Pacifastacus leniusculus | Signal Crayfish | Cráifisc sceide



Conservation status

Least concern

Legal status

Third Schedule listed species under Regulations 49 & 50 in the European Communities (Birds and Natural Habitats) Regulations 2011. (Note: Regulation 50 not yet enacted).

Listed as a schedule 9 species under Articles 15 & 15A of the Wildlife Order (Northern Ireland) 1985 (Article 15A not yet enacted).

Regulated invasive species of Union concern under the European Regulation on the prevention and management of the introduction and spread of invasive alien species [1143/2014].


Invasive species - risk of High Impact

Irish status


Introduction pathways - 1

Release in Nature

Introduction pathways subclass - 1

Other intentional release

Introduction pathways - 2

Transport Contaminant

Introduction pathways subclass - 2

Transport of habitat material, Angling/fishing equipment

Invasive score


NAPRA Ireland risk assessed

Yes. This species underwent a Non-native species APplication based Risk Analysis in 2014.

Overall risk of this species to Ireland is categorised as: MAJOR with a VERY HIGH level of confidence.

Overall conclusion summary: This crayfish species has the potential to seriously threaten the conservation of the native White-clawed Crayfish populations and to a lesser extent other important species in Ireland. Salmonid recruitment may be impacted. The structural integrity of river or pond banks may also be affected in colonised areas.

View the full risk assessment:

Species Biology


Long lived reddish brown coloured crayfish, which can reach 16cm in length. The claws are proportionally large with a bright red under-surface and a turquoise and white patch at the hinge on the upper surface (NNSS, undated).


An opportunistic omnivore which is active during daylight, unlike our native White-clawed crayfish (Austropotamobius pallipes), which is mainly nocturnal (NNSS, undated). Can live for 20 years (Aldridge, 2016). A combination of high fecundity, rapid growth and early sexual maturity means large populations can develop rapidly (Holdich & Sibley, 2009).

Will eat small fish, amphibian eggs, tadpoles, invertebrates, plant material, detritus (Aldridge, 2016), fish eggs and native crayfish (Rushbrook, 2009).

Their feeding activities cause changes in the foodweb and put pressure on numbers of macrophytes, aquatic insects, snails, benthic fishes and amphibian larvae (Johnsen & Taugbøl, 2010). May also compete with fish for shelters from predators (Aldridge, 2016). They outcompete A. pallipes (Peay, 2009) for food and in aggression (Aldridge, 2016) and will eventually replace A. pallipes completely in any given waterway (Peay, 2009).

They are known vectors of the Crayfish plague (Holdich & Sibley, 2009), which does not affect them but decimates populations of A. pallipes.

Will tolerate brackish water (Aldridge, 2016) and periods of drought (NNSS, 2011). Burrows into riverbanks, where it shelters during the winter when it spends much of its time in a 'state of torpor' (NNSS, undated). Its burrowing, which can be up to 2m deep with interconnected tunnels (Aldridge, 2016), causes riverbanks to collapse (Rushbrook, 2009) and therefore increases erosion and damage to bankside structures.

The Signal crayfish uses its outstretched claws to make itself appear to be too big for a fish to take (Peay et al., 2009) but they may be predated in Ireland by coots (Fulica atra) (NNSS, 2011), herons (Ardea cinerea), perch (Perca fluviatilis), pike (Esox lucius), otter (Lutra lutra), mink (Mustela vison) and eels (Anguilla anguilla) (Millane & Caffrey, 2014; Aldridge, 2016).


Mating occurs during autumn. The female produces 200-400 eggs (Johnsen & Taugbøl, 2010), which are carried on her abdomen until hatching in spring (NNSS, 2011). The hatched young, which are miniature crayfish, remain attached to the female for 2 moults (Johnsen & Taugbøl, 2010), until May or June (NNSS, 2011). Males and females are sexually mature at 2-3 years (Aldridge, 2016).

Pathway and vector description

Was originally introduced in Europe (Sweden) in 1960 to replace the native Noble crayfish (Astacus astacus), which had been decimated by the crayfish plague (Johnsen & Taugbøl, 2010). Once established they readily disperse along waterways, across land, sometimes aided by predators (Aldridge, 2016). Typical spread rates are 1-2km per year (NNSS, 2011). Human mediated spread is the main method of dispersal in mainland Europe (NNSS, 2011).

The nearest population to Ireland is in the UK and there is a small potential for them to be introduced to Ireland as a contaminant on angling gear (Millane & Caffrey, 2014). There is also a small risk of introduction as a contaminant of illegal fish stocking from abroad (NNSS, 2011). The main pathway into Ireland though is likely to be deliberate importation for human consumption. From there undersized or surplus crayfish may be dumped in the wild (Millane & Caffrey, 2014).

Mechanism of impact

Competition, Predation, Disease transmission, Grazing/Herbivory/Browsing

Management approach

There are no effective methods for control or eradication of crayfish at present (NNSS, 2011).


Currently trapping is most used but juveniles don't often enter the traps and the adults are very wary making them difficult to trap (Holdich & Black, 2006).


Biocides can be used but they are expensive and cannot be used in flowing water (NNSS, 2011). Adverse effects on human health and the environment.

Broad environment


Habitat description

Freshwater lakes, ponds, rivers, streams, canals (Holdich & Sibley, 2009). Will also tolerate brackish water (Aldridge, 2016).

Species group


Native region

North America

Similar species

White-clawed crayfish (Autopotamobius pallipes). A. pallipes is much smaller (10cm). Its claws are off-white or pink underneath and body colour is brown or olive green (NNSS, undated). Has spines behind the cervical groove, which Pacifastacus leniusculus lacks (Holdich, 2009).


World distribution(GBIF)

Introduced in 23 EU countries (Austria, Belgium, Croatia, the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Italy, Latvia, Lithuania, Luxembourg, the Netherlands, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden and the UK) (European Commission, 2017).

Native distribution

Western North America (Holdich & Sibley, 2009) and Southwestern Canada (Johnsen & Taugbøl, 2010).

Temporal change

Records submitted to Data Centre in 2021

The following map is interactive. If you would prefer to view it full screen then click here.

How can you help

Report any sightings to the National Biodiversity Data Centre.



Aldridge, D. (2016) Signal crayfish, Pacifastacus leniusculus. Factsheet. Non-native Species Secretariat (NNSS). Site accessed 10 October 2017.

Buglife (undated) Crayfish identification, distribution and legislation. Leaflet produced for Buglife and the Environment Agency. Buglife, Plymouth Site accessed 9 October 2017.

European Commission. (2017). Invasive Alien Species of Union Concern. European Union, Luxembourg.

Holdich, D.M. (2009) Identifying crayfish in British waters. Brickland J., Holdich D.M. and Imhoff E.M. (eds) (2009). Crayfish conservation in the British Isles. Proceedings of a conference held on 25th March 2009 in Leeds, UK. Site accessed 9 October 2017.

Holdich, D. & Black, J. (2006) The spiny-cheek crayfish, Orconectes limosus (Rafinesque, 1817) [Crustacea: Decapoda: Cambaridae], digs into the UK. Aquatic Invasions 2 (1) pp. 1-15, Site accessed 6 October 2017.

Holdich D.M. & Sibley P.J. (2009) ICS and NICS in Britain in the 2000s; Brickland J, Holdich DM and Imhoff EM (eds) (2009). Crayfish conservation in the British Isles. Proceedings of a conference held on 25th March 2009 in Leeds, UK. Site accessed 9 October 2017.

Johnsen, S.I. and Taugbøl, T. (2010): NOBANIS – Invasive Alien Species Fact Sheet – Pacifastacus leniusculus. – From: Online Database of the European Network on Invasive Alien Species – NOBANIS Site accessed 10 October 2017.

Millane, M. & Caffrey, J. (2014) Risk Assessment of Pacifastacus leniusculus (Dana 1852) – Signal Crayfish. Prepared for Inland Fisheries Ireland and the National Biodiversity Data Centre Site accessed 10 October 2017.

National Biodiversity Data Centre (undated) Crayfish identification. Handout. Site accessed 9 October 2017.

Non-native Species Secretariat (NNSS) (2011) Rick assessment for Pacifastacus leniusculus – Signal crayfish. Risk assessment carried out for Great Britain.  Site accessed 10 October 2017.

Non-native Species Secretariat (NNSS). (undated). Species Description – Signal crayfish. Identification sheet. Site accessed 10 October 2017.

Peay, S. (2009) Selection criteria for “ark sites” for white-clawed crayfish. Brickland J, Holdich DM and Imhoff EM (eds) (2009). Crayfish conservation in the British Isles. Proceedings of a conference held on 25th March 2009 in Leeds, UK. Site accessed 9 October 2017.

Peay, S.; Guthrie, N.; Spees, J.; Nilsson, E.; Bradley, P. (2009) 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, pp. 394-395. Site accessed 12 October 2017.

Rushbrook, B.J. (2009) Crayfish and River Users. Brickland J., Holdich D.M. and Imhoff E.M. (eds) (2009). Crayfish conservation in the British Isles. Proceedings of a conference held on 25th March 2009 in Leeds, UK. Site accessed 9 October 2017.

CABI Datasheet

Inland Fisheries Ireland

UK Crayfish Hub