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Mass fish deaths will occur more often in rivers as UK gets hotter

Mass deaths of fish in UK rivers – as seen in many parts of the country in recent weeks – are likely to occur more frequently due to climate change, say researchers.

The recent appearance of large numbers of dead fish in rivers has been largely blamed on hot weather, with the UK seeing the hottest June since records began.

High temperatures reduce the amount of oxygen that water can hold, says Robert Britton at Bournemouth University, UK. Cold-blooded fish like Atlantic salmon and brown trout require highly oxygenated waters to survive, he says.

But hot weather may not be the only factor killing fish, says Alistair Boxall at the University of York, UK. He collected water samples from the river Foss in Yorkshire in mid-June, around the same time that fish deaths were beginning to be seen in the river. These samples revealed high levels of ammonia and nitrate as well as extremely high levels of paracetamol (acetaminophen), says Boxall.

“The paracetamol concentration is off the scale,” he says. “It was probably the second highest concentration ever reported in Europe.”

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Paracetamol concentration in rivers is a good indicator of raw sewage being dumped into the water, as sewage treatment works are usually quite effective at removing the drug, says Boxall. Paracetamol isn’t likely to kill fish, he says, but raw sewage can dramatically deoxygenate a river and lead to high fish mortalities.

“The weather in the UK in recent weeks has been very dry and this led to a massive reduction in the [water flow] of the river Foss,” says Boxall. “And then there was this flash flood, which led to sewage systems being overwhelmed by rainwater, leading to the release of raw sewage into this river.”

Boxall suspects that climate change will lead to more mass deaths of fish around the country. “I have a feeling that you’re going to be seeing much more of these short sharp rainfall events after dry periods of weather,” he says. “I think we could see more fish deaths happen because of this.”

Richard ffrench-Constant at the University of Exeter, UK, also anticipates that climate change will lead to more sewage discharges into rivers due to periods of heavy rain. “It’s the perfect storm,” he says.

Britton says the biggest impact climate change will have on fish in UK rivers will be due to high heat and reduced water flow. “But sewage will make a bad situation worse,” he says.

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We need to ensure that fish can find cooler regions in rivers, whether this is by increasing tree cover so that some spots remain under shade, or by increasing the connectivity of rivers so fish can move freely to cooler regions, says Britton.

“Climate change is one of the few things that is arguably going to be existential for some fish populations,” he says. “So, what are we going to do? What will these communities look like in 50 years? I’m not sure that conversation is being had.”

In a statement, an Environment Agency spokesperson said: “We are reviewing information surrounding the recent deaths of fish in the River Foss. The University of York has shared some data with us which we will be considering alongside findings from our own investigations, including the impact of recent hot weather on fish, to determine the probable causes of these issues.”

A spokesperson for Yorkshire Water, the company responsible for wastewater in the area, said they have supported the Environment Agency in its continuing investigation into fish deaths on the river Foss. “They’re confident that the incident is related to warm temperatures and low dissolved oxygen levels in the water,” the spokesperson said. “We are not being investigated in relation to this incident.”

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