The Freshwater Biological Association has been working with the North York Moors National Park for the last 3 years on the ‘Restoring Freshwater Mussel Rivers in England’ project (funded through Biffa Award). Our role has been to rear juvenile mussels ready for reintroduction back into the River Esk at some appropriate time in the future when the habitat is restored and the water quality improved enough to support them.
Rearing juvenile freshwater pearl mussels (FPM) is a 24 hrs a day, 365 days of the year operation with our staff attending the hatchery even on Christmas Day to ensure that the mussels, and the host fish that they rely on as part of their lifecycle, are all okay.
But it’s not a bad job over here on the shores of Windermere. Whilst the wind may funnel up the lake at times, come rain, snow or sunshine, the views are wonderful and we have a great team.
The adult mussels are kept in circular tanks in our hatchery connected to tanks which contain host fish. Keeping fish and mussels in close proximity under controlled conditions improves the chances of the mussels successfully completing their lifecycle. We try to maintain the mussels in as natural an environment as possible so they are kept outside in flowing water provided from Windermere.
We constantly monitor the mussels and fish to record when glochidia (freshwater pearl mussel larvae) are released from the females and when these glochidia have grown sufficiently on the fish gills. Then we set nets to collect juveniles as they drop off their fish hosts and transfer them to trays in our juvenile rearing facility. Whilst other techniques of bank-side encystment and seeded gravel reintroductions have been used for some rivers we believe that rearing juveniles beyond their most vulnerable pedal feeding stage, when they live amongst the gravels before filtering free-flowing water, will yield better reintroduction survival rates and allows us the opportunity to learn more about these fascinating creatures.
Back in 2016, we worked with Simon Hirst (River Esk Project Officer) to translocate more adult mussels from the Esk to the hatchery. This exercise followed a recently commissioned brief from Natural England and was very carefully monitored. It proved successful and we have been collecting the captive bred juveniles in large numbers over the last two seasons. FPM larvae are carried on the gills of specific fish hosts for up to 10 months as part of lifecycle. It is interesting that in recent years the Esk glochidia grow to excystment size (350-400 microns long) and drop of the fish hosts within 2 to 3 months at the hatchery. We are therefore monitoring their survival in the rearing trays carefully.
We have been working with other river populations to prepare juveniles for reintroduction into their native rivers. The trays they are reared in are indoors where the juveniles are well protected. So we have set up flume systems to expose captive-bred juvenile mussels to diurnal and seasonal conditions which better reflect the river situation they will be reintroduced to. Once in the flume we can control flow to ‘teach’ them how to adapt to flood and drought conditions. During these experiments we record how many mussels are deeply buried, how many remain on their side on the surface of the gravels and how many are roaming about using their muscular foot. We also record whether they are siphoning water or not.
We look forward to the time that we can repeat these experiments for the Esk population but in the meantime have a look at some of our other, older juveniles filmed in their flume – click here. Or organise a group visit and come and see us at the Ark to visit the Esk juveniles and learn more about what we do.
Biffa Award is a multi-million pound fund that helps to build communities and transform lives through awarding grants to community and environmental projects across the UK, as part of the Landfill Communities Fund.