Disjunct distribution pattern of Procambarus clarkii (Crustacea, Decapoda, Astacida, Cambaridae) in an artificial lake system in Southwestern Germany

Procambarus clarkii is among the worst invasive species that have invaded Europe. It originates from the central south of the USA and northeastern Mexico and is a hardy, r-selected generalist. There exists a marked gradient in population numbers from southwestern to northeastern Europe: The Iberian Peninsula, Italy and France have the bulk of the populations, while northeastern populations are mainly scattered and isolated from each other. In the present study I report on the distribution of P. clarkii in an artificial lake system along the River Danube in Southwestern Germany, which is within its northeastern range limit. The presence of P. clarkii was confirmed in nine lakes and one canal with stagnant water. Procambarus clarkii was absent from lotic situations, which may indicate that P. clarkii thrives chiefly in stagnant or slow flowing waters. A probable explanation is that those habitats heat up faster and reach higher summer temperatures than most lotic habitats. Since P. clarkii is considered as a `warm water ́ species, lentic habitats probably offer more favorable temperatures. The populations form two disjunct distribution centres, isolated from each other by around seven km linear distance. It is not known for sure when the species was initially introduced, but it may have been present in the western distribution centre since the mid 1970s. The emerging disjunct distribution pattern is a synergistic result of introductions into two lakes and subsequent active spread to surrounding habitats, including migration overland. Procambarus clarkii numbers are often high and the species has become a nuisance to recreational fishery, since it tends to grab on to exposed fish bait. Two noble crayfish (Astacus astacus) populations were wiped out rapidly by P. clarkii, presumably due to crayfish plague transmission. Although eradication may prove impossible, population management, e.g. intense trapping and stocking of predatory fish, is strongly suggested. Early eradication, if necessary relying on extreme control methods (e.g. chemical agents), should be attempted when P. clarkii invades further lakes.