Living on the edge
Despite being officially classed as an endangered species and near extinction in Wales, there is still hope for the future of the freshwater pearl mussel.
Natural Resources Wales (NRW) and the Four Rivers for LIFE Project is on a mission to bring this rare mollusc back from the brink by establishing sustainable populations in Wales once again.
In this blog, Four Rivers for LIFE Freshwater Pearl Mussel Officer Haydn Probert, and Dr John Taylor from Natural Resources Wales outline the complex life cycle of the freshwater pearl mussel, the re-introduction work taking place behind the scenes and their learning on what’s next for the species.
An extraordinarily complex life cycle
Freshwater pearl mussels (FWPM) are critically endangered, having not successfully bred naturally in Wales for the last 40-50 years.
Image: Freshwater Pearl Mussel (credit NRW).
This species requires unique habitat and life cycle conditions. As juveniles they spend 10 years buried in river gravels and the adults can live for more than 100 years, growing up to six inches long.
They need rivers with clean, well-oxygenated water and stable gravels to thrive.
Their complex life cycle also includes a parasitic phase where pearl mussel glochidia (tiny, microscopic larvae approximately a third of the size of a grain of salt) attach to the gills of salmon or trout.
Therefore, healthy populations of salmon or trout are critical for their early life stage. But a drop in the number of juvenile salmonids, coupled with the degradation of habitat over the last four to five decades, has had a detrimental effect on the survival rate of the species and population numbers have declined dramatically.
How have we been helping?
Over the last decade Natural Resources Wales (NRW) has been involved in the rearing of the critically endangered freshwater pearl mussel Margaritifera margaritifera (FWPM).
NRW’s Dr John Taylor has been leading this work and is based at Cynrig Hatchery near Brecon. This facility is managed by NRW and delivers several captive breeding programmes for critically endangered aquatic animals. These include, Arctic char, freshwater pearl mussels, white-clawed crayfish and water voles.
Cynrig Hatchery is one of only two NRW hatcheries in Wales (Clywedog is the other) and one of only four specialist freshwater pearl mussel hatcheries in the UK.
Clywedog is also used for spawning and encystment stages (more on this below), along with growing-on the juvenile mussels ready to be reintroduced back into the wild.
Over the last year, construction work has taken place to upgrade the facility at Cynrig. This work was funded by the EU LIFE funded Four Rivers for LIFE Project run by NRW. Additional financial support was also provided to the project by Welsh Government and Dŵr Cymru Welsh Water.
Working alongside Dr John Taylor, the project has been able to increase the capacity of the FWPM breeding programme at Cynrig, meaning far greater numbers can be reared.
The project aims to release juvenile FWPM into the Rivers Teifi, Tywi, Cleddau and Usk in the future.
A complex and unique breeding programme
The first stage of the FWPM breeding programme is to collect adults. This is done by trained and licensed NRW officers as they go out to survey rivers with known historic populations of mussels.
The officers look for the characteristic habitat requirements of the adults. Once identified, adult mussels are collected and taken to the Clywedog site and placed in a holding tank.
Depending on the river the adults are found in, they will be kept in their own separate closed system tanks to maintain genetic integrity.
During early summer, male mussels will release sperm into the water and females will take this in to fertilise. Between July and September, females will release millions of tiny larvae called glochidia into the water. These are then flushed through into an adjacent tank containing the juvenile trout.
Once flushed through into the tank holding the fish, the glochidia will naturally pass through the mouth of the fish and attach to the gills by a process called encystment.
At this stage the juvenile host fish will be taken to Cynrig where they will grow on for a couple of months, taking in the nutrients and oxygen from their host fish.
If a host fish naturally passes away, the deceased fish is taken into the lab and one side of its gill arches is removed and the number of glochidia on each side of the gill arch is counted.
Image: The dark spots are glochidia attached to the gills of salmon or trout (credit NRW).
The host fish is then placed within a recirculation system and, over the next few weeks, the temperature will gradually be increased. The glochidia usually drop off at around 17 degrees Celsius.
Algae nets are fitted to the water outlets of each tank to catch the glochidia that drop off and daily checks are done to assess the rate of glochidia dropping off along with the maturity and condition.
The glochidia are then taken into the lab where any silt or debris is removed and, under the microscope, every mussel is hand picked out and counted. These are then placed in an incubation tub with a mixture of silt and algae solution for them to graze on.
The aim is to reach approximately 500 per tub to ensure there is no over competing for the food source. Recent collections in May 2024 are totalling approximately 19,000 juvenile mussels.
Each tub is cleaned at least every two days to ensure it has a constant supply of algae and new silt, and to check on the mortality rate. This cleaning regime also helps to prevent the proliferation of predatory flatworms.
Image: Juvenile rearing tubs in the incubator at Cynrig Hatchery (credit NRW)
When the juveniles are large enough, they will be transported back to the Clywedog facility and reared in trays in the cool waters supplied from the reservoir. This also has a natural supply of algae upon which they will feed and grow.
Image: 1000+ juvenile mussels. The pale sandy coloured ones are the mussels, the darker coloured bits are grit or dirt (credit NRW).
What next for the species in Wales?
Although freshwater pearl mussel populations are critically endangered in Wales, and are struggling due to habitat loss and dwindling fish stocks, there is hope.
NRW officers have been searching for suitable reintroduction sites over the last few years.
The sites must be free from potential pollution risks or disturbance from animals and humans and contain clean, well oxygenated stable gravels.
The techniques for releasing large numbers of juveniles back into the wild are still being evaluated but will likely involve a combination of direct release of tagged older mussels (six years plus) or smaller juveniles (two to four years old) in specially designed concrete doughnuts called silos.
In July 2024 the Four Rivers for LIFE project has been surveying for sites on the Eastern Cleddau to reintroduce juvenile mussels and is likely to carry out restoration work similar to the work that has already taken place on the Western Cleddau to ensure the habitat is perfect for the species.
In October 2023 large boulders were introduced into a stretch of the Western Cleddau river. The boulders will create more natural flow diversity and improve the habitat for salmon, a vital element of the growth of the FWPM.
The NRW Freshwater Pearl Mussel Project completed similar work in October 2022 to restore essential habitat for pearl mussels in a north Wales river.
The site in Gwynedd, is one of the last strongholds of pearl mussel populations in the UK. But human activity in past decades, where the river was dredged and the land drained, has degraded the features of the river that are essential for the species to breed and survive.
Later this year, NRW hopes to introduce mussels which have been reared at Cynrig and Clywedog into the site.
Habitat recovery is a very slow process and can potentially take decades to materialise. This work will boost the recovery process by using methods that would naturally occur.
By working in close collaboration NRW the Four Rivers for LIFE Project hopes to see larger populations of this ancient and elusive mollusc in Welsh rivers.
For more information please visit Natural Resources Wales / Four Rivers for LIFE.