Distribution of freshwater crayfish in the British Isles, with particular reference to crayfish plague, alien introductions and water quality



1. The distribution of native and alien crayfish in the British Isles, based on records from 1970–1991, is figured at the 10 km square level and that for Britain is tabulated regionally.

2. Since 1981 crayfish plague has been recorded from six catchments in Britain and one in Ireland, and suspected from an additional four catchments in Britain; 56.2% of native crayfish sites in Britain occur in catchments from which plague has been confirmed.

3. Since the mid-1970s approximately 300 signal crayfish implants have been made in Britain. Only 117 “successful” implants of signals are known to the authors; 39 crayfish farmers were registered in 1986 and 68 by 1990. In 1989 declared production was approximately 7 tonnes.

4. Some mixed populations of natives and disease-free signals exist. Evidence for exclusion of natives is being monitored.

5. Signal crayfish are found in the flowing waters of seven catchments; 58.1% of native crayfish sites are located in catchments which support farmed and wild populations of signals. Some rapidly-expanding Turkish crayfish populations are known, particularly in the Thames catchment.

6. At least 87% of native crayfish records are from good quality waters.

7. Native populations from 88.6% of sites recorded in Britain since 1970 have either been eliminated, or are directly threatened, by crayfish plague infection, and/or habitat invasion by signals, and/or pollution.

8. The native crayfish was added to the list of species protected under the Wildlife and Countryside Act in 1986.

9. Recommendations to assist in the conservation of the native crayfish include a ban on future imports of alien crayfish, a reassessment of records, the setting up of “no-go” areas, the protection of isolated sites, and a restocking programme.

10. There are many restrictedt, healthy populations of native crayfish in areas which could be protected by designating them as Sites of Special Scientific Interest (SSSIs).