Freshwater Pearl Mussel Surveys

Why do a Freshwater Pearl Mussel Survey?
Freshwater Pearl Mussel (Margaritifera margaritifera) is protected under Annex II of the EU Habitats Directive.

This species are extremely sensitive to water pollution and siltation, they are extremely long-lived and unable to move. Their lifecycle is complex and involves a larval stage that resides in the gills of salmonid fish.

This species is vulnerable to even very slight changes in water quality. For this reason, efforts to protect the species focus on ensuring that any proposed works in pearl mussel catchments do not adversely affect this species. If there is any possibility that a proposed development may impact on a river in which there is a population of Pearl Mussel, survey work is often required.

Where are Freshwater Pearl Mussel found?
Pearl Mussel are found in rivers with igneous or sandstone (not Limestone) rocks along the bed over most of the course. The species is mainly in upland areas where there are little polluting activity upstream of population sites.

What is involved in a Freshwater Pearl Mussel Survey?
There are several levels to which Pearl Mussel surveys can be carried out
  • Stage One surveys determine presence or absence.
  • Stage Two surveys estimate population
  • Stage Three investigates age profile and population statistics.

There are two different methodologies used when carrying out a Stage One Fresh Water Pearl Mussel depending on the depth of water.
  •  Bathyscope survey undertaken by wading upstream and searching the riverbed. Carried out in shallow water where wading is possible
  •  Snorkel survey undertaken by snorkeling with the current in deeper water where wading is impractical
As Freshwater Pearl Mussel are protected under Annex II of the EU Habitats Directive all surveys require a licence from the NPWS.

When can Pearl Mussel Surveys be completed? How long do they take?
Pearl Mussel Surveys may be carried out at any time of year and the length of time they take depends on the area to be searched. These surveys do however require good visibility. This means that surveys should be undertaken in bright weather conditions and when water is not carrying a heavy silt loading. These conditions are generally more likely to occur in the summer months.
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