The Environment Agency is investigating the deaths of native white-clawed crayfish in Northumberland.
The endangered species, found in the River Wansbeck, displayed symptoms such as 'unusual orange patches' on their shells.
Similar symptoms were observed in crayfish in the South Tyne, Rede, and Pont rivers.
The Environment Agency, in collaboration with partners, is working to safeguard the future of this 'internationally important' population.
Healthy crayfish will be removed from the River Wansbeck and Hallington Reservoirs for monitoring.
'Berried' females will be kept at Northumberland Zoo to rear and release their young.
Sarah Jennings, area environment manager for the Environment Agency in the North East, said: "There’s a lot we still don’t know about what is happening to the white-clawed crayfish in the area and our enquiries will continue.
"This is a positive step in what is still a concerning and developing situation."
Nick Allen, Wallington National Trust ranger, said: "This latest challenge to one of our country’s native species is a stark reminder of the increasing number of threats nature is facing."
The public is urged to follow the 'Check, Clean, and Dry' advice to prevent the spread of symptoms.
The Environment Agency has ruled out pollution as a cause.
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