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White-clawed crayfish

Background

The white-clawed crayfish Austropotamobius pallipes is Britain’s only native freshwater crayfish. It is readily identified by its whitish underside, and has a brown or olive body with a pitted appearance on the upper surface of its claws.

 

Native crayfish

Adult male white-clawed crayfish (Austropotamobius pallipes) - River Ock, Oxfordshire

 

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 for an extended period. It is also possible that changes in day length may play a role in triggering the breeding response. Females over-winter with a clutch of eggs held beneath the tail. The number of eggs carried may range from 20 to 160, but is usually less than 100. Eggs hatch on the female and juveniles become independent after their second exoskeleton moult, which is normally between June and August in the UK.

Adult white-clawed crayfish may grow to 150 mm in total length and reach 90g wet weight, although they are often much smaller. Males tend to be larger than the females and are more territorial.  They are largely nocturnal, although they can be seen foraging in the shallow margins of lakes as dusk approaches on warm summer evenings. Individuals affected by crayfish plague (see below) may also exhibit daytime activity.  White-clawed crayfish distribution in the UK is largely determined by geology and water quality, with relatively hard, mineral-rich unpolluted water being a fundamental requirement. They are found in a wide variety of aquatic habitats, including canals, streams, rivers, lakes, reservoirs and water-filled quarries.  With respect to flowing water habitats, they typically occur in watercourses ranging from 0.75 m to 1.25 m deep, but can also occur in small streams with depths as shallow as 5cm, and also in deeper, slow-flowing rivers up to 2.5 m deep.  Watercourses that are susceptible to drought do not tend to support white-clawed crayfish populations.

White-clawed crayfish typically occupies cryptic habitats under rocks and submerged logs, among tree roots, algae and macrophytes, although it usually emerges to forage for food. Juveniles in particular may also be found among cobbles and detritus such as leaf litter. Adults may burrow into suitable substrates, particularly in the winter months, and there are records of white-clawed crayfish burrowing into river banks. In flowing water conditions, they are typically found in association with the following features:

  • Undermined, overhanging banks.
  • Reaches with varied flow patterns with refuges.
  • Under cobbles (juveniles) and rocks in riffles, and under larger rocks in pools.
  • Among roots of woody vegetation, accumulations of fallen leaves and boulder weirs.
  • Under water-saturated logs.
  • Submerged man-made features such as bridge abutments and gabions

White-clawed crayfish are able to survive in rivers that exhibit strong flows (e.g. winter flood) as long as suitable permanent refuges that afford shelter in times of spate are present.  They are omnivorous, with worms, insect larvae, snails, small fish, macrophytes and algae being the principal components of the diet.

Conservation status

White-clawed crayfish was once a widespread and common species in English and Welsh rivers, but has suffered severe population decline throughout much of its natural range.  It is listed on the IUCN Red List of Endangered Species, and it is predicted that it will face extinction in much of its former range within the next few decades.

White-clawed crayfish are highly susceptible to competition for food and shelter from three non-native species, in particular the North American signal crayfish Pacifastacus leniusculus, which has spread widely in UK rivers as a result of accidental and deliberate introductions from fish farms since the 1970s.  Signal crayfish are more aggressive than native crayfish, and can dig quite extensive networks of burrow holes in suitable river banks, which over time can lead to river bank collapse.

Non-native signal crayfish

Non-native signal crayfish (Pacifastacus leniusculus)

 

Native and non-native species of crayfish rarely co-occur, and the spread of signal crayfish is one of the most significant threats to the survival of native crayfish in the UK.  White-clawed crayfish are also susceptible to disease, and in particular crayfish plague, a disease caused by the fungus Aphanomyces astaci which is carried by signal crayfish. Spores from the fungus can also be transmitted by a variety of other means, including water, fish and damp fishing equipment. Flood defence works, bridge strengthening, dredging and agricultural activities have also resulted in a loss of crayfish habitat, further contributing to its decline.

 

Signal crayfish burrow holes can under-mine river banks and lead to their failure

Signal crayfish burrow holes can undermine river banks and lead to their failure

 

Legislation

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 under the Act.

 

White clawed crayfish

White-clawed crayfish

 

White-clawed crayfish is also a Priority species in the UK Biodiversity Action Plan, and is a Species of Principal Importance in England under Section 41 of the NERC Act 2006 (Section 42 in Wales) and the Nature Conservation (Scotland) Act in Scotland.

Licensing

A licence is required at sites where there is a realistic expectation that white-clawed crayfish might be present and will be caught and handled during a survey.  Licences are issued by Natural England, the Countryside Council for Wales, Scottish Natural Heritage and (in Ireland) the National Parks & Wildlife Service. In England and Wales, Environment Agency consent is required for the setting of any traps of any design to catch native or non-native crayfish.

Applied Ecology Ltd ecologists have extensive experience in conducting surveys for white-clawed crayfish in standing and flowing water habitats and hold the survey licences for survey work in England.

Survey

Surveys for white-clawed crayfish populations typically involve the use of baited traps, manual searches (for live crayfish and field signs of crayfish) and after-dark torch-light surveys.  In practice the survey methodology employed will be dictated by local conditions, notably water depth and clarity.

Baited crayfish trap set overnight to capture crayfish as part of a native crayfish presence/absence survey

 

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.  In instances where it is not practicable or feasible to retain crayfish habitat, it may be necessary to capture and re-locate animals to suitable alternative habitat under the auspices of a licence from the relevant SNCO. Such operations need to be completed by experienced personnel since the potential for compromising the welfare of individual animals is high.

Typical mitigation measures for white-clawed crayfish could include the removal of individual animals from the watercourse or water-body prior to, or during, construction operations, alongside habitat reinstatement, creation or enhancement as necessary.  During de-watering operations, for example, crayfish can be collected as they emerge from crevices as the water level lowers and relocated up or downstream from where they can then re-colonise the construction area once it has been re-instated.

See Survey Calendar for survey timings.

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