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Native crayfish are feeling the pinch

Posted: Wednesday 12th July 2017 by RiverSearch

American Signal Crayfish

A tasty treat turned into a natural nightmare. The American signal crayfish is now widespread across the UK and causing damage to our native wetland species.

Introduction
The Signal Crayfish (Pacifastacus leniusculus) was introduced from North America to the UK in the 1970s for export to the lucrative Scandinavian market where crayfish stocks had been decimated by a fungal disease known as crayfish plague. Unbeknownst to the UK, the new arrivals also carried the plague but had developed a resistance to it.
Signal crayfish soon escaped from commercial fisheries and began to outcompete our native white-clawed crayfish (Austropotamobius pallipes) for habitat and food, but worst of all they passed on the crayfish plague.
As a result we have seen our native crayfish populations decline by 95% and the species is now classified as 'endangered' on the IUCN red list of threatened species and is at risk of global extinction. In the UK, crayfish plague is still continuing to wipe out the small number of populations we have left in the wild.
On the river Wey catchment in Surrey there are only 3 known sites where our white claw crayfish are still hanging on. These sites are small streams away from people and right at the top of tributaries.

Description
The signal crayfish is lobster-like in appearance and reaches a maximum size of 16-18cm.
Its claws have red undersides with a small turquoise/white blotch on the upper surface at the claw hinge. The upper surface of this species is usually brown to greenish-brown, while the lower surface is often a contrasting bright orange or red. The signal crayfish has a distinctly smooth ridge running along the middle of the rostrum (the foremost projection of the carapace).
In comparison, the native white-clawed crayfish is much smaller (under 12cm), with a brown to olive pitted body. The underside of the claws are usually a dirty-white colour - never red.

Habitat
The signal crayfish occurs both in still and slow-flowing freshwater environments, including rivers, streams, lakes, reservoirs and canals. This species is also known to be able to tolerate slightly salty water.
It takes shelter under rocks and boulders, within tree roots or in burrows and cavities within banks. In the winter, adult signal crayfish shelter in burrows and enter a state of torpor. These burrows are formed of many inter-connecting tunnels, and can be up to two metres deep.

Reproduction
The signal crayfish has a relatively high reproductive potential, with the female of this species laying between 200 and 400 eggs in the autumn. The female then carries the eggs around under her tail through the winter until the spring, when the eggs hatch. Once hatched, the young remain attached to the female's tail until they are released in May or June. Signal crayfish reach sexual maturity at about 2 or 3 years old, and can live for up to 20 years.

Dispersal
The signal crayfish is well established in England and Wales, especially in the south-east of England. They are not as prevalent in Scotland but several well-established populations have been recorded.
Signal crayfish are able to disperse up and downstream, cross most natural and artificial barriers and travel over land to reach water bodies nearby. Dispersal distances of over 300m in 2 days, and several hundred metres over land in one night have been recorded. Dispersal has been aided by human transfer and release of individuals, although this is now illegal.

Other Problems

Environmental
Signal crayfish's extensive burrows can destabilise banks, causing erosion and bank collapse, increasing flood risk and the silt load in the water. Their burrows displace threatened riverside species such as Britain's water vole (Arvicola amphibius).

They feed on fish and amphibian eggs, tadpoles, juvenile fish, aquatic invertebrates, detritus and aquatic vegetation and where present reduce populations of native species and affect food webs.

Economic Impact
Signal crayfish burrowing can cause erosion of riverbanks. Burrows can be up to 2m deep, with many inter-connecting tunnels that weaken the bank. This can contribute to problems with flooding, livestock safety and stability of structures built on the banks.
Crayfish also take refuges from salmonid fish and predate fish eggs, which could reduce the value of commercial fisheries.

Control

Predation
In the UK, the signal crayfish is known to be predated by otters (Lutra lutra), American mink (Mustela vison) and predatory fish such as Atlantic salmon (Salmo salar) and European eels (Anguilla anguilla).

Trapping
Trapping trials have concluded that although numbers may be reduced during the short-term, traps may favour the capture of larger individuals. An unintended consequence of selective harvesting is the increased growth and earlier maturation of juvenile crayfish, which can cause the population to increase. It is not, therefore a sustainable long-term solution.
Anyone wishing to trap signal crayfish from the wild must obtain a trapping licence from their local Environment Agency office.


What Can We Do

CHECK, CLEAN, DRY

The spores of crayfish plague can survive for up to two weeks in damp conditions, but can be killed by drying or disinfecting. The introduction of signal crayfish is usually the source of new outbreaks, however spores may also be carried between waterways on wet fishing equipment, boots and boats.

Anglers –
Dry or disinfect any boots or equipment before moving between waterways.
Avoid fishing different waterways on the same day.
The use of crayfish as bait is illegal in any form.


Boaters –
Check the boat exterior for young.
If removing boat from waterway, check the bilge and interior for any obvious crayfish and flush the engine cooling system through with fresh water from a piped supply

Canoeists -
Drain the boat as usual and check the interior for any obvious crayfish
If possible flush the boat out with fresh water from a piped supply
Under the Wildife and Countryside Act 1981 (as amended), it is an offence to release, or allow to escape, any non-native species into the wild in the UK except under licence.
If a signal crayfish is caught - bring it ashore or into the boat for humane destruction.
 

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