The plight of the pearl mussel

Freshwater pearl mussels

Freshwater pearl mussels

5 October 2012 by Louise Bracken and Liz Oughton

What's killing our mussels, and what can we do about it? Louise Bracken and Liz Oughton are on the case.

Freshwater mussels may not have the obvious charm of pandas, but their future is just as uncertain, and their loss could have serious implications for our rivers.

Our team's research carried out on the River Esk in north-east England, as part of the Rural Economy & Land Use (Relu) programme, could help reverse the decline of this species, which may include individuals up to 120 years old.

Although the freshwater pearl mussel (Margaritifera margaritifera) is found all over Europe, Scandinavia and north-eastern North America, it has suffered serious decline in recent years and is facing extinction throughout its former range. Furthermore, populations in different rivers seem to have their own distinctive features, and this appears to be supported by studies currently being carried out on their DNA. Many of these unique populations are under threat and in need of protection.

Along the Esk, we recorded just 727 pearl mussels, and the vast majority are large, adult shellfish with only a few smaller specimens. The story is similar in many UK rivers. Scotland is the mussels' last European stronghold, supporting functional populations in over 50 rivers. But even here, few juveniles are being found and pearl mussel beds are under severe pressure.

The mussel sits unseen on the river bed. Why should we worry about its fate?

Many people will be quite unaware of the pearl mussel or its plight. So should we be concerned? Ecologists think we should because this aquatic bivalve mollusc isn't just fascinating in its own right: it's an indicator species for the overall health of a river, because its life cycle depends on very specific conditions. Individuals can grow to 140mm long, and burrow into the sandy beds of fast-flowing rivers and streams, often between boulders and pebbles. They reach reproductive maturity at around 12 years old.

To thrive they need cool, well-oxygenated, soft water that's free of pollution. When the mussels spawn, in mid- to late summer, their larvae attach themselves to the gills of salmon and trout, then drop off the following spring to settle in the riverbed gravel. This helps to disperse the young mussels to new areas where they grow to adulthood. Fishing surveys on the Esk suggest that larvae are still found on these fish, but that they are failing to survive after leaving their hosts.

Mussels in the UK have been affected by many factors, including pearl fishing (now illegal), shifts in the populations of salmon and trout, and pollution. Diffuse pollution, where chemicals leach into watercourses from many different sources, is a particularly complex problem.

Freshwater pearl mussel

A freshwater pearl mussel

Changes in land use and farming practices are thought to have contributed to changes in water quality. Livestock bring problems of soil erosion from poaching - animals trampling the ground to mud - and contamination from waste. In areas with a lot of arable farming, more intensive crop production has also played a part, with increased use of fertilizers and greater reliance on herbicides and pesticides. This results in nitrogen and phosphorus from the soil leaching into water courses.

Industrial activities also create diffuse pollution, and chemicals wash from roads into surface-water drains, and eventually into rivers. Septic tanks and sewage-treatment plants also can be sources of pollution. All these can harm the freshwater pearl mussel and the river environment, but the picture is complicated.

We do know that pearl mussels generally live in rivers with very low concentrations of nitrate, phosphate and fine sediment, and increases in any of these pollutants can cause their decline. Yet the Esk has generally been considered a clean river.

Increases in sediment have often been blamed for the decline in population, as the young mussels can become buried. But when we monitored the Esk's water quality and rate of sedimentation, we found that the highest sediment levels do not overlap with the main areas where the pearl mussels live, so this may be less of a problem than previously assumed.

But water quality varied at different monitoring sites and some of these did show levels of nitrates and phosphates that might be damaging the mussels. We also suspect that these monthly water-quality tests were not showing the worst levels, which would probably occur during flooding.

But the freshwater pearl mussel sits unseen on the riverbed. Why should we worry about its fate? The reality is that if we value our rivers and want them to be healthy habitats and places that we enjoy for leisure, we should certainly worry, because a decline in the pearl mussel population can seriously harm the complex wider ecosystem. Mussels feed on small particles from the water, and adult mussels can filter up to 50 litres of water a day to extract these particles, creating within the mussel bed a suitable microhabitat for the tiny invertebrates that provide food for juvenile trout and salmon.

Graph showing changing nitrate concentrations in the River Esk

Nitrate concentrations in the River Esk. The red line is the critical level below which nitrate concentrations are deemed acceptable for freshwater pearl mussels to live and breed.

The Esk Pearl Mussel & Salmon Recovery project is taking on the considerable challenge of reversing the mussel's long-term population decline. To achieve this, we are using an innovative partnership approach, involving several organisations and academic researchers, that aims to address the bigger picture.

Bringing together individuals with varied expertise, including scientific knowledge and practical understanding of farming, from Natural England, the Environment Agency and Durham University and led by the North York Moors National Park, the project has secured funding for a dedicated project officer, and resources to carry out works to improve water quality.

We have set up a demonstration farm to encourage farmers to think about how they might change their working practices to reduce the diffuse pollution problem. By working with the people who manage the land, and engaging the expertise of officers from Catchment Sensitive Farming (an initiative aimed at helping farmers reduce water pollution from their land) they can balance the needs of food production and of the river ecology.

For example, changes that protect the pearl mussel, such as fencing to keep farm animals out of the river, can also protect livestock by reducing foot infections. Providing troughs gives livestock alternative access to clean drinking water, while careful, targeted use of chemicals has the additional benefit of saving the farmer money. This blend of skills and different kinds of knowledge, and the involvement of the local farming community as key partners, has brought about important changes in farming practices along the Esk.

It is too early to judge whether this will eventually lead to the recovery of the pearl mussel population, but the future already looks brighter for this ancient but seldom-seen creature.

Dr Louise Bracken is a reader in the Department of Geography at Durham University.

Dr Liz Oughton is principal research associate at the Centre for Rural Economy at Newcastle University.