Current Status of the Freshwater Pearl Mussel in Ireland

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:-

Also, if you have any queries about issues relating to Freshwater Pearl Mussels please do not hesitate to contact us.

One thought

  1. I come from a small town in Co waterford on the banks of the Blackwater river. Ten years ago I would have told you that the Pearl mussel was over abundant in the river around the town. You could see beds of the Mussel from the bank at low water from Juveniles smaller than my baby finger nail to the great one that were easily 100 years old (almost the size of a mans hand with thick scarred shells). Often you would catch one by accident by just casting a line with a worm in their area while fishing! They sit in the silty deposits with gravel directly underneath protected from the currents and you would see their ‘Tongues’ as we called them hanging out to catch their food.I could easily have named 20 spots where you would find them from the top of my head. In the last few years I have not seen a single Pearl mussel in any of those spots. They seem to have vanished. It’s a terrible shame that something that brought such joy in my child hood seems to have had their population decimated. There are less Salmon and trout around now also so there is even less chance of them recolonising these areas as fewer juveniles complete this phase of their life cycle. Such a shame.

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