Atlantic salmon survival during different life-stages: time to focus on improving marine survival to slow population declines

Abstract

Over the last five decades, the Atlantic salmon Salmo salar has suffered marked population declines across its distribution, attributed inter alia to overfishing, barriers to migration, climate change, aquaculture, and pollution. Conservation measures have been implemented to protect and recover this species, but little improvement in its population status has been observed. Here, we used population monitoring data to estimate the abundance of Atlantic salmon originating from the River Frome in southern England. Subsequently, juvenile (parr-to-smolt) freshwater and adult marine (smolt-to-adult) survival estimates were used to understand associations with environmental and anthropogenic factors. Annual Atlantic salmon trapping and tagging data containing more than 148,000 juveniles were analysed from 2012 to 2024. Over the study period, juvenile freshwater survival varied without a significant temporal trend, whilst marine survival decreased. Annual juvenile freshwater survival rates ranged from 7.2% to 18.3% (mean 11.6%), with their between-year variation explained by mean winter river discharge (40%). In contrast, smolt-to-one- and two-sea-winter adult marine survival rates were significantly lower, varying between 0.4% and 6.1% (mean 2.2%). Sea ice extent within the Norwegian and Greenland Seas was the most important covariate that explained the greatest amount (60%) of variation in smolt-to-adult marine survival rates. Hitherto, Atlantic salmon conservation efforts have primarily focused on protecting juvenile life-stages in freshwater. This study revealed that smolt-to-adult marine survival was 81.6% lower than parr-to-smolt freshwater survival, so much greater emphasis should be given to understanding the factors affecting their mortality at sea to protect this important life-stage. At present, Atlantic salmon are afforded almost no protection at sea, unlike other threatened marine species, and despite population declines observed across their distribution. We discuss measures required to safeguard Atlantic salmon at sea.