Little is known about the phenotypic diversity of lake trout Salvelinus namaycush in large North American lakes outside the Laurentian Great Lakes. This study tested the hypothesis that phenotypic diversity in Great Slave Lake, Northwest Territories, Canada, is associated with water depth, as was observed during similar studies of lake trout in Lake Superior. We describe the association of body size with color, buoyancy, and morphology; compare these phenotypic traits among depth strata; and establish whether lake trout phenotypes occur as discrete groups. Phenotypic diversity increased among fish longer than 43 cm standard length. In water less than 50 m deep, large lake trout (!43 cm) were light in color, buoyantly heavy, and streamlined and possessed short pectoral fins. In water deeper than 50 m, large lake trout were dark in color, buoyantly light, and deep bodied (less streamlined) and had long pectoral fins. Without assigning descriptions to individuals before the analysis, we identified two phenotypic groups. These groups represented nondiscrete phenotypes; lake trout of intermediate shape had intermediate fin lengths, capture depths, and buoyancies. The phenotypic patterns observed in Great Slave Lake were similar to the lean and siscowet forms that currently exist in Lake Superior. Deepwater forms, previously believed to be endemic to the Great Lakes, are more geographically widespread and may represent diversity typical of this species in large, deep lakes. We suggest that the lake trout phenotype associated with deep water (buoyantly light, long pectoral fins) is adaptive for daily vertical migrations.

Resource polymorphisms are widely observed in fishes; however, ontogenetic contributions to morphological and ecological differences are poorly understood. This study examined whether ontogenetic changes in niche partitioning could explain morphological and buoyancy differences between lake trout ( Salvelinus namaycush ) morphotypes in Great Slave Lake (Northwest Territories, Canada). Morphometric analysis, buoyancy, capture depth, diet, and stable isotope data were used in concert to determine whether (i) differences occur in small, as well as large, lake trout, (ii) ontogenetic changes in morphology and buoyancy correlate with shifts in depth or diet, and (iii) a subset of small trout, putatively identified as “humpers”, are distinct from other morphotypes. Ontogenetic changes in lake trout morphology were associated with an ecological shift between benthic and pelagic feeding. Resource partitioning between lean and siscowet-like trout occurred within benthic (small trout) and pelagic (large trout) habitats. The humper subset did not differ from small siscowet-like trout. By combining multiple methods and an ontogenetic perspective, our study provides novel perspectives on resource polymorphisms in large, deep lakes and on existing interpretations of stable isotope data from large lakes in general.