The American crayfish first entered Scottish waters a decade ago

The predator, said to devour natural wildlife and wreck the ecosystem, has been spotted in Leithen Water.

Dr Ronald Campbell, the fisheries biologist responsible for the River Tweed, said the signal crayfish could be wiped out by poison or disease.

Nick Young, the director of the Tweed Foundation, said there was now a real opportunity to eradicate the species.

"We've seen the problems with hedgehogs on South Uist, rats on the island of Canna, as well as mink and grey squirrels," he said.

"This is a new alien and one which we have a real opportunity to do something about before it becomes too widespread.

"We need some agency to take responsibility for it or be given the responsibility by government, so that we can try and eradicate these things for good."

These crayfish are competitors, particularly for cover, but also for food Dr Ronald Campbell

The aggressive signal crayfish got into Scottish waters about 10 years ago and has been in England and Wales since the 1970s.

Dr Campbell said the absence of native crayfish in Scottish waters gave more options for controlling the predator.

"The obvious option is to bring a disease from the crayfish's native homeland in America or to find some crayfish-specific poison," he said.

"These are not options that are available where there is native crayfish."

Dr Campbell said the species had now been found in three locations in the Tweed.

"These crayfish are competitors, particularly for cover, but also for food as well," he said.

"If there's a pair of claws under every stone in the stream, the protection and cover is no longer there when the fish need it."