Ontogenetic dietary shifts of largemouth bass do not increase trophic position in a shallow eutrophic lake in Japan

Abstract

The trophic positions of top predators can provide useful information for estimating the length of food chains and for assessing the impacts of invasive species or the bioaccumulation of harmful compounds. In aquatic ecosystems, large carnivorous fishes may change their trophic positions ontogenetically. We used stable carbon and nitrogen isotope ratios to test the hypothesis that the trophic positions of largemouth bass, top predators in Lake Izunuma, Japan, would increase with total length (TL) as a result of ontogenetic shifts in diet. Results from a stable isotope analysis in R mixing model indicated that largemouth bass were a major food source for largemouth bass ≥200 mm TL. However, the trophic positions of largemouth bass did not increase with the dietary shift from zooplanktivory to piscivory, and actually decreased when larger fish shifted towards feeding on red swamp crayfish. In Lake Izunuma, the trophic positions of small omnivorous fishes and red swamp crayfish were not higher than those of zooplankton. This explains why the largest largemouth bass occupied lower trophic positions than smaller individuals. Our results suggest that the body size of carnivorous fishes should be taken into consideration when using their trophic positions as top predators to evaluate aquatic systems.