Sexual maturation in stream residents from migratory populations of brook trout inhabiting Lake Superior tributaries

There is mounting evidence that populations of brook trout (Salvelinus fontinalis) inhabiting bays and tributaries along the north shore of Lake Superior exhibit partial migration. In this system some fish originate in tributaries and move into the lake for much of the year, grow large and migrate back into tributaries to spawn, whilst other smaller fish reside in tributaries and grow slowly. This study determined whether the stream dwelling brook trout reach sexual maturity in the stream habitats, a criterion needed to distinguish partial migration from other forms of migration that could maintain divergent forms and life histories of brook trout. Maturational development was determined for fish collected in July and August. Sampling these populations during fall spawning was not permitted due to conservation concerns. Male and females sampled from tributaries displayed δ13C signatures and growth histories characteristic of stream residency. Assessment of maturational development suggested that 33% of females would have been expected to spawn in the fall of that year. Our findings demonstrate that a portion of fish residing in tributaries reach sexual maturity without adopting summer residency in the lake providing evidence for partial migration over other forms of migration. Improved understanding of the variation in migratory behaviour in these brook trout populations will contribute to their conservation and management in the face of lake-wide declines of the migrant fish.