Area use of Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) in an Arctic fjord system - a two year acoustic telemetry study

anglais. The Arctic charr Salvelinus alpinus (L.) and the brown trout Salmo trutta L. are fish species with complex and comparable life strategies. However, there are also differences between the two species. The migratory behavior of Arctic charr and brown trout at sea is poorly understood compared to their far more studied behavior in fresh water. Because of the declining populations of anadromous Arctic charr the last decades, this species is particularly important to understand in order to mitigate possible human influence and climate change effects that may affect it in a negative way. The Neiden and Bøk fjord system is a complex fjord system in north-eastern Norway that host both Arctic charr and brown trout. New coastal industrial areas are planned where the marine environment today has a relatively low degree of human impact. In this setting, documentation of the anadromous salmonids behavior in the fjord system is needed. This study aimed to document the fjord area use of Arctic charr and brown trout and the differences between them. From spring 2014 to autumn 2015, the marine migratory behavior of the two species was studied by use of acoustic telemetry. A total of 33 Arctic charr and 86 brown trout were tagged with acoustic transmitters during the study period. The average number of days at sea for the Arctic charr was 32 and 43 for 2014 and 2015 respectively, while the brown trout was documented to stay at sea for 60 and 54 days. The major findings in this study was the difference in fjord area use between the two species. The Arctic charr used the areas far away from the river outlets more frequently than the brown trout. Arctic charr also used the Braselv bay, especially early and late in the season. Large brown trout was found to migrate the furthest. The brown trout which was tagged in the long and narrow Lang fjord in the inner parts of the fjord system, did not migrate out of this fjord to a large degree. Smolt length of brown trout was found to be an important factor in modelled predictions of the fjord area use, but it was not as important as day of season or the size of the fish. Size and day of season was important predictors also for the Arctic charr fjord area use. Both species used the planned harbor areas, but only a low proportion used these areas throughout the whole summer season. However, some individuals of brown trout caught and tagged in the Høybukt bay close to the planned harbor areas used these areas during the entire III summer. To protect the brown trout in the Høybukt bay and the Arctic charr in the Braselv bay, fishing regulations have been proposed. These suggested regulations include a maximum size catch limit and a reduction in fishing pressure in important feeding areas like the estuaries.

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