CAUSES AND CONSEQUENCES OF DIETARY SPECIALIZATION IN SURFPERCHES - PATCH CHOICE AND INTRASPECIFIC COMPETITION

The existence, causes, and consequences of dietary specialization were explored for a temperature reef fish, striped surfperch (Embiotoca lateralis). Analyses of diets of adults from Santa Cruz Island, California, revealed three dietary regimes within the same population: specialization on caprellid amphipods, specialization on gammarid amphipods, or a mixed (generalized) diet composed of both caprellids and gammarids. Individuals that had specialized on caprellids at the time of capture had obtained a greater intake of prey than did generalists or specialists on gammarids. Different diets were associated with marked differences in body size within most adult year classes; specialists on caprellids were the largest and specialists on gammarids were the smallest for a given age. Generalists were intermediate in size. Females in the different dietary categories were projected to experience different expected lifetime reproductive outputs. Fitness was calculated to be almost-equal-to 10% lower for generalists and almost-equal-to 20% lower for specialists on gammarids compared with caprellid specialists. The different diets did not arise from active choice of prey taxa, but from selective use of available foraging patches (i.e., types of benthic algae from which prey are harvested). The degree to which an individual fed from the red alga Gelidium robustum was an excellent indicator of the taxonomic composition of its diet. Gelidium was the only alga harboring substantial numbers of caprellid amphipods; more importantly, it was the most profitable foraging microhabitat. Access to Gelidium was unequal among striped surfperch due to strong interference, and only competitively dominant individuals appeared able to consistently harvest crustaceans from this microhabitat. The least aggressive individuals fed from other, less profitable patches that contained mostly gammarid amphipods. "Dietary" specialization was a passive consequence of patch selection dictated by a strong competitive hierarchy, and was not the result of active choice among available types of prey items.