Spatiotemporal patterns in trophic niche overlap among five salmonines in Lake Michigan, USA

Abstract

Native lake trout (Salvelinus namaycush) and introduced Chinook salmon (Oncorhynchus tshawytscha), coho salmon (Oncorhynchus kisutch), steelhead (Oncorhynchus mykiss), and brown trout (Salmo trutta) are major predators in Lake Michigan’s complex ecosystem and collectively support a valuable recreational fishery, but declines in their primary prey, alewife (Alosa pseudoharengus), have raised ecological and management concerns about competition and prey allocation. We applied niche overlap analysis to evaluate competition among salmonine predators during rapid forage base change in Lake Michigan. δ13C and δ15N stable isotope ratios indicated that lake trout had a unique trophic niche from inclusion of offshore and benthic prey, with <29% lake-wide niche overlap with Chinook salmon, coho salmon, and steelhead. Brown trout had moderate overlap with other species (45%–91%), while Chinook salmon, coho salmon, and steelhead had high overlap (71%–98%). Regional differences in isotopic signatures highlighted the potential importance of subsystem differences in fish diets in large aquatic systems. The uniqueness of the lake trout niche, and broadness of brown trout and steelhead niches, suggest these species may be resilient to forage base changes. This study further demonstrates how niche overlap analysis can be applied to tease apart competitive interactions and their response to ecosystem change.