Mussels left high and dry in drought?

A new report published by NatureScot has found evidence that a drought in 2018 had a detrimental impact on freshwater pearl mussel populations in several Scottish rivers. In today’s blog, river restoration graduate placement Orla Hilton takes a closer look at the impact of drought on this critically endangered species, and what action can be taken to help.

Freshwater Pearl Mussels in very low flows in 2018 ©Iain Sime

Picture this: it’s summer and you’re sunbathing in the garden, drink in hand, enjoying some rare Scottish sun on your browning grass. In your state of bliss, however, spare a thought for the endangered freshwater pearl mussel.

As today’s report shows, in addition to threats from illegal pearl fishers, intensive land use, nutrient pollution and a general reluctance to spawn, these poor creatures are being left high and dry in periods of drought. They are, sadly, the canaries in the coalmines of our rivers – giving us early warning of the threat some watercourses are under.

Unfortunately their hopes for nice wet summers are looking increasingly slim. We’ve all become used to the warmer wetter winters, hotter drier summers mantra and Scotland unfortunately is not exempt. Recent research by NatureScot has shown that we are likely to face an increase in the risk of extreme droughts over the next two decades as a result of climate change.

Freshwater Pearl Mussels suffering in filamentous algae ©Iain Sime

Cast your mind back to summer 2018, when entire mountains in Torridon were in flames and farmers in Speyside were forced to sell cattle they could no longer feed. That year a mortality rate of around 50% was found in part of one river known to be home to pearl mussels. With the species a high priority, globally and in Scotland, we need to take action to avoid this sort of wipe-out.

So let’s dive a little deeper (“if only” the pearl mussels cry!) into what exactly makes these conditions so dangerous for the mussels.

In general, drought periods dry the land out, creating extra quick run-off times when rainfall events happen. These high spate conditions tend to be destructive to the riverbed habitats that mussels and juvenile fish rely on. In drought the river levels drop, leaving some mussels beached and vulnerable to dehydrating or being eaten by opportunistic birds. This also makes potable water scarcer and could result in higher demand for rivers that contain mussels to be managed as water sources. These shallow waters are more vulnerable to unusual and harmful high temperatures – which prove lethal to many riverine species. Finally, the mussels aren’t just more vulnerable to animal predators but also illegal pearl fishers.

Two dead pearl mussel shells following drop in water levels during drought ©Iain Sime

Quite a troubling situation it seems, but the game is not up!

A key solution is riparian planting. With more trees on the banks of our rivers we can protect mussels and fish by offering some shade in summer months. More vegetation in river catchments also slows the flow of water to rivers significantly in high rainfall events, making for more natural and less destructive flow. We can also create more natural hydrology by restoring peatlands and blocking drainage ditches to slow water flow to rivers. The latter also reduces sedimentation in high water events. As today’s report highlights, in certain circumstances local communities can also play an important role in moving mussels in emergency events.

At NatureScot we’re taking action to conserve freshwater pearl mussels, including through the Biodiversity Challenge Fund. We have recently supported two projects in the north of Scotland to plant around 20,000 trees to create riparian woodland. This action has created kilometres of shade and is improving instream habitat allowing biodiversity to recover. Across both of projects barriers to fish movement are being removed to allow the pearl mussel’s important host fish species access to more of their natural habitat.

So as we look towards summer, let’s not forget about this fantastic bivalve and what we can all do to stop them being left high and dry.

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