EXPERTS have released a group of critically endangered freshwater pearl mussels into a Cumbrian river after rearing them at a Lake District facility for a decade.

Scientists from the Freshwater Biological Association set 60 of the filter eaters free at three secret locations yesterday as part of the 10-year project to boost chronically low numbers of the species in the wild.

They have been hand reared under special conditions in an ark kept at the FBA premises at Far Sawrey, near Ambleside.

The mussels will be monitored to ensure they are doing well in their river habitat before a further 40 are released later this year.

Dr Ceri Gibson, the FBA’s mussel project manager, explained the freshwater pearl mussel ark project was set up in 2007 because the species is known to be at 'extremely high risk' of extinction in the wild.

She added: "We reared these native mussels at the FBA’s ark in Windermere, the only facility of its kind in England.

"Freshwater mussels are what we call a ‘keystone species'.

"They may be small, but they have enormous beneficial effects on the lives of other organisms.

"If we can get the water quality and river habitat conditions right for them it will also help other species such as kingfishers and otters."

Because the mussels are so slow growing - taking 12 years to reach adulthood - only the oldest grown in captive conditions have been released.

Their exact locations are being kept secret in a bid to stop them from being disturbed.

Once settled, they will provide benefits to the river ecosystem by filtering out bacteria, algae, or decaying plant or animal matter before passing the clean water back into the river.

The project, which has included a three-year scheme to restore the river environment, has received support from Natural England and the Environment Agency as well as part-funding from Biffa Award.

Andrew Moffatt, chairman of Biffa Award, said: "We are thrilled to fund efforts to augment native freshwater mussel populations in Cumbrian waterways as one of our rebuilding biodiversity themes.

"It is important that we protect and strengthen our natural environment so that it can adapt to threats such as climate change, and is protected for future generations to enjoy."

An exhibition on the Freshwater Pearl Mussel Ark project is now on display at the Aquarium of the Lakes, at Newby Bridge.