An invasion of crayfish at Hampstead Heath’s ponds has left native wildlife struggling to survive, fishermen warn.

The Red Swamp crayfish is a non-native species to the ponds and, without any natural predators, the spindly, sharp-pawed shellfish are taking over in their thousands – allowing other fish little chance of survival.

Doug Slade, chairman of the Anglers’ Association of Hampstead Heath, said the number of crayfish he used to fish out of the ponds a week was around 200.

Now a fisherman has reported catching 180 crayfish in an hour.

“They just breed and breed and breed,” said Mr Slade. “They eat all the fish and nothing can stop them.

“We don’t want them there because they’re destroying the wildlife and they don’t belong. They’re vermin.

“The only way to kill them is to freeze them. I used to put them in my freezer and then give them to the pet shop.”

Nature enthusiast Ron Vester, who captured the crayfish on film, says the shellfish are destroying the ecosystem.

He explained: “They kill the little fish which the big fish need to survive so there’s a shortage.”

Heath by-laws dictate that crayfish cannot be killed without a special licence.

But some, who view the tender, lobster-like meat as a delicacy, cannot be stopped from catching them.

There have been reports from fishermen that the owner of a Chinese restaurant has been catching his fish for free at the ponds.

Over the years, Heath managers the City of London Corporation has granted some licences to catch crayfish for eating.

But the organisation has warned pond users to be wary.

In September 2010, the Corporation issued a formal warning advising people not to swim naked after a swimmer was “attacked” by a crayfish while he was paddling nude in the men’s pond.

The corporation wants people to remain aware of the dangers.

A spokesman said: “We would prefer it if the crayfish had not been released on the Heath in the first place given they are a non-native species.

“Now that they are here, though, we would recommend swimmers to keep their distance to avoid getting into a pinch.”