Record numbers of a blood-sucking fish are being found in a river on the Devon and Cornwall boundary, according to the Environment Agency.
The fishes' mouths latch onto victims
It confirmed primitive sea lamprey had been spotted in the River Tamar, which it said was a sign of clean water.
More than 500 of the fish were registered by the agency's fish counter at Gunnislake.
They are 1m (3.3ft) long, with razor sharp teeth and they survive by drinking the blood of other fish.
Scientists are excited that more than twice the number spotted last year have been captured on camera.
The fish only survive in unpolluted water, which Paul Elsmere, fisheries officer, said: "Is good news because it proves the river is clean."