Seasonal presence, movements and behaviour of Atlantic cod ( Gadus morhua ) in Isfjorden, Svalbard

Atlantification is increasing the inflow of warm, salty Atlantic water into the Arctic region. The fjords on the western coast of Svalbard are experiencing Atlantification, enabling boreal species, such as Atlantic cod ( Gadus morhua ), to expand their distribution northwards. Monitoring the seasonal occurrence and behaviour of species like Atlantic cod in Arctic fjords is therefore key for understanding how ocean warming may restructure Arctic ecosystems and affect native prey species like polar cod ( Boreogadus saida ), supporting improved management under climate change. By using acoustic telemetry, this study investigated the seasonal presence and movement patterns of 27 Atlantic cod in Isfjorden, Svalbard, over the course of 1 year, from August 2023 to August 2024. Atlantic cod showed seasonal changes in vertical distribution in Isfjorden, occurring at greater depths during winter and closer to the surface during summer. Their occurrence was also more frequent along the shoreline of the fjord. Furthermore, we observed that some of the Atlantic cod left the fjord during the winter months, and few individuals were observed returning, suggesting seasonal use of the fjord. Genomic analyses suggest that the Atlantic cod sampled include specimens that are genetically more similar to the non‐migratory Norwegian coastal cod as well as individuals that display genetic signals characteristic for the well‐known migratory Northeast Arctic cod. Remarkably, 2 years after tagging, one of the specimens, displaying a genomic signal most often associated with the non‐migratory coastal cod, was caught in Lofoten. Our findings highlight the potential shift in movement patterns and usage of Isfjorden as a habitat for the iconic migratory ecotype in recent years, but they underscore the need for further studies to illuminate the coupling of the genomic characteristics and their altered migratory behaviour. Finally, our study exemplifies how climate change is reshaping Arctic food webs.