Relationship of baseline and maximum glucocorticoid concentrations to migration propensity: a field test with wild subadult brown trout (Salmo trutta)

There is considerable variation in glucocorticoid (GC) baseline status and stress responses of individuals, yet the cause and consequence of this variation remains ambiguous. Attempts to relate GC levels to fitness and life-history trade-offs have yielded variable results. In this study, we evaluated whether baseline and poststressor GC hormone concentrations predicted migration strategy (i.e., resident or migrant) and successful seaward migration in a partially migrating population of juvenile brown trout (Salmo trutta Linnaeus, 1758). Baseline (N = 99) or poststressor (N = 102) plasma cortisol concentrations were obtained from brown trout and they were tagged with passive integrated transponder (PIT) and released in a natural Danish stream. Subsequently, fish were tracked with PIT reader systems and the stream was resampled for resident individuals. GC levels were not found to be associated with recapture of resident individuals or migration propensity to our first tracking station (S1), but increased baseline (and not poststressor) GC levels were associated with increased passage from S1 to our second tracking station, which anecdotally was an area of high predation or challenge. Our study found no evidence to suggest that cortisol regulates the migration life history in juvenile brown trout, but intermediate increases in baseline GC (and not poststressor GC) levels may favor migration performance.