Trophic ecology and mercury bioaccumulation among Lake Superior lake charr (Salvelinus namaycush) ecotypes.

The Lake Superior lake charr population consists of siscowet, lean, humper and redfin ecological morphotypes or ecotypes. In addition to morphological and physiological differences, these fish occupy different habitats in Lake Superior and partition various invertebrate and fish prey resources. In this study, we measured the stable isotopes of carbon (δC) and nitrogen (δN) and mercury (Hg) concentrations among these ecotypes collected from the waters surrounding Lake Superior's Isle Royale. Average Hg concentrations were highest for redfins (1386 ng/g dry wt) followed by siscowets (826 ng/g), humpers (799 ng/g) and leans (519 ng/g) and differed significantly among the ecotypes (p < 0.05). This pattern in Hg concentrations was generally consistent following corrections for size differences among ecotypes but with length corrected Hg concentrations for humpers (1185 ng/g) being higher than redfins (1075 ng/g). δC and δN results demonstrated a high degree of overlap among ecotypes suggesting that none occupy a unique niche in the waters surrounding Isle Royale. Modeling efforts indicated that differences in growth efficiencies among ecotypes likely contribute to the observed differences in Hg bioaccumulation. Specifically, differences with respect to the extent of insectivorous foraging in the diet likely regulate growth efficiencies and the extent of Hg bioaccumulation realized among ecotypes.