One of Britain's rarest fish is returning in large numbers, the Environment Agency said today.

Scientists have witnessed a massive resurgence in the sea lamprey population in the River Tamar and have recorded five times more fish than the seasonal average.

The Environment Agency (EA) picked up the increase in lamprey numbers while recording migrating salmon at Gunnislake, Cornwall, as they swim upstream.

The EA welcomed the resurgence as it indicates a healthy river environment and said the metre-long sea lamprey is a primitive marine species that comes into rivers to spawn.

The eel-like fish has a jawless mouth and is boneless with a skeleton made of flexible cartilage.

Spokesman for the EA, Paul Elsmere, said: "The sea lamprey is something of an evolutionary throwback, but it is an important indicator species.

"This year more than 500 have passed through our fish counter at Gunnislake which is excellent news as it shows water quality in the Tamar is high."

The EA said the sea lamprey are thought to be more sensitive to pollution than salmon so their presence on any river system is to be encouraged.

For centuries the fish was considered a delicacy in Europe and King Henry I is said to have died from a "surfeit of lamprey".

In Finland, hot-grilled lamprey it still a favourite dish although there is no commercial fishery for sea lamprey in the UK and little is known of its behaviour at sea.

It feeds by attaching itself to other fish using its sucker mouth. Once attached it uses its sharp tongue to rasp a hole in the side of the fish before feeding on its host's blood.

The lamprey later detaches itself and swims off in search of its next meal. The EA said it rarely kills other fish.

Copyright Press Association 2007.

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