Work to save Yorkshire’s last remaining freshwater pearl mussels

Microscopic young of the endangered freshwater pearl mussel were given a helping hand recently by conservationists from the North York Moors National Park and the Environment Agency. The work was carried out as part of the Esk Pearl Mussel and Salmon Recovery Project which aims to reverse the decline of Yorkshire’s last surviving population of these important molluscs.

Environment Agency staff catching fish

Freshwater pearl mussels reproduce by releasing millions of microscopic young (glochidia) into open water in late summer. A few ‘lucky’ glochidia latch onto the gills of young salmon or trout and grow there over winter, causing no harm to the fish. Then, having grown to the size of a pin head, they drop off and settle in the river bed to continue their development.

Watching the fish and mussels

Sam Jones, a biology student from University of York who is nearing the end of a work placement with the North York Moors National Park Authority, said:

“The river Esk is a big place and, as we have so few mussels remaining, the likelihood of the glochidia attaching to the gills of the fish – a process known as encystment – is exceedingly low. Fish surveys earlier this year turned up some good healthy fish populations but unfortunately no young mussels. It is likely that some natural encystment is occurring in the river but at such a low level that it is hard to spot.”

Freshwater Pearl Mussel

To give the young a helping hand in reaching their fish target, staff from the two organisations gathered salmon and trout from the Esk by electro-fishing and put them in containers on the bankside with collected fertile female mussels. The fish and mussels were then returned to the Esk. Electro-fishing will be carried out next spring to measure how effective this work has been.

Allison Pierre, technical officer at the Environment Agency, said:

“This artificial encystment should give the glochidia a better chance of survival. With work such as this and improvements to the river habitat we hope to start finding some young mussels in the river over the next few years thus ensuring the survival of the pearl mussels in the river Esk.”

The freshwater pearl mussel is one of the longest-lived invertebrates known and can live for more than 100 years. Formerly widespread and abundant in England and Wales, its numbers have severely declined with most former populations now on the verge of extinction. The pearl mussels in the river Esk are the last surviving population in Yorkshire and in the main are aged over 60 years. Without action to halt this decline, it is likely that the Esk population will become extinct in the next 40 years.

River Esk near Glaisdale

Pearl mussels are a very important ‘indicator species’, which highlight the health of river systems. The decline in pearl mussel populations is due to a number of factors: habitat degradation caused by sedimentation of river gravels; decline in populations of host fish (salmon and trout); water quality issues and historic pearl fishing.

Habitat restoration work is being carried out along the river Esk to improve conditions for pearl mussels, fish populations and a whole host of other riparian species such as otters, dippers, kingfishers and river invertebrates. The National Park Authority has also worked with over 50 farms to carry out a range of measures including installing fences, planting trees and providing alternative stock watering and crossing points which will benefit habitat and wildlife on the Esk.

A number of freshwater pearl mussels from the Esk are currently residing in a captive breeding facility in the Lake District. It is hoped that juvenile mussels will be reared at the facility and can be re-introduced to the river when the habitat in the Esk has improved and the mussels are large enough for release into the wild.

Funding for the Esk Pearl Mussel and Salmon Recovery Project, which began in 2007, has been obtained from a number of sources including WREN Biodiversity Action Fund, the Heritage Lottery Fund, Yorventure, Environment Agency, North York Moors, Coast and Hills LEADER Programme and the North York Moors National Park Authority.

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