A trophic model for Gulf St Vincent: Balancing exploitation of three fisheries in an EBFM framework Simon D Goldsworthy, Maylene Loo, Anthony Fowler, Michael Steer and Craig Noell April 2017 FRDC Project No. 2013/031 A trophic model for Gulf St Vincent: balancing exploitation of three fisheries ii © 2017 Fisheries Research and Development Corporation and South Australian Research and Development Institute. All rights reserved. ISBN: 978-1-921563-93-5 A trophic model for Gulf St Vincent: Balancing exploitation of three fisheries in an EBFM framework. 2013/031 2017 Ownership of Intellectual property rights Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this publication is owned by the Fisheries Research and Development Corporation and the South Australian Research and Development Institute. This work is copyright. 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Goldsworthy Address: 25 Geils Court Address: SARD Aquatic Sciences Deakin ACT 2600 2 Hamra Avenue Phone: 02 6285 0400 West Beach SA 5024 Fax: 02 6285 0499 Phone: 08 8207 5325 Email: [email protected] Fax: Web: www.frdc.com.au Email: [email protected] In submitting this report, the researcher has agreed to FRDC publishing this material in its edited form. A trophic model for Gulf St Vincent: balancing exploitation of three fisheries iii Contents Contents ................................................................................................................................................. iii Tables ...................................................................................................................................................... v Figures ................................................................................................................................................... vi Acknowledgments ................................................................................................................................ vii Abbreviations ....................................................................................................................................... vii Executive Summary .............................................................................................................................. 1 1 Introduction ......................................................................................................................................... 4 1.1 Background ..................................................................................................................................4 1.2 Need .............................................................................................................................................5 1.3 Objectives ....................................................................................................................................5 2 Methodology ........................................................................................................................................ 6 2.1 Ecopath and mass balance approach ............................................................................................6 2.2 Model area and structure ..............................................................................................................6 2.3 Model fitting ................................................................................................................................7 2.4 Ecosystem indicators ...................................................................................................................8 2.5 Scenario testing ............................................................................................................................8 3 Results................................................................................................................................................ 13 3.1 Trophic structure and flow (Ecopath) ........................................................................................13 3.2 Model fitting (Ecosim) ...............................................................................................................13 3.3 Temporal changes in GSV ecosystem .......................................................................................16 Trends in catch ..........................................................................................................................16 Group biomasses .......................................................................................................................16 Effects of fishing .......................................................................................................................17 3.2 Trophic role of three key species and impact of Snapper ..........................................................25 Diet of three key species ...........................................................................................................25 Predation on three key species ..................................................................................................25 Sensitivity analysis ....................................................................................................................26 Impact of increasing biomass of Snapper on GSV ecosystem ..................................................26 3.3 Optimisation of future ecological and economic performance ..................................................32 Impact of reducing Snapper biomass on high value commercial fisheries and on the GSV ecosystem ..................................................................................................................................32 4 Discussion .......................................................................................................................................... 37 Dynamic management arrangements in the GSV ecosystem ..........................................................37 Trophodynamic indicators of fisheries status and impact ................................................................41 Trophic relationships – role of Snapper in GSV ecosystem ............................................................43 Could a directed fishery for Snapper be used to optimise ecological and economic performance of Blue Crab and prawn fisheries? ..............................................................................45 5 Conclusion ......................................................................................................................................... 46 6 Implications ....................................................................................................................................... 47 7 Recommendations ............................................................................................................................ 48 8 Further development 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