Phenotypic variation among four Lake Trout morphs at six locations in Lake Superior

ABSTRACT Objective The Lake Trout Salvelinus namaycush diversified into multiple morphs in many lakes of northern North America. Four morphs remain in Lake Superior, of many forms that arose since the most recent glaciation of North America. Our objective was to determine if Lake Trout phenotypic diversity was greater within or among four morphs at six geographically distant locations in Lake Superior. Methods Lake Trout were sampled using standardized multi-mesh gill nets in three depth strata at six locations in Lake Superior that were known to have multiple morphs and were assigned to one of four morphs by reconciling statistical (geometric morphometric analysis of head and body shape) and visual (agreement among three experts) assignment rules. The relative importance of morphs, locations, the interaction between morphs and locations, and residual error for describing variation in head and body shape and associated linear traits were compared. Results The same four morphs were present at nearly all locations. Variation in head and body shape, and associated linear characteristics (head depth, preorbital length, body depth, caudal peduncle length, and caudal peduncle depth), was greater among morphs than among locations. This finding supports a hypothesis that Lake Trout morphs were consistent in shape and related linear traits across a large spatial scale within an environmentally diverse large lake, thereby possibly reflecting genetic differences among morphs. In contrast, variation in linear dimensions describing fin lengths, maxillary length, and other linear segments of head and body length was greater among locations than among morphs. This finding suggests that expressions of these morphometric measures was influenced by location-specific abiotic and biotic conditions. Conclusions We conclude that Lake Trout morphs appeared to have a genetic component to their head and body shape and some other linear morphometric characteristics that was differentially expressed in relation to local environmental conditions.