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The freshwater pearl mussel (Margaritifera margaritifera) was once common on many of the river systems in Northern Ireland. However, the species is now in severe decline across most of Europe. The mussel is classed as endangered on the IUCN Red Data List (IUCN, 2008). In addition, it is listed on Annexes II and IV of the EC Habitats Directive and Appendix II of the Bern Convention and is fully protected under Schedule 5 of the Wildlife and Countryside Act 1981 and the Wildlife Order (Northern Ireland) 1985.

Significant numbers of freshwater pearl mussels are known to occur on only 5 rivers in Northern Ireland. Three of these rivers are currently designated as Special Areas of Conservation (SACs). No juvenile mussels (<10 yrs old) have been identified on any of these rivers and anecdotal information would suggest that the populations are no longer recruiting in the wild.

The freshwater pearl mussel has a complex life cycle. They are typically dioecious and reproduce in June-July, when the males release sperm into the water, which is then inhaled by the females. The fertilized eggs develop in a pouch on the gills for several weeks, and are released from July to August as glochidia which are inhaled by juvenile salmonids, where they encyst. Glochidia grow in the gills of host fish until the following summer when they drop off in late May and early June. The absence of juvenile mussels in wild populations may indicate that:

• Adult mussels not reproducing;
• Lack of successful infection of juvenile salmonids;
• Post-glochidial juvenile mussels are not growing in river substrate.

To date conservation efforts in Northern Ireland have been concentrated on the Ballinderry River where fish infected with glochidial freshwater pearl mussels have been periodically released to the river over the last 15 years. In addition, a successful culturing facility has been established at the Ballinderry Fish Hatchery where juvenile mussels have been successfully bred and maintained under hatchery conditions. However, there is no information to verify the reproductive viability of any of the life stages of populations in the wild apart from the obvious absence of juveniles from each of the mussel rivers.

The specific aims of this project are to:

1. Determine gravidity of wild adult mussels in the wild;
2. Verify glochidial infection of juvenile salmonids in the wild