A group of Fenland fishermen found hundreds of fish on the surface of the Forty Foot Drain gasping during Sunday’s heatwave.

Soon after arriving on the banks, anglers from Chatteris Working Men’s Club “realised something was wrong” and called the Environment Agency’s incident hotline number.

The fisheries duty officer was quickly on site and monitoring water quality and a spokesman for the Environment Agency said: “By the time we had arrived, the dissolved oxygen levels had naturally increased (not by much) and only a handful of #fish were gasping at the surface.

“However, at 19 per cent by late afternoon, oxygen levels were unlikely to increase any further and a decline overnight and possible fish mortality the next morning were a real possibility.”

The spokesman added that angling clubs and fishery owners “should be regularly monitoring their water quality at this time of year as part of good fishery management”.

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To prevent a serious incident occurring, the Environment Agency said it “proactively decided to deploy our chemical aeration equipment to give the oxygen levels a much needed boost and see the fish through the night and over the next few days”.

They added that the drain will be “closely monitored this week and we’ll be ready to respond again if required.

“So, if any anglers are out walking the river banks or around stillwaters this week and you see dead or distressed fish, please call our incident hotline number immediately on 0800 807060.