Food resource partitioning among three key fishery species in the Walpole-Nornalup Estuary

Fish dietary studies are crucial for enhancing understanding of the food web dynamics and trophic structure of ecosystems which is integral for conservation purposes. The overarching aim of this study was to assess the extent and nature of any trophic resource partitioning among three co-familial (Sparidae) fishery species (Black bream Acanthopagrus butcheri, Tarwhine Rhabdosargus sarba and Pink snapper Chrysophrys auratus) within an area and time of year in which they all co-occur in the Walpole-Nornalup Estuary, south-western Australia. A total of 227 fish gut samples, 186 of which contained food, were collected across three different size classes (small, 0-150 mm; medium, 150-250 mm; large, >250 mm total length) of the above species and examined for gut content composition. Muscle samples from a subset of those fish were also analysed for their stable isotope (δ13C and δ15N) composition to understand their primary energy sources and relative trophic feeding levels. Overall, dietary composition differences were small to moderate. The largest species differences occurred between the omnivorous and estuarine A. butcheri and the carnivorous and marine C. auratus, and significant ontogenetic shifts were identified only for R. sarba and to a lesser extent A. butcheri, with both species consuming more plant material with increasing size. As expected, the primary energy sources of A. butcheri were more freshwater/terrestrially-derived, while those of C. auratus had the greatest marine influences, but unexpectedly, small-medium A. butcheri had a notably higher trophic level than any other species or size class. The stable isotope results suggested a considerable amount of trophic overlap between A. butcheri and R. sarba. With ongoing climate change effects in south-western Australia, including further salinisation of estuaries and increased abundance of marine species, trophic competition pressures (particularly between A. butcheri and R. sarba) are expected to increase in the Walpole-Nornalup Estuary. Given the subsequent potential impacts on the productivity of these key fishery species, it is suggested that the diets, as well as the abundance and growth of these Sparidae populations, are monitored to help ensure a sustainable recreational fishing experience in the Walpole-Nornalup into the future