Mussels are to have their young harvested and reintroduced into a Yorkshire river in effort to reduce pollution in the first project of its kind.

Endangered freshwater pearl mussels have been taken from the River Esk and moved to a special facility to encourage them to release their young, in a project funded by Yorkshire Water. 

The river’s mussels, which are over 80 years old, are the last remaining in Yorkshire. The young will be kept in the facility, run by charity the Freshwater Biological Association, until they are seven years old.

In the meantime, the North York Moors National Park Authority plans to clean up the river by working with farmers and land managers to reduce runoff from agriculture.

Mussels can live for over a century but their numbers have dropped dramatically in the UK in the past 70 years due to loss of habitat, poaching, pollution and river engineering. 

Once re-released, the growing mussel population will clean the water, making it more habitable for other species, including otters and salmon. One adult freshwater pearl mussel can filter 50 litres of water each day.

Rare freshwater pearl mussels are collected so their young can be reintroduced in a bid to improve the overall health of the River Esk

Credit:  Yorkshire Water/PA

The method has previously been used on commercially farmed mussels in the US, but no successful project has yet been carried out in the UK. 

Mussel larvae grow while attached to fish, and commercial trout are normally used as hosts – but this was not working for the Esk mussels. The funding will be used to experiment with wild sea trout to see whether this is more effective. 

Elizabeth Clements, head of natural environment at the national park, said the authority had not done a survey since 2009 because numbers had dropped so low and they were concerned about disturbing the remaining population. There are thought to be fewer than 500 mussels left.

Using mussels to clean river water was a novel approach, Ms Clements said. "Freshwater pearl mussels are a very important keystone species in our river. We continue to work together with land managers and farmers to get the conditions right for them, and this will have positive knock-on benefits for a wide range of other wildlife," she added.

Louise Lavictoire, mussel reintroductions research officer at the FBA, said: "We’re really excited we can help the Esk population to avoid extinction. 

"We’ve had some success already rearing juvenile mussels from the Esk and this much-needed funding support from Yorkshire Water will help us to build upon this and really help to kick-start the population again."