White-clawed Crayfish

Taxonomic information

Common name: White-clawed crayfish

Latin name: Austropotamobius pallipes

Family: Astacidae

Description

White-clawed Crayfish
sitting on moss
Image by Stephanie Peay

The white-clawed crayfish is our only native species of crayfish. It is now rare and is the subject of a Biodiversity Action Plan at a national and local level. It was formerly widespread across much of the country in a variety of suitable wetland habitats. White-clawed crayfish prefer hard, mineral rich alkaline waters with a high calcium content which means that in the Dales, the crayfish are strongly associated with limestone areas or limestone -influenced shallow streams and deeper, slower flowing rivers. Being a nocturnal species, the white-clawed crayfish can be very difficult to see but survey work has revealed that good populations do still remain in a number of Dales watercourses.

The main threat to the indigenous crayfish comes from the introduced American signal crayfish (Pacifastacus leniusculus) and the crayfish plague which it carries. The signal crayfish are larger and more aggressive than the native species and so can out-compete and even predate the native crayfish. Once in a river system, the signal populations can grow quite rapidly as they will feed on a wider range of food sources than the white-clawed species. The signal will also breed from the age of two (compared to white-clawed that breed from an age of three or four) and female signals also produce far more eggs (up to 500 in contrast to the white-clawed that only lay up to 200 eggs).

White-clawed Crayfish
sitting on stones
Image by Stephanie Peay

The crayfish plague Aphanomyces astaci is a fungal infection carried by the signal crayfish that is thought to have been introduced into British waterways with signal crayfish brought in from Sweden in the 1970s. Once introduced, the fungal spores can disperse rapidly in the water or be transferred on any material that is moved from one river system to another. The fungal infection carried by the signals is lethal to white-clawed crayfish populations and will cause a very high mortality rate that can often lead to localised extinctions. In the Dales, there was a major outbreak of the crayfish plague on the River Ribble in 2000. Thankfully some of the white-clawed crayfish were captured at the onset and are being kept in tanks until conditions are suitable for release back into the Ribble.

The spread of the introduced signal crayfish and crayfish plague has resulted in a significant decline in native crayfish populations across much of the country, with surviving populations in central and northern parts of England now of national importance. Research undertaken in the Yorkshire Dales has shown that once signal crayfish enter a river, they colonise rapidly downstream but are relatively slow to move upstream. It may well be that the strong flow of some of the Dales rivers is slowing the spread of the signals further upstream which, may offer a glimmer of hope to white-clawed crayfish populations in the upper reaches of some of the Dales rivers and streams.