Trophic Relationships among Lean and Siscowet Lake Trout in Lake Superior

Lake trout Salvelinus namaycush occur in several different morphotypes in Lake Superior, including nearshore “leans” and deepwater “siscowets.” Siscowets, which are highly abundant and commercially undesirable, have been hypothesized to constrain populations of the more desirable lean lake trout through competition for prey. The stable isotope ratios (δ¹³C and δ¹⁵N) of leans and siscowets differed in several regions of U.S. waters, implying differences in both production base and trophic level. Spatial differences were partly a result of large-scale variation in food web structure, as stable isotope analysis of fishes in the genus Coregonus revealed different site-to-site patterns. In western Lake Superior, δ¹³C was similar among small leans and siscowets (implying a common production base) but diverged as the fish grew larger. The value of δ¹⁵N was greater for siscowets than for leans at all sizes, further implying that siscowets feed on a different prey assemblage than leans and are preying on another predatory fish, burbot Lota lota. A bioenergetics-based stable isotope model showed that siscowets in western Lake Superior may rely on nearshore prey for up to 25% of their production until they reach relatively large sizes. The large biomass of siscowets thus may exert strong predation pressure on nearshore communities in some regions. Evidence from central U.S. waters of Lake Superior, however, implies that siscowet predation on nearshore prey has not had a direct negative effect on lean lake trout stocks.