Key points
- Most salmon losses now occur at sea, with only around 3% of fish surviving to return after one winter.
- Survival in rivers is relatively stable, but varies between years and is influenced by river flows.
- Overall marine survival has declined over time, with some years seeing returns of less than 1%.
- More fish are now spending longer at sea before returning, suggesting tougher conditions and slower growth.
- Large-scale ocean conditions, rather than river management alone, are now the main drivers of salmon survival.
Background
Migratory fish are among the most threatened wildlife globally because they rely on different habitats at different stages of their lives. Atlantic salmon, which move between rivers and the sea, face multiple pressures including habitat loss, barriers, fishing, predation and climate change.
Despite conservation efforts and reduced fishing pressure, salmon numbers have declined significantly across the North Atlantic in recent decades. Their complex life cycle means survival is needed at every stage, from freshwater growth to marine feeding and return migration.
While conservation has often focused on freshwater habitats, growing evidence suggests that reduced survival at sea is now a major driver of declines, particularly since the 1990s. However, this stage is much harder to study, leaving important gaps in understanding.
This study aims to address some of those gaps by estimating survival rates at key life stages and identifying the environmental and human factors that influence them.
What they did
The study took place on the River Frome in Dorset, a chalk stream with relatively stable flows and temperatures compared to many other UK rivers. These conditions make it a useful system for understanding salmon survival, as they reduce some of the natural variation seen elsewhere.
Since 2005, researchers have conducted annual surveys of juvenile salmon (parr) across hundreds of sites in late summer. Around 10,000 fish each year were captured using electrofishing, measured, and fitted with small electronic (PIT) tags before being returned to the river. These tags allow individual fish to be tracked as they grow and migrate.
In spring, as juvenile salmon (smolts) migrate downstream towards the sea, they are guided into a side channel and captured using traps. Tagged fish are recorded, measured again, and released. Detection stations are also placed in the surrounding area and record when tagged fish leave the river, and when surviving adults return from the sea to spawn. Using this tagging and detection system (capture -mark-recapture methods), the researchers could estimate how many fish survived key life stages: from parr to smolt, and from smolt to returning adult.
The study also examined a wide range of environmental and human variables that might influence survival. These included river conditions such as flow and temperature, fish size and migration distance, as well as marine factors such as sea temperature, climatic variables, fishing pressure and predation. Statistical models were then used to identify the factors most strongly linked to survival at each stage of the salmon life cycle.
What they found
Survival rates in freshwater were relatively variable. On average, 12% juvenile salmon survived from parr to smolt, with survival rates ranging between 7% and 18% depending on the year. There was no clear long‑term trend over the study period, but year‑to‑year variation was evident, with the lowest survival rates (below 9%) recorded in 2012 and 2019, and the highest (above 14%) in 2020 and 2024.
Winter river flows were the only factor significantly linked to this variation, explaining 40% of the year‑to‑year differences in freshwater survival rates. Higher river discharge was associated with increased survival from parr to smolt.
Rates of survival at sea were much lower. On average, only a small proportion of young salmon that head out to sea survive to return. Around 3% come back after one winter at sea (known as one-sea-winter or 1SW fish), and even fewer, about 1.5%, return after two winters at sea (2SW fish). In some years, survival rates were even lower, with as few as 0.4% of 1SW fish and 0.7% of 2SW fish making it back. Overall, rates of survival at sea declined noticeably over the course of the study.
There was also a shift in the type of fish returning. Up until 2021, most returning salmon had spent just one winter at sea. In more recent years, however, fish that spent two winters at sea have become more common. They may suggest that conditions in the ocean are getting tougher, meaning fish grow more slowly and spend longer at sea before returning to rivers to spawn.
Across both stages, adult returns were 82% lower than parr-to-smolt survival, confirming that greater emphasis should be given to protecting them whilst at sea.
Among the environmental factors tested, sea ice extent in the Norwegian and Greenland Seas explained 60% of the variation in adult returns, with greater sea ice associated with lower return rates. This highlights the strong influence of large‑scale ocean conditions on salmon survival once they leave rivers.
What this means
This study shows that while salmon survival in rivers can vary from year to year, the greatest losses occur at sea. Survival from smolt to adult was more than 80% lower than freshwater survival, confirming that the marine phase is now a critical bottleneck in the salmon life cycle.
Although river conditions still matter, with higher flows linked to better survival of juveniles, improving freshwater habitats alone is unlikely to reverse population declines. The results suggest that wider environmental conditions, particularly those linked to climate, exert a much stronger influence once fish leave rivers.
In particular, large‑scale ocean processes, such as sea ice extent and associated changes in temperature, food availability and predator interactions, appear to play a key role in determining whether salmon survive at sea.
Overall, the results point to a need for a more joined‑up approach to salmon conservation. This includes:
- Continuing to protect and improve freshwater habitats to maximise the number of fish reaching the sea;
- Improving water quality across rivers, estuaries and coastal areas; and
- Crucially, placing greater emphasis on conservation measures at sea.
Such marine conservation measures could include better protection of migration routes and feeding areas, reducing pressures such as bycatch, and addressing the long‑term impacts of climate change. While marine management is complex, similar approaches are already used for other threatened and commercially important species, suggesting that action is possible.
In summary, the study reinforces a key message: protecting salmon in rivers remains essential, but without addressing the factors affecting their survival at sea, long‑term recovery will be difficult to achieve.
Atlantic salmon survival during different life-stages: time to focus on improving marine survival to slow population declines Elliott, S. A. M., Aebischer, N. J., Gillson, J. P., Utne, K. R., Beaumont, W. A., Boraiah, K., & Roberts, D. E.