LONDON, Sept. 17 (Xinhua) -- Atlantic salmon in England are facing growing pressures both in rivers and at sea, British scientists warned on Thursday, as stocks fall to their lowest level on record amid deteriorating freshwater habitats and increasingly hazardous marine conditions.
The latest Atlantic Salmon Stock Assessment shows that 95 percent of England's principal salmon rivers are now below the level needed to sustain their populations, the highest proportion on record.
The assessment was produced by the Environment Agency (EA), the Centre for Environment, Fisheries and Aquaculture Science and Natural Resources Wales.
The scale of the decline is particularly evident in the number of salmon returning from the sea. According to the EA, 15 to 20 out of every 100 young salmon that migrated to sea would once return as adults. Now, only one or two do.
The agency said salmon face pressures throughout their life cycle, ranging from pollution, migration barriers and water scarcity in freshwater habitats to aquaculture and bycatch at sea, with climate change intensifying many of these threats.
In rivers, the EA identified water quantity, water quality and damaged habitat as key pressures.
Charles Tyler, professor of environmental biology and toxicology at the University of Exeter, told Xinhua that it was extremely difficult to attribute the decline to any single factor, but said pollution was likely to be an important contributor because salmon depend heavily on clean water to survive and reproduce.
"This is the very sad face of where we are in the UK with most of our salmon rivers," Tyler said.
He said even where pollution was not directly toxic to returning salmon, it could disrupt their sense of smell, which they rely on heavily to return to their natal spawning grounds. Pollution such as eutrophication could also damage spawning beds.
The decline, however, is not confined to rivers. The EA said similar trends are being seen across the North Atlantic, with much of the pressure occurring at sea and beyond the control of any single country.
"Life on the high seas during salmon migrations has always been very risky, but those risks have multiplied over time as the sea has become more overcrowded, overexploited and polluted," Callum Roberts, professor of marine conservation at the University of Exeter, told Xinhua.
Tyler also pointed to pressures on wild fish stocks at sea. He said he suspected that too many salmon were being harvested at sea, citing the scale and technology of modern fishing vessels.
"These figures are stark," EA Chief Executive Philip Duffy said. "Recovery requires collective action."
The EA said it is tackling pollution and illegal water abstraction, restoring rivers and wetlands, and removing barriers that prevent salmon from reaching spawning grounds.
Under commitments running to 2032, the agency also plans to restore peatland, remove barriers to salmon migration and restore habitats along principal salmon rivers.
