Spatiotemporal Patterns in Trophic Niche Overlap Among Five Lake Michigan Salmonine Species: Inference from Stable Isotope Ratios

In Lake Michigan, lake trout, Chinook salmon, coho salmon, steelhead, and brown trout are managed via stocking to support sport fisheries and restore native fishes.  Understanding niche partitioning among salmonines after recent shifts in prey fish abundance is critical to fisheries management.  We evaluated δ¹³C and δ¹⁵N stable isotope ratios from salmonines and their prey to describe spatiotemporal patterns in niche overlap. Samples were collected from four regions of Lake Michigan during early and late summer, spanned gradients of fish size, and separated wild and stocked lake trout and Chinook salmon.  Lake trout had the most unique trophic niche, with <25% overlap with Chinook salmon, coho salmon and steelhead.  Lake trout were enriched in δ¹⁵N, indicating greater reliance on offshore benthic prey than other species.  δ¹³C was enriched for smaller fish of most species, suggesting greater reliance on nearshore energy.  δ¹³C was depleted for wild vs. stocked lake trout, suggesting wild recruitment occurs offshore.  Lake and brown trout isotopes clustered by region, which may indicate regional fidelity; isotopes from Chinook salmon and steelhead did not, possibly due to inter-regional movement.  Our study suggests competition for declining prey fish will be highest among Chinook salmon, coho salmon, brown trout, and steelhead.