Target species
Freshwater pearl mussel
Freshwater pearl mussel (FPM,听Margaritifera margaritifera)听is assessed globally as an endangered species (IUCN). In the EU Habitats Directive, FPM is recognized as a priority species and listed in Annexes II and V, with conservation status assessed as Unfavourable-Bad (U2) across the boreal bioregion. According to the Finnish Nature Conservation Act, freshwater pearl mussel is a threatened species under strict protection. Finnish Ministry of Environment has prepared .
Keystone species
Freshwater pearl mussel is a keystone species in the riverine ecosystem, creating habitats for other species. It thus has a significant role in producing ecosystem services. FPM are filter feeders purifying the water while they are feeding: up to 50 litres of water can pass through one mussel鈥檚 filtering system in one day. Only part of the filtered particles are used as听 nutriment 鈥� the rest are packed into mucous lumps and spat into the river bottom, where they end up as feed for bottom-dwelling animals. As juvenile fish feed on bottom-dwellers, FPM provide food also for salmon and trout 鈥� the host species of FPM larvae. Water mosses, on the other hand, use FPM shell surface as substrate providing shelter for many invertebrates and insect larvae. FPM also mix the bottom gravel with their feet keeping the river bottom rich in oxygen. The latest results show that mussels are even able to filter fish pathogens from the water, and that fish carrying FPM glochidia on their gills survive better from bacterial infection. Thus, a dense mussel population creates a healthy and biologically diverse community. The disappearance of FPM leads to poorer river biota and decreased fish survival. We still may know only a small part of the ecosystem services that FPM provide.
Highly endangered species
Freshwater pearl mussel populations have been declining rapidly throughout Europe. Without action, FPM will become extinct in the vast majority of its current distribution area. Even in Sweden and Finland, which can be regarded as strongholds for the species, the overall status of FPM is unfavourable. There are 646 FPM rivers in Sweden and 129 in Finland, according to the latest estimates. Still, in the project countries, in total up to half of the FPM populations are not recruiting (producing new offspring). This is true for most of the Finnish population. These populations are doomed to extinction. Because of an extremely long lifespan of the species (even over 200 years), disappearance of populations happens slowly: individuals not reproducing can survive in the river for decades even though in the reality the population is already lost. Adequate recruitment (such that will presumably prevent extinction of the population in the foreseeable future) is taking place in only around 10 % of the populations. The only remaining FPM population in Estonia is not recruiting at all and is declining rapidly.
Decades ago, pearl fishing targeting adult mussels caused decline in many FPM populations. Nowadays, however, the biggest problem of FPM is high mortality of young individuals, juveniles. This is caused by siltation of river bottoms with fine sediments due to load from the catchment areas as a result of land use. Siltation is fatal to the juvenile mussels which live their first years burrowed in the riverbed gravel. The fine sediments block water flow in the gravel causing oxygen depletion and mortality of FPM juveniles. Thus, the main threat, impacting approximately 90 % of FPM populations in Fennoscandia, is intensive land use in the catchment.
Atlantic salmon and brown trout
Freshwater pearl mussel larvae live on gills of fish from nine to 11 months before starting their benthic life. Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) are the only suitable (and obligatory) hosts of the parasitic FPM larvae. Our earlier results show that salmon is the only suitable host for salmon river FPM glochidia, and that the local salmon or trout is also the best host. Thus, decline or lack of salmon and trout is a major problem for many FPM populations. River habitat degradation due to channel modifications for timber floating, eutrophication, siltation and hydropower dams and other fish migration obstacles have impaired salmonid populations, affecting FPM negatively.
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