Quick-thinking anglers "saved the day" after more than a dozen big fish came to the surface of a Hampstead Heath pond "gasping for air" then died.

Around 15 carp perished on June 25 due to a reported oxygen crash in the No. 2 Pond caused by algae in the water.

Fishermen on the banks alerted the Environment Agency and staff from the City of London Corporation, which runs the Heath, who brought in oxygen pumps.

Hampstead and Highgate Anglers Society (HaHAS) chairman Bob Gibb said: "Numerous carp weighing 20 pounds or so were swimming on the surface gasping for air. Even the red signal crayfish - mini lobsters - were climbing up the banks for air.”

Mr Gibb blames a “massive” silt accumulation stirred up by unauthorised swimmers caused an algal bloom that left the water like “pea soup”.

"Another few hours would likely have resulted in a total fish kill. Full credit to the anglers who saved the day," he added.

Algae is a primitive plant form that gives off oxygen during the day but absorbs it at night, with some species also producing toxins.

Mr Gibb claimed there had been warnings.

"Climate change and an excessively warm June exacerbated the situation and it's found a lot of the heath kit was not functioning or poorly maintained,” he said.

He believes oxygen pumps could be used “proactively” overnight – but there is a “fear of electrocution” of unauthorised swimmers.

He added: "There are three heath ponds and the lido where people can swim but unauthorised swimming goes on wherever these people can get in. The model boating pond for instance is like Glastonbury on a sunny day so it's dangerous for them and bad for the environment." 

A City Corporation spokesperson said it was working with the Environment Agency and anglers.

They added: “Algae blooms naturally occur in inland waters right across the UK, especially after periods of low rainfall and extreme heat.

“We acted quickly following the algal bloom on Hampstead No.2 Pond last month and have successfully stabilised oxygen levels using aerator units.

“As part of our Wetland and Pond Management Plan, our staff are proactively monitoring oxygen levels and deploying aeration equipment when conditions change.”