The species, which devastates fishing stocks, is the scourge of fishermen and causes river banks to cave in, has been discovered in the River Kelvin.
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According to conservationists, they may have been introduced on purpose by people acting on tips from celebrity chefs, some of whom have recommended the crustacean as a tasty wild treat.
For anglers, however, the crayfish’s arrival could undo years of work stocking the river with salmon and trout.
Fishermen on the Kelvin warned that it was “a serious threat” to Glasgow’s second river, and Dr Colin Bean, fish ecology adviser at Scottish Natural Heritage, described the development as “very disappointing”.
Signal crayfish were first found in Scotland in 1995, he said, and since then have spread rapidly through the country. They have already taken over waters in Dumfries and Galloway, the Borders and Fife, with a presence also recorded in Lothian, Tayside and Grampian.
“They’re like the Steve McQueen of the invertebrate world -- they can escape from anything,” Mr Bean said. “They’re big and can impact fish directly by predating them, or keeping them out of preferred habitats.
“They can also burrow into banks and destabilise them, which presents a danger to livestock if any cows or sheep are walking along the river bank, and to humans as well, to an extent.”
The arrival of the species was first noted earlier this month near the Glasgow University science park in Maryhill, but due to the species’s aggressive expansion habits their territory is likely to extend well beyond that.
One study revealed in The Herald earlier this year found nine crayfish per square metre in Loch Ken, in Dumfries and Galloway, even after a Scottish Government-backed programme that destroyed a million of them. They have now colonised 108 miles of Scottish rivers and several lochs, according to Scottish Natural Heritage.
“It’s clear that these things have been moved about by people; they haven’t been getting there of their own volition,” Mr Bean said. “People like Hugh Fearnley-Whittingstall and Rick Stein catch crayfish and show how to catch these things as a free harvest.”
For anglers, however, the crayfish are a menace. They will put potentially fatal pressure on fish stocks that have only recently bounced back from a century of decline during Glasgow’s industrial period.
A statement from the River Kelvin Angling Association warns members of the dangers crayfish pose.
It says: “The signal crayfish is a serious threat to the native biodiversity of the Kelvin system, and extreme care should be taken to avoid the inadvertent transfer of small specimens on fishing tackle and boots.
“Tackle used on the Kelvin should therefore be disinfected before use in other river systems, including those in other parts of the Clyde catchment.”
It is also illegal to transport crayfish between different bodies of water, and even being caught in possession of one without a licence can carry serious penalties.
But now that the crayfish are established in the Kelvin, Mr Bean said, there was little chance of getting rid of them. He said: “It’s incredibly difficult, if not impossible, to remove them.”
American signal crayfish have no natural predators in Scotland, and so can flourish unchecked, unlike in America. Authorities have variously suggested using electric shocks, or introducing eels to keep their numbers down, but so far nothing has proved successful.
The grey squirrel of the deep
The signal crayfish is a large, lobster-like invertebrate which is native to western areas of North America.
It was introduced to Britain in the 1970s for aquaculture and was considered better than native species because it was less prone to infections.
It has spread quickly since the 1970s and is able to move between neighbouring bodies of water in search of habitats and food.
They are voracious predators and can seriously impact on the salmon population, as well as lamprey and freshwater pearl mussels.
Crayfish also dig tunnels for shelter and can destabilise river banks if present in large numbers, causing livestock to fall into water and drown.
The crayfish are known to be present in at least 108 miles of Scottish river bed, but relatively little is known about their range and numbers.
Trapping has proved ineffective and it is difficult to poison crayfish without harming other species that live in the same habitat.