Spatio-Temporal Patterns of Foraging Success for Fishes in an Illinois Stream

Abstract

Stomach contents of 10 fish species were examined to evaluate several scales of spatio-temporal variation in foraging success within an Illinois stream. Most species fed primarily on aquatic or terrestrial invertebrates, but one species (Pimephales notatus) specialized on fine detritus. Fish ate a wide variety of invertebrate taxa, and diet overlap based on prey taxa was extensive among fish species and age classes. Foraging success, measured by number and size of prey eaten, varied among seasons, years, substrate composition and amounts of woody debris. However, no factor influenced foraging success of all species, and no two species responded identically to all factors. Fish generally fed most successfully in June, and least successfully in October. Some fish ate more or larger prey in a study area with gravel-cobble substrates than in a silt-sand area, and many ate more or larger prey in sites with woody debris than in sites from which debris was removed. In addition, aquatic invertebrates increased in abundance in silt-sand substrates when fish were excluded, but did not increase in gravel-cobble substrates, suggesting a greater potential for resource depression in the silt-sand area. Stream fishes apparently face continuous variation in food availability at several temporal and spatial scales. Because changes in availability are often unpredictable and influence each species differently, the competitive environment of stream fishes may be in a state of constant flux. Anthropogenic modifications of streams may create conditions that favor fishes such as certain cyprinids which have highly opportunistic feeding habits. INTRODUCTION Successful foraging is essential to reproduction of most animals; thus natural selection should favor individuals which are most efficient at obtaining food. Efficiency in foraging probably evolves in response to temporal and spatial variation in foraging success, which can result from inherent fluctuations in resource availability or from resource depression (sensu Charnov et al., 1976) due to coexploiters. Resource depression may, in turn, generate strong competitive interactions and lead to the evolution of food partitioning. Among vertebrates, selective advantages of rapid growth and efficient foraging are perhaps best known for fishes. For example, rapid growth of fish to a relatively large size may be adaptive through increased gamete production (Bagenal, 1978) or immunity from predation (Werner et al., 1983a) or success in dominance disputes. Thus, spatio-temporal patterns of' fish foraging success may reveal information that is useful in understanding their population dynamics. Aquatic or terrestrial invertebrates are the primary food resource of most fishes in small streams of the midwestern United States, whereas detritus, algae and other fish are less commonly consumed (Pflieger, 1975; Smith, 1979; Trautman, 1981). Although diets of most fish species are dominated by only one or two of these food resources, opportunistic feeding is widespread, and seasonal shifts in diet composition are common (Starrett, 1950; Whitaker, 1977). Furthermore, foraging rates often exhibit strong seasonal patterns (Mathur, 1977; Allan, 1981; Angermeier, 1982), which are probably induced by cycles of insect emergence and recruitment (Hynes, 1961; Resh, 1977). In addition to temporal variation, fish foraging patterns should also reflect spatial dynamics of invertebrate availability. For example, production and abundance of stream inverte'Present address: Virginia Cooperative Fishery Research Unit, Department of Fisheries and Wildlife Sciences, Virginia Polytechnic Institute and State University, Blacksburg 24061.