Trophic ecology of juvenile lean and siscowet lake charr (Salvelinus namaycush) in Lake Superior: assessing for potential competition

We investigated the spatial overlap, diet, isotopic niche, and growth of juvenile lean and siscowet lake charr (Salvelinus namaycush) in Lake Superior to address concerns of potential competition with implications to the study of resource polymorphism. Catch data revealed the greatest levels of sympatry in waters from 40 to 60 m. Juvenile lean and siscowet diet changed ontogenetically with Mysis being the dominant prey item for the smallest lake charr but differentiating with onset of piscivory. As ecotypes increased in size, lean diets became dominated by pelagic prey, whereas siscowets had equal proportions of benthic and pelagic prey. Isotopic niche overlap declined between ecotypes coincident with siscowet lake charr shifting to deeper habitats around 400 mm. Lean and siscowet exhibited different growth trajectories. However, length at age-4 declined in parallel for both ecotypes with no trend in condition suggesting that lake charr growth is sensitive to prey biomass and unlikely related to competition. Our findings indicate minimal evidence of competition and support the concept that multiple sympatric ecotypes of lake charr in Lake Superior are maintained by resource polymorphism.