177 © 2011 Deutsche Gesellschat für Herpetologie und Terrarienkunde e.V. (DGHT), Rheinbach, Germany Dice Snake (Natrix tessellata) in the Baltic Sea Drainage Basin (Karvinsko District in Silesia, Czech Republic) Petr Vlček, Vít Zavadil, Daniel Jablonski & Konrad Mebert Abstract. We report on scattered distributional records and a prospering population of the dice snake (Natrix tessel- lata) from Silesia in the northeast of the Czech Republic. Only a few unconirmed records and documented sightings have existed from Silesia untill now. A recently discovered population at Havířov, Silesia, represents the irst docu- mentation of a reproducing population of N. tessellata in the Baltic Sea Drainage area. It is located approximately 145 km straight line from the nearest conirmed population near Brno, southeastern Czech Republic, which is a locality within the Black Sea drainage. here are several factors that characterize the population near Havířov: 1) the popula- tion inhabits a still water system made of ponds, unlike most other river dwelling populations in the Czech Republic; 2) the aquatic habitat is a pond network that drains into the Baltic Sea; 3) the pond system is under heavy anthro- pogenic inluence. he edge of these reservoirs are formed by mullock chips from nearby coal minning activities. In this report we summarize all literature records and verbal information about observations of N. tessellata in Silesia and nearby Poland that indicate its wider distribution in the Baltic Sea Drainage Basin than was previously perceived, pointing toward a natural occurrence of N. tessellata in this area. Key words. Natrix tessellata, distribution, anthropogenic habitat, Baltic Sea Drainage Basin (BSDB), Silesia, Czech Republic, Poland Introduction Natrix tessellata is a semiaquatic snake with a large distribution, extending from central Europe to north- ern Egypt and east as far as northwestern China (Gru- schwitz et al. 1999, Baha El Din 2006). he north- ernmost western and central European populations, considered relicts of a postglacial warm period (Atlanti- kum, see Lenz et al. 2008, Wikipedia 2010), are reported from Germany and the Czech Republic (Gruschwitz et al. 1999). N. tessellata belongs to the rarest reptiles in the Czech Republic and is a law-protected species, iled in the category of nationally critically endangered species. In the atlas of Czech reptiles, Mikátová et al. (2001) summarized the known distribution and biolo- gy of N. tessellata in the Czech Republic. hey includ- ed also records from atypical biotopes in the context of known Czech habitats and extralimital sites in ref- erence to the presumed autochthonous distribution of N. tessellata in this country. he atlas-record from Sile- sia, the northwestern province of the Czech Republic, was based on the discovery of two specimens in 1997 by Vlček (1998) and was classiied as such an extralimi- tal record. By 2009 still only fragment data were known about N. tessellata from Silesia. However, in 2009 a sta- ble and reproducing population was found (V lček et al. 2010). In this report we summarize the published as well as new and unpublished data about the occurrence of N. tessellata in Czech Silesia. Furthermore, we for- mulate preliminary conclusions resulting from observa- tions, which are atypical for this species. Material and Methods An inventory of all published and unpublished data about Natrix tessellata in Czech Silesia was conducted since its discovery in 1997 (Vlček 1998). Data of difer- ent quality and evidence have been collected. Moreo- ver, during the past twelve years the senior author was searching the region for a stable, reproducing popula- tion of N. tessellata. Field search to record N. tessellata was conducted either visually by walking through po- tentially suitable biotopes , or the search was focused on sites, where ishermen reported on having seen „water snakes“. Field work lasted from May to August. Study Area Silesia (Schlesien, Slezsko, Śląsk) is a historical region, extending from Poland into Czech Republic and partly into Germany (Fig. 1). Up until 1742 whole Silesia be- longed to the territory of the Czech Crown with the capital Wroclaw. hereater, Silesia was divided into sev- eral parts of variable size (Czech Silesia, Prussian Sile- sia, Polish Silesia, Austrian Silesia). he relief of Silesia constitutes mainly lowland. For example, the Silesian Lowland covers most of Polish Silesia through which Odra River, the principal Silesian river lows. It rises in northern Moravia (Czech Republic) and lows through western Poland, constituting a 187 km long border be- tween Poland and Germany. Finally, Odra River emp- ties into the Baltic Sea north of the polish town Szczecin (Stettin). he Odra is the second longest river in Poland MERTENSIELLA 18 177-187 20 September 2011 ISBN 978-3-9812565-4-3