1. Section for Freshwater Fisheries and Ecology, Technical Univ. of Denmark; Silkeborg Denmark
2. Dept of Fish Ecology and Evolution, EAWAG Swiss Federal Inst. of Aquatic Science and Technology, Center of Ecology, Evolution and Biochemistry; Kastanienbaum Switzerland
3. Div. of Aquatic Ecology and Evolution, Inst. of Ecology and Evolution, Univ. of Bern; Bern Switzerland
4. Dept of Ecology and Evolutionary Biology, Yale Univ; New Haven CT 6520-8106 USA