Soaring river temperatures and drought risk killing off salmon across England, experts have warned.
Anglers and environmentalists have said the UK faces mass fish deaths after a succession of heatwaves caused oxygen levels to plummet across waterways.
A lack of oxygen can cause cold-water fish to suffer heat stress and suffocate, while lower water levels can make it impossible for wild salmon to swim upriver to reproduce.
Feargal Sharkey, the rock star turned water campaigner, said: “When river temperatures rise to over 20C, you’re in the kill zone. There have been a number of fish kills already. These heatwaves could be the final nail in the coffin.”
Temperatures in some of Britain’s major rivers for salmon fishing have climbed steeply this year.
Environment Agency sensors recorded temperatures of 27.7C on the Wye at the end of June, according to campaigners, 3.7C higher than the same time last year and 1.2C higher than the record for the river.
Temperatures in the River Tweed in Scotland rose to nearly 25C in July, according to live sensor data, and are rising again this month.
Donny Donovan, a river keeper on the Test, in Hampshire, said water temperatures were now at levels he had not seen in six decades.
“It’s fatal for salmon,” he said. “When they swim up into the river from the sea, they are coming into what is in effect a warm bath.”
The Game & Wildlife Conservation Trust, which tracks thousands of wild fish, warned in a study published last month that wild Atlantic salmon survival rates had declined dramatically, with as few as 1pc of salmon from UK rivers returning to spawn.
It previously warned salmon “could be completely gone from our rivers” within 30 years.
Mr Sharkey added that a growing number of rivers risked being closed to anglers to protect fish.
Salmon and trout caught by anglers can become exhausted while hooked on the line, meaning they are more at risk of death when released back into oxygen-depleted waters.
Mark Owen, the head of fisheries at the Angling Trust, said: “Low river flows prevent adult salmon from reaching the cooler water and spawning grounds they need.
“For a species already in crisis, these conditions can be the difference between survival and death.”
WildFish, a campaign group for better rivers, warned earlier this summer that “hot weather is killing our wild fish”.
There have been thousands of fish deaths in UK rivers so far this year across nearly 160 incidents, according to the Environment Agency. That is because of algae blooms, low oxygen levels and pollution.
Officials and volunteers have also conducted dozens of “fish rescues” to save trout and salmon from rivers and lakes amid low water levels.
Salmon had to be rescued from the River Teme, a tributary of the Severn, in Wales last month after they became stranded in rapidly warming, oxygen-depleted pools.
Nick Measham, the chief executive of WildFish, said: “If those fish hadn’t been rescued, they would have died.”
Global sea temperatures climbed to a record level for July, according to the Copernicus Climate Change Service, hitting 20.96C outside of the world’s polar regions.