Environmental teams have been on standby to oxygenate rivers in England to protect fish during the heatwave.
The Environment Agency (EA) said “hundreds of fish kills” had been recorded this year as oxygen levels dropped in rivers and canals.
The Fishing Trust claims there has been a “surge” in fish deaths in recent weeks, with north west England thought to be among the hardest hit.
After the heat of June— Met Office predicts hottest weather on record.
The Environment Agency said its fisheries teams were working “around the clock” to respond to the fish kill, including using aeration equipment to help restore dissolved oxygen levels in rivers and canals.
The agency is already investigating the fish kill in Greater Salford Quays manchesterand a stretch of the Mole Surrey.
It believes the heat and low pressure from thunderstorms – leading to low levels of dissolved oxygen – are the culprits.
However, the Fishing Trust, which has received reports of fish kills in the River Avon in the north-west, south-west and south-east of England and in Warwickshire, warned the approach was “complacent”.
The trust said “poor water quality, contamination and in some places excessive pumping and low flows” were responsible for the incidents.
“While it may be the loss of oxygen that caused the damage, it wasn’t just the hot weather that was to blame,” the trust said.
“We have once again sounded the alarm for EEA and the Department of Environment, Food and Rural Affairs. These fish deaths are further evidence of the appalling abuse our rivers face every day.
“Our rivers are simply not resilient to function in natural freshwater ecosystems as the climate changes.
“A period of hot weather and thunderstorms is just a turning point, not a cause.”
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The Environment Agency’s emergency plans were discussed on Wednesday at a meeting of England’s National Drought Panel, which includes senior policymakers from the agency, government, water companies and major agricultural and environmental organisations.
The team understands that current water levels are higher than 2022, the hottest year on record for the UK.
“However, the natural environment will still take time to recover from last summer’s impact, and the Environment Agency continues to work to monitor how fish and invertebrates are recovering from the drought,” the group said in a statement.