Outdoors: Show some mussels

It has an incredibly complex lifecycle, can live for more than 100 years and is able to filter more water each day than we use in an average shower.

The freshwater pearl mussel is reputed to be one of the reasons why Julius Caesar invaded Britain in 55BC and its fragile ecology has made it one of today’s most threatened creatures.

It may appear to be a rather unremarkable looking mollusc, yet the freshwater pearl mussel commands a pivotal place in our cultural history and is now at the centre of a major campaign in Scotland to prevent it from sliding into extinction.

Caesar coveted the rich pearl mussel beds found in Britain’s clear flowing rivers, and they have been prized for generations since, with the earliest reference to Scotland dating back to the 12th century when Alexander I, King of Scots, was said to have the best pearl collection of any living man. And, of course, locally sourced pearls are embedded in the Scottish crown jewels. But as the exploitation continued, so the population of pearl mussels dwindled, and by the 20th century they had become extinct in many of our rivers. As well as over-fishing, this decline was further fuelled by pollution and engineering works in rivers.

Over the last 100 years, more than one third of Scotland’s rivers that formerly supported freshwater mussels now no longer do so. According to Dr Iain Sime, an ecologist with Scottish Natural Heritage (SNH), we have a very special responsibility to look after our surviving colonies, given that Scotland now supports more than half of the remaining world population.

“Scotland is the global stronghold for the freshwater pearl mussel and this is why we are putting so much emphasis on ensuring its conservation,” he says.

Although now fully legally protected, the surviving remnants are extremely vulnerable from exploitation by pearl fishers. They only very occasionally contain pearls and it can take the devastation of whole colonies through the killing of many hundreds, or even thousands, of the molluscs to have the remotest chance of finding one inside, and even then there is every likelihood that it will be flawed. With no legal market for the pearls, they are also worthless.

The freshwater pearl mussel lives at the bottom of clean, fast flowing rivers and feeds by drawing in water and filtering out fine particles, making it very sensitive to water pollution, nutrient enrichment and siltation. It is now confined to only a few clear flowing rivers south of the Great Glen, as well as in its principal strongholds in north-west Scotland. Even where it does occur, if the conditions are not right, then breeding may stop and never resume again.

SNH, working with other partners such as fisheries trusts and the Scottish Environment Protection Agency, has been co-ordinating and implementing a series of initiatives in recent years to help save the pearl mussel. This is mainly focused around improving water quality and river habitats, including planting riparian woodlands to provide a natural buffer zone between rivers and farmland. Guidance for developers and contractors working on Scotland’s rivers is provided, and a mussel reintroduction scheme into one river is also being trialled. Of crucial importance is the presence of salmonid fish such as brown trout and salmon. The freshwater pearl mussel is totally dependent on these fish for its survival because in the first year of its intricate lifecycle, the larvae live harmlessly on the gills of these fish.

Dr Sime is reasonably optimistic about the future of the pearl mussel in Scotland, but says we can all play an important role in ensuring its survival by keeping a watchful eye out for illegal activities, with the species now a UK wildlife crime priority.

“There is evidence that some stocks are recovering but it can take just one incident involving pearl fishers to decimate a river population,” he says. “People should report to the police any suspicious activity such as dead shells on a riverbank or seeing people working a river with glass-bottomed buckets. The freshwater pearl mussel is important to Scotland, both culturally and ecologically. They are excellent environmental indicators and their presence in a river is a sure sign that it is clean and healthy.”