Amorous mussels spell hope for River Irt

30th May 2023 - Wild Rivers Catchment

Conservationists working to help grow the population of critically endangered freshwater mussels within the River Irt are celebrating this week as they found evidence of the population reproducing for the first time since 2010.

Freshwater mussels have an unusual life cycle. Females release millions of larvae, called glochidia, into the water. To survive they need to be inhaled by fish hosts (juvenile salmon or trout). The glochidia clamp onto the gills of the fish and live there for nearly a year before dropping off. They must land on areas that contain clean and stable sand, or gravels in which they bury and grow. The fish hosts are unharmed during the process which is technically called encystment. The last recorded evidence of encystment was seen by the Environment Agency back in 2010.

The Irt is home to a very small population of around 300 mussels that appeared not to be breeding. Collaborating with partners such as the Environment Agency, Natural England, National Trust and local anglers, West Cumbria Rivers Trust has been working to bolster the population by undertaking habitat improvements throughout the catchment. These works over a number of years have been made possible thanks to funding from the Environment Agency, Natural Course, Natural England, Lake District National Park, Cumbria Community Foundation and Biffa Award, amongst others. In addition, through collaboration with the Freshwater Biological Association, releases of juvenile mussels bred in captivity has been undertaken in the hopes that the population can be increased to a more healthy and self-sustaining number.

Freshwater mussels live for 120 years on average and play an important role in river ecosystems, improving water quality as each adult filters around 50 litres of water per day. Their numbers have declined sharply and only a handful of populations remain in England.

The team were undertaking a routine monitoring visit when the encystment evidence was found. They also found mussels persisting across the sites where the captive bred mussels were released meaning these new introductions have survived two winters in the river. The team also found a mussel introduced during an earlier release programme from 2017. Now classed as a sub-adult, it had grown significantly since it’s release.

Chris West said: “It’s thrilling to find evidence that the population is reproducing. All our previous monitoring has found no evidence of reproduction, so this is heartening to see. It’s also reassuring to see the mussels released into the Irt are thriving. These plus more that we plan to release over the coming years will significantly boost the population number.”