1. Greenland Institute of Natural Resources, Box 570, DK-3900 Nuuk, Greenland
2. Polar Science Center, Applied Physics Laboratory, University of Washington, Box 355640, Seattle, WA 98105, USA
3. Research Unit for Wildlife Population Assessment, Centre for Research into Ecological and Environmental Modelling, University of St. Andrews, St. Andrews, Fife KY16 9LZ, UK