Spencer Gulf Prawn (Melicertus latisulcatus) Fishery 2005/06 Fishery Assessment Report to PIRSA Fisheries September 2007 Dixon, C.D., Roberts, S.D. and Hooper, G.E. SARDI Aquatic Sciences Publication No. F2007/000770-1 SARDI Research Report Series No. 237 Fishery Assessment Report to PIRSA Fisheries Spencer Gulf Prawn (Melicertus latisulcatus) Fishery 2005/06 Dixon, C.D., Roberts, S.D. and Hooper, G.E. September 2007 SARDI Aquatic Sciences SARDI Aquatic Sciences Publication No. F2007/000770-1 SARDI Research Report Series No. 237 This Fishery Assessment Report updates the 2004/05 report for the Spencer Gulf Prawn Fishery and is part of SARDI Aquatic Sciences ongoing assessment program for South Australia’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 2005/06 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: 4 September 2007 SARDI Aquatic Sciences Publication No. F2007/000770-1 SARDI Research Report Series No. 237 Reviewers: Dr. S. Mayfield and Mr. T. Saunders Approved by: Dr. Tim Ward Signed: Date: 3 September 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................................................................................................................... vii ACKNOWLEDGEMENTS .......................................................................................................1 EXECUTIVE SUMMARY..........................................................................................................2 1. GENERAL INTRODUCTION ............................................................................................3 1.1 Overview ..............................................................................................................................3 1.2 Description of the Fishery.................................................................................................4 1.2.1 Fishery location............................................................................................................4 1.2.2 The Spencer Gulf environment.................................................................................6 1.2.3 Nursery areas................................................................................................................8 1.2.4 Commercial fishery .................................................................................................. 10 1.2.7 Recreational, indigenous and illegal catch............................................................. 10 1.3 Management of the Fishery............................................................................................ 12 1.3.1 Management history................................................................................................. 12 1.3.2 Current management arrangements....................................................................... 13 1.3.3 Spencer Gulf Prawn Fishery Management Plan .................................................. 13 1.3.4 Performance Indicators ........................................................................................... 14 1.4 Biology of the Western King Prawn ............................................................................. 16 1.4.1 Distribution and taxonomy..................................................................................... 16 1.4.2 Reproductive biology............................................................................................... 16 1.4.3 Larval and juvenile phase ........................................................................................ 19 1.4.4 Stock structure .......................................................................................................... 22 1.4.5 Growth....................................................................................................................... 22 1.4.6 Length weight relationship...................................................................................... 25 1.4.7 Movement determined from tagging studies........................................................ 25 1.4.8 Natural mortality....................................................................................................... 27 1.4.9 Prawn health.............................................................................................................. 27 1.5 Stock Assessment............................................................................................................. 28 1.6 Current Research and Monitoring Program................................................................. 28 1.6.1 Catch and effort research logbook......................................................................... 28 1.6.2 Stock assessment surveys ........................................................................................ 28 1.7 Discussion ......................................................................................................................... 29 2. FISHERY STATISTICS ....................................................................................................... 30 2.1 Introduction...................................................................................................................... 30 2.2 Catch and Effort .............................................................................................................. 30 2.2.1 Inter-annual trends................................................................................................... 30 2.2.2 Trends among regions ............................................................................................. 31 2.2.3 Trends within years .................................................................................................. 33 2.2.4 Catches during the spawning season ..................................................................... 34 2.3 Catch-Per-Unit-Effort (CPUE)...................................................................................... 35 2.3.1 Inter-annual trends................................................................................................... 35 2.3.2 Trends among regions ............................................................................................. 36 2.3.3 Trends within years .................................................................................................. 37 2.3.4 Trends within fishing periods................................................................................. 38 2.4 Prawn Size......................................................................................................................... 39 2.4.1 Inter-annual trends................................................................................................... 39 2.4.2 Trends within years .................................................................................................. 40 iii 2.4.3 Daily prawn grades................................................................................................... 42 2.5 Discussion ......................................................................................................................... 44 3. STOCK ASSESSMENT SURVEYS................................................................................... 46 3.1 Introduction...................................................................................................................... 46 3.2 Annual Trends.................................................................................................................. 48 3.2.1 Mean catch rate......................................................................................................... 48 3.2.2 Prawn size.................................................................................................................. 49 3.2.3 Sex ratio ....................................................................................................................