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Scientists build traps to manage UK’s rising number of Chinese mitten crabs

It is classified by conservationists as one of the 100 worst invasive alien species in the world. Now, a group of scientists are hoping they have found a way to deplete the UK’s rapidly growing Chinese mitten crab population and prevent the crustaceans, which can grow bigger than a 10-inch dinner plate and have distinctive furry claws, from “eating us out of house and home”.

The group has constructed and installed the UK’s first Chinese mitten crab trap at Pode Hole in Lincolnshire, to catch the voracious predators as they migrate downstream to mate.

“The mitten crab are eating our native fauna,” said Dr Paul Clark of the Natural History Museum, one of the scientists working on the project. “If we start capturing these crabs and depleting their population, we might see changes in our environment for the better.”

This could include an increase in the local population of spined loach, a freshwater fish which is reported to have declined at Pode Hole, along with water voles, bivalve mollusks, small shrimp and snails.

The Chinese mitten crab, so-called because it originated in south-east Asia, was first sighted in the UK in 1935. Since then its population has exploded.

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“Females can spawn two series of eggs, and they spawn somewhere between 500,000 and one million eggs in one go,” said Clark. “There’s millions in the Thames and huge populations in the Medway, the Ouse Washes, the Dee, Tyneside and the Humber. They are eating us out of house and home.”

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He described seeing a mitten crab devouring a snail. “The crab treated it like an ice-cream cone. It nipped off the top, pulled the snail out from its shell – and ate it.”

Similarly, he said, his students have filmed a mitten crab “crushing an amphipod against the underside of its carapace, and with its other claw, it killed the shrimp and ate it”.

Research has shown the crabs also eat salmon and trout eggs – native fish species which are already under threat in the wild.

The scientists plan to freeze the crabs they catch, and then hope to be able to dissect them to find out what they have been feeding on. “If we can, we will do some DNA analysis on the digested guts.”

The “experimental” trap, which has a unique letterbox construction, was designed by Mick Henfrey, a foreman at the Welland and Deepings Internal Drainage Board, and Oscar Jones, a graduate of the Department of Engineering at the University of Sheffield.

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Jones came up with the idea for the project after a Chinese Mitten crab was spotted by a member of the Lincolnshire Wildlife Trust climbing over the weir at Pode Hole. “We got together and decided to build a trap and the Environment Agency gave us permission to set it,” said Clark. “It’s like an extended letterbox that goes across the weir, with two openings, one facing upstream and one downstream. We hope they will climb into the letter box and then up a tube.”

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He said the trap will be inspected every day and hopes that, if it is successful, other traps can be installed in rivers such as the Dee, an important salmon habitat.

The project is being run in conjunction with the Lincolnshire Wildlife Trust and the Welland and Deepings Internal Drainage Board.

Members of the public are asked to report sightings of Chinese mitten crab online at mittencrabs.org.uk.

This article was amended on 16 October 2023. It is Pode Hole in Lincolnshire, not Pode Hill or Pode Hall.