Migration and habitat use of sea trout post-smolts Salmo trutta in a Norwegian fjord system

Abstract

The migration and habitat use of sea trout post-smolts were investigated in a fjord system in Central Norway during the period from 30 April – 26 November 2014. The main aims were to investigate timing of sea entry and freshwater return, return rate, marine residence time, spatial use of the fjord system and migration distance. Fifty sea trout smolts were tagged with acoustic transmitters and tracked with 43 automatic listening receivers distributed throughout the study area. Median seaward migration date was 22 May and median return date to freshwater was 4 July. Of the 40 seaward migrating smolts, 26 returned to freshwater, resulting in a minimum return rate to freshwater of 65%. During transition from the river to the fjord, 80% of the tagged smolts migrated during the night, however, no diurnal pattern was observed during the upriver migration. Mean marine residence time was 38 days, but there was large individual variation ranging from 22-99 days. The innermost parts of the study area were more utilized than the outer part of the fjord system during the sea residency, and with more use of the near shore areas defined as littoral habitat than the open, pelagic areas. However, a widespread distribution was observed, and a large proportion of the post-smolts was observed to utilize the outer part of the fjord system. All of the tagged post-smolts utilized larger marine areas than the river mouth area during the summer, and 67% of the post-smolts had a minimum migration distance of 25 km, 27% had a minimum migration distance of 14 km while the last 6% had a minimum migration distance of 4 km. Eight of the 26 returning individuals (31%) performed a second marine migration during the same summer, after returning to Lake Rovatnet in early June. The spatial use of the fjord system between the first and second marine migration showed two distinctive patterns. The innermost part of the fjord was more often used during the first period, however, more time was spent in the outer part of the fjord during the second migration. Regardless of the high predation risk post-smolts may experience in the marine phase, a widespread distribution and habitat utilization were observed in this study, in addition to a high marine survival rate. Thus, the results indicate that the sea trout post-smolts displayed variable migration behaviour and habitat utilization during their first marine migration. Nevertheless, due to the fact that the bigger post-smolts, to a larger extent, returned to the home river and also utilized areas in the outer part of the fjord, body size seemed to be an important factor in determining the migration behaviour and survival in this study.