Friday, 20 January 2012

Dealing with (Chinese mitten) crabs

I joined a group of volunteers in Dolgellau for a training session on this alien crab. Chinese because it came from China, first recorded in the Thames in 1935, and probably transported in ballast water. Mitten because both claws are covered in fingerless, hairy mitts. No-one seems to know the reason for the mitts.

We see their arrival as a threat because they are multiplying and spreading fast causing problems on the Thames with erosion of riverbanks, caused by their burrowing, and blocking intakes of water to power stations. It’s said they have no predator although I’m sure otters would eat them and like all crabs they are vulnerable when they shed their shells.

A distribution map of records shows them mainly present in the east and southeastern waterways  but worryingly they are knocking on the door of Wales. There are many records from the Mersey and the Dee. A solitary male was recorded in the Conwy about 4 years ago and an individual was recorded in the English side of the Severn near Worcester. If they get a hold in Wales, the fear is they will eat the eggs of salmon and trout and encroach on species within Special Areas of Conservation (SACs).

They are highly mobile travelling up to 1500 km inland in China and quite happy to travel overland. On the Dee they can be seen walking round the weir and in Yorkshire there’s a lake, without any connecting stream, in which they are present. Maybe they will get to the source of the Dee and go cross-country to the Mawddach?

They are the only crab in the UK that lives in freshwater but they need the marine environment for breeding. In the autumn (September on the Thames) the 4 to 5 year old males migrate downstream to the brackish / saltwater and a little later are joined by females. The males die after mating and the females overwinter in deeper salt water returning to the estuary in February / March to lay between 250,000 to a million eggs over a three month period! After one or two months the larvae / juveniles migrate upstream to freshwater between March and May  and spend the next few years there until it’s time to breed. In China it all happens with a couple of years, maybe because it's warmer.

As volunteers we were being told to look out for their burrows; elliptical holes not far above the waterline, with a downwards angle (so they have a puddle beneath the water table) and signs on the outside of mud / silt that has been scraped out. Other techniques include looking out for skeletons and, in the spring, lifting (and carefully replacing) boulders in the intertidal area to look for juveniles. Next autumn we will try putting  out fyke nets to catch the adults going downstream to breed.

The hope is that if we can identify the crabs early enough we may be able to do something about them before they become a big problem as on the Thames.

At our training event we were lucky to have the company of Marvin, now resident in the UK, but originally from near Shanghai. When asked how they dealt with the problem in China he explained their problem was somewhat different; not enough crabs! For a thousand years or so they have been a highly valued delicacy but in the 1970s to 80s were fished into rarity. These days the national craving is only satisfied through crab farming whereby they are raised in pens within lakes from which they can’t escape.  

There was a young man in Marvin’s village who was the local expert at hunting wild crabs, he knew how to differentiate between burrows for crabs and those for snakes! He was also very adept at getting them out without too much personal pain; plate size crabs give quite a nip.

It strikes me that this, eating the invasive species, is a classic Natural Environment Framework type of solution.

If you’d like to know more about the subject visit the Chinese mitten crab website. If you’d like to join us volunteers  checking out the Dee, Clwyd, Conwy and Mawddach contact Rhian Hughes at the North WalesWildlife Trust.

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