The propensity for animals to migrate can vary markedly among species, populations within species, and individuals within populations. Many taxa displaying variation in migratory behaviour and life history are attracting conservation concern as migratory populations decline in abundance. I developed a conceptual framework for delineating the nature of the broad phenotypic variation observed among and within migratory populations and apply this to Lake Superior brook trout (Salvelinus fontinalis). My literature review of variation in the migratory behaviour of fishes revealed that subsets of the predictions outlined in my framework have been tested for most study systems, but few systems had tested all five predictions. My field studies of Lake Superior brook trout (i) compared isotopic signatures of brook trout sampled from Lake Superior and tributary streams to demonstrate that the fish sampled from the lake and stream differ consistently in habitat use and trophic ecology, (ii) applied age and growth analyses to demonstrate that fish sampled from the lake live longer and grow faster than fish sampled from the streams, with the differences in growth apparent by the end of the first year of life, (iii) used histological measures to demonstrate that fish residing in streams, and exhibiting a stream growth history, reach sexual maturity, and (iv) provided no evidence based on growth histories to suggest that individuals change migratory behaviours later in life. Together, these field studies suggest that partial migration is the best hypothesis to describe the variation observed in the migratory behaviour of brook trout in the Nipigon Bay area of Lake Superior. Although the possibility remains that some individuals could complete their life cycle in the lake. My review and field studies demonstrate that the conceptual framework can be used to help delineate variation in the migratory behaviour exhibited by fishes whose lifetime movements can be difficult to track. / Funding was provided by the Ontario Ministry of Natural Resources through the Canada-Ontario Agreement Respecting the Great Lakes Basin Ecosystem, Great Lakes Fishery Commission Fishery Research Program, a Natural Science and Engineering Research Council (NSERC) Postgraduate Scholarship, and a NSERC Discovery Grant.