Spatio-temporal distribution of Atlantic salmon (Salmo salar) post-smolt in Sognefjord

Abstract

Atlantic salmon (Salmo salar) is an anadromous fish that migrates from fresh to salt water, facing many challenges and threats throughout their life cycle, particularly during post-smolt migration through fjords. This study investigates the spatio-temporal distribution of Atlantic salmon post-smolts in Sognefjord, using data from the national monitoring program of salmon lice on wild salmonids (NALO) collected over three years between 2022 and 2024. The study aims to determine (I) the main migration routes out of Sognefjord, (II) the rivers of origin for post-smolts and how capture timing differs between these (III) if post-smolts migrate in groups, and how these changes within the migration period and (IV) whether these groups consist of post-smolts from several rivers. The findings indicate that the majority of post-smolts migrate through the southern opening of the fjord. Analysis determined that post-smolts from rivers in the inner Sognefjord regions arrived later than post-smolts from rivers in the outer Sogn region. The study provides the first evidence that post-smolts migrate in groups within the fjord and analysis determined that post-smolts tend to migrate individually or in small groups early in the season and in larger groups towards the peak migration period. It also provides the first evidence that post-smolts from different tributary rivers in Sognefjord migrate together in groups of mixed rivers of origin. These findings have important implications for Atlantic salmon management and highlight the need for extended monitoring and targeted mitigation measures of salmon lice during peak migration to minimize post-smolt mortality.