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Freshwater pearl mussel

What action is going on for this species?

The following will give you information on what work has been taking place through the Species Action Framework:

Species background

The freshwater pearl mussel (Margaritifera margaritifera) is a large, long-lived, bivalve mollusc of rivers and streams.

Why is this on the Species Action List?

It meets criterion 1a of the Species Action Framework, as a species for conservation action.

Scotland is now a global stronghold for the freshwater pearl mussel, having up to half the world's known recruiting populations, but it has declined substantially in the last 100 years due to human activities.  However, there are a number of management actions that can be employed which should help to turn around this decline. 

Extensive programmes of survey and research have been undertaken on the species in Scotland so there is a good knowledge base.  A wide range of other species will benefit from management targeted at pearl mussel because it requires high quality riverine habitat.  Management measures aimed at enhancing salmonid fish stocks are also important as these fish are an essential link in the pearl mussel life cycle.

It is a UKBAP Priority Species and is included on the Scottish Biodiversity List. It is listed on Annexes II and V of the EC Habitats Directive and is fully protected under the Wildlife and Countryside Act 1981, as amended.

Habitat, distribution and abundance

The mussel inhabits coarse sand and gravel beds of fast-flowing, non-calcareous streams and rivers.  There are approximately 69 recruiting or 'viable' populations (i.e. with juveniles present) in Scotland, mostly in the north and west, with scattered records of the species elsewhere.  Few viable populations occur elsewhere in Britain or even the rest of Europe.

General ecology

The mussel feeds by filtering out fine organic debris in river water. Growth is slow - this species does not reach reproductive maturity until at least 12 years old and some animals may reach 80-100 years.  Mussel larvae ('glochidia') are released by the females in summer and a small proportion of these will attach themselves to young trout or salmon where they live as ectoparasites before dropping off and settling on the substrate the following spring.

History of decline, contributory factors and current threats

The pearl mussel is now extinct, or there is no evidence of recent juvenile recruitment, in approximately two thirds of 155 Scottish rivers occupied 100 years ago.  Most of the remaining populations have also suffered substantial declines as a result of pearl fishing, pollution and habitat loss, and salmonid declines.  Despite recent legislation on pearl fishing, a range of pressures continue to operate, including:

  • Habitat removal and alteration through development, drainage schemes, flow regulation and fisheries management.
  • Declines in populations of salmon and trout, which act as the larval hosts.
  • Destructive pearl fishing and illegal pearl trade, aided by improved accessibility.
  • Poor water quality, including nutrient enrichment (which also affects the numbers of host fish).
  • Conifer planting, exacerbating the effects of river acidification.
  • Sedimentation from soil erosion, affecting the suitability of gravel and sand beds for juvenile mussels.

The 'Species Lead' at Scottish Natural Heritage

Iain.Sime@snh.gov.uk  Tel 01463 725000