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."