Volunteers are being asked to help test water for the presence of water voles, mink and critically endangered white-clawed crayfish.
The Eden Rivers Trust said it was looking to find “safe havens”, where crayfish in particular “might still thrive, free from invasive species”, in the North Pennines fellsides.
Tests involve sending samples containing blood, poo, fur or skin cells to a laboratory to be analysed, with the aim of identifying all the species in a stretch of water.
The trust said the Eden catchment in Cumbria was “one of its last strongholds for white-clawed crayfish in England, but populations were rapidly declining due to water pollution and the invasive American signal crayfish”.