According to current knowledge, the noble crayfi sh (Astacus astacus) made its latest invasion into the Nordic aquatic ecosystem after the last retreat of the ice shelf, roughly 10,000 years ago. The species inhabited freshwaters up the 62° parallel of northern latitude (Helle 1904, Cukerzis 1988, Holdich 2002, Souty-Grosset et al. 2006) and it has been speculated that its spread was assisted by man (Lehtonen 1975, Souty-Grosset et al. 2006). This remained the northernmost limit until the commercial value of the noble crayfi sh was discovered and noble crayfi sh stockings started during the 19th century (Lehtonen 1975, Jussila and Mannonen 2004). As a result, the noble crayfi sh distribution expanded, the rivers in western coast of Finland became stocked and were very productive, with the northern limit in Finland reaching up to the Arctic Circle, i.e., 67°30’ parallel of northern latitude (Järvi 1910, Holdich 2002, Westman 1973, Souty-
1 Department of Biology, The University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, SuomiFinland.