The study, led by the Arctic University of Norway, involved a cooperative research study by ten Universities and Institutions across Europe, including Inland Fisheries Ireland. The study involved tagging 204 salmon kelts with satellite tags from seven European countries and the east coast of North America, including salmon from the Barrow, Nore, Suir and Blackwater Rivers in Ireland, and tracked them during their oceanic migration.
Salmon travelled to oceanic fronts, but with specific patterns. Norwegian and Danish salmon rapidly migrate north and north-west toward the North Atlantic Ocean between Iceland and Svalbard. In contrast, Irish salmon migrated primarily westward towards South and East Greenland. Despite the variation in migration patterns among populations, most individual salmon migrated to polar ocean frontal areas.
The study found that salmon released further south tended to cover longer migration distances, with a straight-line distance tracked as far as 2,400 km for one salmon tagged from River Suir in Ireland. Tagged salmon spent 80% of their time foraging at the surface and performed occasional dives of up to 870m.
Overall, populations closest in proximity tended to converge in their oceanic feeding area, but taken together the salmon populations exploit a very large part of the ocean. Given that Atlantic salmon from different geographic locations feed in distinct areas at sea, they experience different temperature regimes. For example, Irish salmon experienced much warmer temperatures, ranging from 5 to 16°C, than Norwegian and Danish salmon which experienced temperatures ranging from 0 to 11°C. These differences not only contribute to variation in growth and survival across populations, but also are likely to affect Atlantic salmon populations differently with changing climate.
Southernmost populations, like those of Ireland, are more at risk than northernmost populations as migration distances are likely to become longer, or more variable, thereby decreasing feeding time, with important consequences for the marine survival and productivity of different populations.
Taken together, these findings suggest that a common marine factor responsible for the decline in Atlantic salmon is unlikely. Importantly, this means conservation efforts should be focused locally, such as during the freshwater phase.
The full study, ‘Redefining the oceanic distribution of Atlantic salmon,’ can be found at Nature.com at https://doi.org/10.1038/s41598-021-91137-y