Invasives Blog

As one of Europe’s five native crayfish species, the white clawed crayfish (Austropotamobius pallipes) has suffered a huge decline in numbers in the last couple of decades. This docile crustacean is usually found hiding under rocks in streams, rivers and lakes, only emerging at night to avoid predators. A fortnight ago it’s IUCN status was upgraded from ‘vulnerable’ to ‘endangered’ on the Red List of Threatened Species, with experts predicting it could be extinct within 30 years. It has recently been documented that 50 to 80% of the populations in England, Italy and France have disappeared in the last 10 years. What could be causing such a devastating loss in the U.K.?

NNSS_Pacifastacus leniusculus

Pacifastacus leniusculus (Image courtesy of Trevor Renals)

Introducing the American signal crayfish, Pacifastacus leniusculus. Following introduction from North America to Sweden in the 1960s to replace the declining European noble crayfish (Astacus astacus) as a food source, it was introduced to the U.K. in 1976. Merely a few years later this aggressive alien had escaped from the safety of its enclosures and invaded the water bodies the white-clawed crayfish calls home. The negative impacts of this exotic crayfish are manifold, ranging from ecological to structural.

The signal crayfish has many characteristics which render it more competitive than our modest native species. As well as being twice the size, it has a much greater reproductive potential than the white-clawed crayfish; the females produce far more eggs which hatch as much as a month before our native species. The juveniles can therefore establish themselves in a more favourable position, leading the native to an early disadvantage before they even hatch. They share a similar diet although the signal is much less fussy; A. pallipes may find themselves the victims of cannibalism at the hands of the introduced species. Their broad diet is reported to create problems for anglers due to loss of bait and even predation on stock.

Another less obvious impact which is becoming increasingly common is accelerated river bank erosion. P. leniusculus extensively burrows into soft river banks causing significant degradation. This negatively modifies the environment for native flora and fauna, and is very costly to rectify.

The most critical impact of this aggressive species is that it is a vector of another invasive alien species; the deadly crayfish plague. The plague was initially introduced to Europe in 1860, possibly from a ballast water discharge in Italy. It then spread across Europe resulting in a widespread loss of native European crayfish. When the signal was introduced to Sweden as the replacement, authorities were unaware that it was a carrier of the plague and the introduction spread the plague to unaffected areas. The signal crayfish is resistant to the plague which is caused by a parasitic fungus-like organism, Aphanomyces astaci. This disease is fatal to European species, and is chiefly responsible for the decline in the largest population density of white-clawed crayfish  in Europe, present in Great Britain. A. astaci attacks its host by colonising the soft tissue under the abdomen; American crayfish have developed a response that inhibits the parasite from entering past the cuticle. The white-clawed crayfish may live only between 5 and 50 days after the initial infection. The crayfish plague is also carried by other American crayfish; the red-swamp crayfish and spiny-cheek crayfish, which although invasive, are not as widespread as the signal in the U.K.

Control remains tricky. Physically removing individuals is moderately effective and legislation in the U.K. states that a licence is required to trap any species of crayfish. There have been trials using pesticides to eradicate signal crayfish however these would have negative impacts on other invertebrates in the water. Advice to anglers is that all equipment must be sterilised to prevent the spread of spores from plague infected areas. As for the white-clawed crayfish, isolated ‘Ark’ sites are being established in the U.K. in areas known to be free of non-native crayfish. In these refuges, new populations can establish away from the risk of crayfish plague.

Sonal Varia


Bohman, P., Nordwall, F., Edsman, L. (2006) The effect of the large-scale introduction of signal crayfish on the spread of the crayfish plague in Sweden Bull. Fr. Peche Pisic. (380-381) 1291-1302

Freeman, M. A., Turnbull, J. F., Yeomans, W. E., Bean, C. W. (2010) Prospects for management strategies of invasive crayfish populations with an emphasis on biological control. Aquatic Conservation: Marine and Freshwater Ecosystems 20:2, 211-223

Bubb, D. H., Thom, T. J. and Lucas, M. C. (2006), Movement, dispersal and refuge use of co-occurring introduced and native crayfish. Freshwater Biology, 51: 1359-1368.

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