L. Forró szerk.: Miscellanea Zoologica Hungarica 10. 1995 (Budapest, 1995)

Nesemann, H., Pöckl, M.; Wittmann, K. J.: Distribution of epigean Malacostraca in the middle and upper Danube (Hungary, Austria, Germany)

A species introduced from eastern North-America into Germany and France at the end of the 19th century (1894 in the Oder system) to support fisheries, as native crayfish were decimated by the parasitic fungus Aphanomyces astaci. North of the Pyrenees and Alps, O. limosus is a common and widespread crayfish in most of the river systems flowing into the Baltic and North seas as well as into the Atlantic Ocean (Holthuis 1979). Living stocks were also transferred to localities in Bavaria and the vicinity of Budapest (Thuránszky 1960). Up to 1985, free living populations of O. limosus in the Danube system were not known. It was first recorded in a large secondary branch of the Hungarian Danube, called Téli kikötő, near Újpest at the Népsziget (rkm 1654; Thuránszky & Forró 1987) and in the Bavarian Danube near Ingolstadt (Nesemann 1987). Hence, in the 1980s two disjunct populations in the upper and middle course of the Danube could be distinguished. In the Bavarian Danube near Ingolstadt, O. limosus was the only decapod crustacean species and was found infrequently and sparsely. Since that time it spread downstream as far as Donaustauf. The second Danubian population of O. limosus in Hungary and in the river Morava is denser and is associated with a further decapod crustacean, A. leptodactylus. In 1991, O. limosus was caught frequently in the main course of the Danube River between Leányfalu (rkm 1673) and Dunaföldvár (rkm 1562), indicating a progressive spreading in the last six years. From the Rhine River an advance of five km per year was reported (Schweng 1968). Under optimal conditions, this species may reach high population densities, e.g. 20 individuals per square meter in the Rhine. In the Danube, 1-2 specimens were caught per square meter at Csillaghegy. In September 1991, the first population of O. limosus in the Austrian part of the Danube was observed. The crayfishes live abundantly in the port Ölhafen near Vienna (rkm 1918). This isolated locality may be the effect of an unknown importation by ship (Fig. 7). O. limosus prefers the shallow water of the stony banks of large lowland rivers and seems to be perfectly adapted to these types of running waters with periodical floods and droughts, always strictly following the water's edge. Discussion and conclusions The number of species of higher crustaceans tends to increase discontinously in downstream direction. The Bavarian section of the Danube River, which is a typical lowland river with meanders, is inhabited by six Ponto-Caspian species: J. istri (Janiridae), D. villosus, D. haemobaphes, N. hrabei (Gammaridae), C. curvispinum (Corophiidae) and the recently immigrated L. benedeni (Mysidae). Locally, some of these are of high abundance, density and biomass. In the eastern part of Lower Austria, three species are added to the previously named ones: C. ischnus, S. ambuláns (Gammaridae) and A. leptodactylus (Astacidae). Downstream of the Dunakanyar (rkm 1700), the structure of the community is completely changed and the diversity of the fauna increases markedly. Three further species are frequently found, O. obesus (Gammaridae), O. cavimana (Talitridae) and the introduced crayfish O. limosus (Cambaridae). Corresponding to important geological and hydrological changes, the following distribution limits can be distinguished: the Dunakanyar at rkm 1700, the "Wiener Pforte" (rkm 1940), the narrow valley of the "Wachau" (rkm 2006), and the "Weltenburger Enge" (rkm 2420). Correspondingly, several species show approximately the same distribution pattern. D. haemobaphes and C. curvispinum are frequently collected throughout the whole upper reach of the Danube as far upstream as Kehlheim, or even further. Before the explosive upstream