WAMSI Node 4.3.1b - Determination of the diets of Snapper and Silver Trevally and construction of a food web for the demersal fish community in South-western Australia

Brief description

From June 2009, for two years Snapper (Chrysophrys auratus) and Silver Trevally (Pseudocaranx georgianius) stomachs were sampled for their contents to create food webs. An understanding of the dietary composition of these two species is crucial for developing a food web involving the main demersal fish species found along the coast of south-western Australia. The production of ecosystem models using these food webs will facilitate the development of effective management strategies to sustain these species within the context of EBFM. Currently, there is little known of the dietary composition of these two species. Knowledge of these diets during each life cycle stage and each season will provide, in conjunction with data for other demersal fish species, an understanding of how food resources are partitioned among and within the different species and life stages. By including these data with large existing data sets for approximately 40 other demersal fish species from the lower south west coast, a holistic food web can be constructed. Such a food web will be the first of its kind in Western Australia and will provide an invaluable tool in fisheries management and evaluation. Samples were collected seasonally over two years from inshore waters ranging from Kalbarri to Esperance.

Lineage

Statement: Fish are being collected through line fishing, trawling, and spear fishing and from recreational fishers. Fish stomachs are being cut open and their contents identified to the lowest taxonomic level possible. Attention is being placed on obtaining representatives of all size classes and each season for each species over a two-year period. Specifically, percentage volumetric dietary data for Pagrus auratus were collected on both the mid west (Kalbarri) and the lower west coasts (Perth metro region), and for Pseudocaranx georgianus on the lower west and south coasts (Esperance). The extent to which the diets of these two species change with respect to increasing body size, season and region were then elucidated. Furthermore, the extent to which the two species compete for resources when the co-occur on the lower west coast was examined. The data were then added to an existing Microsoft access data base, 'Guts', along with other dietary data collected by the Centre for Fish and Fisheries. Variables in the database include: - Project I.D. (Investigator, start date, finish date, data custodian) - Funding bodies - Amounts (c. 16,000) - Trip I.D. (1649) - Dietician (name) - Species - Fish number - Total length - Fish ID - Total weight - Sex - Gonad stage - Gonad weight - Gut fullness - Gut portion - Contact details - *Prey I.D.* (this category includes another c. 500 variables measuring the prey species ingested, this is the real ‘data’ so to speak) - Volume of prey - Wet weight - Dry weight - Method code (description of fishing gear) - Predator type (Species, genus and family) - Site codes (117 different sampling locations) - System codes (12 different systems) Research such that the percentage volumetric dietary data for 35 important, co-occurring species from the lower west coast of Australia were in a common format for further analysis. The dietary data for these 35 species were then used to construct a qualitative food web that linked predator groups to prey groups, displaying the relative magnitude of each link.

Notes

Credit
Margaret Platell

Credit
Bob Clarke

Credit
Norman Hall