| Summary: | Ecosystems are significantly impacted by activities from human settlements. This is especially the case for aquatic ecosystems as many large cities sit on coastlines or along major rivers. Species like salmonids are especially affected since they migrate between freshwater and saltwater habitats during their lifetime. To study the relationship between human settlements and salmonids, the Oslo fjord in Norway is notable both as a destination for Atlantic salmon (Salmo salar) and sea trout (S. trutta) migration and as a major metropolitan area. Human activity in freshwater habitats affects the salmonids in particular since they use this environment for spawning and as a nursery. Sandviksvassdraget is one of the most important salmonid-carrying watercourses around the Inner Oslo fjord and the reported high catches of salmonids in Sandviksvassdraget are the result of continuous hatchery activity since 1857. However, there are characteristics that degrade the habitat of salmonids in the watercourse consisting of man-made dams, which reduce the available freshwater habitat for salmonids, as well as surrounding roads and construction sites that decrease coverage of riparian vegetation and introduce particle pollution. The salmonids in the watercourse, their habitat, and their main food source, macroinvertebrates, have been studied in past years, but the variables that significantly impact the local population of salmonids and macroinvertebrates are still not well understood. Neither is the actual output of smolt to the Oslo fjord from the Sandviksvassdraget well known. This study found overall lower densities in the upstream stretches, where salmonid juveniles from the hatchery are released, than in the downstream stretches, where wild juvenile salmonids are found. Neither the tested environmental variables, nor the total abundance of macroinvertebrates had a significant effect on the salmonid densities. The macroinvertebrate diversity and abundance differed significantly between the rivers and upstream and downstream ...
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