Atlantic salmon Salmo salar and sea trout S. trutta have experienced significant declines. To mitigate the negative impacts of salmon farming, Norway has designated 29 marine protected areas, known as national salmon fjords, to safeguard key S. salar populations. We investigated the behavior of S. salar and S. trutta post-smolts relative to the spatial boundaries of one such protected area. Species-specific differences were observed, with important implications for conservation efficacy. S. trutta remained primarily within the inner fjord, hence within the protected area during their marine residency, while S. salar passed close to open-net fish farms beyond the protected zone on the way to offshore feeding grounds, highlighting the critical need to maintain low lice levels at aquaculture facilities. S. salar post-smolts migrated faster in the outer fjord than the inner fjord, indicating an acceleration of movement with increasing distance from the river mouth, potentially reducing exposure time in unprotected areas associated with elevated pathogen and parasite risk. Although 2 potential migration pathways existed from the river to the ocean, 93% of S. salar selected the same route across both study years. In conclusion, our findings indicate that the national salmon fjord provides effective spatial protection for S. trutta post-smolts but only partially fulfills its objective of safeguarding S. salar, which must transit aquaculture-intensive areas during a critical early marine life stage. These findings underscore broader concerns for other national salmon fjords, many of which share similar geographic constraints and adjacency to aquaculture operations situated along key migratory corridors.
Atlantic salmon Salmo salar is an anadromous species that exhibits a wide range of life history strategies. One of the least understood and documented strategies involves adult salmon ascending rivers in the autumn or early winter and remaining there without feeding for a full year before spawning. This study provides the first scientific documentation of year-early migratory salmon in Norway. About 5% of Atlantic salmon captured during tagging studies in the river Alta arrived a year prior to spawning. Tracking of 8 tagged fish in the river Alta showed that they entered the river a year prior to spawning and overwintered in the river after spawning. The longest recorded stay in freshwater was 21 mo (640 d). Scale analysis revealed that year-early migrants had smaller sizes at age during their ocean years but entered rivers with higher relative energy reserves. Genetic analysis showed that the year-early migrating fish belong to the river Alta and were not strayers from Russian populations. The factors shaping this strategy are still unclear but may involve balancing the loss of ocean growth and the costs of extended river residency against the advantage of escaping high ocean mortality rates.
