Distinguishing between juvenile anadromous and resident brook trout (Salvelinus fontinalis) using morphology

Phenotypic variation linked to habitat use has been observed in fish, both between and within species. In many river systems, migratory and resident forms of salmonids coexist, including anadromous (migrant) and resident brook trout, Salvelinus fontinalis. In such populations, juvenile anadromous (migrant) brook trout, prior to migration, inhabit regions of higher current velocity than residents. Because it is more costly to occupy fast currents than slow currents, differences in morphology minimizing the effects of drag were expected between the two forms. As predicted, migrant brook trout were found to be more streamlined (narrower and shallower bodies) than resident brook trout, and these differences persisted into the marine life of the fish. Migrants also exhibited shorter pectoral fins, which facilitate pelagic swimming, indicating that migrants, prior to their migration to the sea, possess the appropriate morphology for swimming in open water habitats. The reported differences between migrants and residents were powerful enough to derive discriminant functions, using only five of the seven measured traits, allowing for accurate classification of brook trout as either migrants or residents with an overall correct classification rate of 87%. Importantly, this study contributes to the notion that a link exists between morphology, habitat use, metabolic costs and life-history strategies.