Fashionably early or simply premature: Atlantic salmon ascending the river Alta a year prior to spawning

Abstract

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.