1. Technical University of Denmark, National Institute of Aquatic Resources; Kavalergaarden 6 DK-2920 Charlottenlund Denmark
2. University of Gothenburg, Department of Biological and Environmental Sciences; P. O. Box 463 405 30 Gothenburg Sweden
3. University of Copenhagen, Faculty of Health and Medical Sciences; Stigbøjlen 7 DK-1870 Frederiksberg C Denmark