Lampreys are extremely selective with their spawning sites
The discovery of a rare blood-sucking fish in the River Wear is proof of high water quality, conservationists said.
Seven adult sea lampreys, which have toothed, funnel-like sucking mouths, have been found in the river near Chester-le-Street, County Durham.
Only three species of lampreys remain in Britain, and they are protected under European law.
The Environment Agency said the creatures only breed in water which is very clean.
So far the agency has identified twelve spawning sites, known as redds.
Fisheries officer Paul Frear said: "We were thrilled to discover lampreys back in the River Wear, as these rare blood-suckers show us that the water quality in the river is very high.
"Lampreys are extremely selective with their spawning sites and will only nest where the water quality is optimal."
Scientists are continuing to search for more lampreys, and anyone who spots one is asked to report the sighting to the Environment Agency.