Partitioning of food resources by four abundant, co-occurring elasmobranch species: Relationships between diet and both body size and season

The present study has tested statistically the hypothesis that the diets of four abundant and co-occurring elasmobranch species differ and change with body size and season and has determined the extent of any differences, to ascertain their potential for reducing competition for food resources. Non-metric multivariate analyses of volumetric contributions of dietary categories to stomach contents demonstrated that the dietary compositions of the rays Myliobatis australis and Aptychotrema vincentiana and the sharks Heterodontus portusjacksoni and Squatina australis in south-western Australian waters differed. M. australis fed predominantly on benthic invertebrates, whereas A. vincentiana consumed large volumes of teleosts. The durophagous H. portusjacksoni ingested a wide variety of prey, including gastropods, cephalopods, bivalves, echinoderms and teleosts, whereas S. australis ingested mainly teleosts and cephalopods. The extent and pattern of change in the diet with increasing body size varied among species. For example, the diet of H. portusjacksoni changed abruptly to larger, harder-bodied prey at ∼400-mm total length, whereas that of S. australis underwent small, gradual changes with increasing body size. The diets of each species changed seasonally. Inter- and intraspecific variations in dietary composition reduce the potential for competition between and within these abundant elasmobranch species in south-western Australian waters.