Life history tactics in cohorts of a partial migratory brown trout (Salmo trutta L.) population

We monitored temporal changes in body size for three cohorts of a partial migratory, lake-migrating brown trout population.Wetested if body mass differed between nonmigratory males, migrants, and other members of the cohort (females and immaturemales).We hypothesized that large-sized individuals would mature as nonmigratory males or migrate at younger ages than smallsizedindividuals. As previous studies have shown that female fecundity is influenced by body size and that more trout from thedownstream section (D) of the stream migrated than from the upstream section (U), we hypothesized that there would be a greaterproportion of mature males in D than U.We found that body size of males that reproduced was similar to migrants that migratedthe subsequent spring and larger than other cohort members. Reproducing males had a larger body size than equal-aged malesthat delayed reproduction. Similarly, individuals that migrated had a larger body size than equal-aged individuals that migratedsubsequently. The proportion of mature males was greater in D than in U. The fact that body size differentiation occurred late inontogeny and that age of maturation and migration varied within cohorts suggests that the decision to mature or migrate mightbe conditionally dependent