Indigenous white-clawed crayfish in the River Wharfe are almost certain to be wiped out, a nature group has been warned.

The crayfish is falling victim to the impact of its cousin, the American signal crayfish, which was introduced to British waters towards the end of the last century.

As well as winning the battle for the same food against their smaller local hosts, the American invaders are spreading a deadly fungus. They are immune to the disease but it has fatal effects on the white-clawed crayfish.

And now according to an expert, the signal crayfish are spreading up and down the River Wharfe – spelling disaster for the white-clawed variety.

Speaking at a meeting of the Wharfedale Naturalists’ Society, ecologist and renowned crayfish specialist Stephanie Peay said latest studies revealed a bleak prognosis.

It is believed the effect of the American crayfish has spread further along the River Wharfe at least as far south as Ilkley and also northwards, especially into some of the small tributaries. The American animal can grow up to 17cm long but there are no real figures on how many of them are in the Wharfe. Wharfedale Naturalists’ president, Peter Riley, said things are not looking good for the English crayfish. “The signal crayfish are spreading south and also spreading north including up the tributaries,” he said.

“It’s a bit like red and grey squirrels where it’s not necessarily direct competition, it is the disease the grey squirrels carry that they pass on,” he said. “It’s been demonstrated by practical application that controlling them physically, by moving them or trying to kill them or whatever, isn’t going to work. It’s fairly simple I’m afraid.”

Mr Riley said the only faint hopes were research suggesting the American crayfish cannot get past some physical barriers such as waterfalls or nature taking a hand.

“It is a pretty bleak picture,” said Mr Riley.