Helping the critically endangered freshwater mussel

11th December 2020 - Wild Rivers Catchment

The ancient and internationally protected freshwater mussel is critically endangered in Europe with only a handful of populations remaining in England. The River Ehen in West Cumbria is home to the only one of these populations still showing signs of reproducing. We’re working on two projects to help secure the future of this iconic species.


Freshwater mussels in the River Ehen

Freshwater mussels clean our rivers; one adult filters more water each day than we use in an average shower! They can live for over 100 years and have a fascinating lifecycle. Between July and September, females release several million tiny larvae, called glochidia, into the water. The tiny shells of the glochidia are held open until they are inhaled by fish hosts, then they snap shut on the host’s gills. Glochidia need juvenile Atlantic salmon or trout for this process, and will grow on their gills until the following spring, when they drop off. The juvenile mussels must land and burrow into clean, sandy or gravelly substrates in order to settle and start to grow.


Glochidia (the white specks) on fish gills

The process of juvenile mussels latching onto fish gills is known as encystment and does not appear to harm the fish. Atlantic salmon and trout are critical to the freshwater mussel’s lifecycle, so mussel conservation efforts also involve work to boost fish numbers.

River Ehen: Habitat improvements

The River Ehen flows from the west end of Ennerdale Water and is a European Special Area of Conservation and a Site of Special Scientific Interest for freshwater mussels and Atlantic salmon. It has the largest population of mussels left in England and the only one showing evidence of juveniles, but not yet in the numbers that are required to prevent the extinction of the species in this river.

We’re working with United Utilities, the Environment Agency and Natural England to deliver conservation measures to improve the habitat for mussels, fish and all the other beasties that benefit from a healthy river ecosystem. The project encompasses the upper catchment with particular focus on land owned by United Utilities along the river. The project also forms part of the Wild Ennerdale Partnership.

Thanks to funding from United Utilities, Project Officer Diane O’Leary has been working on this project for the last four years, and this funding has now been extended for a further four years, providing a fantastic opportunity to continue to improve the chances of securing the future of mussels and salmon in the Ehen.

In recent months, with the help of volunteers and contractors, work has included:

Surveys indicate there has been recent reproduction which is extremely positive. However, more work is required to ensure they are healthy, thriving, sustainable populations for future generations to enjoy and benefit from.

River Irt: Reintroduction and artificial encystment

The River Irt flows from the south-western end of Wastwater. Also in partnership with United Utilities, Natural England and the Environment Agency, and working closely with the Freshwater Biological Assocation (FBA), we’re working on an exciting project to improve habitat for freshwater mussels and release juvenile mussels reared at the FBA’s captive breeding programme to revive an existing population.

Our work with mussels on the Irt is led by Project Officer Chris West, who has spent a number of years working on other projects on the Irt.

In the wild, encystment is a numbers game. Mussels release millions of glochidia into the water in the hope that a percentage are inhaled by host fish. Every year we inspect host fish gills in May to see if this process is happening. Worryingly, we found encystment in the River Ehen but not the River Irt, which would mean the population declines over time, so we decided to undertake artificial encystment.

The full process is complex, but in essence artificial encystment brings mussels and host fish closer together when the glochidia are released. This significantly increases the chances of encystment. In the Irt we’ve moved mussels to areas of better habitat with more fish.

We’re also taking Irt mussels to a hatchery at the Freshwater Biological Association where juveniles can be reared in controlled conditions to increase survival rates. We released 70 juveniles from this programme in 2017 as part of a separate project and they appear to be surviving well. This year we’ve been testing new release sites on the Irt using mussel silos. These safely hold juvenile mussels in one place so we can monitor their rates of growth and survival and the water quality. This will help us identify the best sites for next year, when we plan to release 2,000 juvenile mussels equipped with tiny electronic tags to help us monitor them.


A mussel silo where juveniles can be released and monitored

Finally, we’re improving habitats on the Irt in partnership with local anglers. One really successful recent piece of work was restoring and reconnecting a palaeochannel (a remnant of an old stream) in Bibby Dubb. This has allowed the Irt to access an area of wet woodland, helping to slow the flow downstream and also creating new habitat for salmon and trout. Just a couple of months later we spotted trout redds in the area!


Restored channel at Bibby Dubb where trout redds have now been spotted