CARDIFF U. (UK)—Pollution, habitat loss and over-fishing have all been blamed in the past for declines in trout and salmon populations, but new evidence shows that climate change could be a major factor, putting both species at risk.

A study of the salmon and trout populations in the River Wye in Wales found that salmon numbers fell by 50 percent and trout numbers by 67 percent between 1985 and 2004, even though the river itself was cleaner, says Steve Ormerod, professor of biosciences at Cardiff University.

The fish were hit hardest following the hot, dry summers of 1990, 2000, and 2003, suggesting that warmer water and lower river levels combine to affect both species. Details of the study have been published in the journal Global Change Biology.

As both trout and salmon favor cool water, they face potentially major problems if climate warming continues as expected in the next two to three decades.

“Huge efforts have been put into bringing salmon back into Europe’s formerly polluted rivers such as the Taff, Thames, Clyde, Seine, and Rhine, so these results are a major worry,” says Ormerod.

“Salmon and trout fishing also generate many jobs and large economic benefits. In Wales alone, salmon fishing contributes around £90 million annually. Any risk of eventually losing these species to climate warming is therefore one we must consider very seriously.

“We suggest measures to ensure that river levels are maintained in hotter conditions alongside the use of riverside trees to create shade and protect against the highest temperatures.

The team used data on fish population collected each year by the Environment Agency at more than 50 locations spread throughout the Wye. Stream temperatures increased over the study period by 0.5–0.7 degrees Celsius in summer and 0.7-1.0 degrees Celsius in winter, with the latter effects apparently affecting the fish at low flow.

Water temperature is known to affect growth and susceptibility to disease in these fish, while lower water levels restrict their access to cooler habitats.

“We recognize that climate warming is probably already affecting many elements of our natural environment, including salmon, trout, and sea trout, and this detailed analysis of our long-term data is extremely interesting and, if proven correct, would be of great concern, adds Peter Gough, fisheries scientist with Environment Agency Wales.

“We are currently examining these and other data further. There is a suggestion that earlier migration to sea of salmon smolts might account for at least part of the apparent decline, but this doesn’t explain the reduction in trout numbers. More work is needed to clarify some important issues.”

The comparison between trout and salmon is important because, unlike salmon, trout from the Wye never migrate to the sea. Only factors affecting the river could explain their decline.

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