Distribution and dispersal of two invasive crayfish species in the Drava River basin, Croatia

The aim of this work is to explore the current distribution and dispersal rates of two nonindigenous crayfish species (NICS) recorded in Croatia: the signal crayfish (Pacifastacus leniusculus) and spiny-cheek crayfish(Orconectes limosus). Both NICS have been recorded in the Drava River basin, with signal crayfish spreading downstream from the north-west along the Drava’s tributary the Mura River, and spiny-cheek crayfish spreading upstream from the east from the Danube River throughout the Drava River. Signal crayfish distribution in the Mura River has been recorded up to 3 km from the confluence with the Drava River. Based on literature data and the current recorded distribution front, the downstream dispersal rate was between 18 and 24.4 km·yr−1. Spiny-cheek crayfish distribution has been recorded 15 km upstream of the Drava River mouth into the Danube River. Its upstream dispersal in the Drava River has been calculated at 2.5 km·yr −1. Both NICS could have an impact on native crayfish populations recorded within the Drava River basin in Croatia: the noble crayfish (Astacus astacus) and the narrow-clawed crayfish (Astacus leptodactylus). In the Mura River no noble crayfish have been recorded since 2007, and the watercourse is at the moment dominated by the signal crayfish. Spiny-cheek crayfish populations have been found in coexistence with narrow-clawed crayfish populations, with O. limosus dominating by 16:1.