Born to be Wild

Dr Ceri Gibson – Pearl Mussel Projects Manager, Freshwater Biological AssociationFBA logo

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.

Proof that the FPM are looked after even on Christmas Day. Copyright FBA.

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.

View looking north over Lake Windermere. Copyright FBA.

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.

Rainbow from Mitchell Wyke. Copyright FBA.

Biffa Award logoBiffa 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. 

 

Ageing Mussels

Simon Hirst – River Esk Project Officer

As mentioned previously, the River Esk in the North York Moors is the only river in Yorkshire with a Freshwater pearl mussel population Margaritifera margaritifera. The population is estimated to be comprised of approximately 1,000 individuals and is in drastic decline, so much so that it is on the verge of extinction. The decline is due to a number of linked causes such as water pollution, choking of the river bed by sediment build-up, deterioration in fish numbers and habitat degradation.

A dense bed of healthy adult mussels in Scotland. Copyright Sue Scott - SNH,

We’re working to improve the riparian habitat and so help secure the local population of Freshwater pearl mussel in the River Esk. We recently sent a sample of mussel shells from the Esk* over to the Swedish Natural History Museum in Stockholm, in order to determine the age of the mussels in the River Esk. The maximum age of Freshwater pearl mussels in the wild has been shown to vary considerably, from a low of 35 years in Spain (warmer, lower latitude rivers) to over 200 years in arctic areas (colder and high latitude rivers). Information from the ageing study would tell us how long we have left to save the Esk population from extinction and help identify the approximate time when the River Esk mussel population went into decline.

Dr Elena Dunca from the Swedish Natural History Museum sectioned (cut though) the shells supplied and then counted the growth lines on the mussel shell using a high powered microscope.

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Growth lines visible on the freshwater pearl mussel shell.

Esk FWPM - Age and length graph - Swedish Natural History Museum

This age/length graph will allow us to age fairly accurately any mussels we find in the wild in the future just by measuring them.

A total of 10 shells were aged by Dr Dunca, and the graph below shows that the mussels sent to Sweden ranged in age from 45 to 88 years of age.  The mussels in the River Esk also showed normal growth rates.

Esk FWPM - Length frequency graph - Swedish Natural History Museum

Length frequency graph of mussels in the River Esk

The smallest live mussel we have found in the Esk up to now was 75mm (approximately 28 years of age). This means the last time the Esk mussels reproduced successfully in the wild was in the late 1980s. The largest mussel we have found in the Esk was 156mm (approximately 100 years of age), which means it was born around the time of the First World War. The vast majority of the mussels are around the 130mm-140mm size range (approximately 80 years of age). We now know for scientific certainty that the Esk has an ageing population in need of help!

The best hope for our mussels is for them to start to successfully reproduce again. We’re working with the Freshwater Biological Association (FBA) who are carrying out captive breeding work in the Lake District. We hope to re-introduce the captive bred young Esk mussels from the FBA Facility back into the Esk once the riparian habitat is restored enough to sustain them, and so ultimately stop this species from becoming extinct in the wild (of Yorkshire).

* Please note – No mussels were harmed in the making of this study! We used empty shells that were found on the banks of the Esk.

Thanks to our funders at Biffa Award, for their support to carry out this vital research work.

Biffa

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.

In Extremis update

From Eloy at the Freshwater Biological Association Facility:

The Esk mussels are settling in. They are buried and stable, not moving that much, as if they had found their space in the cage.

Recently translocated mussels from the Esk now at the FBA Facility - copyright FBA

 

In Extremis

Simon Hirst – River Esk Project Officer

As mentioned before on this Blog, the River Esk in the North York Moors is the only river in Yorkshire with a Freshwater pearl mussel (Margaritifera margaritifera) population. Our population is in drastic decline – currently it is estimated there are approximately 1,000 individuals, all adults.

We’re part of a national project (funded by Biffa Award) to conserve the populations of Freshwater pearl mussel in England. Nine years ago a number of the adult mussels were collected from the Esk and moved to the Freshwater Biological Association’s Facility in the Lake District. A couple of weeks ago, early one morning, Eloy from the Association and I collected another 20 and took them across to join the others at the Facility 130 miles away.

Eloy preparing the FPM transportation unit ie a cool box - copyright Simon Hirst, NYMNPA

River Esk substrate to take to the FBA Facility - home comforts for the Esk mussels - copyright Simon Hirst, NYMNPAThe lid of the cool box had to be left open so oxygen exchange could occur during the 3 hour journey - so the box was thoroughly jammed in - copyright Simon Hirst, NYMNPA

Safely arrived at the FBA Facility - acclimatising the Esk mussels to their new home - copyright Simon Hirst, NYMNPAThe idea is the translocated adults breed and the Facility then rear the captive juvenile mussels with the ultimate idea of reintroducing the young mussels back into their native rivers. In the meantime we’re continuing to tackle the problems that have had and continue to have such a detrimental effect on the mussels. So on the Esk we’re working in partnership to improve the riparian habitat in order to increase water quality and vitally reduce the amount of suffocating sediment so that the river becomes a suitable release site and juvenile mussels have a chance of surviving. Improving the river habitat also benefits the migratory fish which are such a vital part of the mussels’ lifecycle.

We’re trying everything we can to help the Esk population and give it the best chance of survival. Mussels can live to be over 100 but if there are no juveniles, slowly but surely the Freshwater pearl mussel will become locally extinct.

Biffa