Factors affecting year-to-year and within river variability of one-sea-winter Atlantic salmon in Norwegian rivers

Many Atlantic salmon, Salmo salar, populations are decreasing throughout its distributional range due to several factors acting in concert. A number of studies have documented the influence of freshwater and ocean conditions, climate variability and “man-made obstacles” including dams and aquaculture. However, most of the historical research has focused on single or few river analyses, and quantifying isolated effects rather than handling factors in conjunction. By using a multi-river mixed-effects model we estimated impacts of oceanic and riverine conditions, as well as human threats on both year-to-year and within river variability across 60 time series of recreational catch of one-sea-winter salmon (grilse) from Norwegian rivers over 29 years (1979-2007). Warm coastal temperatures at the time of smolt entrance into the sea and increased water discharge during upstream migration were associated with higher rod catches of grilse. When hydropower stations are present in the course of the river systems the strength of the relationship with runoff is reduced. Moreover, catches of grilse in the river increased significantly following the discontinuation of the harvesting of this lifestage at sea. However, a general decreasing temporal trend was still detected being stronger with the presence of salmon farms in the migration route of smolts in coastal/fjord areas. These results suggest that both ocean and freshwater conditions in conjunction with diverse human impacts contribute to shape interannual fluctuations and within river variability of wild Atlantic salmon