Download The white clawed crayfish

The white clawed crayfish
Why does the white clawed crayfish need help?
The white clawed crayfish (also known as the Atlantic
stream crayfish) is the only freshwater crayfish native to
the British Isles. The species is widely distributed
nationally, although naturally absent from western Wales,
Scotland, and parts of western England and Ireland. It was
once widespread across much of Europe but has suffered
a serious decline across its range and is considered to be
a globally threatened species.
How do I recognise a white clawed crayfish?
• Crayfish look rather like small lobsters growing to
about 10 or 12 cm in length.
• The body is smooth, pitted and brown to olive in
Natural England
colour.
• They have large fearsome-looking claws have pale
undersides (hence the common name) but are darker
and rough above. These are larger in the male.
White clawed crayfish
• They walk using their four pairs of legs and can propel
themselves quickly through the water with vigorous
flicks of their fan-like tails.
Where do you find white clawed crayfish?
In Britain the white clawed crayfish requires clear, welloxygenated waters that contain enough calcium to form
their outer (exo) skeletons. This means they are usually
found in fast flowing streams, brooks, and rivers, but can
also be found in reservoirs and quarry pools that are
relatively free of fine sediment. Crayfish are active mainly
at night, hiding by day under stones, tree roots,
submerged plants or in burrows and crevices in the
riverbank.
What do they eat?
Native crayfish are omnivorous, feeding on both plant and
animal matter. They prey on snails, insect larvae and
scavenge for dead fish and vegetable matter. Like other
species of crayfish they are sometimes cannibalistic –
occasionally feeding on the softer-bodied young of their
own species.
Suffolk Wildlife Trust
What are its predators?
Juveniles are particularly prone to predation by fish, such
as trout and perch, dragonfly and aquatic beetle larvae,
and mature crayfish. Adults fall prey to rats, mink and
herons, especially after moulting when their bodies are
soft. In some parts of the country they are an important
source of food for otters.
Lifecycle
Crayfish take 3—4 years to reach reproductive maturity
and mate in October or November. Around 100 fertilised
eggs become attached to the underside of the female’s
abdomen and spend the winter in this state. The eggs
hatch into tail-less, miniature crayfish in the late spring
where they continue to cling to their mother before
moulting into a second stage with a rounded hairy tail fan
about three weeks later. The juveniles then leave their
mother and undergo up to five more moults before
reaching adulthood after their first year. In subsequent
years, the adult crayfish undergo an annual moult. After
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moulting, crayfish are soft and particularly vulnerable to
predation. Crayfish may have a life span of between 7 and
12 years taking up to six years before they reach full size.
Threats to white clawed crayfish
• Poor water quality
White clawed crayfish are susceptible to pollutants
such as fertiliser run-off and sewage which lowers the
oxygen concentration in water. They are also
vulnerable to certain pesticides especially synthetic
pyrethroid insecticides.
• Site and catchment management
Stream bed management can destroy or damage their
habitat and watercourses may become unsuitable for
crayfish through erosion of soil within the catchment,
which reduces water quality and clogs streams.
• Crayfish plague
This disease is caused by the fungus Aphanomyces
astaci and is a serious contributory factor to the
national decline of the white-clawed crayfish. Many
populations of the native species, especially in the
south, were lost to the disease from the 1980’s
onwards. It is thought that the disease is mainly spread
by the non-native species which appear to be
unaffected. Spores of the fungus can spread through
water and can become attached to fish scales, mud
and angling equipment. The fungus causes abnormal
behaviour (eg walking about during the day) and
blotching of the exoskeleton before death. There is no
means of eradicating crayfish plague.
• Competition for food and habitat from introduced
crayfish species
Four introduced crayfish species have become
established in the wild in the UK, two of which, the
Turkish crayfish and the North American signal
crayfish, have become widespread and abundant in
British waters. Both of these species, which arrived in
the 1970’s, can out-compete the native species for
resources and may even prey upon it.
Legal protection
White clawed crayfish are protected under Schedule 5 of
the Wildlife and Countryside Act (1981) and cannot be
killed, kept, sold or handled without a licence. The same
Act also prohibits the release of exotic crayfish into
the wild.
How can I help native crayfish?
• Equipment used in waters where non-native species
are present, or where native crayfish mortality has
occurred, should be thoroughly dried before use and
treated with a proprietary disinfectant before
further use.
• Report sightings of native or non-native crayfish to the
project (see the address below).
Other relevant SWT factsheets:
• Creating an ‘otter haven’
• The water vole
• The European otter
• Relevant conservation grants
For further advice, contact Suffolk Wildlife Trust on:
01473 890089
[email protected]