Monday, 11 December 2006

Red signals danger for our crayfish

With the recent rain it is hardly surprising that the river is running so high. Still, it is a bright start to the day and I am optimistic that we will catch up with some crayfish, thanks to our trap and a punctured tin of cat food. We have just one native species of crayfish here in Britain, the white-clawed crayfish, but our chances of seeing this species are pretty slim on this stretch of river. Instead we are likely to see a good number of the larger signal crayfish, a menacing introduction from North America. While our native is a relatively small species, usually less than 10cm in length, the signal crayfish can reach 30cm. Bigger, aggressive and able to produce more young, this American interloper poses a real threat, something that is made worse by the fact that it can carry a virulent disease called crayfish plague. This disease seems to have little effect on the signal crayfish but it has contributed to drastic losses of native crayfish from many of our rivers.

A number of non-native crayfish species have been farmed here since the late 1970s and, perhaps inevitably, they have broken out and established themselves in the wild. Here they can occur in such incredible densities that their burrowing activities undermine river banks and flood defences. The threat from these alien crustaceans has now been recognised; the white-clawed crayfish has been listed as being globally threatened and research is underway to establish how the non-native species can be controlled and removed.

The Brecks Countryside Project is coordinating the research work being carried out locally. This work examines ways by which introduced species can be controlled through trapping; exploring different types of traps and bait, and looking to see if regular trapping has any impact on the size or structure of the crayfish population. The traps that we were on our way to examine the other morning were part of this project. Hauling them out of the water, it was clear that the signal crayfish were still very active at the site even this late in the year. The two traps held 29 crayfish, all signal. Each individual was sexed and measured, and then notes were made of the water temperature and flow rate. The mild conditions overnight had prompted the crayfish to venture out in search of food and the cat food would have proved highly attractive. This local scheme is a partnership between the Lark Angling & Preservation Society, the Brecks Countryside Project and The Environment Agency. Just coming to the end of its first year, the project has already generated some very useful information and it seems likely that more will follow next year.

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