Migratory behavior and habitat use of Atlantic salmon Salmo salar and sea trout S. trutta smolts in relation to a marine protected area

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.