Stable isotope analysis of western Lake Superior predatory fishes, part one: Trophic niche overlap

Characterizing food web interactions between introduced and native fish predators is important to quantify niche overlap, assess predator-prey balance, and support fisheries management. Lake Superior provides an ideal setting to investigate potential trophic overlap between native top predators, lake trout (<i>Salvelinus namaycush</i>) and walleye (<i>Sander vitreus</i>), and a diverse assemblage of introduced salmonids. Angler-caught fish were used to measure trophic position and niche overlap between predatory fishes in the western arm of Lake Superior based on carbon and nitrogen stable isotope ratios (δ¹³C, δ¹⁵N). Within a species, we did not find consistent differences in either δ¹³C or δ¹⁵N values by sex, collection year, capture location, or length. Among species, the range in δ¹⁵N values was >5 ‰, indicating that these species occupy ~2 trophic positions, with dietary reliance ranging from primarily invertebrates to primarily fish, whereas the range in δ¹³C values indicated dietary reliance on a mix of pelagic and benthic prey. Combined, the δ¹³C and δ¹⁵N values revealed siscowet lake trout as the apex predator having a distinct trophic niche, while walleye and lean lake trout were intermediate trophic position predators with overlapping isotopic niches, and introduced salmonids were the lowest trophic position predators with overlapping trophic niches. Overlapping trophic niches indicated similar resource reliance and habitat occupancy among native lake trout and walleye, as well as among introduced salmonids, but less so between native and introduced predators.