The food polar bears consume when they're living off the ice doesn't give them much more energy than if they were eating nothing at all, new research co-authored by a University of Alberta biologist has shown.

With the ice on Hudson Bay freezing later and melting earlier, the study disputes the contention that polar bears could adapt to a shorter ice season by eating more land-based calories, such as caribou carcasses, goose eggs or plants.

"It comes down to this idea that polar bears tank up their energy stores when they're on the sea ice feeding on the blubber of seals," said Andrew Derocher, a U of A professor of biological sciences and study co-author.

"This (study) suggests there is not enough energy from those terrestrial resources to make it work for them."

The research, which appeared in the journal Physiological and Biochemical Zoology, examined 142 bears at the polar bear holding facility near Churchill, Man. between 2009 and 2014.

Bears are housed there if they get too close to the townsite during the ice-free period in Hudson Bay. The bears are released once the ice freezes again.

The scientists weighed the bears when they entered the facility and when they left. The bears were given snow and water at the facility.

The scientists found that, on average, the bears lost approximately one kilogram per day, equivalent to the estimated weight loss in free-ranging bears during that season.

Polar bears have adapted to survive for long periods with limited caloric consumption. But since the 1980s, the Hudson Bay ice freeze has started happening approximately two weeks later and the ice break-up about three weeks earlier, said Derocher.

Polar bears now have less time on the ice to bulk up on seal blubber.

'Young and growing bears will be more vulnerable to the loss of sea ice.' - Andrew Derocher, University of Alberta biologist

"Once you cross a threshold of about 180 days without food, we could see over 50 per cent of the sub-adult animals in the population dying and around 20 per cent of the adult males dying," said Derocher.

"We're not there yet in the Hudson Bay ecosystem, so it's really a precautionary approach looking forward, that once we get to those (levels) we'll have a lot of hungry bears looking for things to eat."

The bears are not fed at the holding facility, which is run by the Manitoba government. When scientists tried feeding the bears during a study in the 1980s, they found they returned the following year.

"Polar bears never forget," said Derocher.

Polar bear population down 30 per cent in 35 years

The Western Hudson Bay polar bear population is approximately 800 bears and is currently stable, as the ice-free season has shown recent short-term stability. However, the population is down by about 30 per cent from the 1980s. 

Polar bears can vary in weight dramatically; an adult male might weigh between 300 and 500 kilograms. A cub might weigh 40 kilograms and rely on its mother's milk to sustain it. With bears losing approximately one kilogram per day during the ice-free season, younger bears are particularly vulnerable.. 

"Intuitively, you understand that young and growing bears will be more vulnerable to the loss of sea ice … what surprised me is how much more vulnerable these younger bears are," said Derocher.