AN endangered species at risk of extinction has been given a helping hand by scientists committed to saving them.

Freshwater pearl mussels were once widespread across the country but their numbers are in severe decline and in danger of disappearing completely.

Those in the River Esk on the North York Moors are the last surviving population in Yorkshire and in the main are aged over 60 years.

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With the invertebrates living for in the region of 100 years, it is likely the Esk population will become extinct in the next 40 years unless action is taken.

Freshwater pearl mussels reproduce by releasing millions of microscopic young – glochidia -into open water in late summer, a few of which latch onto the gills of young salmon or trout.

They grow there over winter, causing no harm to the fish. Then, having grown to the size of a pin head, they drop off and settle in the river bed to continue their development.

But with so few mussels remaining, the likelihood of such an attachment is very low – which is where the conservationists have now stepped in.

Staff from the National Park and the Environment Agency gathered salmon and trout from the Esk by electro-fishing and put them in containers with collected fertile female mussels to encourage attaching – known as encystment – to take place.

Then the fish and mussels were returned to the river – where more electro-fishing will be carried out next spring to measure how effective the work has been.

Environment Agency technical officer Allison Pierre added: “This artificial encystment should give the glochidia a better chance of survival.

“With work such as this and improvements to the river habitat we hope to start finding some young mussels in the river over the next few years, thus ensuring the survival of the pearl mussels in the river Esk.”

Pearl mussels are regarded as an important “indicator species” which highlights the health of river systems.

Their decline is due to a number of factors: habitat degradation caused by sedimentation of river gravels; decline in populations of host fish; water quality issues and historic pearl fishing.

Habitat restoration work is being carried out along the river Esk to improve conditions for pearl mussels, fish populations and a whole host of other species including otters, dippers and kingfishers.

The National Park Authority has also worked with more than 50 farms on measures including installing fences, planting trees and providing alternative stock watering and crossing points which will benefit habitat and wildlife on the Esk.

A number of mussels from the Esk are also in a captive breeding facility in the Lake District. It is hoped juvenile mussels will be reared there and can be re-introduced to the river when the habitat has improved.