According to a report in New Scientist, the estimates were done by Seth Cherry of the University of Alberta, Canada, and colleagues, who monitored the health of polar bears in the ice-covered Beaufort Sea region of the Arctic during April and May in 1985, 1986, 2005 and 2006. They immobilized the bears using tranquillizer darts and measured the ratio of urea to creatinine in their blood.
A low ratio means that nitrogenous waste material is being recycled within the body and indicates the animal is fasting - a state which usually only occurs temporarily in males during the spring breeding season.
In 1985 and 1986, the proportion of bears fasting was 9.6 and 10.5 per cent respectively. By 2005 and 2006, this had risen to 21.4 and 29.3 per cent. Cherry's team believes that the increase in fasting bears is explained by warmer temperatures and earlier spring melts.
Polar bears use sea ice as a hunting platform, catching seals by sitting next to their breathing holes and waiting to pounce.
Spring is usually a time of feasting for polar bears, filling up before summer when the ice retreats. "It is clear that the changes in the sea ice are affecting the hunting opportunities available to the bears," said co-author Andrew Derocher of the University of Alberta. The early melting may also be resulting in a lack of prey, according to researchers.
Sea ice is important to seals because they build dens for their pups in the overlying snow, so their numbers may have dropped, explained Cherry.
Anecdotal evidence backs up the team's conclusions, with many more sightings of polar bears swimming in open water and resorting to eating other food, such as fish. According to Rick Steiner, a marine conservationist at the University of Alaska in Anchorage, "If the ice continues to contract, which seems inevitable, polar bears will become even more nutritionally disadvantaged. The study proves polar bears are in serious trouble."