Variation in surfperch diets between allopatry and sympatry: circumstantial evidence for competition.

Characteristics of the diet of black surfperch, Embiotoca jacksoni (Embiotocidae), were compared between islands where it occurs with and without a congener, the striped surfperch, Embiotoca lateralis. Several hypotheses were considered to explain observed dietary differences between allopatric and sympatric populations of black surfperch. Some differences could be readily attributed to interisland differences in the availability of prey items. Remaining dietary differences were examined in view of optimal foraging and competition hypotheses. Observations did not conform to predictions of optimal foraging theory, but did fit predictions regarding interspecific competition. There was a shift in the types and sizes of gammarid amphipods consumed between allpatric and sympatric populations of black surfperch. Gammarid amphipods are the most important prey taxon to both Embiotoca species, comprising more than 70% by number of all prey items in the diet. Further, allopatric E. jacksoni displayed patterns of selectivity for prey of various sizes that were qualitatively different from those displayed by black surfperch sympatric with E. lateralis. The prey size selectivity of allopatric black surfperch was qualitatively very similar to that found for E. lateralis. The differences in black surfperch diet that were not explained by differences in resource bases among islands were in the direction predicted by competition theory. The dietary breadth of E. jacksoni was also broader in allopatry than in sympatry. As a result of these dietary differences, there was an apparant divergence in prey use by co-occurring Embiotoca species.