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. We tested if body mass differed between nonmigratory males, migrants, and other members of the cohort (females and immature males). We hypothesized that large-sized individuals would mature as nonmigratory males or migrate at younger ages than small-sized individuals. As previous studies have shown that female fecundity is influenced by body size and that more trout from the downstream section (D) of the stream migrated than from the upstream section (U), we hypothesized that there would be a greater proportion of mature males in D than U. We found that body size of males that reproduced was similar to migrants that migrated the subsequent spring and larger than other cohort members. Reproducing males had a larger body size than equal-aged males that delayed reproduction. Similarly, individuals that migrated had a larger body size than equal-aged individuals that migrated subsequently. The proportion of mature males was greater in D than in U. The fact that body size differentiation occurred late in ontogeny and that age of maturation and migration varied within cohorts suggests that the decision to mature or migrate might be conditionally dependent. 1. Introduction For salmonids, migration is normally associated with movements between feeding, refuge, and reproductive areas [1]. By exploiting better feeding habitats (i.e., the sea or a lake), migration normally enables individuals to attain high growth rates, size-at-age, and fecundity [2]. There are also drawbacks to migration as the migratory journey itself often is associated with delayed reproduction, high mortality, and energy and time losses [3, 4]. Females should predominate among migrants [5–8] because high fecundity is more closely associated with large body size in females than in males [9–11]. Females may also benefit from large body size as they may be more attractive to males and better able to defend and obtain high-quality spawning nests than small-sized females [12, 13]. Thorpe’s [14] studies of Atlantic salmon suggest that migratory and resident tactics can be viewed as two conflicting developmental processes, where individuals that optimize for residency (i.e., delay or exclude migration) focus their energy on maturation, whereas migrants focus their energy on somatic growth, with the potential of obtaining high payoffs by delaying reproduction. Maturation should be the first priority for all individuals, occurring if a series of genetically determined threshold