Rising river temperatures and ongoing drought conditions threaten to devastate wild salmon populations across England and parts of the United Kingdom, experts and environmental groups have warned. Successive heatwaves this year have led to critical drops in oxygen levels in waterways, increasing the risk of mass fish deaths and disrupting salmon spawning cycles.

Cold-water species such as wild Atlantic salmon are particularly vulnerable to elevated water temperatures. When river temperatures exceed 20°C, fish experience heat stress and face a higher likelihood of suffocation due to diminished oxygen availability. Anglers and conservationists report that some of Britain’s key salmon rivers have seen unprecedented temperature spikes. For instance, the River Wye recorded levels as high as 27.7°C at the end of June—3.7°C above last year's figures and surpassing previous historical highs. Similar trends have been observed in Scotland’s River Tweed, where temperatures neared 25°C in July and are reportedly rising again.

Donny Donovan, a river keeper on Hampshire’s River Test, described the current water temperatures as unprecedented in his 60 years of experience. “It’s fatal for salmon,” he said, explaining that the warm conditions pose significant challenges for salmon migrating upstream to spawn. Reduced water flow exacerbates the issue, restricting access to cooler spawning grounds vital for reproduction.

A recent study from the Game & Wildlife Conservation Trust highlighted the steep decline in wild Atlantic salmon survival rates, noting that as few as 1% of salmon are returning to British rivers to spawn. The organization has cautioned that wild salmon could disappear entirely from UK rivers within three decades unless conditions improve.

These environmental stresses have also prompted concerns among anglers. Fish caught and released in low-oxygen waters often become exhausted and face increased mortality risks. Mark Owen, fisheries head at the Angling Trust, emphasized how reduced river flows prevent salmon from reaching suitable spawning habitats, a critical factor for a species already classified as being in crisis.

The Environment Agency reported approximately 160 incidents of fish kills across UK rivers in 2023, resulting from a combination of algal blooms, low oxygen levels, and pollution. Efforts to mitigate the losses have included numerous fish rescue operations where stranded salmon and trout have been relocated to more favorable conditions. In Wales, salmon were recently rescued from oxygen-depleted pools in the River Teme, as these fish faced imminent death without intervention.

Underlying these freshwater impacts, global sea temperatures have increased to record highs, with July sea surface temperatures outside polar regions reaching 20.96°C, according to the Copernicus Climate Change Service. This broader warming trend further complicates efforts to conserve vulnerable fish populations already subjected to climate-induced stresses in their freshwater habitats.