Freshwater Pearl Mussels (Margaritifera margaritifera) are a species of mussel can live up to 140 years old. These mussels have a multi-stage life cycle which requires salmonoid fish for incubation at their larval stages, before burrowing into the riverbed for up to four years, before coming to the surface of the riverbed to filter feed. The average age of sexual maturity for this species is 20 years old, and as such ascertaining details of the population, age-structure is vitally important when assessing population viability. The presence of juveniles (a feature essential to the long-term sustainability of mussel populations) is a clear indicator of the structural and functional features of the habitat required for the survival and reproduction of the species. Currently, the species is found in just 5% of its previously known distribution and is listed as ‘endangered’ on the IUCN Red List.
Whilst in the UK this species stronghold is in Scotland, populations are also present within Wales, England, and Northern Ireland. These species are known to be distributed throughout a number of sites in South Wales and they are protected under the Protected under the EC Habitats Directive (Appendix II and Appendix V), Bern Convention (Appendix III) and Wildlife and Countryside Act 1981 (Schedule 5). In addition to this, it is also a UK BAP Priority species and a Species of Principal Importance in Wales.
If development works are planned to be completed within, or at close proximity to a watercourse or waterbody, it is important to determine if Freshwater Pearl Mussels are present and whether the proposed works will impact on the species.
Surveys of the riverbed can be carried out using bathyscopes (glass bottomed buckets). It is possible to survey for the larval form of the species through electrofishing surveys of salmonoid populations in the river.