Sympatric Polymorphism in Lake Trout: The Coexistence of Multiple Shallow-Water Morphotypes in Great Bear Lake

Polymorphism in northern fishes is common, but the extent to which polymorphism occurs in Lake Trout Salvelinus namaycush, a species generally associated with low intraspecific variation, is not well known. This study examined the polymorphism of Lake Trout inhabiting the shallow-water zones (30m) of Great Bear Lake, Northwest Territories. We combined an analysis of classical morphometric and traditional linear measures with shape analysis (geometric morphometrics) to quantify morphological differences in body shape, head shape, and fin and body length measurements among 558 adult and 55 juvenile shallow-water Lake Trout from Great Bear Lake. A UPGMA cluster analysis on adult Lake Trout distinguished three different morphs that coexist in the shallow-water habitat. The most important differences among adult morphotypes were associated with head and fin measurements, whereas body shape variation was less distinct. A fourth, albeit rarer, morph was supported by a multivariate ANOVA that indicated significant differences in head and fin characteristics among the four groups. The divergent morphologies among the shallow-water Lake Trout of Great Bear Lake are consistent with traits generally associated with feeding and swimming. In contrast to adult trout, no consistent patterns were found for juveniles, suggesting that phenotypic differences develop at a later stage. This unusual level of endemic diversity in the shallow-water habitat expands our knowledge of Lake Trout diversity beyond the predominant focus on shallow- versus deepwater forms. Received July 16, 2012; accepted December 29, 2012