Foraging Efficiency and Body Size: A Study of Optimal Diet and Habitat Use by Bluegills

A foraging model was developed to predict the optimal diet breadth and maximum energetic intake of a given—sized fish foraging in each of three aquatic habitats: the open water, vegetation, and bare sediments. Model parameters of prey encounter rates and prey handling times were quantified as functions of fish size, prey density, and prey size through a series of laboratory feeding experiments using the bluegill sunfish (Lepomis macrochirus). Results of these experiments show both searching ability and prey handling efficiency to increase with increasing fish size. Predictions of prey size selection and optimal habitat use based upon maximizing energetic gain were then examined in a small Michigan lake for three size classes of bluegills. Bluegills > 100 mm standard length were highly size selective in their feeding and their diets closely matched predictions of an optimal diet model. From two estimates of relative prey visibilities I show that these fish selected larger prey items than would be predicted if prey were consumed “as encountered.” Habitat use of large bluegills was also shown to maximize foraging return as fish switched from utilizing vegetation—living prey to utilizing open—water zooplankton as relative foraging profitabilities in the two habitats changed across the summer. Bluegills <100 mm standard length were restricted in their habitat use, remaining in or near the vegetation despite demonstrated increases in foraging return available in the open—water habitat. Size—related predation risk apparently accounts for differences in habitat use between bluegill size classes, and its consequences for intra— and interspecific competition between size classes are discussed.