Sea ice and polar bear den ecology at Hopen Island, Svalbard

The maternity denning of polar bears Ursus maritimus was studied at Hopen Island, Svalbard, Norway, using information collected by direct observation and live-capture of females and cubs during den emergence in spring of 1994 to 2008. The number of maternity dens observed annually varied from 0 to 36. The arrival of sea ice at Hopen Island in autumn shifted from late October to mid-December during the period 1979 to 2010. Fewer maternity dens were found on Hopen Island in years when sea ice arrived later in the autumn. There were no significant differences in body mass or litter size between female polar bears denning on Hopen Island and females caught elsewhere in Svalbard; however, females denning on Hopen Island were significantly younger than females denning elsewhere in Svalbard. Later arrival of sea ice in the autumn at Hopen island was correlated with lower body mass of adult females and their cubs at emergence. The timing of arrival and departure of sea ice is highly variable but a trend of later arrival in autumn may be affecting the denning ecology of polar bears at the southern extent of their range in Svalbard.