Large numbers of native white-clawed crayfish have been affected in the River Ure and its tributaries near Aysgarth, a location thought to be one of the last key remaining sites for the indigenous species in England.
Tom Pagett, from the Environment Agency, said surveys had just been completed in order to understand the current extent of the infection.
“We are moving quickly to protect the remaining white-clawed crayfish in the catchment,” he said.
“White-clawed crayfish are our largest native, freshwater invertebrate but are now rare and protected in Britain.”
He said it appeared the infection had been transferred unknowingly from another area through water users moving between catchments. The outbreak is likely to continue until next spring with the priority now being on stopping the disease spreading to other rivers and catchments.
The white-clawed crayfish have no natural resistance to the disease, which is carried by the invasive North American signal crayfish which arrived in UK rivers in he early 1980s.
Mr Pagett said steps must be taken to minimise contamination. “To protect our rivers, and the unique wildlife that call them home, we all need to ensure that we follow good biosecurity principles.”
The Environment Agency is urging river users, including anglers, to follow steps to minimise the potential transfer of the disease including cleaning equipment thoroughly and not using the same footwear or equipment in different rivers or steams without treating it first.
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