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Twenty-nine of the mussels have been taken to special tanks in the Lake District where they should breed and produce juvenile mussels which can be put back into the Esk between Danby and Glaisdale in five years.

It's hoped that by then the problems which have threatened this species will have been tackled. At the moment there's just too much sediment in the river.

Special tanks for the Esk mussels

"When they breed, the female pearl mussel will release thousands of what are called glochidia - which are tiny little pearl mussels - and they attach themselves to the gills of fish and they live on the fish over the winter," said Fraser Hugill, senior farm conservation officer at the North York Moors National Park Authority.

"Then in the spring they drop off into nice clean gravels and they bury down into the gravels where they live for five or six years and gradually grow and spring out of the gravels.

"But unfortunately in the Esk we believe that the gravels are far too silty so in effect when they're burying down into just thick silt they can't breathe and hence they die and that's why we believe we don't have young pearl mussels in the Esk."

Fraser Hugill and Pete Dowson

There are now about 200 freshwater pearl mussels in the Esk and all of them are large and old. They haven't bred for decades. If the situation continues there won't be any left in about 25 years.

The increased sediment in the river has been caused by changing agricultural practices introduced from the end of the Second World War.

"The answer we're looking at is: can we reduce the numbers of stock actually physically going into the river?" asked Mr Hugill.

"When they're going into the river they're grazing the river banks, they're loosening the river banks, eroding the river banks and hence then the river is taking the sediment away and depositing it on the gravels.

The River Esk near Danby

"The simple answer is to try and fence off the river banks so the stock aren't going in and giving the stock alternative places to water."

One farmer is showing how it can be done. Pete Dowson has Furnace Farm at Fryup and probably has the most land of any farmer bordering the River Esk. He's putting about 1,000 metres of fencing along the riverbank to prevent his 50 suckler cows from getting into the Esk.

"It makes stock management a load easier if you can keep them from wandering about the river," said Mr Dowson.

"A river is not a particularly good boundary for animals. They love to wander and wallow in the river and they do make a tremendous mess.

A damaged river bank on the Esk

"But also I'm looking to improve the banks and the vegetation because it looks very raggy when you get a lot of livestock puddling in and out of the river. It just looks an absolute mess and we don't like that, we like to see things looking good."

Some 50 letters have been sent to farmers and landowners along the Esk between Glaisdale and Danby encouraging them to be involved in the project. Mr Dowson says it's worthwhile.

"I didn't particularly understand the pearl mussels until this came along and it's opened my eyes and I thought, yeah, let's do something. I can remember years ago as children, we'd wander into the river and we'd pick these things out.

A project officer is to be appointed to oversee the rescue of the mussels for the next three years and 拢70,000 of funding has come from the Heritage Lottery Fund and Yorventure.

"Freshwater pearl mussels are as important a part of our heritage as a Rembrandt," said Fiona Spiers, regional manager for the Heritage Lottery Fund.