Recent trends in relative abundance of two dreissenid species, Dreissena polymorpha and Dreissena bugensis in the Lower Don River system, Russia

We sampled sites in the lower Don River system, Russia between 1977 and 2004 (5 sites) or between 1999 and 2004 (10 sites) to determine relative trends in two dreissenid species, Dreissena bugensis and Dreissena polymorpha. The sites were located in the main river, in connecting reservoirs, and in a major tributary, the Man- ych River. For sites sampled beginning in 1977, D. bugensis was first found in the lower river in 1980 and then more upstream in 1991. The relative proportion of D. bugensis increased to reach a maximum of 30-50% of the dreissenid population by 1999. After 1999, this species decreased at 14 of the 15 sites. At sites in the Don River, the proportion that D. bugensis comprised of the total dreissenid population after 1999 declined from 25-50% to 10-18%, whereas at sites in the Manych River the propor- tion declined from 65-75 % to 33-43 %. The decline of D. bugensis relative to D. polymorpha is unique; in most other water bodies D. bugensis displaces D. poly- morpha over time because of its superior physiological attributes. Reasons for the rela- tive decline of D. bugensis are unclear, but we speculate that selective predation by fish may be a potential factor.