Does cortisol manipulation influence outmigration behaviour, survival and growth of sea trout? A field test of carryover effects in wild fish

For anadromous brown trout (Salmo trutta), the transition from life in freshwater to the marine environment is an inherently challenging and dangerous period characterized by high levels of mortality. As such, smoltification is a relevant life-history phase to examine how physiological state, in particular glucocorticoids, influences fitness-oriented endpoints such as migration timing and survival. We experimentally assessed the effect of cortisol by combining passive integrated transponder (PIT) telemetry with a physiologically relevant exogenous cortisol manipulation (i.e., intracoelomic injection) in juvenile sea trout in the Gudsø Stream in Denmark. Individual survival, migration behaviour (timing and speed), and growth were assessed for four treatment categories: control (CO), sham (SH), and low- (LW; 25 mg/kg) and high-dose (HI; 100 mg/kg) cortisol. There was no difference in the timing of migration among treatments, but trout in the HI treatment had lower survival rates to the lower station (41.6%) when compared to the CO (53.9%) and SH (52.3%) groups. After migration, the system was electroshocked again to contrast growth of trout that remained in the system. HI, LW and SH individuals recaptured in the stream had lower growth rates for length than the CO treatments; HI and LW also had significantly lower growth rates for mass than CO trout. Future monitoring of this population may demonstrate the long-term repercussions of chronic stress as trout return from the ocean and further contribute to our understanding of the relationship between organismal condition and fitness while elucidating the potential for carry-over effects (lasting effects that influence future success)