Stable isotope analysis of western Lake Superior predatory fishes, part two: prey source contributions

Niche partitioning of time, space, or resources facilitates the coexistence of competitor species. The Lake Superior food web includes several piscivores that are important to fisheries, including native lake trout (Salvelinus namaycush, both lean and siscowet morphotypes) and walleye (Sander vitreus), and introduced Chinook salmon (Oncorhynchus tshawytscha), coho salmon (Oncorhynchus kisutch), pink salmon (Oncorhynchus gorbuscha), brown trout (Salmo trutta), and rainbow trout (Oncorhynchus mykiss). Here we quantify specific prey source contributions to these predators in the western arm of Lake Superior using dual stable isotope mixing models based on carbon and nitrogen stable isotope ratios. For each species, three mixing models were constructed to examine predator size effect: one with all fish sampled, one with the 10 smallest fish sampled, and one with the 10 largest fish sampled. We found that native and introduced species had distinct diets from one another; siscowet lake trout primarily relied on burbot (Lota lota) and invasive rainbow smelt (Osmerus mordax), native lean lake trout and walleye both relied on invasive rainbow smelt, and introduced salmonids relied on native invertebrates Mysis diluviana and Diporeia spp. Additionally, siscowet lake trout, lean lake trout, Chinook salmon, and coho salmon exhibited size-based shifts in prey contributions. The low piscivory in introduced salmonids and high reliance of native predators on invasive rainbow smelt is likely due to low abundance of cisco (Coregonus artedi). Our study reveals complex, potentially adaptive responses of both native and introduced nearshore predators to both native and invasive prey resources in Lake Superior.