| Summary: | Based on earlier migration studies of lake-dwelling anadromous Arctic charr Salvelinus alpinus (L.) and sea trout Salmo trutta L. in northern Norway, both species are believed to solely overwinter in fresh water and spend only one to three months in the sea during summer. This is an assumption upon which all current management of the species in the area is based, although no studies have focused on the migration behaviour of the riverine populations of these species in the area. The main objective of the present study was therefore to map the winter migratory behaviour of riverine and sympatric adult anadromous Arctic charr and sea trout in the River Skibotn in northern Norway. A combination of acoustic telemetry (one year study) and Data Storage Tags (DST) (two year study) were used to track fish migrations between the river, estuary and sea. The acoustically tagged fish were tracked both manually and automatically within the river and estuary, while temperature experienced by the fish were obtained from the recaptured DSTs and used as indicator if the fish had been in fresh or salt water or in the estuary. In total, 30 fish were tagged acoustically and 157 fish tagged with DSTs, where acoustic data was obtained from a total of 9 Arctic charr and 15 sea trout, and temperature preference data from 10 of the recaptured charrs and 22 of the recaptured sea trout. The results surprisingly showed that large proportions of both the tagged Arctic charr and sea trout descended the river for estuarine and/or marine waters as early as October, with increasing numbers of fish entering the estuary and sea throughout the winter. Both the estuarine and marine waters thus seem to play an important role during overwintering for both species. Fish moved actively between habitats throughout the winter, and there was large individual variation in time of migration and time spent in different habitats. No clear differences between and within species could be detected. These results thus contradict the earlier assumption that both species solely overwinter in fresh water. The seemingly difference in behaviour between lake-dwelling and riverine populations may be related to the harsh and unstable overwintering conditions within the river (e.g. low water levels and drifting ice) compared to the more stable winter conditions within lakes. The river is regulated for hydropower, which may have affected the behaviour of the fish. However, based on unpublished results from other undisturbed rivers in the area, it is reason to believe that the behaviour observed in the present study may also be common for populations in unregulated rivers. In order to fully verify if this behaviour is common among the northernmost riverine populations, further studies has to bee conducted on other northern populations of both species, as well as detailed studies of their habitat use and dispersal at sea during winter.
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