Population size and structure of three mussel species (Bivalvia: Unionidae) in a northeastern German river with special regard to influences of environmental factors

Abstract

Unionid mussels are of great ecological importance in running and standing waters, however their populations declined continually during the 20th century. In order to collect more data on the situation of these mussels in running waters, the populations of Anodonta anatina, Anodonta cygnea, and Unio tumidus (Unionidae) were investigated in the northeastern German river Ryck from 1996 to 1998. At three sampling stations along the river, the mussel stocks as well as sediment and water properties were analyzed. In this river abundances of up to 138.7 indiv./m2 were detected. The average population densities varied from 10.9 to 34.9 indiv./m2. More critically however, the age structures showed significant signs of irregular reproductive success and overaging. Consequently, the mussel stocks are going to decline, and furthermore, U.tumidus should be listed as an endangered species in the Ryck river. Negative influences on bivalves can be expected from sporadically occurring low oxygen and high ion concentrations in the water. But after all, the conditions within the sediments seem to be decisive for the survival of juvenile mussels and thus for the age structure of the populations.