Endangered Species Day – freshwater pearl mussels

Each year Endangered Species Day shines a light on some of our most threatened species. Here Orla Hilton from NatureScot’s South Highland team takes a look at the critically endangered freshwater pearl mussel and what can be done to help this struggling species.

Freshwater pearl mussels ©Sue Scott/NatureScot

The freshwater pearl mussel Margaritifera margaritifera L. is a long-lived mollusc (some live for over a century) that is under great pressure from river pollution and modification as well as illegal exploitation from pearl fishers. Scotland holds a significant global population of the species and hence our important role in conserving our populations.

Freshwater pearl mussels are also vulnerable to changing climate and drought.  This can be alleviated by riparian (riverside) planting. Initiatives such as the Pearls in Peril project have carried out extensive riparian planting and other management. In one population in the Hebrides this led to a pearl mussel population resuming breeding again for the first time in decades. As well as providing valuable shade from the sun, trees create more diverse water courses with more varied habitats for mussels and their host salmonids.

Woodland near Dunkeld. ©Lorne Gill/NatureScot

Currently fine sediment in rivers has been highlighted as a key pressure on pearl mussels. It can affect their filter feeding and smother the spaces in the gravel where young pearl mussels tend to live.  Together with the Scottish Government, we are using a technique called ‘sediment fingerprinting’ to identify the sources of fine sediment in key catchments, allowing us to tackle any problematic areas.  In upland areas some of this sediment can come from degrading peat bogs so one way of safeguarding our pearl mussel is to restore our peat bogs by blocking dams, reprofiling exposed peat hags and scrub removal.

It is also important when any forestry works are going ahead to ensure proper measures are in place to prevent any sediment from machinery or felling works from smothering pearl mussels in the adjacent burns. This can be achieved by increasing the buffer between streams and plantation trees and by damming any pre-existing ditches. There are several examples where this kind of work has helped improve conditions – particularly a successful scheme in Sutherland by Forestry & Land Scotland.

Blocked ditches and newly planted riparian trees along a pearl mussel watercourse in Sutherland (trees funded by the Biodiversity Challenge Fund). ©Kyle of Sutherland Fisheries Trust 2020.

In some rivers the cause of decline is much more mysterious. An unexplained and catastrophic decline in pearl mussels has been observed in two burns in Sutherland over the last two decades. Despite a lot of research the reason for the decline remains a frustrating mystery.  But just recently it has been suggested that a pathogen may be behind this mass mortality.  Unlike our battle with a very well-known pathogen (ahem, COVID-19, looking at you) the source of our mussels’ plight is yet unconfirmed. Marine Scotland Science have begun a study to test for possible pathogens, to try and better understand and address this worrying situation.

Until recently the ideal known habitat for this invertebrate has always been shallow fast flowing, well-oxygenated rivers. In exciting developments a recent study has found a significant population (containing juveniles!) dwelling in an Irish lough. In the future we hope to look into our own lochs in Scotland to have a clearer idea of the populations we have!

Could there be Freshwater Pearl Mussels in Loch Ness? ©Lorne Gill/NatureScot

This is encouraging news for such an iconic species. Alongside our efforts to protect and improve the current known river populations the opportunities to loch-ate (!) more populations offers tantalising hope for future safeguarding of this key species.

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