Determination of the diets of Snapper and Silver Trevally and construction of a food web for the demersal fish community in south-western Australia B. French, I.C. Potter, M.E. Platell and K.R. Clarke Project No. 2009/006 1 2 Determination of the diets of Snapper and Silver Trevally and construction of a food web for the demersal fish community in south-western Australia B. French, I.C. Potter, M.E. Platell and K.R. Clarke April 2013 Centre for Fish and Fisheries Research Murdoch University, South Street, Murdoch, Western Australia, 6150 3 Title: Determination of the diets of Snapper and Silver Trevally and construction of a food web for the demersal fish community in south-western Australia. Authors: B. French, I.C. Potter, M.E. Platell and K.R. Clarke FRDC Project No 2009/006 Date: April 2013 © Copyright Fisheries Research & Development Corporation and Murdoch University, 2013. This work is copyright. 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The Fisheries Research and Development Corporation plans, invests in and manages fisheries research and development throughout Australia. It is a statutory authority within the portfolio of the federal Minister for Agriculture, Fisheries and Forestry, jointly funded by the Australian Government and the fishing industry. ISBN: 978-1-921877-06-3 Fisheries Research and Development Corporation Report FRDC Project 2009/006 4 TABLE OF CONTENTS 1. NON-TECHNICAL SUMMARY ........................................................................................................... 7 2. ACKNOWLEDGEMENTS .................................................................................................................. 10 3. BACKGROUND ................................................................................................................................. 11 3.1 RANKING OF LENGTH-CLASS, SEASONAL AND REGIONAL EFFECTS ON DIETARY COMPOSITIONS OF THE CO-OCCURRING PAGRUS AURATUS (SPARIDAE) AND PSEUDOCARANX GEORGIANUS (CARANGIDAE) ............................................................................................................................................................ 12 3.2 AN INNOVATIVE STATISTICAL APPROACH TO CONSTRUCTING A READILY COMPREHENSIBLE FOOD WEB FOR A DEMERSAL FISH COMMUNITY ............................................................................................................ 13 4. NEED ................................................................................................................................................. 15 5. OBJECTIVES .................................................................................................................................... 16 6. METHODS ......................................................................................................................................... 17 6.1 RANKING OF LENGTH-CLASS, SEASONAL AND REGIONAL EFFECTS ON DIETARY COMPOSITIONS OF THE CO-OCCURRING PAGRUS AURATUS (SPARIDAE) AND PSEUDOCARANX GEORGIANUS (CARANGIDAE) ............................................................................................................................................................ 17 6.1.1 Sampling localities and regimes .................................................................................................................... 17 6.1.2 Preparation of jaws and analyses of diets ..................................................................................................... 18 6.1.3 Multivariate analyses ...................................................................................................................................... 19 6.2 AN INNOVATIVE STATISTICAL APPROACH TO CONSTRUCTING A READILY COMPREHENSIBLE FOOD WEB FOR A DEMERSAL FISH COMMUNITY ............................................................................................................. 21 6.2.1 Sampling of fish and treatment of gut samples .............................................................................................. 21 6.2.2 Structure of data ............................................................................................................................................. 21 6.2.3 Initial screening of dietary data ...................................................................................................................... 23 6.2.4 Identification of predator guilds ...................................................................................................................... 24 6.2.5 Identification of prey guilds............................................................................................................................. 25 6.2.6 Food webs ...................................................................................................................................................... 26 7. RESULTS .......................................................................................................................................... 27 7.1 RANKING OF LENGTH-CLASS, SEASONAL AND REGIONAL EFFECTS ON DIETARY COMPOSITIONS OF THE CO-OCCURRING PAGRUS AURATUS (SPARIDAE) AND PSEUDOCARANX GEORGIANUS (CARANGIDAE) ............................................................................................................................................................ 27 7.1.1 Jaws and dentition ......................................................................................................................................... 27 7.1.2 Overall dietary compositions .......................................................................................................................... 27 7.1.3 Relationships between diets and body size ................................................................................................... 31 7.1.4 Analyses of dietary compositions of Pagrus auratus on the lower west and mid west coasts ...................... 33 7.1.5 Analyses of dietary compositions of Pseudocaranx georgianus on the lower west and south coasts .......... 38 7.1.6 Comparisons between the diets of Pagrus auratus and Pseudocaranx georgianus on the lower west coast ................................................................................................................................................................................. 40 7.2 AN INNOVATIVE STATISTICAL APPROACH TO CONSTRUCTING A READILY COMPREHENSIBLE FOOD WEB FOR A DEMERSAL FISH COMMUNITY ............................................................................................................. 43 7.2.1 Identifying predator guilds and their typifying prey species ........................................................................... 43 7.2.2 Identifying prey guilds and their relationships to predator guilds ................................................................... 49 7.2.3 Food webs ...................................................................................................................................................... 49 8. DISCUSSION ..................................................................................................................................... 57 8.1 RANKING OF LENGTH-CLASS, SEASONAL AND REGIONAL EFFECTS ON DIETARY COMPOSITIONS OF THE CO-OCCURRING PAGRUS AURATUS (SPARIDAE) AND PSEUDOCARANX GEORGIANUS (CARANGIDAE) ............................................................................................................................................................ 57 8.1.1 Influence of body size and season on the dietary composition of Pagrus auratus ....................................... 57 8.1.2 Influence of body size and season on the dietary composition of Pseudocaranx georgianus ...................... 58 8.1.3 Regional comparisons of dietary compositions of Pagrus auratus and Pseudocaranx georgianus ............. 59 8.1.4 Comparisons between the dietary composition of Pagrus auratus and Pseudocaranx georgianus on the lower west coast ...................................................................................................................................................... 60 5 8.2 AN INNOVATIVE STATISTICAL APPROACH TO CONSTRUCTING A READILY COMPREHENSIBLE FOOD WEB FOR A DEMERSAL FISH COMMUNITY ............................................................................................................. 61 8.2.1 Relationships between predator guilds and prey taxa ................................................................................... 61 8.2.2 Food webs, including identification and characteristics of prey guilds .......................................................... 62 9. BENEFITS AND ADOPTION............................................................................................................