Lake trout (Salvelinus namaycush) in Lake Mistassini, Quebec, were investigated to determine whether they resembled the lean and siscowet morphotypes of Lake Superior and Great Slave Lake. Lake trout caught in deep water were predicted to resemble the siscowet morphotype and to be better adapted for vertical migration (i.e., low percent buoyancy) than those caught in shallow water. The research objectives were to 1) identify groups based on shape, and 2) determine whether shape was associated with other morphological traits (fin length, buoyancy, color), ecology (habitat depth, diet), and life history (size at adulthood). Eighty-five lake trout were collected from three depth zones. At least two phenotypes exist in Lake Mistassini. A shallow-water form (< 50-m depth), identified by its streamlined shape, was dark in color and high in percent buoyancy. A deep-water form (> 50-m depth), identified by a deep anterior-body profile, was light in color and lower in percent buoyancy than the shallow-water form. Absolute buoyancies were relatively high in both forms; therefore, the deep-water form did not appear well-adapted for vertical migration. Opossum shrimp (Mysis relicta) were more frequent and abundant in stomachs of deep-bodied trout. All deep-bodied trout (minimum 32-cm SL) had reached adulthood, whereas immature streamlined individuals were as long as 49 cm in SL. The deep-bodied form resembled humper lake trout, a lesser-known third morphotype from Lake Superior. A humper-like morphotype in Lake Mistassini, and the apparent absence of a siscowet-like morphotype, challenges the previously-held hypothesis that humpers resulted from an introgression of leans and siscowets.
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