Comparison of growth physiology, morphology, and smolt indicators in juvenile Lake Superior brook trout (Salvelinus fontinalis) strains in reference to life history variation

Lake Superior supports fluvial, adfluvial, and lacustrine populations of brook trout ( Salvelinus fontinalis ). Adfluvial and lacustrine populations (termed coasters) are known for their large size and are coveted by anglers; however, little is known about their migratory habits or physiology. This study examined physiology and morphology of age 1+ lacustrine, adfluvial, and fluvial strains of brook trout in a laboratory setting. All strains in the study grew; however, there were no differences in growth rate, condition, relative mass, morphology, white muscle metabolic enzymes, or gill Na+,K+-ATPase that clearly associated with putative life history strategy. Both thyroxine and triiodothyronine varied over the study period, and the fluvial (resident) strain consistently showed lower thyroid hormone levels than the three coaster strains. We conclude that the populations compared differed at the strain level, but do not show physiological or morphological variability that clearly associates with life history strategy; the exception was that populations demonstrating the coaster phenotype had increased concentrations of plasma thyroid hormones, which may be linked to growth potential or other coaster-related characteristics such as migration.