Mitten crab could deliver a knockout punch to river life

Mitten crabs, which can grow to the size of a dinner plate, have swept through Europe, crawling as far as the River Tyne, in Northumberland.

Now Dr Matt Bentley, a researcher at Newcastle University, has predicted that mittens could come as far as Scotland.

The mitten crab - so called because its claws are coated with clumps of dark-brown fur - would cause damage as it feeds on salmon eggs and destabilises river banks by burrowing boreholes 50cm long into them.

The invaders, considered a delicacy in China and Japan when eaten raw, can survive in sea and fresh water and are adept at moving over land. They are already present in waterways such as the Thames and Humber, and parts of the North Sea and Channel coasts.

They were brought into Europe in ships' ballast water. The UK spread is most marked along the east coast northwards to the River Tyne and on the south coast west to the River Teign.

Dr Bentley, a member of the research team, said: "The pattern of the spread in the UK since the 1970s mirrors the spread in mainland Europe and in the Baltic region, which experienced a major outbreak. This is a fairly good indication that the UK is set for a similar situation."

And he said the crabs could come as far as Scotland. "There's every likelihood. They will almost certainly spread up the coasts of the UK. Temperature is not likely to be a barrier to them; they have become well established round the Baltic Sea."

Dr Bentley said it was unlikely that the crabs would colonise Scotland in the next decade, but they have the capacity to do great damage if they do come.

He said: "If they are present in those sort of numbers, clearly they could cause a great deal of destruction. The question is whether or not a river like the Spey could support a crab population of that level. "It's unlikely they would be up around the salmon spawning grounds, but they do have the capacity to walk 1,200km [upstream]."

The study, published in the academic journal Biological Invasions, recommends that a nationwide monitoring and trapping system for the crabs should be introduced before it is too late to control the population. The team also asked anglers to look out for mittens, so their spread can continue to be monitored.

Dr Bentley said: "With most invasive species, such as the grey squirrel, the problem is not recognised until it is too late to do anything and you cannot eliminate it without taking drastic environmental measures."

Charlie Duncan, a keen salmon fisherman and conservationist, said there were also threats to Scottish salmon.

"We have to have a plan of action before this becomes a real threat to Scottish salmon fisheries," he said. "We know already the degradation caused by the American signal cray fish and it is very desirable we start a campaign on both fronts soon."