Life-history variation among four lake trout (Salvelinus namaycush) morphs at six locations in Lake Superior

Life-history variation among four lake trout Salvelinus namaycush morphs was quantified at six geographically distant locations in Lake Superior (similar to 30 to 250 km apart), one of the largest freshwater lakes in the world (82,100 km(2)). Lake trout were sampled using standardized multi-mesh gillnets in three depth strata at six locations in Lake Superior that were known or thought to have multiple morphs. Life-history traits were estimated using length-age analysis of back-calculated growth from sagittal otolith increments. Morphs, assigned using statistical and visual assignment rules, included 122 humpers, 646 leans, 86 redfins, and 1154 siscowets. Density (CPUE) varied 11-fold among morphs, 7-fold among locations, and 3-fold among depths. Morphs seemed to fill the same ecological niche at all locations, because life-history traits related to weight (body condition, buoyancy, mean weight), age, and growth rate varied more among morphs than locations. However, abiotic and biotic variation among locations also seemed to exert control over life-history variation, because life-history traits related to length, maturity, and early life history varied more among locations than morphs. We conclude that lake trout morphs appeared to have a genetic component to their life history that was differentially expressed along environmental gradients.