This study is aimed at identifying the key primary producers, which support abundant fish species with different feeding modes, in a permanently-open (Swan-Canning) and a seasonally-closed (Wilson Inlet) estuary in south-western Australia.
The species studied were the sparid Acanthopagrus butcheri (omnivore), the atherinid Leptatherina wallacei (pelagic feeder) and the gobiid Pseudogobius olorum (benthivore).
Three complementary, quantitative approaches were used:
1) Stomach content analyses to determine the dietary compositions of the above three species and how they vary with body-size and season;
2) Stable isotope ratios of C13/C12 and N15/N14 for fish and their prey; and
3) Fatty acid biomarkers in fish and dietary items.
Stomach content data demonstrated that size-related changes in diet were evident and that dietary compositions of each species differed among estuaries. Stable isotope ratios of N15/N14 showed that the food web in each estuary contained three trophic levels, while the C13/C12 ratio enabled differentiation of the food chains based on detrital material and plankton. Fatty acid data revealed trophic markers for dinoflagellates in A. butcheri and L. wallacei and for diatoms in P. olorum.
These findings suggest that:
1) the food resources are partitioned within and among species,
2) the dietary compositions differ between estuaries and
3) different sources of organic material support pelagic and benthic food chains
