In former times Unio crassus was abundant in many European running waters. Its distribution area stretches over northern and central Europe to the complete area of the Black Sea and Mesopotamia (Falkner 1990). Even in the beginning of the 20th century farmers shoveled Unio crassus out of the streams to feed them to ducks and hens (Israel 1910). However, since then, the stocks have decreased by 90% as a result of pollution, eutrophication and ditching activities (Engel and Wächter 1990; Bauer et al. 1991). The remnant populations are mostly superannuated, with only few or even no juvenile mussels present (Bauer et al. 1991; Hochwald 1997).
The distribution range of the thick shelled river mussel (Unio crassus) in Sweden. Empty circles = confirmed findings of Unio crassus (before 1950’s). Black/red circles = confirmed findings of Unio crassus after 1950. The map has been produced by Ted von Proschwitz and Torsten Nordander, The natural history museum, Gothenburg.
Numerous Unio crassus populations have become extinct in Sweden, presumably due to large-scale anthropogenic disturbances, causing habitat degradation and fragmentation. The remaining Unio crassus populations have a fragmented (totally c. 140 sites) and narrow south-eastern distribution in Scandinavia. Accordingly, Unio crassus is threatened (EN) and constitutes a major conservation concern in Sweden. Moreover, Unio crassus is declining rapidly and there is an urgent need of concrete conservation actions if the species should be preserved for the future, in Sweden as well as in Europe.
Stefan Lundberg, Naturhistoriska riksmuseet, Jakob Bergengren, Länsstyrelsen i Jönköpings län, Ted von Proschwitz, Göteborgs Naturhistoriska Museum, Lea Scheider Karlstad University författare.