Hundreds of young freshwater pearl mussels – one of the UK’s rarest aquatic creatures – are to be released into a river at a secret spot in north Wales this summer as part of a project to save the molluscs from extinction.
The juvenile mussels have been bred at a hatchery in the Brecon Beacons, in the south of the country, and will be carefully moved to the river in Gwynedd after it was restored to create the sort of conditions they are able to thrive in.
Pearl mussels can live for more than a century, meaning that some of the adults in the hatchery tanks may have been alive when Queen Victoria was on the throne, but young mussels have not successfully reached maturity in the wild in Wales for decades.
Loss of habitat, pollution, the climate emergency and human exploitation – they have been harvested for the pearls they sometimes produce since Roman times – have caused populations to crash.
In the coming weeks, the unnamed river will be surveyed again by experts from Natural Resources Wales (NRW), and if the necessary permits are obtained, the first release will take place.
The site is being kept secret in case anyone is tempted to break the laws in place to protect them and search for pearls. Even if they did find the spot, they would almost certainly be disappointed as vanishingly few mussels actually produce pearls.
“They are fabulous things,” said Tristan Hatton-Ellis, a freshwater ecologist with NRW. “We are hoping that putting back the riverbed will be massive not just for the pearl mussels but for salmon, trout and invertebrates.”
Only about 1,000 ageing adults survive in the wild in Wales. The adults, which are up to 14cm long, like clean, fast-flowing water. They produce glochidia (the larval stage), which attach to the gills of salmon or trout. Then as juveniles they spend up to 10 years buried in river gravels, but none have reached maturity in recent years.
Their complicated life cycle means they need rivers with a diverse flow of water, but human activity such as dredging, building embankments and draining the nearby land has degraded the features that pearl mussels thrive in.
Over a period of six months, more than 850 tonnes of boulders and cobbles were carefully placed back into the river in Gwynedd along with more than 330 tonnes of fresh gravel.
Embankments were removed to reconnect the river with its floodplain, and drainage ditches, which had been cut through an area of peatland, were filled in.
Fencing has been installed around the river to keep livestock out and trees planted to provide shade for the river and, hopefully, the mussel beds that will develop.
Meanwhile, NRW has been upgrading its Brecon site where the mussels are bred. It is one of two specialist freshwater pearl mussel hatcheries in the UK, the other being in Cumbria, in north-west England.
Katie Fincken-Roberts, a biodiversity specialist from NRW, said there were currently about 32,000 juveniles in the hatchery. She said: “We need them to grow to a certain size, about 25mm, before we think they’d be robust enough to be released. We have a couple of hundred that would potentially go back this year and then the following year there would be pushing a thousand and hopefully it will ramp up.”
