The Labrador Current cold front shaping the Atlantic salmon homing migration routes from the waters off Southern Greenland to eastern North America

Abstract

Atlantic salmon (Salmo salar) experienced drastic population declines from the mid-1970 s to the early 1990′s throughout their range. The survival of the salmon while at sea is considered as the main driver of these declines, even though the underlying mechanisms are poorly understood. This study aims at improving the general understanding of the ocean distribution and migration timing of sub-adult salmon returning to North America from the waters off southern Greenland that could help determine the drivers of marine mortality. We used animal biotelemetry and numerical modeling to improve our knowledge of Atlantic salmon migratory behavior. We used data from 43 North American Atlantic salmon tagged at West Greenland, of which 5 individuals migrated towards their native rivers, and developed an individual-based model to simulate homing migration from their feeding grounds in South Greenland towards the coastal areas of their native rivers. The tagged and simulated salmon exhibited similar behavior when they encountered the cold water front formed by the southward flowing Labrador Current. The salmon either crossed the Labrador Current near the Newfoundland shelf break or continued their route southward along the warmer side of the cold front. These two pathways emerged as the migration routes split where the shelf slope is less steep. This discontinuity along the shelf break leads to a highly dynamic region, a high sea surface temperature variability, and occasional breaches in the thermal front that favor on-shelf fish migration. The salmon trajectories appear to be deflected when the front temperature is 1 °C or less. The 1 °C isotherm would thus bound the thermal distribution of North American Atlantic salmon and shape the species migration routes.