A study into the health of populations of Britain's only native crayfish is taking place in Northamptonshire and Lincolnshire.
The Environment Agency is carrying out surveys of white-clawed crayfish in the two-week project.
The surveys will involve searching under stones at the bottom of the rivers and using an underwater camera.
The native species is under threat from the American signal crayfish which is bigger and more aggressive.
Richard Chadd, of the Environment Agency, said the American species is believed to have escaped from fish farms, or was deliberately introduced into the wild, in the 1980s.
He added: "The signal crayfish is bigger and more aggressive than the white-clawed and carries a fungus known as crayfish plague.
"Signals are immune to the fungus but the white-clawed is not and populations have been devastated as a result."
The surveys will monitor the health of the population of white-clawed crayfish to see if populations remains.