| Summary: | Native populations of the stone crayfish Austropotamobius torrentium in Tyrol (Austria) are restricted to the region of Außerfern, where the lake “Haldensee” and the river “Archbach” are known to hold intact populations. Both records are of biogeographical importance because they are the southernmost located stock of the Bavarian-Tyrolean distribution area. In addition, the “Haldensee” is one of the highest locations holding stone crayfish in Austria. “Plansee”, a lake within the Archbach catchment contains a dense population of Austropotamobius pallipes that was introduced about 80 years ago. During recent field surveys a gradual immigration of A. pallipes into the habitat populated by A. torrentium was noticed. We characterised the native populations of A. torrentium in Haldensee, its immediate outflow and in the river Archbach by means of population parameters and crayfish morphometry. The potential influence of the non-indigenous A. pallipes as intruder in the A. torrentium population is discussed.
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