Investigating the phenology of seaward migration of juvenile brown trout (Salmo trutta) in two European populations

Recent evidence supports the existence of a downstream autumn-migratory phenotype in juvenile anadromous brown trout (Salmo trutta), however the precise timing, extent and ecological significance of such behaviour remains ambiguous. We investigated the phenology of downstream migration of wild juvenile trout using passive integrated transponder (PIT) telemetry over an eight-month period in two European rivers; the River Deerness, north-east England, and the River Villestrup, Denmark. The incidence of autumn-winter seaward migration was greater in the Deerness than the Villestrup, with at least 46% of migrating juveniles detected prior to spring smoltification in the Deerness. Timing of migration was strongly regulated by factors associated with river discharge in both systems. While autumn and spring downstream migrants did not differ in size at the time of tagging in either system, evidence that spring migrants were of better condition, travelled faster (autumn: 11.0 km day(-1); spring: 24.3 km day(-1)) and were more likely to leave the Deerness suggests that autumn and spring migrant conspecifics respond to different behavioural motivations. Further investigation into the sex of autumn migrant juveniles, as well as the temporal and geographical variability in the incidence and fitness consequences of autumn emigration by juvenile trout would be beneficial.