Influence of historical exploitation and recovery of biological resources on contemporary status of Margaritifera margaritifera L. and Salmo salar L. populations in Northwestern Russia

The influence of the historical exploitiation and recovery of biological resources on populations of freshwater pearl mussel and salmon was studied in the Kozha river (Onega River Basin). The freshwater pearl mussel was characterized by a low population density (0.09 ind./m 2 , on the average) and number of recruits. The role of the anthropogenic factor in the population is negligible; the effect of the pearl fishing is not observed. The dramatic decrease in the number of Atlantic salmon in the Onega River Basin in the late 20th century is a major reason for the slowed down reproduction of the freshwater pearl mussel. The second factor is the reconstruction of the Onega salmon hatchery in 1984, which led to a decrease in the juvenile salmon population density below the critical level on spawning and nursery grounds. As a result, mollusc reproduction has stopped. An artificial increase in the number of recruits must be developed in order to preserve the salmon population in the Onega River Basin. It is also necessary to forbid salmon fishing, including fishing migratory paths (with the exception of trapping for hatchery needs and traditional harvesting performed by locals in historical settlements).