White-clawed crayfish

Latin name: Austropotamobius pallipes

Family: Astacidae

The white-clawed crayfish is the only native species of crayfish and was formerly widespread across much of the country in a variety of suitable wetland habitats.  They 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.  The signal crayfish are larger and more aggressive than the native species and so can out compete and even predate the populations of native crayfish.  Once into 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.  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).      

The crayfish plague Aphanomyces astaci is a fungus 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 be rapidly spread to other areas in the water or on any material that is moved from one river system to another.  The fungus carried by the signals is lethal to white-clawed crayfish populations and will cause very high mortality that often leads to localised extinctions.  In the Dales, there was a major outbreak of the 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 spread 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.

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Wharfedale in Yorkshire is a valley favoured by nature and enriched by romance. From 'Wharfedale' by Ella Pontefract & Marie Hartley

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