1. Department of Biological and Environmental Science, P. O. Box 35, University of Jyväskylä; 40014 Jyväskylä Finland
2. Finnish Game and Fisheries Institute, Survontie 9/Technopolis; 40500 Jyväskylä Finland
3. Department of Biology, University of Turku; FI-20014 Turku Finland
4. Pyhäjärvi Institute; Sepäntie 7 27500 Kauttua Finland