Distribution and status of the freshwater pearl mussel Margaritifera margaritifera in northern Fennoscandia

The present distribution and status of the freshwater pearl mussel (Margaritifera margaritifera) was investigated in the northern cross-border areas of Sweden, Norway, Russia, and Finland in the period of 2003-2007. Location of the populations was done in two steps, first by gathering all the existing data from earlier studies, historical pearl fishing, and from local people through interviews. After this preliminary data collection, the populations were located more precisely by field studies. The results of the investigations varied depending on the catchment area. Abundant and viable populations were found in some of the catchments, while in the others M. margaritifera had disappeared or were near extinction. Besides the absence or low densities of host fish, other threats to M. margaritifera in these vast and remote areas are ground preparation as part of the felling operations, ditching of forests and bogs, building of forest roads, and mining. During the field surveys, the mussels were found in habitats not reported earlier, e. g., in rivers totally covered by big boulders, on the roots of the water plants, and in extremely shallow brooks.