Alien invasion in the fens could destroy our wetlands experts warn

BOB Riches had never seen anything like it. Plodding through the mud was the strangest creature he d ever set eyes on, in all his 79 years in the Fens. What looked like something from another planet turned out to be an alien invader which can threaten na

BOB Riches had never seen anything like it. Plodding through the mud was the strangest creature he'd ever set eyes on, in all his 79 years in the Fens.

What looked like something from another planet turned out to be an alien invader which can threaten native species and even increase the risks of flooding.

Retired farmer Mr Riches from Denver, near Downham Market, saw what looked like a giant crab when he went to check his cattle on the Ouse Washes, near Welney.

"We own some grazing land out near the washes I went to check all was well when something moved strangely in the mud," he said.

"It was a frosty morning, I thought that's something strange. I could see it was some peculiar sort of crab, so I popped it in a potato sack to take home to show my grandchildren."

Mr Riches' two young grandchildren wouldn't go near the crab. His daughter Sandra Florido said: "We were shocked when he brought it home, it was still alive. It looked like a horrible, gruesome thing."

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Mrs Florido called Hunstanton Sea Life Sanctuary, to see if they could identify the creature. When experts saw its furry claws, they realised it was a Chinese mitten crab.

Sanctuary manager Nigel Croasdale said: "We think the crab must have scuttled up one of the underground tunnel ways used to let water from the River Ouse wash over the cattle's grazing land every summer.

"We're extremely concerned at what this could mean for the Ouse Washes nature reserve. Chinese mitten crabs are an alien species to our British waters, and multiply extremely quickly, causing chaos and landslides when they burrow in their hundreds into riverbanks."

Mr Croasdale said if the crabs multiplied in the area, they could threaten rare birds such as the terns which breed on the washes. But conservationists said the creatures' burrows posed a greater danger.

Lee Marshall, reserve warden for the Wildfowl and Wetland Trust's reserve at Welney, said: "They're on the washes, they're in the river and they're in quite a few other places.

"We get the odd one or two, they're not causing a problem at the moment but we're monitoring the situation."

While the crab's wanderlust helps it rapidly colonise new pastures, its fondness for a stroll on dry land would also make it vulnerable to the otters and birds of prey which also inhabit the washes.

Graham Elliott, the RSPB's Fens area manager, said the crabs were not believed to pose a threat to birds. "The main concern is that they can undermine the banks and damage watercourses," he added.

Environment Agency surveys have found the crabs as far inland as Holywell, near St Ives. After spending half their lives marching upstream, the crabs do a U-turn and head for the sea to spawn.

Natural England marine specialist Paul Gilliland said: "By burrowing into muddy banks, they end up causing the banks to collapse if there are loads of them. They end up like a bar of Aero with holes everywhere."

A mitten crab caught by an angler on Heacham Beach last year was carrying thousands of eggs. Mr Croasdale urged anyone seeing one to report it.

"This could be an isolated incident, but we would ask those visiting the area to be vigilant and get in touch if they see any more Chinese mitten crabs out on the washes," he said.

"If the tunnels are providing the crabs a route to invade, we will need to examine our options as a community."


Eriocheir sinensis - the Chinese mitten crab - is among the world's most invasive species.

It colonised the Thames after being carried in the bilges of cargo ships from the Far East.

The first recorded sighting was in 1935.

Since then, the crabs have spread as far north as the Humber and are becoming increasingly common in the Great Ouse and its feeder rivers.

Last year an army of the crabs, several hundred strong, was found marching along the Thames.

They can survive walking several miles across dry land in search of food or fresh rivers to populate.

They've even been found in swimming pools.


Mitten crabs burrow deep into riverbanks with their powerful claws, to hide from predators and avoid drying out in warm weather at low tide.

There are fears this could weaken the banks which protect low-lying areas of the Fens from being flooded.

Banks being breached - by the weather, not by crabs - caused thousands of hectares to flood in the winter of 1947, ruining the potato crop and sparking food shortages.

Pike anglers have had large fish baits ripped in half by greedy crabs. Coarse fishermen have been warned not to rinse their hands in the river after mixing groundbait.

Naturalists fear mittens will eat fish spawn, small mammals and amphibians; upsetting the natural ecology of our rivers.

Scientists are investigating whether the invasion can be stemmed by trapping the crabs commercially.

In China they are called moon crabs and considered a delicacy. One solution could be exploiting the interlopers as a food source - like their fellow invader the American signal crayfish.