Habitat use and depth preferences of sea trout (Salmo trutta) in a Norwegian estuary

The brown trout (Salmo trutta) is a freshwater fish found in habitats ranging from small streams to rivers, lakes, estuaries and the coastal marine waters. The anadromous form, often termed sea trout, exhibits large variation in migratory strategies and habitat use. Many major estuaries in Norway are highly affected by human activity, as cities are often built around larger river outlets. This can potentially have negative consequences for the sea trout, as estuaries are the transition zone for their migrations between freshwater and sea. To gain more knowledge on the importance of estuaries as a habitat for sea trout, a two-year study was conducted in Gaulosen estuary in the Trondheimsfjord, Central Norway. This estuarine area has a relatively low level of impact from humans, making it an ideal place to study the sea trout’s natural use of estuaries. From March 2018 through September 2019, 43 sea trout were tagged and tracked using acoustic telemetry in order to test the following hypotheses: (i) that the sea trout utilised the estuary year-round, (ii) that individuals with smaller body lengths, higher body condition, and especially males showed a greater utilisation of the estuary than other sea trout and (iii) that swimming depth changed throughout the year, with greater depths during the summer.
The sea trout used the estuary during all months of the year, but the number of individuals varied between seasons. The number of fish was highest in the period from December to May, while only 11% of the sea trout registered in the estuary at some point stayed there during the summer months of June to September. The tendency of remaining in the estuary during summer was influenced by sex, age, condition factor and body length (LT), where females, fish with higher body condition, larger and older fish were more likely to remain resident in the estuary throughout the summer. The swimming depths in the estuary from March 2018 to August 2019 were generally surface-oriented, with a mean of 2.4 m (range 0.1 m – 25.6 m). It was influenced by season and body length, with smaller individuals staying deeper in the water and a deeper swimming depth during spring and summer. When comparing swimming depths between the estuary and the fjord for the period December 2018 to June 2019, it was significantly shallower in the estuary (mean = 2.9 m, range 0.4 – 25.6 meter) than in the fjord (mean = 5.7 m, range 0.5 – 14.0 meter). The tagged fish were found to visit nearby watercourses, and 18% of the sea trout captured and tagged in Gaulosen estuary during the spring, moved to either River Nidelva or River Orkla during the summer.
The results from this study illustrate that estuaries are important habitats for sea trout, where some individuals utilise it all year round, while other use it only as an overwintering habitat or as a transition zone between freshwater and the marine environment. The decline in sea trout populations seen in large parts of Norway makes it crucial to conserve habitats of high value for the sea trout. Given the importance of estuarine areas, protective measures should be taken to ensure that they are kept in good condition.