Invasive Alien Species in Northern Ireland

Pacifastacus leniusculus, North American Signal Crayfish

Pacifastacus leniusculus (Dana, 1852)
The signal crayfish is a lobster-like freshwater invertebrate originating in western North America; it is larger, hardier and more prolific than the native white-clawed crayfish.
Signals live in both running and still water of reasonable quality; some lime is necessary. Adults, being large and nocturnal, need plenty of hiding places such as stones, tree roots or weed beds.
The signal crayfish was introduced to England for aquaculture, but it quickly escaped into watercourses and has spread over much of the country. Regulations prevented its introduction to Ireland, but because it is now widespread in Great Britain, the likelihood is that it or other exotic crayfish will reach Northern Ireland.
Signals reach about 15cm long and 100gm in weight, and look like a small lobster. They have a rigid carapace supporting antennae and five pairs of walking limbs, the front ones ending in large claws, red underneath and with a white flash on the upper side. The segmented, muscular abdomen or tail is flexed for swift backward movement, and bears delicate swimming legs.
Males grow larger than females and have larger claws. The females may bear up to 400 eggs attached to the abdominal legs, between November and April or May.
Country of origin
A native of western North America, from British Columbia to California.
Current distribution
Signal crayfish were first introduced to Sweden in 1960 and England in 1976 and are now found in almost every European country except Ireland.
Location in Ireland
Not yet present.
Life cycle
Signals become sexually active and pair in autumn; the male pinning down the female by their claws before transferring a semi-solid ‘spermatophore’ or block of sperm to the bases of her legs, near the oviduct. Later she will find a shelter, secrete a sticky substance (‘glair’) and lay her eggs, which become attached by the glair to the hairs on her abdominal limbs. She incubates the eggs into late spring; when the juveniles hatch they remain attached for one moult, and then become free-living at about 8mm long, at which time the female loses her feeding inhibition and may treat them as prey. Signal crayfish moult their skeletons a number of times each year until they reach breeding size (about 6cm long) in 2-3 years, thereafter usually moulting once (females) or twice (males) annually. The lifespan is up to 20 years.
Wildlife and habitat impacts
Signals compete successfully with the smaller native crayfish for food and shelter. Mixed populations may rarely persist for a few years, but the signal, like other American crayfish, is a carrier of a fungal ‘crayfish plague’ which is lethal to all non-American crayfish. Signals themselves rarely develop signs of the disease unless they are stressed.
Human impacts
Signals were introduced to Great Britain for farming, ignorant of the fact that a native species was still widespread. Most farm populations have escaped, leading to a loss of revenue, although wild catches of up to 10 tonnes per year are still made. There are no known human health problems, although other American crayfish now in Great Britain may transmit the lung fluke Paragonimus to humans if improperly cooked.
Key vectors
Man is the usual vector of introduction and spread between river catchments, although birds can catch and transport crayfish over short distances. Signal crayfish may spread naturally upstream or downstream, several km a year.
What you can do as an individual
Since the economic benefits of farming signal crayfish have been discounted, and the ecological impacts could be disastrous, do not attempt to purchase or introduce them to Northern Ireland. Never release unwanted crayfish or other aquatic pets into the wild. If you come across what you think is a signal crayfish in the wild immediately notify EHS and CEDaR, Botany Department, Sciences Division, National Museums Northern Ireland, 153 Bangor Road, Cultra, Holywood, County Down BT18 0EU. Tel: 028 9039 5256, email[at]
Any fishing gear used in British or continental waters containing signals may be infested with fungal plague spores, and must be sterilised before using them in Northern Irish waters. There have already been outbreaks of crayfish plague in Ireland, believed to be from this source.
Advice to specific groups relevant to this species
Crayfish are hardy creatures, capable of surviving up to days out of water.
Aquarists may exchange different species, but NEVER release unwanted pets to the wild, even to an isolated pond.
Pet shop owners and garden centres should have clear notices explaining the dangers of releasing exotic crayfish.
The water holding fish for restocking may contain fungal spores, so never release imported stock directly.
Anglers should be aware of the dangers of crayfish plague, and how to decontaminate their keep-nets, boots, etc.
Management measures
Once established, it is considered impossible to eradicate this species. In Great Britain there are attempts to keep a few catchments as ‘no-go’ areas for signals, but the populations of natives continue to decline. This approach would not work in Ireland given its basin-like structure. It is therefore vital that the authorities contain any introductions as control efforts are unlikely to work even at an early stage.
Chemical control has been tried in isolated Scottish ponds, but it involved deoxygenating the water and then poisoning all forms of aquatic life, and it is not certain that all crayfish were removed.
Management groups
Management expertise exists in Great Britain; both privately and within the Nature Conservancy, Environment Agency and regional Water Authorities. Experienced consultants include David Holdich (Nottingham) and Stephanie Peay (Leeds-Bradford).
Further information
Gherardi, F. and Holdich, D.M. (eds.) (1999). Crayfish in Europe as alien species. How to make the best of a bad situation? A.A. Balkema, Rotterdam (Crustacean Issues, 11).
Hiley, P.D. (2003). The slow quiet invasion of signal crayfish (Pacifastacus leniusculus) in England — prospects for the white-clawed crayfish (Austropotamobius pallipes), pp. 127-138. In: Holdich, D.M. and Sibley, P.J. (eds.). Management and conservation of crayfish. Proceedings of a conference held in Nottingham on 7 November 2002. Environment Agency, Bristol.
Holdich, D.M. and Domaniewski, J.C.J. (1995). Studies on a mixed population of the crayfish Austropotamobius pallipes and Pacifastacus leniusculus in England. Freshwater Crayfish 10: 37-45.
Holdich, D.M., Rogers, W.D. and Reynolds, J.D. (1999). Native and alien crayfish in the British Isles. pp. 221-236. In: Gherardi and Holdich (1999).
Peay, S. and Rogers, W.D. (1999). The peristaltic spread of signal crayfish (Pacifastacus leniusculus) in the River Wharfe, Yorkshire, England. Freshwater Crayfish 12: 665-676.
Souty-Grosset, C., Holtich, D.M., Noel, P.Y., Reynolds, J.D. and Haffner, P. (eds.) (2006). Atlas of crayfish in Europe. Muséum national d’Histoire naturelle, Paris, 187p. (Patrimoines naturels, 64).
See the Invasive Species Ireland web site for further information -
Text written by:
Dr Julian Reynolds