White Clawed Crayfish Surveys

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White Clawed Crayfish
© Matteo Photos

Background

The white-clawed crayfish Austropotamobius pallipes, is Britain’s only native freshwater crayfish.

It is an olive-brown colour with a pitted appearance on the upper surface of its claws. It can grow up to 12cm long but sizes below 10cm are more common.

White-clawed crayfish can live for more than 10 years, and usually reach sexual maturity after three to four years. Breeding takes place in autumn and early winter (September to November) when the water temperature drops below 10°C.

Females hold their eggs beneath their tail over the winter. The exoskeleton of the white-clawed crayfish moults as they grow and juveniles become independent after their second exoskeleton moult, which is usually between June and August in the UK.

White-clawed crayfish typically live in rivers and streams approximately 1m deep where they hide among rocks and submerged logs. In flowing water, they are normally associated with overhanging banks, submerged rocks, woody vegetation and areas with varied flow patterns with refuges.

White-clawed crayfish are omnivorous, mainly feeding on worms, insect larvae, snails, small fish, macrophytes and algae.

White-clawed crayfish are highly susceptible to competition for food and shelter from three non-native species including the North American signal crayfish Pacifastacus leniusculus. Introduced in the late 1970s and 1980s, the signal crayfish spread quickly across the UK resulting in almost a complete loss of the native crayfish through disease and direct competition. Signal crayfish are more aggressive than native crayfish, and can dig quite extensive networks of burrow holes in suitable river banks, which can lead to river bank collapse.

The white-clawed crayfish is listed in Appendix III of the Bern Convention, and Annexes II and IV of the EC Habitats Directive, and is protected under Schedule 5 of the Wildlife and Countryside Act (1981) in respect of taking from the wild and sale.

Schedule 9 of the Act makes it an offence to release or allow to escape into the wild all three non-native species of crayfish in the UK, and the use of white-clawed crayfish as bait is also illegal. In the UK it is an offence to keep any crayfish without a license, except in some parts of southern England.

Survey

Surveys for white-clawed crayfish involve the use of baited traps, manual searches (both for live crayfish and for evidence of crayfish) and after-dark torch-light surveys. Trapping is the most robust way of finding out if crayfish are present, particularly if the water is deep.

Trapping surveys use a plastic mesh trap with funnel entrance and are baiting with scraps of oily fish, cat food or other suitable bait and are inspected the day after setting.

Mitigation

The presence of white-clawed crayfish on a site would be a material planning concern, and efforts should be made to ensure that crayfish and their habitat are retained and suitably protected within a development wherever practicable.

If this is not feasible, it may be necessary to capture and re-locate animals to suitable alternative habitat under a licence from the relevant body. Exclusion of crayfish can be carried out at any time of year if water temperature is 4°C or more, as crayfish are more likely to stay in refuges if water is cold.

Additionally, crayfish should not be exposed to air temperatures below 0°C and late May and June should be avoided to avoid disturbance to females carrying eggs.

Contact us to discuss your site and requirements in more detail.

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