Trophic Variation in Coastal Plain Stream Predatory Fishes

Unique morphologies along with associated differences in habitat use and feeding behavior can result in fish at the top of piscine food chains differing in trophic level. Broad size ranges inherent within large species provide opportunity for size-related trophic shifts. Such relationships between size and trophic level can be species specific. Furthermore, individual-based diet variation can bring about differences among similar-sized organisms. A challenge to aquatic ecologists is deciphering these patterns of trophic change both between and within species. Stable isotope analysis has emerged as a powerful tool for evaluating such patterns. Employing stable isotope analyses, we assessed trophic differentiation in 4 large predatory fish species from a coastal-plain stream. We established the trophic base by including 2 herbivorous invertebrates in the analysis and identified a trophic hierarchy among species, with 2 specialized, generally open-water piscivores, Lepisosteus osseus (Longnose Gar) and Micropterus salmoides (Largemouth Bass), occupying the highest trophic position. The largest-bodied and generally benthic-oriented species, Ictalurus punctatus (Channel Catfish), occupied the lowest trophic level among the fishes studied. Trophic position of Largemouth Bass and Longnose Gar increased linearly and gradually with size within the broad size ranges collected. In contrast, Channel Catfish exhibited a more abrupt shift in trophic position with size and much individual variation associated with the shift. Additionally, groups of Longnose Gar had belonged to distinctly different food chains, despite coexisting in a relatively small stream when collected. Differences between the observed patterns and other published accounts indicate further evaluation of trophic patterns of these fishes among habitats is warranted.