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What caused more than 100 fish to die in the River Cam in Cambridge?





The deaths of more than 100 fish in the River Cam are most probably the result of “a cascade of causes”, say Cambridge-based environmentalists.

Alarm bells rang yesterday when dead fish were spotted floating on the surface at Ditton Meadows. The Environment Agency was alerted and experts attended the scene.

Dead fish in the River Cam, June 22, 2023. Picture: Clara Todd
Dead fish in the River Cam, June 22, 2023. Picture: Clara Todd

A spokesperson for the Environment Agency said: “Our thanks go to the local members of the public who reported a number of fish deaths to us on the River Cam. Specialist fisheries officers inspected the river where it is suspected that low levels of dissolved oxygen after heavy rainfall were the cause of the fish deaths, not pollution.

“If you see any fish in distress, here or in other water bodies, or have other environmental concerns, please call us on our free 24-hour incident hotline, 0800 807060.”

Dead fish in the River Cam, June 22, 2023. Picture: Clara Todd
Dead fish in the River Cam, June 22, 2023. Picture: Clara Todd

An Anglian Water spokesperson said: “Anglian Water, along with The Environment Agency, attended an incident yesterday afternoon (20 June) at Ditton Meadows. Anglian Water inspected the site with The EA and took samples and from this both ourselves and The EA can confirm that the issue resulting in the death of the fish is not related to Anglian Water in any way.”

Stephen Tomkins, emeritus fellow at Homerton College and previously chair of Cam Valley Forum, advised people to alert the Environment Agency during such incidents.

He told the Cambridge Independent. “They have a team and are very alert to the problem.

“I’m pretty sure deoxygenation is the reason for the deaths. It could be disease or poisoning but both unlikely given hot weather.

“The deaths are upstream of Anglian Water’s Milton sewage treatment works discharge. There is a possibility that another source of water pollution has raised the biochemical oxygen demand for oxygen - the BOD.”

Mr Tomkins added: “Fish may get into trouble in hot weather and the Environment Agency will have done the treatment - to use hydrogen peroxide to oxygenate the water.

Dead fish in the River Cam, June 22, 2023. Picture: Clara Todd
Dead fish in the River Cam, June 22, 2023. Picture: Clara Todd

“H2O2 - hydrogen peroxide - breaks down in water and liberates its oxygen as it does so.”

He added: “This is not a new phenomenon in lowland rivers. The Environment Agency is there to fix these things and they seem to have come straight away with a sticking plaster.”

The situation on the Cam is mirrored in fish die-offs in other parts of the country including Greater Manchester, Merseyside, Cheshire, Leicestershire, the north west and Warwickshire. They include the Avon, the Oxford Canal, Salford Quays and the Mole.

“It’s nothing to do with the sewage treatment works in Milton,” agreed Clara Todd, a director at Water Sensitive Cambridge CIC, who has been assessing the situation on the Cam today (June 22). “There’s no algae indicated in the incidents. The Environment Agency is saying there’s a lack of oxygen in the water and rapidly fluctuating temperatures with sudden rains then the hot sun, so the temperature is fluctuating, which is changing the flow of oxygen in the water.

“There have been fluctuations in temperature before so why is this happening now? The fact is that rivers are under massive stress. It’s a complicated system which is facing multiple crises and feedback loops that are causing a cascade of impacts. The fish are not in the best of health, perhaps because of surface run-off putting microplastics, oil, bacteria, fertilisers and pesticides directly in the river, as well as low natural flows. There’s lots of compacting and compound issues - it’s not just one thing.

Dead fish in the River Cam, June 22, 2023. Picture: Clara Todd
Dead fish in the River Cam, June 22, 2023. Picture: Clara Todd

“The big picture – of multiple smaller changes that are all compounding to cause a bigger change of state – balances away from the reductionism that happens in views among people on all sides of the situation.

“It’s all of it happening at the same time, that’s the problem.”

The photos of the fish shown here were taken this morning (June 22) by Clara Todd. The fact that their bodies have not decayed indicates they died in the last 24 hours. It is estimated that more than 100 fish have died so far.



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