Chinese mitten crabs threaten wildlife

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Non-native crabs from China are spreading through the UK and placing our native species at risk, says new research.

Chinese mitten crabs, Eriocheir sinensis, were first spotted near Lots Power Station in the River Thames in 1935 and are believed to have arrived in the UK as stowaways in the ballast water of large ships.

Reaching up to 8cm across the carapace, with legs about 12m in length, the big crabs are entering estuaries and freshwater rivers up and down the country where they are predating on native crayfishes, the eggs of fishes, and damaging river banks by digging enormous burrows in them.

The results of a new study undertaken by Dr Matt Bentley and a team of co-workers at the University of Newcastle are predicting that the crabs will spread to all of Britain's estuaries.

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

Bentley says that the crabs pose a particular threat because they are capable of migrating up to 1000km and can even travel across dry land.

Spreading northThe study looked back at historical records showing the presence of Mitten crabs in the UK and found that they were moving north, as well as south west.

Mitten crabs have spread north through the UK over the past few decades.

The crabs are already present in the Thames, the Humber and Tyne, as well as parts of the North Sea and English Channel coasts; Bentley says if you ever find a crab in freshwater it's almost certainly a Mitten crab.

He says that something needs to be done to continue monitoring the spread of the alien crabs before it's too late to eliminate the species:

"This study demonstrates the importance of a monitoring programme for the mitten crab, even if its appearance is just a rare occurrence in an estuary. Records demonstrate the crab's ability to rapidly expand once the local population reaches a critical density and or conditions become favourable.

Chinese mitten crab, Eriocheir sinensis.

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

"This study shows there is a need for a monitoring system for the mitten crab which could help manage the spread at an early stage. Low cost options could include a public awareness campaign where anglers and other users of rivers and the coastline are encouraged to report sightings of crabs. Measures which are currently used to monitor fish in our rivers, such as the electrical fish counters, could also be adapted to include monitoring of mitten crabs."

The Chinese mitten crab is a delicacy and is eaten by the Chinese and Japanese. It gets its common name from the "fur" that grows on its claws, and it's scientific name roughly translates as "wool hand".

For more information see: Bentley M; Clare T; Rushton S and Herborg LM (2006) - The invasion of the Chinese mitten crab in the United Kingdom and its comparison to continental Europe. L-M Herborg et al, Biological Invasions 7:959-968.