HOPES have been raised for the future of an endangered species following a survey which has proved that the creatures are still breeding.
The rare pearl mussel is regarded as a globally endangered species with its numbers dwindling around the world.
But a small population of the freshwater bivalve is clinging on in the River Esk on the North York Moors although most are more than 50-years-old.
The lack of juvenile mussels could have been regarded as a sign that the creatures were no longer breeding.
But the freshwater pearl mussel has a complicated life cycle - with the larval stage living as a parasite on salmon and trout for about nine months during its development.
And an electric fishing survey has indicated that salmon and trout in the Esk are infected with pearl mussel larvae.
During the survey the fish were temporarily stunned so they could be examined before being released unharmed - and experts have been delighted by the result.
"It was great to find out that the mussels in the Esk still have some get up and go about them," said project officer Simon Hirst.
"Given the age of the existing pearl mussels however, it would seem that the juveniles are not surviving in the river gravels after they drop off the host fish.
"Juvenile mussels need very clean and well-oxygenated gravels to survive and it is therefore thought that sedimentation in the Esk is the reason for the mussel’s decline.
"If we can reduce levels of sedimentation, hopefully we can create an environment where the juveniles will survive and the population will begin to increase."
The survey was carried out by the Esk Pearl Mussel and Salmon Recovery Project, a joint project between the National Park and the Environment Agency which is being funded by the National Lottery and Yorventure.
It is working with landowners and farmers along the river to carry out environmental improvements such as fencing off river banks, tree planting, river bank stabilisation and providing alternate stock watering facilities.
Environment Agency biodiversity officer Allison Pierre said: "These improvements will provide habitats for wildlife and reduce the amount of silt going into the river.
"This is a long-term project and over the coming decade we hope to see continued progress, which will benefit the pearl mussel and other species."
The freshwater pearl mussel is one of the longest-lived invertebrates known, and can reach an age of more than 100 years.
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