Between 1984 and 2009 the weight of female bears in Ontario fell by over 10% while climate change meant they had 30 fewer days a year to hunt seal on ice

Three decades of melting sea ice has led to significant weight loss among the worlds southernmost population of polar bears, new data from Canadian researchers suggests.

Its a red flag, said Martyn Obbard, a scientist with the Ontario provincial government and co-author of a recently published study in the journal Arctic Science.

Researchers looked at a population of about 950 polar bears who live along the northern coast of Ontario, stretching from James Bay to the provincial border with Manitoba. As they live further south than any other population of polar bears, the group is particularly vulnerable to the effects of climate change.

Using data from 900 bear captures carried out at varying points between 1984 and 2009, researchers found the average weight of males had dropped by 45kg (99lb) and females by 31kg.

Its a significant change, Obbard said, given that male polar bears typically weigh about 500kg (1100lb) and females around 250kg (550lb).

The drop in weight was strongly correlated with melting sea ice. Since 1980, the ice season in Hudson Bay has fallen at a rate of about one day a year, forcing polar bears to spent more time on dry land rather than hunting seals their main source of food on sea ice.

Polar bears now get some 30 fewer days each year on ice than previous generations. Thats 30 days less out on the sea ice when theyre actively hunting seals, Obbard said.

He highlighted the drop in weight among females as worrisome. Its more than a 10% decline for adult females, he said, and could ultimately affect the survival of the population. We could see females having cubs less often and with less success.

Some scientists have suggested that polar bears may supplement their diet by foraging for food on land, turning to sources such as caribou and geese. But the data suggest this is not providing enough sustenance for the polar bears to maintain their weight, Obbard said.

The findings echo research previously done on polar bears who live on the western shores of the Hudson Bay. Melting sea ice has led to a spike in polar bear sightings in the town of Churchill, Manitoba, while scientists say the starving bears are resorting to risky and atypical behaviour, such as cannibalism and wandering far inland in search of food.

Migrating north is an improbable option for the polar bears studied by Obbard, as the northern areas are unable to support greater numbers of bears. A few individual bears may be able to migrate and survive but for most of them its going to be a slow eventual loss, said Obbard.

While the dramatic loss of weight has not led to a decline in the population of polar bears, that could change as a warming Arctic continues to melt away sea ice. These changes that were seeing now are a warning that there are major changes happening in that whole marine system of Hudson Bay, said Obbard. Who knows when its going to happen it might be 50, maybe 100 years and the bears will be gone from Hudson Bay.

Read more: http://www.theguardian.com/environment/2016/apr/05/polar-bears-losing-weight-arctic-sea-ice-melts-climate-change-canadian-study