7 kg)

Figure 2.11 Mean daily prawn size estimated from prawn grade data provided in commercial logbooks during 2002/03, 2003/04 and 2004/05.

47 2.5 By-product

Prawn fishers are permitted to take two “by-product” species, the slipper lobster (Ibacus spp) and southern calamary (Sepioteuthis australis). Whilst it has been mandatory to report retained catches of slipper lobster and calamary in daily logbooks since 2002, it has not been mandatory to retain them. Hence, catch and effort data collected since 2002/03 do not reflect the total catch of these species, but only the retained catch that was reported by fishers. As such, analyses of this dataset disregarded any zero values that some fishers reported.

2.5.1 Inter-annual trends

In 2004/05 total reported catches of slipper lobster and calamary were 3.2 t and 26.7 t, respectively (Figure 2.12). Between 2002/03 and 2004/05, total reported catch of calamary was approximately ten times the catch of slipper lobsters in these fishing years. The total reported catch of slipper lobster increased since by-product reporting in daily logbooks was made mandatory (slipper lobster 1.9 t to 3.2 t) while calamary catch peaked in 2003/04 (24.0 to 30.9 t). The number of reported days where by-product catch was retained has increased since 2002/03 for both species, although the number of vessels that reported retained catches has decreased (slipper lobster: 30 to 29 vessels; calamary: 37 to 36 vessels). Comparatively, Carrick (2003) reported an estimated calamary catch of 46.6 t from the entire fleet in 2000/01.

Figure 2.12 Reported catch of by-product (slipper lobster and calamary) from the Spencer Gulf Prawn Fishery between 2002/03 and 2004/05. Numbers indicate reported days of retained by-product catch, while numbers in brackets indicate vessels that reported retained catch.

48 2.5.2 Trends within years

Average daily reported catches of slipper lobster were relatively consistent throughout the fishing year between 2002/03 and 2004/05 (2.9 to 5.1 kg/day; Figure 2.13). Average daily reported catches of calamary were more seasonal, peaking between March and June in 2002/03 and 2003/04 (12.6 to 22.3 kg/day), with lower catches pre-Christmas. In contrast, reported calamary catches peaked pre-Christmas in 2004/05 (up to 18.8 kg/day) with lower catches in other months (10.6 to 15.8 kg/day).

Figure 2.13 Average daily reported catch of by-product (slipper lobster and calamary) from the Spencer Gulf Prawn Fishery during fished months between 2002/03 and 2004/05.

49 3. STOCK ASSESSMENT SURVEYS

3.1 Introduction

Stock assessment surveys, using industry vessels (with observers) have been undertaken by SARDI Aquatic Sciences since February 1982. A summary of the number of survey trawl shots conducted within regions (see Figure 1.2) of Spencer Gulf is provided in Table 3.1.

Survey shots were done at semi-fixed sites. Each survey shot is initiated as close as possible to a fixed Global Positioning System (GPS) position and then continues in a particular direction for a specified length of time (usually 30 minutes). The distance trawled depends on trawl speed (generally 3–5 knots), which is influenced by vessel power, tide and weather conditions. The accuracy of distance measurements and starting positions improved when GPS and computer technology were introduced into the fishery.

The data collected during surveys include total catch, trawl time, trawl distance, water temperature and a length frequency sample from the catch. Samples were processed to provide sex-specific length, sex ratio and mean prawn weight data. During 1994, bucket counts were introduced as means for rapid estimation of mean prawn weight. Prawns used for the bucket counts were then measured to obtained length frequency data. Therefore, mean weight data obtained from these two measures were not independent. Information on tidal flow was collected from 1997, although is not assessed in this report. GPS data for the start and end of trawl shots have been collected from November 1998. Length-frequency data are not available, while only limited catch rate and prawn size (bucket count) data are available, for November 2003, February 2004 and April 2004.

The timing, location and number of surveyed shots varied considerably between 1982 and 1993 (Table 3.1). Since 1993, surveys have been regularly conducted prior to fishing (October/November), immediately after the major recruitment to the fishery (February/March), and toward the end of the fishing year (April). These three comprehensive surveys, referred to herein as November, February and April surveys, aim to determine the relative prawn biomass prior to fishing, the extent of recruitment to the fishery and the exploitation rate of the Spencer Gulf Prawn Fishery, respectively.

50 Table 3.1 Number of stock assessment survey shots done in fishing regions of Spencer Gulf from February 1982 to April 2005.

Middlebank/ South West Corny Year Month North Cowell Gutter Wallaroo Wardang Total Shoalwater Gutter Gutter Point

1982 Feb 52 3 55 Mar 52 3 36 91 Apr 53 6 59 Nov 13 13 1983 Jun 53 6 38 97 Oct 36 24 60 Nov 34 34 1984 Oct 56 6 62 1985 Jun 66 15 39 120 Oct 27 27 1986 Apr 74 15 89 Jun 74 14 42 130 Oct 70 15 18 103 1987 Feb 74 15 40 27 3 18 177 Jun 75 15 36 27 3 156 Oct 76 15 27 3 18 139 1988 Feb 77 15 42 27 3 11 175 Jun 68 15 38 121 Nov 75 15 32 27 16 165 1989 Feb 75 15 42 27 16 175 Apr 37 16 17 70 Jun 59 1 21 81 1990 Jun 68 17 85 1992 Feb 65 16 46 27 3 20 177 1993 Feb 73 16 34 27 20 170 Apr 3 15 18 May 19 19 Jun 68 9 77 1994 Feb 58 12 40 27 3 17 157 Apr 68 20 30 11 24 4 157 Jun 82 4 36 16 1 139 Nov 94 17 35 22 18 4 190 1995 Feb 68 15 36 23 18 160 Apr 82 18 36 10 3 149 May 80 7 36 123 Nov 80 6 37 12 11 146 1996 Feb 75 16 2 93 Apr 76 18 9 103 Jun 74 9 52 30 165 Nov 61 9 50 28 16 164 1997 Feb 69 13 49 26 2 21 10 6 196 Nov 69 12 48 25 6 21 10 3 194 1998 Feb 74 15 61 29 5 22 10 6 222 Apr 74 18 22 114 Nov 78 12 52 25 1 21 10 199 1999 Feb 86 16 41 27 2 22 8 6 208 Nov 77 16 44 25 4 23 10 6 205 2000 Feb 76 15 11 27 4 22 155 Oct 62 16 24 27 2 27 158 2001 Feb 76 16 36 30 8 27 10 12 215 Apr 80 18 7 32 137 Nov 82 19 35 29 6 38 10 219 2002 Feb 82 17 5 25 2 43 174 Apr 71 21 7 31 130 Nov 82 17 36 30 9 43 10 12 6 245 2003 Mar 80 18 36 30 8 42 10 12 7 243 Apr 77 21 40 138 Nov 82 16 36 29 6 31 9 209 2004 Feb 81 21 36 27 10 40 2 12 8 237 Apr 80 22 28 13 40 183 Nov 78 22 38 23 1 29 10 201 2005 Feb 78 22 42 24 7 29 9 12 223 Apr 78 22 22 19 5 41 3 190

51 3.2 Annual Trends

3.2.1 Mean catch rate

Catch rates from fishery-independent surveys have been highly variable among years (Figure 3.1). Since 1996/97, survey catch rates have oscillated between 89 and 170 kg/h. Annual survey catch rates since 1996/97 significantly correlate to annual fishing catch rates (Dixon et al. 2005a) although the latter are generally lower. Survey catch rates are determined from shots throughout the gulf, generally on the new moon, and annual trends are influenced by the timing and location of shots. Fishing catch rates are determined from harvested regions only and are influenced by moon phase and fishing mortality.

Catch rates calculated as kg/hr correlate highly to those calculated as kg/km (Linear Relationship: r2 = 0.948, df = 7700, P<0.001; Figure 3.2). Catch rate calculations determined from trawl time (kg/hr) are mainly influenced by vessel power, speed, tidal strength and weather, while catch rate calculations determined from trawl distance (kg/km) are mainly influenced by variability in GPS equipment and error in GPS data provided by survey vessels.

Figure 3.1 Mean annual catch rate obtained during surveys and throughout the fishing year in Spencer Gulf from 1981/82 to 2004/05.

52

Figure 3.2 Correlation between survey catch rates calculated as kg/h and kg/km between 1981/82 to 2004/05.

3.2.2 Prawn size

The mean size of female prawns has consistently exceeded that of male prawns (range: 3–6 mm CL) (Figure 3.3). Between 1981/82 and 1985/86 the mean size of both female (34.2–39.2 mm) and male (31.1–34.5 mm) prawns increased substantially. Since 1985/86 the mean size of each sex has fluctuated among years with no obvious long-term trend. Mean prawn size in 2004/05 was at its lowest since 2000/01 (females 38.9 and 40.1 mm; males 34.1 and 35.6 respectively).

Figure 3.3 Mean sex-specific prawn size (mm, CL) from surveys conducted in Spencer Gulf between 1981/82 and 2004/05.

53 3.2.3 Sex ratio

Since 1981/82 the proportion of males in the survey catch has varied annually between 51 and 58%. Whilst the ratio of males to females was generally close to 1:1, results from 59 chi-square tests on length frequency data obtained since 1981/82 showed that the ratio of male:female prawns was significantly different during 57 of 59 surveys.

3.3 November Surveys

3.3.1 Catch rates and prawn size

Mean catch rates from November surveys, standardised as catch (kg) per hour trawled, ranged between 36.6 and 207.4 kg/h between 1982/83 and 1988/89 (Figure 3.4). During the same period, mean prawn size also increased from 33.6 to 38.5 mm CL. November surveys were not done during 1989/90–1993/94. Since 1994/95, survey catch rates have ranged between 67.9 and 135.9 kg/h, and mean size has varied between 35.8 and 39.7 mm CL, with no obvious trend over time. The catch rate for the November 2003/04 survey was the lowest recorded since 1985/86. Mean catch rate and prawn size in 2004/05 was 93.4 kg/h and 36.3 mm CL, respectively.

Figure 3.4 Mean (+ SE) catch rate (kg/h) and mean (± SE) prawn size (mm, CL) from November surveys conducted in Spencer Gulf between 1981/82 and 2004/05.

54 3.3.2 Correlation between November surveys and prawn catch

During 1982/83–1988/89, November fishery-independent survey catch rates were highly variable and there was no obvious correlation with commercial prawn catch. However, since 1994/95 November survey catch rates were highly correlated with subsequent catches, particularly with the pre-Christmas periods (November and December) (LR: r2 = 0.75, df = 10, P<0.001; Figure 3.5). This correlation is important because it shows how data from the November surveys have been used to determine the harvest strategies and catch levels for November and December.

During 1995/96, 1998/99 and 2001/02 survey catch rates were higher than the previous year, and catches of 539, 781 and 678 t respectively were harvested before Christmas. On each occasion, the November survey catch rates were substantially lower in the following two years, possibly in response to this large pre-Christmas catch. As a result, pre-Christmas catches were reduced for the two lower catch rate years and survey catch rates increased the following year.

Nov FIS 2500 250 Other months Nov-Dec 2000 200

1500 150

Catch (t) Catch 1000 100

500 50 Nov FIS catch rate (kg/h) rate catch Nov FIS

0 0 80/81 83/84 86/87 89/90 92/93 95/96 98/99 01/02 04/05 Fishing-year

Figure 3.5 Mean November fishery-independent survey (FIS) catch rate (kg/h), pre-Christmas catches and total catches from 1980/81 to 2004/05.

55 The percentage change in mean November survey catch rates was correlated with both pre- Christmas and annual prawn catches using non-parametric Spearman’s Rank Correlations (Quinn and Keough, 2002). Mean catch rates were not used for correlations as there were substantial progressive changes made to the number and location of sites surveyed.

Changes in the mean November survey catch rates were significantly correlated to pre-Christmas catches 1, 2 and 4 years prior (Table 3.2). This result suggests that the extent of pre-Christmas fishing affects recruitment to the fishery. That this effect is significant over two years is consistent with our understanding of recruitment, that spawning during November-January results in recruitment to the fishery the following May-June, and that later spawning during February-March results in over-wintering and recruitment to the fishery in the following February (Carrick 2003). The significant correlation after 4 years is related to the 3-year cycle of high-low-low pre-Christmas catch.

Correlations with annual catch were only significant after 2 years, suggesting that the total annual catch does affect egg production the next year. Mean survey catch rate is not affected 3 years after annual catches, and this may be reflective of the greater proportional changes in pre- Christmas catches rather than annual catches.

Table 3.2 Spearman’s Rank Correlations between commercial fishing catches (November/December and annual), and the percent change in mean catch rate during November Fishery-independent Surveys (FIS) for the fishing years between 1994/95 and 2004/05.

FIS November & December catch Annual catch Time lag n rs P n rs P Year +1 10 -0.84 <0.01 10 0.10 0.78 Year +2 9 -0.77 <0.05 9 -0.70 <0.05 Year +3 8 -0.10 0.82 8 -0.48 0.23 Year +4 7 -0.86 <0.05 7 -0.11 0.82

56 3.4 February Surveys

3.4.1 Catch rates and prawn size

February surveys were rarely conducted prior to 1991/92, but have been conducted annually since then (Figure 3.6). Since 1981/82 catch rates have ranged from 91.0 to 182.3 kg/h, with the 2002/03 catch rate being the lowest recorded. Mean size has ranged from 34.2 to 38.3 mm, CL, during the same period. Since 1995/96 a trend of increased catch rates corresponding to decreased prawn size was apparent, which may be influenced by patterns in recruitment.

Figure 3.6 Mean (+ SE) catch rate (kg/h) and mean (± SE) prawn size (mm, CL) from February surveys conducted in Spencer Gulf between 1981/82 and 2004/05.

3.4.2 Recruitment

Carrick (2003) suggested an appropriate measure of recruitment was the square root transformation of the numbers of prawns (males <33 and females <35 mm carapace length) per nautical mile trawled from 39 stations in the north of the gulf during February surveys. This was to replace the existing recruitment index set out in the Management Plan (MacDonald 1998).

Recruitment data were available for 17 February surveys conducted since 1982. During the 1982 surveys only 34 of the 39 recruitment shots were surveyed. In all other years at least 36 were completed. The recruitment index was lowest during 2000 (~30/nm; Figure 3.7) and greatest during 2001 (~60/nm), which equates to a 4-fold increase when data are not square root transformed. During 2002 and 2003 recruitment decreased to ~41/nm and ~35/nm respectively, and has steadily increased to ~46/nm during 2005.

57

Figure 3.7 Mean (+ SE) recruitment index for up to 39 stations surveyed in February in the northern region of Spencer Gulf from 1982 to 2005. The line represents the target reference point (40/nm).

3.5 April Surveys

April surveys have been conducted sporadically since 1981/82 (Figure 3.8). Catch rates and mean size have fluctuated inter-annually with no long-term trends (ranges 135.8–255.2 kg/h, 33.4–38.3 mm, CL). The catch rate during 2002/03 was the lowest that has been recorded.

Figure 3.8 Mean (+ SE) catch rate (kg/h) and mean (± SE) prawn size (mm, CL) from April surveys conducted in Spencer Gulf between 1981/82 and 2004/05.

58 3.6 Comparison of catch rates and prawn size between months

Catch rates increased between surveys throughout the fishing year, being lowest during November and highest during April in all years (Figure 3.9). The differences in survey catch rates are similarly observed during commercial fishing (see Section 2.3.3). The factors that influence these differences include seasonal effects on catchability, recruitment, growth, natural mortality and fishing mortality.

Figure 3.9 Mean (+ SE) catch rate (kg/h) from November, February and April surveys conducted in Spencer Gulf between 1993/94 and 2004/05.

From 1994/95–2002/03, prawn size was determined for all November and February surveys and four April surveys (Figure 3.10). Prawn size was generally smaller during February than it was during the previous November survey. However, this trend was not evident in February 1994/95 and 2004/05 when mean prawn size slightly increased after the November survey. Comparisons with April surveys provided no obvious trend. The general reduction in mean prawn size between November and February surveys in most years, combined with the increase in catch rates between the surveys, is likely to indicate recruitment to the fishery, rather than a decline in mean prawn size due to the harvesting of large prawns in November and December.

59

Figure 3.10 Mean (+ SE) prawn size (mm, CL) during November, February and April surveys from 1994/95 to 2004/05.

3.7 Estimates of egg production from survey catch

Egg production was estimated for Spencer Gulf surveys conducted in November 2004 from data that included catch rate, weight of prawn grades and the proportion of females in the catch. These estimates reflect the potential egg production of female prawns captured on all survey shots. They are useful to assess the relative contribution toward egg production from female prawns of various size grades. Similar estimates derived from November surveys in future years will be useful for comparison of egg production between years.

The model utilises much of the current knowledge of the biology of M. latisulcatus (see Section 1.4). It should be noted that, as this model is in its infancy of development, there is considerable uncertainty associated with the outputs. These outputs are likely to change as more data becomes available. The model is underpinned by a range of assumptions: • the catchability of prawns was constant during the survey, • female prawns spawned three times during the spawning period, • spawning frequency does not vary with size, • natural mortality was zero, • the % of females within each grade does not vary over time, • the % of females with fertilized eggs doesn’t vary with time or abundance of males, and • sex-specific length frequency data from surveys were representative of the population.

60

Data on the biology of prawns (see Section 1.4) and on prawn grades obtained from commercial processors were used in the model. The following steps (1–10) describe the estimation of egg production and Table 3.3 presents the associated values: 1) The mean weight of prawns for each prawn grade was obtained from commercial processors. 2) Data from 1) were used to calculate the mean size (mm, CL) of prawns in each grade. 3) Data from 2) were used to calculate the mean number of eggs produced per female prawn for each prawn grade. 4) The proportion of female prawns with fertilized eggs for each prawn grade was estimated from the logistic equation provided by Carrick (1996). 5) Spawning frequency was assumed to be 3 for all prawn grades. 6) Mean (SE) catch weight per grade per shot was calculated directly from prawn grade weight data collected during November 2004. 7) Data from 6) and 1) were used to calculate the mean (SE) number of prawns captured per shot. 8) The proportion of female prawns in each prawn grade was calculated from sex-specific length-frequency data collected from all shots surveyed during November 2004. 9) Data from 7) and 8) were used to calculate the mean (SE) number of female prawns captured per shot. 10) Data from 3), 4), 5) and 9) were used to calculate the number of potential fertilized eggs (million) per shot that captured females could have contributed to egg production.

Table 3.3 Egg production from female prawns captured from surveys conducted during November 2004.

Prawn grade 21+ 16-20 10-15 U10 U8 Mean individual weight (g) 16.7 26.3 37.0 50.0 62.5 Mean CL (mm) 31.3 37.2 42.3 47.3 51.5 Eggs per female 119745 216987 338534 500303 668900 % fertilized 19.4% 55.2% 83.5% 95.3% 98.5% Spawning frequency 3 3 3 3 3 Mean catch (kg) per shot 16.2 (1.2) 18.2 (1.0) 10.4 (0.8) 1.7 (0.2) 0.3 (0.1) Prawns per shot 900 (68) 673 (39) 273 (20) 34 (3) 5 (1) % females 18.1% 28.7% 39.7% 92.7% 97.8% Females per shot 163 (12) 193 (11) 108 (8) 31 (3) 5 (1) Eggs (x107) per shot 4.6 (0.3) 14.1 (0.8) 13.2 (1.0) 2.6 (0.3) 0.5 (0.1)

61 Approximately 96% of the total survey catch by weight was comprised of the prawn grades 10– 15, 16–20 and 21+ (Table 3.3). Mean catch weight was highest in the 16–20 size grade (18.2 kg per shot), followed by the 21+ grade (16.2 kg per shot) and the 10–15 grade (10.4 kg per shot). Despite the difference in mean catch of 10–15 and 16–20 grade prawns, egg production from these grades was similar (Figure 3.11). Whilst 21+ grade prawns comprised 35% of the catch by weight, they contributed <10% of eggs produced. These discrepancies were due to a) the proportion of female prawns in each prawn grade (21+=18.1%, 16–20=28.7%, 10–15=39.7%), b) higher fecundity of larger females, and c) increased insemination rates of larger females. These results have implications for the development of harvest strategies during the spawning season. Whilst the capture of smaller prawns during the early spawning period would clearly reduce the immediate impact on egg production, the effect on future recruitment by removing smaller individuals is unknown, and is a major source of uncertainty in these analyses. More complex analyses that incorporate natural and fishing mortality rate may improve this understanding.

20

Mean catch (kg) per shot

Fertilized eggs (x10 ) per shot 15

10

5

0 u8 u10 10_15 16_20 21+ Prawn grade

Figure 3.11 Mean (SE) graded catch (kg) per shot (males and females) and the estimated number of fertilized eggs (eggs % 107) produced by females captured from survey shots during November 2004.

62 3.8 Spot Surveys

Spot surveys have been conducted since May 2002 to determine whether it may be appropriate to open areas that were closed to fishing in the previous harvest strategy. Spot surveys have been undertaken in the Northern, Middlebank/Shoalwater, Cowell, Gutter and Wallaroo regions (Table 3.4). In all instances, where more than 10 survey shots had been conducted in a region and subsequently more than 10 t had been harvested from that region, the survey catch rate was consistently higher than the fishing catch rate, primarily because spot surveys were conducted on the new moon. Despite the higher catch rates, there was a high correlation between spot survey and subsequent regional fishing CPUE (r2=0.88, n=11; Figure 3.12). This indicates that when industry survey methods incorporate more than 10 shots in a region, catch rates are indicative of subsequent fishing CPUE.

Table 3.4 Mean (SE) catch rates and prawn size from spot surveys, and regional total catch and CPUE in the remaining of the fishing month immediately subsequent to spot surveys. ‘C’ indicates confidential data.

Mean (SE) size Mean (SE) Subsequent Subsequent reg. Year Month Region Shots (prawns/kg) rate (kg/hr) reg. catch (t) CPUE(kg/hr) 2002 May North 1 45.7 (0.0) 353 (-) – – 2002 May M’bank/S’water 15 31.4 (1.1) 220 (18) 146.7 130 2002 May Cowell 9 35.0 (5.1) 103 (31) – – 2002 May Gutter 9 26.4 (2.1) 77 (26) 4.9 112 2002 May Wallaroo 16 35.4 (1.1) 174 (17) 86.4 111 2002 Dec North 46 39.0 (0.7) 101 (7) 0.3 35 2003 Mar North 11 62.0 (3.6) 160 (13) – – 2004 Mar Gutter 4 30.5 (4.6) 204 (57) 26.2 115 2004 Mar Wallaroo 32 32.7 (1.2) 214 (28) 142.1 129 2004 May North 17 33.4 (1.3) 187 (16) 3.0 144 2004 May M’bank/S’water 14 39.9 (2.4) 233 (25) 243.0 135 2004 May Wallaroo 22 33.3 (1.2) 96 (10) 130.5 82 2004 Dec Gutter 13 40.1 (1.4) 81 (12) C C 2004 Dec M’bank/S’water 3 32.4 (2.0) 151 (18) 85.9 70 2004 Dec North 24 42.1 (1.4) 148 (14) 8.7 88 2005 Mar Gutter 5 38.0 (1.8) 140 (38) 6.5 69 2005 Mar North 5 33.6 (4.1) 422 (174) – – 2005 Mar Wallaroo 27 35.9 (1.8) 200 (23) 30.1 112 2005 May Gutter 12 39.2 (2.9) 146 (28) 28.9 113 2005 May M’bank/S’water 19 34.4 (0.6) 258 (18) 59.3 137 2005 May South Gutter 5 30.9 (3.4) 75 (27) 15.5 71 2005 May Wallaroo 24 34.2 (0.8) 143 (9) 138.7 95 2005 June M’bank/S’water 21 34.2 (0.8) 192 (16) 163.9 104 2005 June North 13 34.1 (1.0) 167 (18) – – 2005 June Wallaroo 14 36.0 (2.0) 85 (12) 30.3 81

63 Figure 3.12 Correlation between regional spot survey CPUE (when >10 shots) and subsequent regional fishing CPUE (when >10 t harvested) in the same month (r2 = 0.88, n=11).

64 4. SURVEYS, CATCH AND PRAWN SIZE FOR 2004/05 HARVEST PERIODS

4.1 Introduction

There were six harvest periods for the SGPF during 2004/05. Each harvest period was preceded by either a “Stock Assessment” or “Spot” survey on the dark of the moon. Survey results were used to develop harvest strategies for the following harvest period. In this report harvest periods were defined as all fishing activities conducted between surveys, and spanned across two fished calendar months on occasions.

The following section comprises twelve figures, two for each harvest period. The first figure presents survey information on catch rate and mean size prior to commercial fishing in each harvest period. The second figure presents commercial catch and mean size data by fishing block for each harvest period. Both figures contain the initial harvest strategy closure lines adopted during the fishing period. Comparison of these figures enables assessment of the effectiveness of the harvest strategy developed, particularly with respect to prawn size. Figures are presented alternately as survey results, followed by commercial catch and mean size, sequentially for each harvest period.

Fishery-independent “stock assessment” surveys were conducted during November 2004, and February and April 2005 (see section 3.1). Fishery-dependent “spot” surveys were conducted during December 2004, and May and June 2005 (see section 3.6). Data on catch rate and mean sizes are presented for each site surveyed prior to each harvest period. The start and end dates of the survey and the number of nights surveyed, are also presented.

Data on commercial catch and mean size were determined from commercial logbooks (see section 2). Data from fishing blocks with catches from <5 fishers for that harvest period are not presented due to confidentiality issues. The start and end dates of the harvest period, the number of nights fished, the total catch for the harvest period, are also presented.

All results are discussed in terms of the regions defined in Figure 1.2. In the following section, catch rates <4 lb/min are referred to as “low”, 4–10 lb/min as “medium”, and >10 lb/min as “high”. Also, mean size <220 prawns per 7 kg are referred to as “large”, 220–240 prawns per 7 kg as “medium” and >240 prawns per 7 kg as “small”. Commercial catches are reported in ranges that vary with respect to the total catch for that period.

65 4.2 Survey and Catch Data by Harvest Period

4.2.1 Harvest Period 1

Stock assessment survey catch rates were generally medium to high in the North, northern Cowell and the Main Gutter regions, and low in all other regions. Prawns were generally small in the North and Main Gutter regions, large in the Wallaroo region and mixed in the Cowell region (Figure 4.1). Catches from 5–60 t were obtained from 9 fishing blocks in the Wallaroo and Middlebank/ Shoalwater regions during harvest period 1. Large prawns were harvested from all but 1 fishing block (Figure 4.2). Both the magnitude of commercial catches and the size of prawns harvested correlated well with survey results.

4.2.2 Harvest Period 2

Spot survey catch rates were generally medium to high in the North and northern Middlebank/Shoalwater regions, and low in the Main Gutter region. Prawns were generally small in the North and Main Gutter regions, and mixed in the Middlebank/Shoalwater region (Figure 4.3). Commercial catches were generally <20 t in most blocks fished in the Wallaroo and Middlebank/Shoalwater regions. Large prawns were harvested from all but 1 fishing block (Figure 4.4). Catches were reported (confidential results) from three blocks that were completely within the closed region off Cowell.

4.2.3 Harvest Period 3

Stock assessment survey catch rates were generally medium to high in the North, northern Middlebank /Shoalwater and Main Gutter regions, and low to medium in all other regions. Prawn size was generally small in the North, large in the northern Wallaroo and Corny Point regions, and mixed in all other regions (Figure 4.5). Catches >30 t were obtained from 2 blocks in the Corny Point region, and 1 block in each of the South Gutter and Wardang regions. Catches 10–30 t were obtained from blocks in the Main Gutter, South Gutter, Cowell, Wardang and Wallaroo regions. High catches of small and medium prawns were harvested from adjacent blocks in the Cowell, Main Gutter, West Gutter and South Gutter regions. Large prawns were caught in all other blocks (Figure 4.6).

Of note, the catch of small and medium sized prawns comprised 34% of the total catch during this period. Whilst several shots of high catch rate of small and medium sized prawns were observed in the opened area during February surveys, these areas also comprised shots of high and medium catch rates of large prawns. The discrepancy between survey catch rates and commercial catches was likely to have been exacerbated by the extended duration between the conduct of surveys and the commencement of fishing (24 days). Also noteworthy, closure lines

66 were slightly modified after the initial harvest strategy was determined, but were too late to prevent catches of 10–30 t of small and medium sized prawns being harvested from adjacent blocks in Cowell. Finally, catches were reported (confidential results) from two blocks that were completely within the closed regions.

4.2.4 Harvest Period 4

Stock assessment survey catch rates were generally medium in the North, Middlebank/ Shoalwater and Main Gutter regions, mixed in the Wallaroo region and low in other regions. Prawn size was generally small in most regions, with medium and large prawns interspersed throughout the Wallaroo and Main Gutter regions (Figure 4.7). Catches >40 t were obtained from several blocks in the Wallaroo and Main Gutter regions. Most of these high catches were medium sized prawns, with high catches of large prawns in northern part of the Wallaroo region (Figure 4.8). A catch of 10–40 t of small prawns was obtained from one block in the south of the Wallaroo region, a block not surveyed during April 2005.

The high catches of medium sized prawns were obtained in blocks where large prawns were generally observed during surveys. It is noteworthy that very high catch rates of small prawns were observed in all areas immediately adjacent to these blocks and thus it is possible that this discrepancy may have resulted from prawn movement.

4.2.5 Harvest Period 5

Spot survey catch rates were generally medium to high in the Middlebank/Shoalwater region and mixed in other regions. Prawn size was mixed throughout surveyed regions (Figure 4.9). Catches >20 t were obtained from several blocks in the Wallaroo, Middlebank /Shoalwater and Main Gutter regions. Large prawns were harvested from all but 1 fishing block (Figure 4.10).

4.2.6 Harvest Period 6

Spot survey catch rates were generally medium in the North region, medium to large in the Middlebank/ Shoalwater region and low in the Wallaroo region. Prawn sizes were predominantly medium and small (Figure 4.11). Catches >20 t of medium sized prawns were obtained from 3 blocks in the Middlebank /Shoalwater region. Catches of 5–20 t of large and medium sized prawns were harvested from blocks in the Wallaroo and Middlebank /Shoalwater regions (Figure 4.12). The sizes of prawns harvested and the magnitude of catches generally reflected the results observed during spot surveys.

67 November

Stock Assessment Survey

11 Nov 2004 to 13 Nov 2004 !( µ !( !(!( !( !(!(!( Nights surveyed = 2 !(!(!( !(!( !(!( !(!( !(!( !( !(!( !( !( !(!(!(!(!( !( !( !(!( !( !( !( !( !( !(!( !( !(!(!(!(!(!( !( !(!(!(!( !( !( !(!( Catch rate Prawn size !(!( !(!( !( !( !(!( !( !( (lb per min) (Prawns per 7 kg) !( !(!( !(!(!(!(!( !(!(!(!( !( !(!(!( (! < 221 < 4 !(!(!( 4 - 10 (! 221 - 240 !( !( !(!( * !( !(!(!( !( (! !( > 10 > 240 !( !(!(!( !( !( !(!( !( !( !(!( !( !( !( !( !(!(!( !( !( !( !(!( !( !(!( !( !( !( !( !(!( !( !( !( !( !(!( !(* !( !(!( !( !( !( !( !( !( !( !( !( !( !( !( = Fishing closure !( !( !( !( !( !( !( !( !( !(!( * !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !(!( !(!( !( !(!( !(!( !( !(!( !(!( !(!( !(

*

012.5 25 50 75 100 Kilometers

Figure 4.1 Catch rate and mean size during the November 2004 Stock Assessment survey, prior to harvest period 1. Black dotted lines represent harvest strategy closure lines.

68 Harvest Period 1

14 Nov 2004 to 11 Dec 2004

Nights fished = 10 µ Total catch = 276 t Confidential catch = 16 t (6%)

Catch Prawn size

Confidential (Prawns per 7 kg) (! < 5 t !( < 221 5 - 20 t !( 221 - 240 (! * (! (! !( > 240 (! 20 - 60 t (! (! (! * (! (! (! * = Fishing closure

*

012.5 25 50 75 100 Kilometers

Figure 4.2 Commercial catch and mean size from blocks fished during harvest period 1. Black dotted lines represent harvest strategy closure lines.

69 December

Spot survey µ 12 Dec 2004 !( !( !( !(!( !( !(!( !( !(!(!( Catch rate Prawn size !(!( !(!(!( (lb per min) (Prawns per 7 kg) !(!( !( !(!(!( !(!( !( < 4 (! < 221 4 - 10 (! 221 - 240 * > 10 (! > 240 * !(!( !(!( !( !( !( = Fishing closure !(!( !( * !( !(

*

012.5 255075100 Kilometers

Figure 4.3 Catch rate and mean size during the December 2004 Spot survey, prior to harvest period 2. Black dotted lines represent harvest strategy closure lines.

70 Harvest Period 2

13 Dec 2004 to 19 Dec 2004

Nights fished = 7 µ Total catch = 154 t Confidential catch = 5 t (3%)

Catch Prawn size (! Confidential (Prawns per 7 kg) (! (! (! <5 t !( < 221 5 - 20 t !( 221 - 240 ! (! * ( (! !( (! 20 - 50 t > 240 (! (! (! (! * (! (! = Fishing closure * (!

*

025507510012.5 Kilometers

Figure 4.4 Commercial catch and mean size from blocks fished during harvest period 2. Black dotted lines represent harvest strategy closure lines.

71 February

Stock Assessment Survey µ 9 Feb 2005 to 10 Feb 2005 Nights surveyed = 2 !( !( !(!( !( !(!( !(!( !(!( !(!( !(!(!(!(!( !( !(!( !(!( !(!(!(!(!( !(!( !(!( !(!( !(!( !(!(!( !( Catch rate Prawn size !(!(!(!(!(!( !(!(!(!( (lb per min) (Prawns per 7 kg) !(!(!(!(!(!( !(!( !(!( !( !(!(!( (! !(!(!( !(!(!( < 4 < 221 !(!(!(!( !( !(!(!( 4 - 10 (! 221 - 240 !(!(!( > 10 (! > 240 !(!(!(!( !( !( !( !( !( !( !( !(!(!( !( !( !(!( !( !( !(!( !( !( !( !(!( !(!(!( !( !( !( !( !( !(!( !( !( !( !(!( !( !(!( !( !( !(!( !( !( !(!(!( !( !( !( !( !( !( !( !( !(!( !( !( !( !( = Fishing closure !( !( !( !( !(!(!( !( !( !( !( !( * !( !( !( !( !( !( !( !(!( !( !(!( !(!( !( !(!( !(!( !(!( !( !(!( !( * !(!( !( !(!( !(!( !( !(!(!( !( !( !( !(

!( !( !( !( !( !( !( !( !(!( !( !(

012.5 255075100 Kilometers

Figure 4.5 Catch rate and mean size during the February 2005 Stock Assessment survey, prior to harvest period 3. Black dotted lines represent harvest strategy closure lines. Grey line represents an amendment to the closure.

72 Harvest Period 3

6 Mar 2005 to 8 Apr 2005

Nights fished = 14 µ Total catch = 373 t Confidential catch = 16 t (4%)

Catch Prawn size

Confidential (Prawns per 7 kg) <10 t !( < 221 10 - 30 t !( 221 - 240 !( > 240 30 - 90 t (! (! (!

(! = Fishing closure (! * (! (! (! (! (! * (! (! (! (! (! (!

(!

(!

(!

(! (!

012.5 25 50 75 100 Kilometers

Figure 4.6 Commercial catch and mean size from blocks fished during harvest period 3. Black dotted lines represent harvest strategy closure lines. Grey line represents an amendment to the closure.

73 April

Stock Assessment Survey µ 9 April 2005 !(!(!( !(!( !(!(!( !(!(!( !(!( !(!( !(!(!( !( !( !( !(!( !( !(!( !(!( !( !(!( !( !( !( !(!( !(!( !(!(!( !( Catch rate Prawn size !(!(!(!(!(!( !( !(!(!(!( (Prawns per 7 kg) !(!( !( (lb per min) !(!(!(!(!(!(!( !(!(!( !( ! !(!(!(!(!(!(!( < 4 ( < 221 !(!(!(!( !(!( !( (! !(!( 4 - 10 221 - 240 !(!(!( !(!(!( > 10 (! > 240 !( !( !(!(!( !( !( !( !(!( !(!( !( !(!(!(!(!(!( !( !( !( !( !( !(!(!(!( !( !( !( !( !(!( !( !( !(!( !( !(!( !(!( !( !(!( !(!(* !( !( !( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !( !( !( !( !( !(!( !( !( !( !( !( !(!(!( !( !( = Fishing closure !( !( !( !( * !( !( !( !( !( !( !( !( !( !( !( !( !( *

012.5 255075100 Kilometers

Figure 4.7 Catch rate and mean size during the April 2005 Stock Assessment survey, prior to harvest period 4. Black dotted lines represent harvest strategy closure lines.

74 Harvest Period 4

10 Apr 2005 to 7 May 2005

Nights fished = 11 µ Total catch = 583 t Confidential catch = 32 t (5%)

Catch Prawn size

Confidential (Prawns per 7 kg) <10 t !( < 221 10 - 40 t !( 221 - 240 (! !( 40 - 140 t > 240 (! (! * (! (! (! (! = Fishing closure (! * (!

(! (! *

012.5 25 50 75 100 Kilometers

Figure 4.8 Commercial catch and mean size from blocks fished during harvest period 4. Black dotted lines represent harvest strategy closure lines.

75 May

Spot survey µ 8 May 2005

Catch rate Prawn size (lb per min) (Prawns per 7 kg) (! !(!(!( < 4 < 221 !(!(!( 4 - 10 (! 221 - 240 !(!(!(!( !( !(!(!( > 10 (! !( > 240 !(!( !( !(!(!( !( !( !(!( !(!( !( !( !(!(!( !(!( * !(!(!( !( !(!(!( !(!( = Fishing closure !(!( * !( !(!( !( !( * !( !( !( !(

!( !( !( !( *

012.5 255075100 Kilometers

Figure 4.9 Catch rate and mean size during the May 2005 Spot survey, prior to harvest period 5. Black dotted lines represent harvest strategy closure lines.

76 Harvest Period 5

9 May 2005 to 6 Jun 2005

Nights fished = 11 µ Total catch = 356 t Confidential catch = 16 t (5%)

Catch Prawn size

Confidential (Prawns per 7 kg) <5 t !( < 221 5 - 20 t !( 221 - 240 (! !( (! 20 - 60 t > 240 (! (! (! (! * (! (! (! (! * = Fishing closure (! * (!

(! (! (!

(! * (!

(! (! (!

012.5 25 50 75 100 Kilometers

Figure 4.10 Commercial catch and mean size from blocks fished during harvest period 5. Black dotted lines represent harvest strategy closure lines.

77 June

Spot survey µ 7 June 2005

!( !( !( !( !( !(!( !(!( !( Catch rate Prawn size !( (lb per min) (Prawns per 7 kg) !(!(!(!( !( !(!(!( (! < 221 < 4 !(!(!( 4 - 10 (! 221 - 240 !(!(!(!( * !(!(!(!( > 10 (! > 240 !( !( !( !( !( !( !( !(!( !( !(!( !( !( * !(!(!( !( * = Fishing closure

*

025507510012.5 Kilometers

Figure 4.11 Catch rate and mean size during the June 2005 Spot survey, prior to harvest period 6. Black dotted lines represent harvest strategy closure lines.

78 Harvest Period 6

8 Jun 2005 to 12 Jun 2005 µ Nights fished = 5 Total catch = 197 t Confidential catch = 13 t (7%)

Catch Prawn size

Confidential (Prawns per 7 kg) ! !( < 221 (! ( * <5 t (! 5 - 20 t !( 221 - 240 (! (! (! 20 - 60 t !( > 240 (! (! * (! * = Fishing closure

*

012.5 25 50 75 100 Kilometers

Figure 4.12 Commercial catch and mean size from blocks fished during harvest period 6. Black dotted lines represent harvest strategy closure lines.

79 5. PERFORMANCE INDICATORS

In this section, performance of the fishery in 2004/05 was assessed against the PI’s outlined in the Management Plan (MacDonald 1998) (Table 5.1).

Table 5.1 Summary of Performance Indicators for the 2002/03 and 2003/04 fishing years of the Spencer Gulf Prawn Fishery (MacDonald 1998). PI Target Limit ’02/’03 ’03/’04 ’04/’05 RP RP Effort (days) 70–80 85 71.8 74.5 80.0 Size at capture (prawns/kg) <40/kg >40/kg 29.4 31.6 30.6 Recruitment indices 40 35 35.2 41.2 45.6 % Virgin spawning biomass 50% 40% NA NA NA

5.1 Fishing Effort

The nominal days of effort were adjusted by a coefficient of fleet power (inferred from Carrick 2003) to calculate the effective effort for each fishing year. Effective effort was first calculated in 2001/02 and the same coefficient of fleet power was used in this assessment.

During 2004/05 the effective effort was 80.0 days for 58 days of actual effort. Effort in 2004/05 was the highest in the last 3 fishing years. Effective effort during 2004/05 was at the maximum value for the target range for this PI and well below the limit RP.

5.2 Size at Capture

Average size at capture, calculated from logbook grade data (see section 2.4.3), was below the target and limit RP for this PI. During 2004/05 average size at capture was 30.6 prawns per kg. The limit reference of 40 prawns per kg was exceeded on 24 individual vessel nights during 2002/03, on 66 occasions during 2003/04 and on 24 occasions during 2004/05.

5.3 Recruitment Indices

Recruitment indices were calculated as the square root transformation of the numbers of prawns (males <33 and females <35 mm carapace length) per nautical mile trawled (after Carrick 2003). Length-frequency data were not available for all recruitment sites in February 2004. Thus, recruitment indices for February 2004 were determined from relative catch rates at recruitment sites during February 2002 and 2004 surveys, assuming that the proportion of recruits in the

80 catch was equal. Average weight data from these sites during February 2004 suggests that this is likely to be a conservative estimate of the recruitment index (mean prawn size 20 g in 2002, 17 g in 2004).

The recruitment index of 45.6 during 2004/05 was above the target RP and limit RP.

5.4 Proportion of Virgin Spawning Biomass

As outlined in the Management Plan (MacDonald 1998), the percentage virgin spawning biomass is calculated from the reduction in prawn abundance between April and November surveys, after accounting for seasonal mortality and catchability (see Carrick 2003). As such, these estimates reflect fishing mortality from the April survey to the end of the fishing year, not the percentage of virgin spawning biomass remaining. Thus, the actual virgin spawning biomass remaining cannot be calculated.

81 6. DISCUSSION

6.1 Assessment of the Spencer Gulf Prawn Fishery

Fishery assessments for the Spencer Gulf Prawn Fishery aim to determine whether prawns are being harvested sustainably and include evaluation against the performance indicators listed in the Management Plan.

Harvest strategies have been a key factor in the success of the Spencer Gulf Prawn Fishery. Harvest strategies aim to limit effort to ecologically sustainable levels for each fishing period and are primarily based on data collected from fishery-independent surveys. There are two aspects of harvest strategies that require assessment: harvest strategy development and harvest strategy management. Harvest strategy development is based on data obtained from the surveys and involves the establishment of closure lines to exclude fishers from areas where prawn size is sub- optimal. Harvest strategy management is the tool that makes the Spencer Gulf Prawn Fishery unique. The Committee at Sea refines (in “real time”) the closure lines during the fishing period to maximize the economic performance of the fishery.

Assessments of the fishery have been regularly conducted since 1998 (Carrick and McShane 1998; Carrick and Williams 2000, 2001; Carrick 2003; Dixon et al. 2005a). These assessments have been substantially augmented by reports on by-catch (Carrick 1997, 1999; Dixon et al. 2005b; Svane 1998, 2005) and biological information (Carrick 1996, Roberts et al 2005). Previous reports have not addressed harvest strategy development or management. Currently, assessment of both harvest strategy development and management are impeded by a lack of documentation of the methods and rationale by which data are used to inform decisions.

6.2 Information Available for Assessment of the Fishery

Annual fishery assessments are underpinned by fishery-dependent catch and effort data, fishery- independent survey and biological data. There are considerable fishery-dependent data for the Spencer Gulf Prawn Fishery, with annual catch and effort information being available for the entire period since the inception of the fishery in 1968. Daily logbook data are available for most fishing periods since the early 1980’s. Whilst there are relatively few historical data on prawn size, daily prawn grade data have been obtained from commercial logbooks since November 2002.

Fishery-independent surveys have been conducted since February 1982. Analysis of inter-annual trends in prawn abundance and size in the period up to the mid-1990s is impeded by large

82 variations in the timing and design of surveys. Over the last decade, surveys have been conducted mainly in November, February, April and May over the new moon and with increased spatial consistency. Hence, fishery-independent data collected over this period appear to provide a reasonable basis for monitoring inter-annual trends in the status of the stock and fishery. These fishery-independent surveys also provide the primary source of information for assessing the fishery against the Performance Indicators outlined in the Management Plan.

Additional data are available on several other aspects of the fishery. A recent report documented the patterns of distribution and abundance of juvenile prawns (Roberts et. al. 2005). Substantial data were obtained from tagging studies done in Spencer Gulf, providing valuable information on growth, mortality and movement. Considerable studies were also conducted on the biology of M. latisulcatus in Spencer Gulf, including reproductive biology and morphometric relationships.

Uncertainty in the assessment of stock status arises from: temporal variations in the timing and design of historic fishery-independent survey data; limitations in the data available on historic prawn sizes; problems associated with the use of commercial CPUE and survey catch rate data as an index of abundance (resulting from variations in prawn catchability and fishing efficiency); and, perhaps most significantly, inadequacies with the current Performance Indicators.

Harvest strategies are developed for each fishing period in November, December, March, April, May and occasionally in June. Whilst the primary aim of fishery-independent surveys is to determine estimates of relative biomass for annual assessment, they are also the main source of information for the development of harvest strategies. During months when fishery-independent surveys are not conducted, “spot” surveys are used to refine harvest strategies. Spot surveys are done at smaller and varying spatial scales to determine regions that contain areas of prawns of target size. Spot survey data are not used for annual assessment. This report is the first to directly assess the effectiveness of harvest strategies determined from spot and fishery-independent surveys with regard to the capture of prawns of an appropriate size.

The Committee at Sea manages harvest strategies during the fishing run, with the aim of maximizing the economic performance of the fishery. These changes usually involve a reduction in the fished area by moving closure lines to exclude areas of prawns below the target size. These decisions are based on data volunteered by fishers during commercial fishing. These data are not available for this assessment.

Performance assessment of both harvest strategy development and management are substantially impeded by a lack of documentation of the methods and rationale by which data are used to

83 inform decisions. Further, assessment is also impeded at these finer scales by the coarseness of the spatial scales of catch and effort data (i.e. fishing block) and issues regarding confidentiality of catch and effort data.

6.3 Current Status of the Spencer Gulf Prawn Fishery

6.3.1 Annual stock assessment

As indicated in the 2005 stock assessment report (Dixon et al. 2005a), there are several lines of evidence that suggest the Spencer Gulf Prawn Fishery is being fished within sustainable limits: catches have been relatively stable since 1987/88, effort has reduced to less than half of the 1978/79 peak, and mean harvested prawn size was larger in recent years than in 1978/79. This success can be largely attributed to the fishery-independent surveys conducted since 1982 that have established an understanding of the distribution and abundance of prawns in Spencer Gulf and enabled the ongoing refinement of harvest strategies.

There are several lines of evidence to suggest that the biomass of prawns in Spencer Gulf during 2004/05 was similar to that during the previous year. Firstly, survey catch rates in the two years were generally similar during November, February and April. Secondly, total catch in 2004/05 (1,939 t) was the same as that obtained in 2003/04 (1,939 t). Also, the slight decrease in CPUE during 2004/05 (91 kg/hr) compared to the previous year (96 kg/hr) was balanced out by the larger average size of harvested prawns during 2004/05 (214 prawns per 7 kg compared to 221 prawns per 7 kg in 2003/04), as smaller prawns are usually associated with higher catch rates. Encouragingly, the recruitment index during 2004/05 (45.6 recruits/nautical mile) was higher than the previous year (41.2) and above the target reference for recruitment (40).

Trends in catch during 2004/05 were also similar to the previous 5-year average. Total catch (1,939 t), catches during the early spawning period (430 t) and CPUE (91 kg/hr) were all slightly lower than the 5-year average (2,006 t, 469 t and 97 kg/hr, respectively). During 2004/05, the majority of catch was obtained from the Wallaroo region (48%), as it was in the previous 5 years (45%). Catches in 2004/05 were slightly higher in the Main Gutter and South Gutter regions (11% and 4%, respectively, compared to 9% and 2%) and substantially lower in the North region (1% compared with 5%). Again encouragingly, the recruitment index during 2004/05 (45.6 prawns/nautical mile) was higher than the previous 5-year average (41.5).

84 6.3.2 Assessment of harvest strategy development and management for 2004/05

Approximately 50% of the 2004/05 catch was harvested in 33 fishing nights during periods 1, 2, 5 and 6. Predominately large prawns were harvested during periods 1, 2 and 5, and a mix of sizes was harvested during period 6. The size of prawns harvested correlated well with the size of prawns observed during surveys. Approximately 30% of the total catch was removed during 11 nights fishing in harvest period 4. High catches of medium sized prawns were obtained from the Wallaroo and Main Gutter regions during this period, and again the size of harvested prawns generally reflected those observed during surveys.

Fourteen nights were fished during harvest period 3 to obtain 20% of the total catch for 2004/05. During this period, substantial catches (>10 t) of small and medium sized prawns were obtained from several adjacent blocks in the Cowell, Main Gutter and South Gutter regions. February stock assessment survey results showed medium-high catch rates (>4 lb/min) of a mix of prawn sizes in these fishing blocks. The relatively poor correlation between survey and harvest prawn sizes was likely due to the extended period between February surveys and the commencement of the March harvesting period (24 days). Conducting a spot survey prior to March fishing may have resulted in the development of a refined harvest strategy and a subsequent reduction in the harvest of small prawns.

This report is the first to compare catch rate and prawn size data from surveys and subsequent fishing. These analyses can be used to assess the effectiveness of harvest strategy development and management at a variety of spatial and temporal scales. Whilst the mean size of harvested prawn was well within performance measures for the fishery, it is clear that high catches of small prawns were harvested on a number of occasions throughout the year. This approach taken here augments assessment of the fishery, and provides a valuable tool for improving decision-making in the real time management process. However, critical assessment of harvest strategy management is currently limited by a lack of documentation of the decision rules used by the “committee at sea”, the coarseness of the spatial scales of catch and effort data (i.e. fishing block), and issues regarding confidentiality of catch and effort data.

6.4 Performance Indicators

As identified in the 2005 stock assessment report (Dixon et al. 2005a), the current suite of Performance Indicators (PIs) do not adequately assess the performance of the SGPF. The development of new and appropriate PIs is essential to ensure the sustainable management of the fishery into the future. Furthermore, the development of a suite of performance measures that

85 relate directly to harvest strategy development and management are needed to assess the fishery in a “self-management” framework. It is critically important that these performance measures are established at appropriate temporal and spatial scales.

Each of the four PIs for annual assessment need to be redefined. The PI for prawn size is currently calculated as the mean size of prawns harvested throughout the year. Whilst this is a useful measure, it does not reflect the variability in the size of prawns harvested at various times. Also, the current target and limit Reference Points for mean size (i.e. 40 and >40 prawns/kg) do not match the current size criteria adopted by the industry in the development of harvest strategies. The value of size data presented in this report would be improved by research to investigate the size distribution of prawns within the commercial grade categories reported in fishery logbooks.

As it is currently defined, the PI for effort (days) is of limited use. For many years, the level of effort applied in the SGPF has been limited through the development of harvest strategies informed from results of fishery-independent surveys, not on the quantitative limitation of effort across temporal scales. Further, the approaches used to convert measures of nominal effort into measures of effective effort have not been documented or updated since their inclusion in the stock assessment process (Carrick 2004). Useful measures of effort for the fishery would be more appropriately expressed in terms of catch per unit effort (CPUE). Also, the need to limit the amount of prawns harvested during the early spawning season, as highlighted by Dixon et al. (2005a), may be most appropriately managed by catch rather than effort controls.

The PI for recruitment is based on data from a limited number of sites surveyed in northern Spencer Gulf during February. Whilst it is clear that northern Spencer Gulf produces the highest recruitment, particularly during February, it is likely that only a subset of sites was selected because of the inconsistent spatial distribution of the sites surveyed (historically). Given the consistent spatial and temporal distribution of the current fishery-independent survey methodology, and that recruitment to the fishery occurs throughout most months of the year and in a variety of locations, it would seem appropriate to base recruitment indices on results from all sites and each fishery-independent survey.

There are several reasons why the PI for percentage virgin spawning biomass is problematic and should be replaced. Firstly, the “virgin spawning biomass” of this fishery is unknown and cannot be calculated reliably. Prior to this report, values presented for this PI have reflected estimates of fishing mortality from April surveys to the end of the fishing year (approximately 60% of annual catch). Despite the apparent stability of these estimates in previous stock assessments (Carrick

86 and Williams 2000, 2001, Carrick 2003), Dixon et. al. (2005) suggested that heavy fishing in November and December may have reduced egg production in recent years, and that “percentage virgin spawning biomass” as currently defined may not be an effective index of egg production. In addition, the relevance of percentage virgin spawning biomass is questionable, especially given the lack of information regarding the percentage that is needed to ensure adequate recruitment. A better option would be to establish PIs for abundance that are based explicitly on the results of the fishery-independent surveys conducted three times each year.

Another annual performance measure that may be considered is the estimate of egg production from survey catch presented in this report. The estimate reflects knowledge of the reproductive biology of Western King prawns and the abundance and size distribution of female prawns during November surveys. Unfortunately, the inconsistent spatial replication of November surveys conducted prior to 2004 prevents meaningful historical comparisons of egg production.

As discussed previously in this report, the decision rules that underpin harvest strategy development and management need to be documented. Once formalised, appropriate performance measures need to be determined and included in the Management Plan.

6.5 Future Research

Annual assessments of the SGPF have shown that sustainable catches of prawns have been obtained with increasing efficiency since the early 1980’s. However, problems encountered in the neighbouring prawn fisheries in GSV and on the West Coast, as well as analyses for the SGPF that show the influence of early season catches of spawning adults on recruitment, demonstrate the vulnerability of this short-lived and valuable species to overfishing.

The collaborative relationship that has developed between researchers, industry and policy- makers in managing the fishery has been critical to its current success, particularly in the light of the problems with the current Performance Indicators, and the lack of decision rules for harvest strategy development and management. The development of appropriate Performance Indicators and clear decision rules for annual stock assessment and auditing of harvest strategies, are essential to ensuring the continued sustainability of this resource, particularly in the face of the increasing role industry is playing in management of the fishery.

Several areas of future research would substantially augment assessment of the ecological sustainability of the SGPF. In particular, studies on the reproductive biology of prawns in Spencer Gulf would advance the understanding of the relationship between egg production and

87 recruitment, with implications for harvest strategies during the spawning season. Also, understanding the changes in fishing efficiency over time (i.e. standardisation of CPUE data) would substantially improve the interpretation of changes in prawn abundance over time from both commercial CPUE and, most importantly, survey CPUE data.

Appropriate methods for assessing fishery performance at finer temporal scales must also be established. Preliminary analyses presented in this report identified two impediments to these assessments: the discrepancy between the spatial scale of catch and effort reporting blocks and the development and management of harvest strategies, and; confidentiality issues of catch and effort data. An improved understanding of prawn grade data, including studies on the size composition of prawns within various grades, the variability between commercial grading machines, and validation of commercial catch and effort data on prawn grades, would reduce the uncertainty associated with prawn size data. Finally, assessment of the performance of the Committee at Sea cannot be made without the 1) documentation of the decision rules governing harvest strategy management and 2) collection of the data that underpin these decisions.

88 7. REFERENCES

Anon (2003) Ecological assessment of the South Australian Spencer Gulf Prawn Fishery, Gulf St Vincent Prawn Fishery and West Coast Prawn Fishery. South Australian fisheries management series, Primary Industries and Resources of South Australia.

Anon. (2004) Assessment of the South Australian Spencer Gulf Prawn Fishery, Gulf St Vincent Prawn Fishery and West Coast Prawn Fishery. Department of the Environment and Heritage, Commonwealth of Australia.

Arnstein D.R. and Beard T.W. (1975) Induced maturation of the prawn, orientalis Kishinouye in the laboratory by means of eyestalk removal. Aquaculture 5: 411–412.

Benzie J.A.H., Frusher S.D., Kenway M. and Trott L. (1995) Utility of streamer tags to assess survival and growth of juvenile tiger prawns (Penaeus monodon) in aquaculture environments. Aquaculture 136(1–2): 57–69.

Broadhurst M.K., Larsen R.B., Kennelly S.J. and McShane P. (1999) Use and success of composite square-mesh codends in reducing bycatch and in improving size-selectivity of prawns in Gulf St Vincent, South Australia. Fish. Bull. 97: 434–448.

Broadhurst M.K., McShane P.E. and Larsen R.B. (2000) Effects of twine diameter and mesh size in the body of prawn trawls on bycatch in Gulf St. Vincent, Australia. Fish. Bull. 98: 463–473.

Brown D.E. and Holthuis L.B. (1998) The Australian species of the genus Ibacus (Crustacea: : Scyllaridae), with the description of a new species and addition of new records. Zool. Med. Leiden 72(10): 113–141.

Bryars S. (2003) An inventory of important coastal fisheries habitats in South Australia. Fish Habitat Program, Primary Industries and Resources South Australia.

Caddy, J. and Mahon, R. (1995) Reference points for fisheries management. FAO Fisheries Technical Paper 347, 1-83.

Carrick, N.A. (1982) Spencer gulf prawn fishery – surveys increase our knowledge. SAFIC vol 6(1) pp 3-32.

89 Carrick, N.A. (1996) Key factors which affect prawn recruitment and implications to harvesting prawn stocks. Final report FRDC 91/3.

Carrick, N.A. (1997) The by-catch from prawn trawlers in Spencer Gulf, South Australia – a study of spatial distribution and the effects of trawling. SARDI Report Series 97/2.

Carrick, N.A. (1999) The Spencer Gulf Penaeid shrimp fishery: a case for an exemption to the US shrimp embargo (P.L. 101-162, section 609). SARDI Aquatic Sciences internal report.

Carrick, N.A. (2003) Spencer Gulf Prawn (Melicertus latisulcatus) Fishery. Fishery Assessment report to PIRSA. SARDI Aquatic Sciences RD03/0079-2.

Carrick, N.A. and Mc Shane, P. (1998) Spencer Gulf and West Coast Prawns. South Australian Fisheries Assessment Series 98/8.

Carrick, N.A. and Williams, H. (2000) Spencer Gulf and West Coast Prawns. Fishery Assessment Report to PIRSA. South Australian Fisheries Assessment Series 00/08.

Carrick, N.A. and Williams, H. (2001) Spencer Gulf and West Coast Prawn. Fishery Assessment Report to PIRSA. South Australian Fisheries Assessment Series 01/08.

Coleman N (1984) Molluscs from the diets of commercially exploited fish off the coast of Victoria, Australia. J. Malac. Soc. Aust. 6(3-4): 143–154.

Courtney A.J. and Dredge M.C.L. (1988) Female reproductive biology and spawning periodicity of two species of king prawns, Penaeus longistylus Kubo and Penaeus latisulcatus Kishinouye, from Queensland’s east coast fishery. Aust. J. Mar. Freshwater Res. 39: 729–741.

Courtney A.J., Cosgrove M.G. and Die D.J. (2001) Population dynamics of Scyllarid lobsters of the genus Thenus spp. on the Queensland (Australia) east coast. I. Assessing the effects of tagging. Fish. Res. 53: 251–261.

Courtney A.J., Montgomery S.S., Die D.J., Andrew N.L., Cosgrove M.G. and Blount C. (1995) Maturation in the female eastern king prawn Penaeus plebejus from coastal waters of eastern Australia, and consideration for quantifying egg production in penaeid prawns. Mar. Biol. 122(4): 547–556.

90 Dixon C.D. and Roberts S.D. (2006) West Coast Prawn (Melicertus latisulcatus) Fishery. Fishery Assessment report to PIRSA Fisheries. SARDI Aquatic Sciences Publication no. RD03/0076–4, SARDI Research Report Series No 122.

Dixon C.D., Roberts S.D. and Ward T.M (2005a) Spencer Gulf Prawn (Melicertus latisulcatus) Fishery. Fishery Assessment report to PIRSA. SARDI Aquatic Sciences RD03/0079–3.

Dixon C.D., Roberts S.D., Hooper, G.E., Steer M.A. and Ward T.M (2006) Gulf St Vincent Prawn (Melicertus latisulcatus) Fishery. Fishery Assessment report to PIRSA. SARDI Aquatic Sciences RD03/0063-4. SARDI Report Series No 159.

Dixon C.D. Svane I. and Ward T.M (2005b) Monitoring and assessment of by-catch and by- product species of the Spencer Gulf Prawn Fishery. Report to PIRSA. SARDI Aquatic Sciences RD04/0249. SARDI Report Series No. 102.

Edyvane K. (1999) Coastal and marine wetlands in Gulf St. Vincent, South Australia: understanding their loss and degradation. Wetlands Ecol. Manag. 7: 83–104.

Gales R., Pemberton D., Lu C.C. and Clarke M.R. (1993) Cephalopod diet of the Australian Fur Seal: variation due to location, season and sample type. Aust. J. Mar. Freshwater Res. 44: 657– 671.

Grey, D.L., Dall, W. and Baker, A. (1983) “A guide to the Australian penaeid Prawns”. Department of Primary Production, Northern Territory, Australia.

Harvell D., Aronson R., Baron N., Connell J., Dobson A., Ellner S., Gerber L., Kim K., Kuris A., McCallum H., Lafferty K., McKay B., Porter J., Pascual M., Smith G., Sutherland K., and Ward J. (2004) The rising tide of ocean diseases: unsolved problems and research priorities. Front. Ecol. Environ. 2(7): 375–382.

Hudinaga, M. (1942). Reproduction, development and rearing of Penaeus japonicus Bate. Jap. J. Zool. 10: 305-393.

Kangas M. (1999) Postlarval and juvenile western king prawns Penaeus latisulcatus Kishinouye studies in Gulf St Vincent, South Australia with reference to the commercial fishery. PhD thesis, University of Adelaide.

91 Kangas M. and Jackson B. (1995) A pilot study of mesh selectivity of prawn trawl nets in Gulf St. Vincent. SARDI Research Report Series No. 2.

Kangas M. and Jackson B. (1997) Gulf St Vincent Prawn Fishery. South Australian Fisheries Assessment Series 99/05.

Kangas M. and Jackson B. (1998) Sampling juvenile Penaeus latisulcatus Kishinouye with a water-jet net compared with a beam-trawl: spatial and temporal variation and nursery area in Gulf St. Vincent, South Australia. Mar. Freshwater Res. 49: 517-523.

King M.G. (1977) The biology of the western king prawn Penaeus latisulcatus Kishinouye and aspects of the fishery in South Australia. MSc Thesis, University of Adelaide.

Lee S.Y. (2004) Relationship between mangrove abundance and tropical prawn production: a re- evaluation. Mar. Biol. 145:943–949.

Linnane A., Ward, T.M., McGarvey R., Xiao, Y. and Feenstra J. (2005). Northern Zone Rock Lobster (Jasus edwardsii) Fishery 2003/04. South Australian Research and Development Institute, Publication No. RD03/0142–2.

McShane P., Broadhurst M., Carrick N. and McDonald B. (1998) Transfer of by-catch reduction technology to South Australian prawn fisheries. FRDC report No. 96/254.02.

MacDonald, N. (1998) Management plan for the Spencer Gulf and West Coast prawn fisheries. Internal document, Primary Industries and Resources, South Australia.

Menz A. and Blake B.F. (1980) Experiments on the growth of Penaeus vannamei Boone. J. Exp. Mar. Biol. Ecol. 48: 99–111.

Nash G., Anderson I.G. and Shariff M. (1988) Pathological changes in the tiger prawn, Penaeus monodon Fabricus, associated with culture in brackishwater ponds developed from potentially acid sulphate soils. J. Fish Dis. 11: 113–123.

Nunes R. and Lennon G. (1986) Physical property distributions and seasonal trends in Spencer Gulf, South Australia: an inverse estuary. Aust. J. Mar. Freshwater Res. 37: 39–53.

92 Penn J.W. (1975) Tagging experiments with the western king prawn Penaeus latisulcatus Kishinouye. I. Survival, growth, and reproduction of tagged prawns. Aust. J. Mar. Freshwater Res. 26(2): 197–211.

Penn J.W. (1976) Tagging experiments with the western king prawn Penaeus latisulcatus Kishinouye. II. Estimation of population parameters. Aust. J. Mar. Freshwater Res. 27: 239–250.

Penn, J.W. (1980) Spawning and fecundity of the western king prawn, Penaeus latisulcatus Kishinouye, in Western Australian waters. Aust. J. Mar. Freshwater Res. 31: 21-35.

Penn, J.W., Hall, N.G. and Caputi, N. (1988) Resource assessment and management perspectives of the Penaeid prawn fisheries of Western Australia. In: J. Caddy (Ed) The scientific basis of Shellfish Management. John Wiley and Sons, New York.

Poiner, I., Glaister, J., Pitcher, R., Burridge, C., Wassenberg, T., Gribble, N., Hill, B., Blaber, S., Milton, D., Brewer, D. and Ellis, N. (1998) Final report on effects of trawling in the far northern section of Great Barrier Reef: 1991-1996. CSIRO Division of Marine Research, Cleveland.

Potter, I.C., Manning, R.J.G. and Loneragan, N.R. (1991) Size, movements, distribution and gonadal stage of the western king prawn (Penaeus latisulcatus) in a temperate estuary and local marine waters. J. Zool., Lond. 223: 419-445.

Primavera, J.H. and Lebata, M.J.H.L. (2000) Size and diel differences in activity patterns of Metapenaeus ensis, Penaeus latisulcatus and P. merguiensis. Mar. Fresh. Behav. Physiol. 33: 173-185.

Quinn, G. P. & M. J. Keough. 2002. Experimental design and data analysis for biologists. Cambridge: Cambridge Univ. Press. 537 pp.

Rasheed, M.A. and Bull, C.M. (1992) Behaviour of the Western king prawn, Penaeus latisulcatus Kishinouye: Effect of food dispersion and crowding. Aust. J. Mar. Freshwater Res. 43: 745-752.

Roberts, S.D. (2000) The feeding ecology of the blue swimmer crab Portunus pelagicus, as it relates to discarded by-catch from Spencer gulf prawn trawlers. Honours thesis, the Flinders University of South Australia.

93 Roberts, S.D, Dixon C.D. and Ward T.M. (2005) Assessment of juvenile prawn (Melicertus latisulcatus) surveys in Spencer Gulf, South Australia. Report to PIRSA Fisheries. SARDI Aquatic Sciences RD04/0211–2, SARDI Research Report Series No 95.

Sang H.M. and Fotedar R. (2004) Growth, survival, haemolymph osmolality and organosomatic indices of the western king prawn (Penaeus latisulcatus Kishinouye, 1896) reared at different salinities. Aquaculture 234: 601–614.

Shokita, S. (1984) Larval development of Penaeus (Melicertus) latisulcatus Kishinouye (Decapoda, Natandtia, ) reared in the laboratory. Galaxea 3: 37-55.

Sluczanowski, P. (1980) The modelling and optimal control of the Spencer Gulf Prawn Fishery. PhD thesis, Imperial College, London, 1-237.

South Australian Coast Protection Board (2003) Coastline: a strategy for implementing CPB policies on coastal acid sulphate soils in South Australia. Department for Environment and Heritage, Report No. 33, January 2003.

Steer, M.A., Lloyd, M.T. and Jackson W.B. (2006) Southern calamary (Sepioteuthis australis) Fishery. Fishery Assessment report to PIRSA. SARDI Aquatic Sciences RD05/0006-2, SARDI Research Report Series No 149.

Stewart J. and Kennelly S.J. (1997) Fecundity and eff-size of the Balmain bug Ibacus peronii (Leach, 1815) (Decapoda, Scyllaridae) off the east coast of Australia. Crustaceana 70(2): 191–197.

Stewart J. and Kennelly S.J. (1998) Contrasting movements of two exploited Scyllarid lobsters of the genus Ibacus off the east coast of Australia. Fish. Res. 36: 127–132.

Stewart J. and Kennelly S.J. (2000) Growth of the Scyllarid lobsters Ibacus peronii and I. chacei. Mar. Biol. 136: 921-930.

Svane, I. (1998) Prawn fishery by-catch and discards: fates and consequences for a marine ecosystem. FRDC report 1998/225.

Svane I. (2003) Gulf St Vincent Prawn Fishery (Melicertus latisulcatus). Fishery assessment report to PIRSA for the Prawn Fishery Management Committee, October 2003. SARDI Aquatic Sciences RD03/0063-2.

94

Svane I. (2005) Occurrence of dolphins and seabirds and their consumption of by-catch during prawn trawling in Spencer Gulf, South Australia. Fisheries Research 76: 317-327.

Svane I. and Roberts S.D. (2005) Gulf St Vincent Prawn Fishery (Melicertus latisulcatus). Fishery assessment report to PIRSA. SARDI Aquatic Sciences RD03/0063–3.

Tanner, J.E. and Deakin, S. (2001) Active habitat selection for sand by juvenile western king prawns, Melicertus latisulcatus (Kishinouye). J. Exp. Mar. Biol. Ecol. 261: 199-209.

Vijayan K.K., Balasubramanian C.P., Jithendran K.P., Alavandi S.V. and Santiago T.C. (2003) Histopathology of Y-organ in Indian white shrimp Fenneropenaeus indicus, experimentally infected with white spot syndrome virus. Aquaculture 221: 97–106.

Wallner B. (1985) An assessment of the South Australian West Coast western king prawn (Penaeus latisulcatus) fishery. Department of Fisheries, South Australia.

Winstanley R.H., Potter M.A. and Caton A.E. (1983) Australian cephalopod resources. Memoirs of the National Museum Victoria. 44: 243-253.

Wu, J. (1990). Environmental factors affecting the survival and growth of western king prawns, Penaeus latisulcatus, under aquaculture conditions in Spencer Gulf, South Australia. Masters Thesis, University of Adelaide.

Xiao Y. and McShane P. (2000) Estimation of instantaneous rates of fishing and natural mortalities from mark-recapture data on the western king prawn Penaeus latisulcatus in the Gulf St. Vincent, Australia, by conditional likelihood. Trans. Am. Fish. Soc. 129: 1005-1017.

95 8. APPENDIX

Recommendations from the Executive Summary of the DEH Assessment of South Australia’s Prawn Fisheries (Anon 2004).

1. PIRSA to advise DEH of any material change to the management arrangements for the Spencer Gulf Prawn Fishery, Gulf St Vincent Prawn Fishery and the West Coast Prawn Fishery that could affect the criteria on which EPBC decisions are based, within 3 months of that change being made.

2. The current review of South Australia’s Fisheries Act 1982 should provide for the inclusion of general community and conservation interests on fisheries management committees. PIRSA to ensure that general community and conservation interests are provided the opportunity to engage regularly with the Prawn Fisheries Management Committee. Greater efforts should also be made to increase conservation and general community involvement in stock assessments and research priority setting processes.

3. By December 2005, PIRSA to develop and implement a compliance strategy for the South Australian Prawn Fisheries that is reviewable and publicly available, to address compliance and enforcement risks identified in the compliance risk assessment of the prawn fisheries.

4. By the end of 2005, PIRSA to ensure that information on the size composition of prawn catch is collected and monitored on an ongoing basis in the Gulf St Vincent Prawn Fishery and the West Coast Prawn Fishery.

5. By December 2005, PIRSA to review the performance indictors and performance measures used in the assessment of Western King prawns stock status in the Gulf St Vincent Prawn Fishery and the West Coast Prawn Fishery to ensure that they are appropriately precautionary and sufficient to detect significant changes in stock status.

6. PIRSA to ensure that the recreational harvest of Western King prawns is monitored and factored into the management of the Spencer Gulf Prawn Fishery and the Gulf St Vincent Prawn Fishery.

7. PIRSA to develop and implement harvest strategies for all by-product species taken in the Spencer Gulf Prawn Fishery, Gulf St Vincent Prawn Fishery and the West Coast Prawn Fishery within two years. Harvest strategies should clearly articulate performance indicators and performance measures and any specific management arrangements to be applied to by-product species permitted in the fisheries.

96 8. By December 2006, PIRSA to develop a system for the ongoing collection and monitoring of information on bycatch and by-product species taken in the Gulf St Vincent Prawn Fishery and the West Coast Prawn Fishery sufficient to enable identification of long-term trends in bycatch and by-product. In the event that catch levels of any bycatch or by-product species change, PIRSA will investigate suitable management responses.

9. PIRSA to conduct a bycatch risk assessment in the Gulf St Vincent Prawn Fishery and the West Coast Prawn Fishery and develop suitable management responses for any species identified as high risk.

10. PIRSA to continue to pursue reduction in the amount of bycatch taken in the Spencer Gulf Prawn Fishery, Gulf St Vincent Prawn Fishery and the West Coast Prawn Fishery through the adoption and refinement of bycatch mitigation technology and to investigate methods for increasing the survivability of bycatch species. Any suitable methods identified will be implemented in a timely manner.

11. By December 2008, PIRSA to specify bycatch indicator species for the Gulf St Vincent Prawn Fishery and the West Coast Prawn Fishery to be used to monitor the impact of the fisheries on bycatch species and/or species groups.

12. By December 2005, PIRSA to introduce mandatory structured reporting of all interactions between the Spencer Gulf Prawn Fishery, Gulf St Vincent Prawn Fishery and the West Coast Prawn Fishery and endangered, threatened or protected species. To complement such a reporting system, PIRSA to also ensure that an education program for fishers is developed and implemented to promote the importance of protected species protection and accurate incident reporting.

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