Abstract
This study aims to identify the key primary producers that support abundant fish species with different feeding regimes, i.e. Acanthopagrus butcheri (omnivore), Leptatherina wallacei (pelagic feeder) and Pseudogobius olorum (benthivore), in the Swan-Canning estuary. Although there is extensive knowledge of stomach content composition for many fish species in this highly important urban estuarine system, there is poor understanding of the energy sources and trophic interactions.
Three complementary, quantitative approaches were adopted. 1) Analysis of the dietary items consumed by the fish and variation in diet with body size, habitat and season. (2) Determination of stable isotope ratios of C13/C12 and N15/N14 for fish and their prey. (3) Determination of fatty acid biomarkers of fish and dietary items. Stomach content data indicated that the overall diets differed significantly between species.
For each of the three species, ontogenetic changes were apparent and seasonal differences were most pronounced in spring. Stable isotope ratios of N15/N14 suggested that three trophic levels exist in the system, while the C13/C12 ratio made a differentiation possible between food chains based on detrital material or plankton. Fatty acid data revealed trophic markers for dinoflagellates in A. butcheri and L. wallacei, while trophic markers characteristic for diatoms were present in P. olorum. These findings suggest that inter- and intraspecific resource partitioning occurs among the three fish species and different organic matter sources support both pelagic and benthic food chains in this estuary. The resultant reliable data can be used in ecosystem modelling.