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

Brown trout (Salmo trutta) exhibit facultative partial migration and the factors regulating spring downstream smolt migrations have been well documented in many river systems. Comparatively few studies have investigated movement outside the typical smolt migration period, but recent evidence supports the existence of an alternative autumn downstream migratory phenotype. This study is the first to provide a direct comparison of juvenile autumn downstream migration between two populations. The phenology of downstream migration of wild juvenile trout was examined using passive integrated transponder (PIT) telemetry over an eight month period in two European rivers; the River Deerness, England, and the River Villestrup, Denmark, exhibiting contrasting proximity to the marine environment. Additionally on the Deerness, indices of trout population dynamics were monitored using repeat sample and mark-recapture techniques. Site fidelity was strong (83%) for Deerness trout in summer 2014, but a large degree of local redistribution was evident by March 2015. The incidence of autumn-winter seaward migration was greater in the Deerness (46 % of migrating juveniles detected prior to spring smoltification) than the Villestrup (25 %). In both systems, the probability of migration was positively correlated with water level. 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 than autumn migrants suggests autumn and spring migrant conspecifics responded to different behavioural motivations. Variation in migration timing and overwintering habitat use could have ecological consequences relating to trophic cascades. 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 migration by juvenile trout would be beneficial to salmonid population management.