Foraging and Habitat Use by the Bluegill Sunfish

Abstract

Patterns of resource utilization by different species have provided fundamental inferences for the theory of the ecological niche and community organization (e.g., Schoener 1974). Consequently, how an animal chooses to use different resources, and how this use is related to the profitability or net energy return in various habitats are critical questions bearing on the mechanisms underlying the theory-e.g., the specific mechanism of competition between species. Our study interprets habitat utilization and switching behavior of the bluegill sunfish (Lepomis macrochirus) from laboratory measures of foraging efficiency or energetic return in different habitats. Learning or training bias appear to be an important determinant of individual specialization and habitat switching. We suggest that theoretical and empirical studies of foraging behavior that concern the utilization of different habitat and prey types must incorporate the action of learning or experience. We measured foraging rates of the bluegill in two common habitats: the open water column and sediments. Aquaria were constructed to mimic these habitats, and several combinations of prey size and density were presented to the fish. We measured search and handling times for the bluegill in each treatment combination and related search time to prey size and density using stepwise multiple linear regression. Foraging rates increased in both habitats with experience, in some cases as much as fourfold over the course of eight trials. Both prey size and density also strongly influenced foraging rates. These relationships enable us to estimate return rates for a bluegill foraging in a natural habitat whose resource abundance and size-distribution are known. Subsequently, we introduced a population of 225 bluegills into a pond (29 m in diameter, 1.8 m deep) consisting primarily of open water and exposed sediments. Both invertebrate resources and the fish were sampled at oneto two-week intervals. Habitat use of the fish was readily determined from stomach contents because three prey species obligate to specific habitats accounted for >80% of the diet. Following their introduction, the fish exhibited a rapid switch from feeding in the small amount of vegetation habitat in the pond to feeding in either the open water or sediment habitats. The majority of fish specialized on a single habitat (Figure 1). Generalists were less successful foragers than specialists; those individuals with a mixed diet (i.e., 80% of diet) on one habitat or the other.