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 <i>Myliobatis australis</i> and <i>Aptychotrema vincentiana</i> and the sharks <i>Heterodontus portusjacksoni</i> and <i>Squatina australis</i> in south-western Australian waters differed. <i>M. australis</i> fed predominantly on benthic invertebrates, whereas <i>A. vincentiana</i> consumed large volumes of teleosts. The durophagous <i>H. portusjacksoni</i> ingested a wide variety of prey, including gastropods, cephalopods, bivalves, echinoderms and teleosts, whereas <i>S. australis</i> 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 <i>H. portusjacksoni</i> changed abruptly to larger, harder-bodied prey at ∼400-mm total length, whereas that of <i>S. australis</i> 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.