Warming seas again highlighted as the key threat to wild salmon

The Game & Wildlife Conservation Trust (GWCT) has highlighted the need for better protection of salmon marine migration routes and feeding areas after a new study showing the survival of wild Atlantic salmon at sea has declined dramatically.

In recent years, on average, only 3% of fish leaving the River Frome in Dorset have returned to spawn and this figure has regularly dropped below 1%.

The research, based on the Trust’s long-term data over more than 20 years, also revealed that overwinter survival of salmon in the river in the first phase of life was 80% higher than at sea, confirming that the marine environment is now a critical bottleneck in the salmon life cycle.

Warmer waters

Warming seas that affect feeding opportunities for salmon are thought to be causing the decline, while accidental capture of the fish by boats fishing for mackerel, herring and whiting in the commercial fisheries in the North Atlantic – although low – is also causing concern.

“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,” write the authors of the study, which can be read in full here.

“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.” 

10,000 parr tagged

The GWCT is one of the founding members of the Missing Salmon Alliance, a group of organisations that includes Fisheries Management Scotland, which is calling on Government to better enforce protection of marine migration routes and feeding areas, reduce pressures such as bycatch, and fund research into addressing the long‑term impacts of climate change on the ocean environment.

Since 2005, researchers at the GWCT’s monitoring station at East Stoke on the River Frome have tagged around 10,000 juvenile salmon (parr) annually, allowing individual salmon to be tracked as they grow into smolts, which migrate out to sea in spring. On their journey downstream the tagged fish are then caught again, measured and released. Detection stations record when they leave the river, and when surviving adults return from the sea to spawn.

So far, this year numbers are higher than average, but still only 111 have been recorded as returning.

Whereas around 3% come back after one winter at sea (one-sea-winter fish), even fewer, about 1.5%, return after two winters at sea. In some years, survival rates were even lower, with as few as 0.4% of one-sea-winter fish and 0.7% of two-sea-winter fish making it back. Overall, rates of survival at sea declined noticeably over the course of the study.

GWCT head of fisheries Dylan Roberts, who oversaw the project, said: “We believe declining marine survival is driven by warming seas affecting feeding opportunities for salmon at sea.

“We’re also very concerned about the potential high risk of young salmon being accidentally caught in commercial factory fishing nets.

“In this context, improving river habitats for salmon is even more crucial. We know from our research that the largest, fittest smolts are three times more likely to return as adults. So, if you get the habitat right, this can help improve the very low marine survival we have been witnessing in recent years.”

In the river system, 12% juvenile salmon survived the transition from parr to smolt, with no clear long‑term trend over the study period. However, in recent years the number of parr being caught for tagging has dropped dramatically from 10,000 in 2023 to 3,226 in 2025.

Melting sea ice

While the reasons for the decline of salmon in freshwater environments are comparatively well understood, problems at sea are harder to ascertain. The GWCT said it is likely that ocean-scale processes, such as melting sea ice with associated changes in temperature, food availability and predator interactions, are playing a key role. It added that its new research suggests that as well as improving water quality and habitats in rivers, greater legal protection at sea is vital to prevent the extinction of wild Atlantic salmon, which is now a red listed species.

“What we really need is a concerted national effort to save this much-loved species before it’s too late,” said Roberts.

“As well as restoring our rivers, we need to look at the risk of salmon being caught by boats fishing for mackerel, herring and whiting in the commercial fisheries in the North Atlantic. Without placing greater emphasis on protecting them both at sea and in rivers, we will never reverse their decline.”