Content
White-clawed crayfish scoping survey
An initial white-clawed crayfish scoping survey is generally carried out as part of a Preliminary Ecological Appraisal. A desk top study is carried out to identify any records of white-clawed crayfish and other invasive alien crayfish species close to site, with a field survey undertaken to assess any onsite and adjacent water bodies for their suitability to support crayfish.
This scoping survey can be carried out at any time of year, however a dedicated presence/absence crayfish survey may be recommended if water bodies upon or adjacent a site are suitable to support crayfish and a proposed development or an activity is likely to have an impact upon white-clawed crayfish or the places they use for breeding, shelter or protection.
White-clawed crayfish presence/absence survey
Crayfish prefer unpolluted, calcareous fresh waters with limited sediment and plenty of shelter in the form of rocks, aquatic plants and tree roots to hide from predators and to avoid being washed downstream in times of heavy flow. Therefore, white-clawed crayfish presence/absence surveys primarily involve actively searching for crayfish in suitable habitat during times of low water flow. This involves a hand search under suitable refuges such as river bed rocks. It is less effective when bankside refuges are abundant, or where there is soft substrate.
Another method used is night searching. This involves searching water bodies at night using a high powered torch when crayfish are more active, and is a suitable option where deep water means active searches are not feasible.
Finally, baited or un-baited traps can be deployed; although these can be poor at detecting low populations and have animal welfare risks in certain circumstances.
Timing of surveys
- Scoping surveys can be undertaken at any time of year and can be undertaken as part of a Preliminary Ecological Appraisal;
- The recommended time for carrying out white-clawed crayfish presence/absence surveys is between July and October. Surveys should be avoided between April and June, to reduce disturbance to females whilst carrying eggs or releasing young.
Mitigation and licencing
Dependent upon the scale and type of impacts identified, measures to prevent harm to white-clawed crayfish should be undertaken. Potential impacts could include direct disturbance of the habitat such as would occur in river re-profiling or indirect impacts e.g. a new water outflow that will affect water chemistry.
In some cases white-clawed crayfish may need to be removed and relocated under a Natural England and/or an Environment Agency licence. Where feasible, any works to a water body supporting white-clawed crayfish should ensure the habitat is suitable for crayfish post-works or an alternative habitat is enhanced for this species. The provision of suitable refuges is therefore critical, as well as creating areas of low flow.
Legislation
White-clawed crayfish are afforded protection under Schedule 5 of the 1981 Wildlife and Countryside Act (WCA) (as amended) making it illegal to kill, injure or take individuals of this species.
White-clawed crayfish are also listed in Appendix III of the Bern Convention, and Annexes II and IV of the EC Habitats Directive.
Note
The above information is based on standard guidelines and current UK legislation. Every site is unique so please contact a Torc Ecology consultant who will advise on the best survey approach in relation to the project requirements.
Facts about white-clawed crayfish!
The white-clawed crayfish is the UK’s only native freshwater crayfish. It can live for more than 10 years and grows up to 12cm long, although sizes below 10cm are more common. It lives predominately in clean freshwater rivers and streams where the water depth is below 1m and the river bed contains stones, rocks and/or small crevices under which it can shelter. The white clawed crayfish diet is primarily carnivorous including small invertebrates and carrion, although it is an omnivore and will eat water plants, particularly those with a high calcium content which they will use to grow their shells.
Once widespread and common, the white clawed crayfish has suffered a large population decline. Non-native crayfish species, particularly the North American signal crayfish, have introduced disease to which the white clawed crayfish have no natural resistance. It is also considerably larger than the white-clawed crayfish and directly out-competes it for food and shelter. Other factors in the decline of the white clawed crayfish include pollution; it is highly vulnerable to biocides and silage which can cause large scale mortalities.
