The return to the river Wear of an unusual jawless fish known for sucking the blood of other species appears to suggest a high water quality in the river, according to scientists.
Experts are continuing to search for more of the particularly rare breed, called lampreys, believing their presence to be a positive signal for the river's water quality.
Environment agency fisheries officer Paul Frear said he was thrilled to welcome the fish back to the river Wear.
"Lampreys are extremely selective with their spawning sites and will only nest where the water quality is optimal," he said.
So far 12 spawning sites have been discovered in the area near Chester-le-Street in County Durham, along with a total of seven adult lampreys.
The tip-off regarding the curious creatures came from local anglers, who the environment agency encourages to report sightings.
Meanwhile, a company based in Cornwall was recently fined for polluting the river Fal in south-east England with liquid clay on three occasions.
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