Polar bears shift from thinning ice

A long-term study by the US Geological Survey (USGS) suggests that recent changes in the quality and availability of sea ice in northern Alaska are the most likely explanation for a decrease in female polar bears bringing up cubs on sea ice and an increase in basing themselves on land. Researchers from the USGS used satellite telemetry to evaluate changes in the distribution of female nesting sites in the northern Alaska region between 1985 and 2005. They found that the proportion of female sites on sea ice declined from 62 per cent in 1985–1994 to 37 per cent in 1998–2004. The researchers evaluated whether the significant shift in distribution of ‘dens’, sites where females raise their cubs, from sea ice to land could be explained by changes over the study period in three factors: patterns in hunting of polar bears, sea ice patterns and characteristics or the availability of subsistencehunted bowhead whale remains on shore. The USGS researchers, Anthony Fischbach, Steve Amstrup and David Douglas, found that neither change in availability of whale carcasses or in hunting patterns were determining factors in the observed shift in nursery sites. “Our findings suggest that sea ice changes offer the most plausible explanation for the observed shift in maternal denning sites”, said Fischbach. “In recent years Arctic pack ice has formed progressively later, melted earlier and lost much of its older and thicker multi-year component. Together, these changes have resulted in pack ice that is a less stable platform on which to give birth and raise