Deadly crayfish close in on rare pearl mussels
THEY have dwelt in Scotland’s waterways since the last Ice Age and were, according to Roman historians, the reason why Julius Caesar invaded Britain.
But the freshwater pearl mussel, already critically endangered, is facing a potentially terminal threat from another foreign invader.
American signal crayfish released into the wild have spread through the nation’s rivers in recent decades and now directly threaten the remaining colonies of the rare molluscs.
Scottish Natural Heritage (SNH), the government’s countryside protection agency, says crayfish are now just 20 miles away from a prime colony of freshwater pearl mussels in the Tay.
Anglers are being encouraged to protect the mussel beds from being destroyed. The Tay Fisheries Board is urging fishermen and other river users to avoid anything which could help spread the crayfish, such as eggs which have attached to fishing gear. They also stress the need to kill any adult crayfish found and never return them to the water.
The warning follows SNH-supervised experiments which have demonstrated that crayfish will attack colonies when they eventually invade mussel habitat.
Scottish Natural Heritage’s freshwater adviser, Dr Colin Bean, said they now had evidence of a “mortal” threat to the mussel beds.
“Upstream in the River Earn, there are crayfish around Comrie,” he said. “You get mussels as far down as Perth. They haven’t clashed yet on the Tay, but there is a threat given how fast they spread. It’s not far away.”
Scotland is home to half the world’s population of freshwater pearl mussels. They have been harvested close to extinction on the off-chance they might contain a pearl, and are sensitive to pollution.
The precise location of surviving colonies is publicised as little as possible to give the mussel numbers a chance to recover.
But Bean said that after identifying the risk, SNH wanted to look into what would happen when the creatures met.
He said: “We put the two of them in tanks, looking at the behaviour of the crayfish.
“What we saw was them disturbing the mussels, throwing them around.”
Bean said this could harm the mussels, as it separated them from the river sediment where they thrive.
“The potential for mortality is greater than if they are left alone,” he said.
“In Germany it was found that crayfish had damaged the valves of pearl mussels, suggesting they did try to get into them,” said Bean.
Zara Gladman, a PhD student at the University of Glasgow, carried out the studies in a series of tanks at Stirling University.
She said: “The mussels are very endangered. As far as I know, crayfish haven’t invaded where they are now, but they are in the same river sections.”
Crayfish have colonised around 100 miles of Scottish rivers since they were accidentally introduced 17 years ago. Until now, salmon were considered the primary species at risk.
Bean said: “Signal crayfish were first brought to Britain largely for agrarian purposes, but they spread very quickly. They were first recorded in Scotland in 1995. Now they occupy 170km of streams and rivers.”
He admitted: “It’s going to be impossible to eradicate them, but we are still looking at ways to control them.”
Dr Peter Cosgrove, of Aberdeen-based ecological consultancy Alba Ecology, said it would be a “disaster” if crayfish invaded pearl mussel colonies.
Having studied freshwater pearl mussels for 16 years, he said: “It wouldn’t take the crayfish long. On the day they’re introduced they will forage prominently on the bottom of the river.
“On day one you would see mussels being clobbered by crayfish. The juveniles would be directly killed because they can crack their shells open. The adults would die slowly from being pinched and cracked. They would hurt their breeding capabilities.”
On average, three new populations of crayfish have been found annually since 1995, and government-sponsored drives to eradicate them have been met with failure.
An experimental bid to eradicate crayfish from a loch in 2009 failed to clear the crustaceans from the water.
The smaller females slipped through trappers’ nets, meaning they were free to breed again and spread their young throughout the loch.
Various eradication methods have been tried. A barrier was set up last year to prevent their spread along the River Annan in Dumfriesshire, but it is not yet known how successful it has been.
Signal crayfish have distinctive red claws and grow up to 25 centimetres long, about five times the size of native crayfish.
The American invaders, brought in to stock ill-fated crayfish farms, can be eaten by humans with the tails and claws the most edible parts.
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