1. Leibniz-Institute of Freshwater Ecology and Inland Fisheries; Müggelseedamm 310 12587 Berlin Germany
2. Department of Zoology and Fisheries; Czech University of Life Sciences Prague; 165 21 Praha 6 - Suchdol Czech Republic
3. Département de Géographie; Université de Namur; Rue de Bruxelles 61 5000 Namur Belgium