Distribution, status and threats of the freshwater pearl

Dolmen D and Kleiven E. 2008. Distribution, status and threats of the freshwater pearl mussel Margaritifera margaritifera (Linnaeus) (Bivalvia, Margaritiferidae) in Norway. Fauna norvegica 26/27: 3-14. The distribution of Margaritifera margaritifera (Linnaeus) in Norway is mainly along the coast and in the lowland. Based on a questionnaire sent to county governors’ offices and municipal administrations in Norway, about 430 former and existing localities have been recorded, of which about 300 are still-existing. However, the real number is probably more than twice as large. Central Norway has the highest number of documented sites. The northernmost locality is at Berlevåg (Finnmark) at 70°50’N lat., and the highest verified record is near Snåsa (Central Norway) at 472 m a.s.l. The great majority of localities are associated with Cambro-Silurian volcano-sedimentary rocks or situated below the postglacial marine limit, i.e. in areas not too poor in calcium. The species is an early immigrant, and a landlocked population in Central Norway has probably existed since 8900 14C-yr B.P. The pearl mussel has become extinct during the past few decades at as many as 30% of its localities, mostly due to urbanisation and pollution. There is a high correlation between the density of people in a county (or the proportion of cultivated land) and the “density” of extinct pearl mussel populations (r=0.91). Other threats considered were hydropower regulations, excavations and constructional work in the watercourse, fishing for pearls, acidification and natural droughts or floods. In spite of the negative trend, some large populations still exist, with local densities >100 ind. m-2 and numbers of up to 1 mill. ind. km-1 river stretch. The Norwegian stock probably consists of >300 mill. ind.