P íspěvek k poznání vodních měkky evropsky vyznamné lokality Byst ice se zamě ením na populaci velevruba tupého (Unio crassus) A contribution to the knowledge of aquatic molluscs of the Byst ice SCI focused on the population of Unio crassus

This paper brings results of a malacological survey of the Byst ice River on the territory of the Site of Community Importance (SCI) Byst ice (Eastern Bohemia, Czech Republic). The research was focused on the population of endangered bivalve Unio crassus which is the main object of conservation of this SCI. Twenty one species of aquatic molluscs (11 gastropods, 10 bivalves) were found in 2010. Most of the molluscs belong to common and widespread species. Altogether 26 species have been found during research in the Byst ice River since 2000. Expansion of non-native species Potamopyrgus antipodarum and Physella acuta is the most visible change in comparison with the previous research. The research in 2010 confirmed existence of the occurrence of Unio crassus all over the SCI and also upstream of this site. The population density documented in 2010 was higher than that found during the previous research. In the upper part of the SCI, the density of U. crassus population was at least 1 ind./1 m of the river stream while in the lower part it did not exceed 1 ind. per 10 m of the river. The reason was the canalisation of the river and the absence of suitable sediments. The occurrence of the bivalve was not recorded downstream of the SCI.