Distribution and morphological variation of the slimy sculpin (Cottus cognatus) in the north central United States

Slimy sculpins (Cottus cognatus) in the north central United States are found primarily in three areas: Lake Superior and its tributaries, Lake Michigan, and small cold streams in the "Driftless Area" of southwestern Wisconsin, northeastern Iowa, and southeastern Minnesota. In the region between the Driftless Area and the Great Lakes, populations exist only in a few small streams and two small deep lakes, Trout and Big Green. Slimy sculpins from the north central United States display extensive within-population morphological variation. Males and females appear to differ in morphometry (shape) but not in meristics. Patterns of morphological variation among populations have little correspondence to patterns from other parts of the species' range, and do not match predictions based on the likely degree of reproductive isolation among populations. Some morphometric differences are evident among populations in different habitats, primarily because of the distinctiveness of slimy sculpins from Trout and Big Green lakes. Slight clinal differences exist in meristic variables from Great Lakes to Driftless Area populations, although Big Green Lake deviates from the cline. Hybridization with the mottled sculpin (C. bairdi) may contribute to morphological variation within some populations.