Abstract
Estuaries are highly productive and often urban ecosystems. It is of great interest to managers and ecologists to understand the trophic linkages within a system. In this study, the traditional stomach content analysis approach was used to identify predator-prey relationships. This was complemented with biochemical methods to elucidate the pathway of energy transfer from primary producers into three abundant and important fish species with different feeding regimes, i.e. Acanthopagrus butcheri (omnivore), Leptatherina wallacei (pelagic feeder) and Pseudogobius olorum (benthivore), in a permanently-open (Swan-Canning) and a seasonally-closed (Wilson Inlet) estuary. Three complementary, quantitative approaches are being 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. Size-related changed in dietary composition were evident for each of the three species. Stable isotope ratios of N15/N14 suggested that three trophic levels exist in each system, while the C13/C12 ratio enabled differentiation between food chains based on detrital material or plankton. In the Swan-Canning estuary, 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 intra-specific resource partitioning occurs among the three fish species and between estuaries, and different organic matter sources support both pelagic and benthic food chains. The resultant data can be used in quantitative ecosystem modelling.