The Pearl Mussel is listed under Annex II and V of the Habitats Directive (92:43: EEC). It is legally protected in Ireland under Schedule 1 of the Wildlife Act (1976 (Protection of Wild Animals) (Statutory Instrument No. 112, 1990) and the European Communities (Natural Habitats) Regulations (Statutory Instrument No. 94, 1997).
The Republic of Ireland is estimated to hold 46% of all the pearl mussels in the European Union but only one of its populations is in favourable condition and none of the rest have sustainable juvenile recruitment. This species is evaluated as being of overall ‘Unfavourable to Bad” conservation status nationally (NPWS 2008). The pearl mussel rivers in Ireland that are known to have recruited young recently are generally in remote areas, with short rivers and small catchments that have not historically been subject to intensive fertiliser inputs. They are typically areas of low human population density, with few urban areas, any habitation being located low down in the catchments. They are mainly below lakes, which provide an even, buffered source of water through the river. Many of the SAC watercourses for Margaritifera margaritifera fall into this category.
Recovery of a mussel population from unfavourable to favourable condition becomes more difficult when adult numbers are reduced, as the life history of the mussel relies on very large numbers of glochidia (juvenile mussels which attach to the gills of salmonids during their early life stage) in the cleanest of waters to result in adequate juvenile survival. Thus, early detection of river management problems and fast remedial action is very important. The survival rates for glochidia in salmonid gills has been found to be in the region of 5%; with a further 5% survival rate for juvenile mussels in gravels in rivers capable of supporting recruitment.
The decline of pearl mussel populations in Ireland has mostly occurred from the continuous failure to produce new generations of mussels because of the loss of clean gravel beds, which have become infiltrated by fine sediment and/or over-grown by algae or macrophytes. Macrophytes smother the juvenile habitat even further, and trap more sediment, exacerbating the problem in the long term. Filamentous algae can lead to the death of juvenile mussels, through blocking oxygen exchange with the sediment.
If the river water remains strongly turbid for a number of days, mussels can die from oxygen starvation, either from remaining clammed, or from ingesting contaminated water while stressed. During a time of year when water temperatures are high, oxygen depletion in the body occurs more rapidly, and mussels die more quickly.
NPWS (2008). The Status of EU Protected Habitats and Species in Ireland. Conservation Status in Ireland of Habitats and Species listed in the European Council Directive on the Conservation of Habitats, Flora and Fauna 92/43/EEC.
For further information on Freshwater Pearl Mussel please see the following links:-
- Freshwater Pearl Mussel Article 17 Assessment Ireland
- NPWS Freshwater and Terrestrial Invertebrate Research
Also, if you have any queries about issues relating to Freshwater Pearl Mussels please do not hesitate to contact us.