Freshwater Pearl Mussel

Factors causing the decline of the freshwater pearl mussel

Despite some large populations of adults, the species is expected to go extinct within the current generation. The main reason for this is the fact that not enough young mussels are surviving to replace the dying adult mussels. The young mussels bury into clean gravel/cobble on the riverbed for about 5 years and therefore require these gravels to remain clean with well-oxygenated water flowing easily between pore spaces between the stones. If these pore spaces become clogged with fine sediment (e.g. silt) or algal growth, the young mussels become starved of oxygen and die. Increased silt can be caused by erosion and soil disturbance (e.g. due to machinery or livestock damage) and nutrients can often come from inappropriate management of fertilisers and manures. Changes to the hydrology of the river (e.g. excessive land drainage leading to increased river floods or changes to riverbanks causes changes to flow regime) can also result in mussels being scoured out or having material deposited on them.

The three primary problems can be summarised as:

  1. Fine sediment inputs to its habitat
  2. Nutrient inputs to its habitat
  3. Changes to the habitat hydrology

Other losses of adult mussels can occur through kills from major pollution incidents, such as toxic poisoning (e.g. from sheep dip), trampling, eutrophication (through smothering of adult mussels by filamentous algae or macrophyte growth). Killing freshwater pearl mussels in search of pearls is believed to have caused widespread damage to adult freshwater pearl mussels in the past but is generally less of an issue now because (a) it is illegal (see below) and (b) the pearls have little/no economic value and are not found in many mussels

Protection of freshwater pearl mussels

The freshwater pearl mussel is protected under Annex II and V of the European Community Council Directive on Conservation of Natural Habitats and of Wild Fauna and Flora (92/43/EEC). It is listed on Appendix 3 of the Bern Convention. Under Irish law, it is illegal to interfere with M. margaritifera (Statutory Instrument No. 112, 1990). This in turn conferred protected faunal species status for the species under the fifth schedule of the Wildlife Act (1976), and other subsequent protections under the Wildlife (Amendment) Act 2000.


Conservation of freshwater pearl mussels generally requires sustainable land management on farms and forests.  Sustainable farming and forestry practices can contribute to this by reducing silt and nutrient losses to water. Avoiding changes to river hydrology (e.g. increased land drainage leading to larger river floods) is also important, as is avoiding direct damage to mussels or their habitat (e.g. from livestock access or machinery).  There are a number of actions being trialled in the KerryLIFE project for effectiveness in conservation of freshwater pearl mussels and the project is working closely with farmers and foresters to implement these actions within the project area:

C1 Drain management
C2 Stabilising riparian sediment sources using broadleaf planting
C3 Establishment of in-field buffer strips
C4 Grazing and livestock management
C5 Reduction of farm nutrient inputs
C6 Alternative drinking water facilities for livestock
C7 Restructuring of commercial plantations to long-term retention woodland
C8 Transformation of conventional clearfell managed commercial forests to continuous cover forestry
C9 Firebreak management