Signal Crayfish

Species Description

Scientific name: Pacifastacus leniusculus

 Identification sheet

Native to: Pacific Northwest

Habitat: Small streams to large rivers, and lakes from the coastal to the sub-alpine regions. In the UK, they occupy a similar ecological niche to the white-clawed crayfish, and so compete with them for habitat.

The signal crayfish were brought to the UK for aquaculture and have escaped or been introduced into rivers and streams as a food source.  They are carriers of crayfish plague which is deadly for the native white-clawed crayfish and has caused a serious decline in numbers.  They also compete for habitat with the white clawed crayfish.  They are voracious predators, eating aquatic invertebrates, plants, fish eggs and even young fish.  Work by the Ribble Rivers Trust and others has shown the serious impact signal crayfish have on fish populations*.  Their burrows in river banks also lead to greater erosion and sediment pollution.

Further Information


Signal crayfish are not always easy to distinguish from white-clawed crayfish. Although the main identification factor of signals is the large red claws, these are not always obvious on juveniles.  One key feature is the absence of spines on the cervical groove. See the EA leaflet describing crayfish identification in more detail.

Dispersal and Reproduction


  • Breeds from the age of two (one in exceptional circumstances)
  • Can hybridise with native crayfish, however offspring are sterile
  • Females produce up to 500 eggs (some research has shown survival from egg to 2 years old is 10-52%)
  • Mating and egg laying occurs during autumn, mainly in October.
  • After egg laying the female carry the eggs under the tail until hatching.
  • Hatching time varies greatly depending on temperature. But generally young hatch April to May
  • The eggs hatch into miniature crayfish that stay with the mother for three stages (two moults). In the third stage the juvenile crayfish adopt a solitary life.
  • Size at maturity is usually 6-9 cm total-length (from tip of head to edge of tail-fan) at an age of 2-3 years.
  • Competition and cannibalism can greatly affect survival in dense populations.
  • Maximum age and size are reported to be approx. 20 years and 16-18 cm, but these sizes are rare.


  • Humans are the most important vector for dispersal and most new introductions are due to deliberate release.
  • Within the watercourse the signal crayfish can spread by own migration. Upstream migration rates of more than 1 km per year are reported.
  • Downstream spread can be faster.
  • Signal crayfish may colonise new water bodies by walking overland to either migrate or to escape unfavourable conditions.

Environmental problems
  • They are known to burrow into banks up to 1.2m which causes damage and affects the habitat of bank-dwelling species such as the threatened water vole.  It also speeds up erosion, causing changes in the bank-side vegetation and increases the silt load in the water.
  • Signal crayfish predate on fish eggs laid below the gravel and they will attack small fish such as bullhead and salmon fry.
  • They are also capable of outcompeting fish, through their consumption of plants and insects. The loss of plants and insects has further knock on effects for both aquatic and terrestrial species.
  • As well as preying on insects, signals will also eat juvenile crayfish (thus trapping must be done in an appropriate manner as the removal of larger specimens who self thin can increase the population).

Crayfish plague (Aphanomyces astaci)

This oomycete disease is fatal to the endangered white-clawed crayfish. It spreads by waterborne spores which are released from infected crayfish. According to the International Union for Conservation of Nature, this is one of the 100 worst invasive species in the world. Crayfish plague was first detected in the Ribble catchment in 2000.  

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.  River users have a key role in reducing the risk of spreading the disease by:

  • Drying, or disinfecting any boots or equipment before moving between rivers. 
  • It is best to avoid fishing different rivers on the same day.
  • Not using any crayfish as bait (this is illegal).


Trapping has been shown (in some cases) to make the dispersal worse, due to reduced intraspecific competition.  It is for this reason that trapping should only be done under licence from the Environment Agency and by an expert.  Even the most intensive trapping programs have not successfully eradicated crayfish populations. Biosecurity is currently the best defence against further spread of the signal crayfish.

* Peay, S., Guthrie, N., Spees, J., Nilsson, E. & Bradley, P. (2010) The impact of signal crayfish (Pacifastacus leniusculus) on the recruitment of salmonid fish in a headwater stream in Yorkshire, England. Knowledge and Management of Aquatic Ecosystems, 12, 394-395.
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