Migratory and resident forms of salmonid fishes, including brook trout (Salvelinus fontinalis), coexist in many river systems, but little is known about the ecological basis of these important variations in life history. This thesis elucidates the bioenergetic basis of anadromy (migration from freshwater spawning sites to the sea) in populations of brook trout. By focusing on the early stages, I provide support for the idea that variation in energy allocation leads to the adoption of migratory or resident strategies. More specifically, I demonstrate that juvenile anadromous brook trout, in the year(s) prior to migration, exhibit higher food consumption rates but lower growth efficiencies compared to residents indicating that they have higher metabolic costs. Higher metabolic costs of migratory fish are associated with the exploitation of higher current velocity habitats that provide more food but at a higher cost. This conclusion is supported by differences in delta13C (migrants have more negative delta13 C compared to residents), morphology (migrants are more streamlined than residents), and field observations (brook trout inhabiting streams with both forms exploit a wider range of habitats than those inhabiting 'pure' resident streams). Brook trout thus appear to migrate in response to energetic limitations in their local habitat. The estuary to which they migrate has better feeding opportunities, as the prey spectrum at sea is both larger and wider than that found in freshwater. This permits them to undergo diet shifts to larger prey, reducing their foraging costs, and thus most likely contributes to the trout's rapid growth rates experienced at sea. Importantly, the results of this thesis indicate that the persistence of migrant and resident strategies in the same system suggest a trade-off between local adaptability and the ability to exploit large-scale environmental heterogeneity.

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