A “rescue mission” for native crayfish is under way in Northumberland after a population were found dead with mysterious patches on their shells. The Environment Agency is working to save the internationally important species of native white-clawed crayfish in the River Wansbeck by searching for females with eggs to be taken and reared in a hatchery. At least 100 individuals of the endangered species, which is the UK’s only native freshwater crayfish, have been found dead since the end of September and environment experts are perplexed as to what is causing the “concerning” mass die-off. Extensive surveys have found the orange patches on the crayfish shells are widespread across the Wansbeck catchment area, as well as in that of the River Tyne and the River Blyth, with some crayfish also found with white, opaque muscles in the tail. Thirty affected crayfish are being monitored in tanks in an Environment Agency lab to understand more about how the symptoms develop and how deadly the condition is. The Environment Agency has deployed a team to remove healthy crayfish from the Wansbeck at the National Trust’s Wallington estate and from Northumbrian Water’s Hallington reservoirs. Females carrying eggs, known as “berried” females, will be kept at Northumberland zoo in specially prepared tanks where the eggs can be hatched and the young eventually released back into the rivers. Other crayfish will be quarantined at Wallington by the National Trust and monitored for a minimum of three months to ensure they do not develop symptoms, before also being released. “There’s a lot we still don’t know about what is happening to the white-clawed crayfish in the area and our inquiries will continue,” said Sarah Jennings, the area environment manager for the Environment Agency in the north-east. “But what we can do now is build on work that has already taken place through the Northumberland Crayfish Partnership to protect the future of this species. “With the support of our partners we will carry out rescues to move some crayfish into quarantine so they can be monitored and, if healthy, released back into the wild in future. This is a positive step in what is still a concerning and developing situation.” Nick Allen, a National Trust ranger, said: “Here at Wallington we are focused on playing our part in the rescue mission alongside other organisations from the Northumberland Crayfish Partnership. This latest challenge to one of our country’s native species is a stark reminder of the increasing number of threats nature is facing.” Common diseases have been ruled out including crayfish plague, which is spread by the invasive American signal crayfish and has caused the populations of native crayfish to struggle since the 1970s. Water-quality testing has ruled out pollution.