The River Ehen in Cumbria is home to the largest and only breeding population of mussels left in England but they are still declining and at critical risk of extinction. The Pearls in Peril Project trialled an innovative technique, termed, bankside artificial encystment to try to boost the mussel population in the River Ehen. The Compensatory Measures Project has been continuing this work.


The lifecycle of the freshwater pearl mussel is extraordinary. They spend the first few months of their life growing on the gills of a young fish (Atlantic salmon or trout).

The adult mussel releases several million larvae, called glochidia, in the summer. The glochidium looks like a tiny mussel, measuring less than a 10th of a millimetre long – that’s less than the thickness of a piece of paper!

Photo from FBA - microscopic glochidia

The tiny shells are held open until they are inhaled by the fish, then snap shut on the gill filaments.This association does not appear to harm the fish. The chances of a glochidium meeting a suitable fish are very low; only four in every million will do so. Nearly all are swept away by the river.

The glochidia remain on the gills of the fish and grow in this oxygen-rich environment until the following spring, when they drop off. They must land and burrow into clean, sandy or gravelly substrates in order to survive; if they land in silt or mud they will suffocate.

Due to the essential role young fish play in the life of the mussel, the conservation of Atlantic salmon and trout is central to the survival of the freshwater pearl mussel.

Bankside Artificial Encystment

Both the Pearls in Peril Project and Compensatory Measures Project has intervened for five years at the stage in the lifecycle when the mussels release their glochidia into the water column with the hope of being inhaled by a juvenile fish. A special protected species license was essential to be able to undertake this work.

The first step was to determine when glochidia were about to be released into the river which some literature states is a highly sychronised event potentially triggered by water temperature and oxygen levels. Using specialist equipment, an otoscope and needle, a sample of glochidia was extracted from a small number of mussels which were returned quickly to the river unharmed. The sample was examined using a field microscope. Just before their release, the glochidia resemble miniature mussels and their shells begin to snap in readiness to attach to fish. When they reach this stage, the date of their release is imminent.

Once the date is determined, a team is organised to collect young fish from the river using a technique called electrofishing. The fish are placed in an aerated tank on a shady section of the riverbank to ensure they are kept cool and healthy.

While the electrofishing team are collecting fish, several mussels are collected (under license) and placed in a bucket of river water. Warm water was added to raise the temperature a couple of degrees to simulate a temperatre increase in the river and to induce the females to release the glochidia, the mussels are then returned to the river

The glochidia were then added to the tank with the fish. This maximizes the potential for the glochidia to attach to the gills of the fish in a confined space. After a few minutes the fish were counted, measured and returned to the river, in the hope that they carry the next generation of mussels. This has been termed bankside artificial encystment as it has occurred on the riverbank in a controlled environment.

Around 10 months later, the glochidia have grown large enough to be visible as white blobs on the gills of the young fish. It is at this point that they drop off and bury into the bed of the river to continue to live and grow. 

The attached mini film shows this work.

In May every year, WCRT will collect fish and their gills will be checked for glochidia which will be big enough to see with the naked eye. Currently, for the River Ehen, there is no known method available to determine whether the glochidia on the gills are from natural encystment, from artificial encystment or a combination of the two. There is evidence prior to the commencement of artificial encystment in 2013, that natural encystment does occur in the river.

Successful natural and artificial encystment requires suitable juvenile salmon and freshwater mussel habitat. The Compensatory Measures Project provides a rare opportunity to make improvements to the habitat for the survival of both these species which also benefits the overall river ecosystem.

Given that mussels grow very slowly and spend the first few years completely buried in the riverbed, it will take decades to demonstrate improvement in mussel population numbers in the River Ehen from both catchment management projects and bankside artificial encystment. However, it is essential to try as many strategies as possible to ensure that juvenile mussels and salmon have the best chance of surviving to adulthood.


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