Sea lampreys – which are snake-like creatures with a circular disc of razor-sharp teeth instead of jaws – are slowy returning to the rivers Derwent and Ouse after the Environment Agency installed new aids to allow them to swim upstream using their sucker-like mouths.
The fish predate dinosaurs by 200 million years and can grow up to 3ft long.
Industrial pollution and man-made barriers had reduced their numbers over the past 200 years, the agency said.
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Environment Agency fisheries expert Simon Toms said: “For the last 200 hundred years, some rivers have not been capable of supporting lamprey species as a result of water quality, poor habitat and manmade barriers.
“Now that water quality has improved and some of these barriers have been removed we are seeing lampreys return to the upper reaches of rivers such as the Ouse and Derwent, where they were absent as recently as 30 years ago.
“These are fascinating fish, living fossils, which have a special place in the history and traditions of this country.
“We hope that with a helping hand from us they will be able to thrive in England’s rivers once again.”
During the Middle Ages lampreys were widely eaten by the upper classes throughout Europe. It is thought King Henry I died from overindulging on lampreys.
Lamprey pie also features in the TV series Game of Thrones. In The Prince of Winterfell episode, Tyrion Lannister dines on lamprey pie while talking over plans for the Battle of the Blackwater with his sister Queen Cersei.