Until recently the river Allen’s native White-clawed crayfish population, one of the few remaining in Dorset, has managed to remain free from disease but dead and distressed crayfish were recently spotted in the river in July.
Samples were sent to the Centre for Environment Fisheries and Aquaculture Science for disease analysis and they have now confirmed that the crayfish are infected with crayfish plague.
Andy Martin for the Environment Agency said:
The crayfish plague is carried by non-native American signal crayfish, which are not susceptible to it and are present in many of our Dorset Rivers. It is transferred to new waters either through the movement of the signal crayfish or by water and/or damp equipment that has come from waters that contain signal crayfish. It is not clear yet how the disease reached the River Allen.
American Signal Crayfish look similar to small lobsters, they are red-brown in appearance with large, smooth claws. They are far larger than the endangered native white-clawed crayfish.
They carry a fungal disease called crayfish plague, which can kill native crayfish. They are active during the summer and hibernate in winter, usually in burrows in riverbanks.
They outcompete our native White-Clawed Crayfish for food. They also cause damage to riverbanks by deep burrowing, impact on river fly populations and can reduce fish stocks by eating large amounts of fish eggs.
The Environment Agency is working closely with the Dorset Wildlife Trust and local landowners to monitor the situation and determine the extent of the outbreak.
Dorset Wildlife Trust Conservation Officer, Amanda Broom, said:
It is very sad that White Clawed Crayfish have been infected with the crayfish plague on the River Allen, as this was one of just three populations remaining in Dorset. However, by remaining vigilant and observing biosecurity procedures such as cleaning and drying equipment and shoes that have been near the river, hopefully we can limit the amount of crayfish being affected by this disease.
Whilst we can’t be sure of the fate of the surviving crayfish, the work we are doing on the River Allen with the Environment Agency, such as providing cover for crayfish to hide in, will provide any surviving white clawed crayfish a good habitat to thrive in.
Andy Martin, for the Agency, added:
We are urging river users to ensure that any equipment they use in the River Allen is clean and dry before entering the river, when moving between locations and at the end of the day. This will hopefully limit the rate of spread of the disease through the river and reduce the risk of it being spread to other rivers that still have white-clawed crayfish populations.
The White-clawed crayfish is the UK’s only native crayfish species and is designated as “endangered” on the International Union for the Conservation of Nature (IUCN) Red list of threatened species. Non native invasive species of crayfish, mainly the American signal crayfish, have been the main reason for the rapid decline in our native White-claws. This is mainly due to their ability to out-compete for habitat and food and because they carry a deadly mould called crayfish plague (Aphanomyces astaci) which the natives are not immune to. This disease can be spread between rivers and lakes on damp and muddy equipment. It is vital that any river / lake user follows the Check, Clean and Dry biosecurity measures before entering a watercourse.
In response to the severe decline of native White-clawed crayfish in South West England, The South West Crayfish Project was set up. The project is a partnership between the Environment Agency, Buglife, Avon Wildlife Trust and Bristol Zoo. The project involves re-homing at-risk populations of White-clawed crayfish to new safe havens known as Ark sites, monitoring existing populations and captive breeding populations at Bristol Zoological Gardens. Last year good numbers of White-claws from the River Allen were translocated to a safe isolated stream away from the threat of any non-native crayfish species. This population will be monitored in future years to determine the success of the project. Other opportunities are also being investigated for any remaining potentially healthy White-claws in the River Allen.