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Invasive non-native species (UK) – Signal crayfish

GBNNSS ©Trevor Renals

This week’s article, the seventh in our invasive non-native species series, focuses on the signal crayfish…

Signal crayfish (Pacifastacus leniusculus) were introduced into the UK by the British Government in the 1970’s, intended to be farmed for food, primarily for the Scandinavian market. Signal crayfish escaped the fisheries and swiftly spread across watercourses and land, where they began to outcompete the native white-clawed crayfish (Austropotamobius pallipes) for both food resources and habitat.

Photo credit GBNNSS ©Crown Copyright 2011

Photo credit GBNNSS ©Crown Copyright 2011

Identification & Ecology
The signal crayfish has a varied diet which includes fish, frogs, invertebrates, plants and sometimes other signal crayfish.

Identification features of signal crayfish include:

  • Red underside to claws.
  • Upper surface is a greenish brown colour.
  • Lower surface is an orange/red colour.
  • Smooth ridge running along the middle of the rostrum.
  • Can grow up to 16-18 centimetres in size.

In contrast, the white-clawed crayfish is brown to olive colour and the underside of the claws are usually a white colour – not red. The white-clawed crayfish is also much smaller in size, growing to a maximum of 12 centimetres.

Habitat suitable for signal crayfish includes both still and slow-flowing freshwater environments, including rivers, streams, reservoirs and canals. Signal crayfish are also able to tolerate slightly salty water. Crayfish take shelter under rocks, within tree roots or in burrows and cavities within banks.

Signal crayfish have caused a number of environmental problems and can have a significant economic impact. They carry the crayfish plague, which is fatal to the native white-clawed crayfish. The plague is a fungus-like disease which spreads by waterborne spores. The spores can survive for up to 2 weeks in damp conditions making it important to disinfect and dry any equipment used during crayfish surveys.

The burrowing of the signal crayfish can cause erosion of banks, as they can create a network of tunnels that can go up to 2 metres into the bank. This causes both an economic and environmental impact, with collapse of banks causing increased flood risk, and silt load in the water. They can also reduce the value of fisheries by predating on fish eggs and taking habitat from the fish.

Legislation which attempts to control the distribution of signal crayfish, includes Schedule 9 of the Wildlife and Countryside Act, 1981 (as amended), which makes it illegal to distribute or allow the release of signal crayfish into the wild.

Control Measures
Control of signal crayfish includes predation, trapping and biosecurity.

  • Signal crayfish are predated by American mink, otters, and Atlantic salmon.
  • Trapping reduces numbers in the short term; however, trapping has been shown to favour the larger crayfish, resulting in an increase in overall numbers because there is reduced competition for the juvenile signal crayfish, leading to a population boom in the subsequent year.
  • The best defence against signal crayfish is biosecurity, ensuring that individual crayfish are not transported between waterbodies and that the spores from crayfish plague are killed prior to entering another waterbody. This includes all equipment, for example wellies/waders, boats, canoes, and nets which have been in the water.


  • The Inland Waterways Association HERE.
  • Non-Native Species Secretariat (NNSS) factsheet HERE.
  • Buglife UK Crayfish Hub HERE.

Header Image: Photo credit GBNNSS ©Trevor Renals

About the Author: Elizabeth Kimber works for an ecological consultancy based in Dorset (Lindsay Carrington Ecological Services Ltd). As part of the role she manages the ecological works for a multi-phase development. She conducts protected species surveys and holds a class 1 bat licence, smooth snake and sand lizard licence. She can be contacted via email: liz (at)