Invasion routes, current and historical distribution of the Chinese mitten crab (Eriocheir sinensis H. Milne Edwards, 1853) in Sweden

Single specimens of the Chinese mitten crab, Eriocheir sinensis H. Milne Edwards, 1853, have been regularly reported along the western and eastern coasts of Sweden since the 1930's. The crab has most likely been brought from overseas via the transfer of ship's ballast water or secondarily introduced from its key European distribution areas. Since 2001 a sharp increase in the occurrence of the mitten crab has been noticed in Swedish inland waters, but the dispersal routes and distribution of the species into Sweden remain poorly known. Here we document the current and historical distribution of the Chinese mitten crab in Sweden and assess possible invasion routes. A special focus is put on the historical occurrence of crabs in Lake Vanern, which empties into the Skagerrak/North Sea and Lake Malaren, which connects to the Baltic Sea. The existing time series available for the mitten crab from the two lakes shows a large variation in dispersal pattern between different areas within each lake. In order to detect and monitor outbreaks of the mitten crab in Sweden, an internet based reporting system was created in 2007. Museum collections and reports from the general public throughout Sweden were compared with the known occurrence of mitten crabs in Lake Vanern and Lake Malaren. During the study, the web-based reporting system was not advertised and the general public did not get paid for reporting the information. Population densities of the crab occurred in peaks and unevenly over the last decade In both lakes, suggesting a pulse invasion instead of a constant supply by migration. Significant difference in male and female occurrence together with few transport possibilities between lakes suggests different invasion routes for the crabs. Reports from the general public throughout Sweden coincide with the observations from local fishermen in Lake Malaren and Lake Vanern. Data on occurrence and abundance arc discussed in relation to a potential reproduction area on the western coast of Sweden.