1. Institute for Pharmaceutical Biology, Technical University of Braunschweig, Mendelssohnstrasse 1, D-38106 Braunschweig, Germany, Institute for Air, Water and Soil Hygiene, Corrensplatz 1, D-14195 Berlin, Germany, and Institute of Freshwater Ecology and Inland Fisheries Department IV: Biology and Ecology of Fish, Mueggelseedamm 310, 12587 Berlin, Germany