VITAL work is underway to safeguard one of the world’s most critically endangered species – with the creation of a new “ark”.

The River Esk on the North York Moors has Yorkshire’s last remaining population of freshwater pearl mussels – but the 1,000 that are left are in drastic decline.

Pollution, choking of the river bed by sediment build-up, a deterioration in fish numbers and habitat degradation are all behind the fall in numbers.

Now 20 adult mussels have been taken from the river to a special captive breeding facility at the Freshwater Biological Association in the Lake District.

This captive breeding facility provides an “ark” to conserve the dwindling populations and to rear juvenile mussels through captive breeding for reintroduction to their native rivers.

The national park’s River Esk project officer Simon Hirst said: “Hopefully the adult mussels will breed in the Lake District, and in seven or so years we will be able to re-introduce these mussels back to the River Esk.

“Freshwater pearl mussels are an important indicator species; if we get conditions right in the river for them, it will have positive knock on benefits for a range of other wildlife such as otters, Atlantic salmon, dippers and kingfishers.”

As well as the captive breeding work, the project is working with farmers to reduce sediment and nutrient input into the Esk.

By 2018, it is hoped sections of the river will have been restored and suitable habitat will be available for the return of juvenile mussels.

The work has been given a helping hand in the shape of a £300,000 grant from Biffa Award, part of the Landfill Communities Fund.

The grant forms part of a larger £1.5m Biffa Award project led by the Freshwater Biological Association that will also see river restoration carried out in mussel habitats in Cumbria and Devon

Head of grants Gillian French said: “This project is an exciting opportunity to save one of the most long-lived animals from extinction; the freshwater pearl mussel can live for more than 100 years and is internationally protected.”

The freshwater pearl mussel has a complex reproductive cycle. Millions of microscopic young are released into open water with a few latching onto the gills of young salmon or trout – a process known as encystment.

They grow on the fish over winter and when the size of a pin head, they drop off and settle in the river bed to continue their development as juvenile mussels.