Polymorphism in lake trout in G reat B ear L ake: intra‐lake morphological diversification at two spatial scales

G reat B ear L ake is the most northerly lake of its size and provides unique opportunities for intraspecific diversification. Despite increasing attention to intraspecific polymorphism, several knowledge gaps remain (e.g. determining the extent of intraspecific diversification in large relatively pristine lakes and at which spatial scale it can occur). We focused on geographical patterns of morphological differentiation within lake trout ( S alvelinus namaycush ) to describe two levels of intralake diversification in G reat B ear L ake. We used a combination of geometric and traditional linear measurements to quantify differences in body shape, head shape, and fin and body lengths among 910 adult lake trout from the five distinct arms of G reat B ear L ake. Although head and fin linear measurements discriminated the three common morphotypes at the whole‐lake level, inter‐arm variation in body shape was observed within each morphotype. A comparison of genetic and morphological distance matrices revealed the lack of an association between the two sets of data, although both comparisons revealed an association in the inter‐arm variation patterns among morphotypes, suggesting a phenotypically plastic response to distinct environments. The whole‐lake and inter‐arm morphological variation observed within lake trout demonstrates the importance of considering scale, especially across large lakes that exhibit marked complexity and a variety of freshwater habitats. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society , 2014, 114 , 109–125.