The continuing spread of

Crayfish plague and introductions of non-indigenous freshwater crayfish species (NICS) have had major consequences for the survival of autochthonous crayfish populations in Europe. Beside habitat loss, the invasive signal crayfish Pacifastacus leniusculus is currently responsible for the decline of indigenous crayfish species (ICS) in Carinthia (Austria). Here, we studied the distribution of P. leniusculus and native Astacus astacus and Austropotamobius torrentium in selected catchments to assess the ongoing colonisation with signal crayfish and to monitor the existence of ICS localities. Our results showed that P. leniusculus is widespread in Carinthia and many areas with native crayfish have disappeared within nine years. However, whereas populations of A. astacus became extinct in running waters, those of A. torrentium seemed to be more protected by occupying headwaters. In contrast to the ICS, P. leniusculus was found in a broad range of freshwater habitats and water quality conditions. We hypothesise that the fast expansion of P. leniusculus and the decline of ICS was caused by human-mediated stockings of NICS, followed by active signal crayfish invasions from established populations into new watercourses