Fisheries Research Report No. 306 Blue Swimmer Crab (Portunus armatus) and Mud Crab (Scylla serrata and Scylla olivacea) Resources in the North Coast and Gascoyne Coast Bioregions, Western Australia Johnston, D., Yeoh, D., Harris, D. and Fisher, E. July 2020 Correct citation: Johnston, D., Yeoh, D., Harris, D. and Fisher, E. (2020). Blue Swimmer Crab (Portunus armatus) and Mud Crab (Scylla serrata and Scylla olivacea) Resources in the North Coast and Gascoyne Coast Bioregions, Western Australia. Fisheries Research Report No. 306. Department of Primary Industries and Regional Development, Western Australia. 156pp. Enquiries: WA Fisheries and Marine Research Laboratories, PO Box 20, North Beach, WA 6920 Tel: +61 8 9203 0111 Email: [email protected] Website: www.fish.wa.gov.au A complete list of Fisheries Research Reports is available online at www.fish.wa.gov.au Important disclaimer The Chief Executive Officer of the Department of Primary Industries and Regional Development and the State of Western Australia accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it. Department of Primary Industries and Regional Development Gordon Stephenson House 140 William Street PERTH WA 6000 Telephone: (08) 6551 4444 Website: dpird.wa.gov.au ABN: 18 951 343 745 ISSN: 1035-4549 (Print) ISBN: 978-1-921258-64-0 (Print) ISSN: 2202-5758 (Online) ISBN: 978-1-921258-65-7 (Online) Copyright © State of Western Australia (Department of Primary Industries and Regional Development) 2020 ii Fisheries Research Report [Western Australia] No. 306 Table of Contents Table of Contents ................................................................................................................... iii List of Abbreviations .............................................................................................................. vi Executive Summary ................................................................................................................. 7 1. Scope ................................................................................................................................... 10 2. How the Department Operates ........................................................................................ 10 3. Aquatic Environment ........................................................................................................ 11 3.1 North Coast Bioregion ................................................................................................ 12 3.2 Gascoyne Coast Bioregion .......................................................................................... 12 4. Resource Description ........................................................................................................ 14 4.1 Pilbara and Exmouth Gulf Blue Swimmer Crab Resource ......................................... 14 4.2 Kimberley Mud Crab Resource ................................................................................... 15 5. Species Description ............................................................................................................ 16 5.1 Blue swimmer crab (Portunus armatus) ................................................................... 16 5.1.1 Taxonomy and Distribution ......................................................................... 16 5.1.2 Stock Structure ............................................................................................. 17 5.1.3 Life History .................................................................................................. 18 5.1.4 Inherent Vulnerability .................................................................................. 32 5.2 Green mud crab (Scylla serrata) and Brown mud crab (S. olivacea) ...................... 34 5.2.1 Taxonomy and Distribution ......................................................................... 35 5.2.2 Stock Structure ............................................................................................. 37 5.2.3 Life History .................................................................................................. 37 5.2.4 Inherent Vulnerability .................................................................................. 46 6. Fishery Information .......................................................................................................... 47 6.1 Commercial Fishery ................................................................................................. 47 6.1.1 Overview ...................................................................................................... 47 6.1.2 History of Development ............................................................................... 48 6.1.3 Current Fishing Activities ............................................................................ 54 6.1.4 Fishing Methods and Gear ........................................................................... 66 6.1.5 Susceptibility................................................................................................ 71 6.2 Recreational/Charter sector ...................................................................................... 72 6.2.1 History of Development ............................................................................... 72 Fisheries Research Report [Western Australia] No. 306 iii 6.2.2 Current Fishing Activities ............................................................................ 74 6.2.3 Fishing Methods and Gear ........................................................................... 75 6.2.4 Susceptibility................................................................................................ 76 6.3 Customary Fishing ................................................................................................... 77 6.4 Illegal, Unreported or Unregulated Fishing ............................................................. 78 7. Fishery Management ........................................................................................................ 78 7.1 Management System ................................................................................................ 78 7.2 Harvest Strategy ....................................................................................................... 79 7.3 External Influences .................................................................................................. 81 7.3.1 Environmental Factors ................................................................................. 82 7.3.2 Introduced Pest Species ............................................................................... 86 7.3.3 Market Influences ........................................................................................ 86 7.3.4 Non-WA Managed Fisheries ....................................................................... 87 7.3.5 Other Activities ............................................................................................ 87 8. Information and Monitoring ............................................................................................ 87 8.1 Range of Information ............................................................................................... 87 8.2 Monitoring ............................................................................................................... 89 8.2.1 Commercial Catch and Effort ...................................................................... 89 8.2.2 Recreational/Charter Catch and Effort......................................................... 92 8.2.3 Customary Catch .......................................................................................... 95 8.2.4 Illegal, Unreported or Unregulated Catch .................................................... 95 8.2.5 Fishery-Dependent Monitoring .................................................................... 95 8.2.6 Fishery-Independent Monitoring ................................................................. 98 8.2.7 Environmental Monitoring........................................................................... 98 9. Stock Assessment ............................................................................................................... 99 9.1 Assessment Principles .............................................................................................. 99 9.2 Assessment Overview .............................................................................................. 99 9.2.1 Peer Review of Assessment ....................................................................... 100 9.3 Analyses and Assessments ..................................................................................... 101 9.3.1 Data Used in Assessment ........................................................................... 101 9.3.2 Catch and Effort Trends ............................................................................. 101 9.3.3 Catch Distribution Trends .......................................................................... 111 9.3.4 Fishery-Dependent Data Analyses ............................................................. 122 iv Fisheries Research Report [Western Australia] No. 306 9.3.5 Trends in Age / Size Structures ................................................................. 126 9.3.6 Productivity Susceptibility Analysis .......................................................... 132 9.4 Stock Status Summary ..........................................................................................