09/12/2016 14:25 EDT | Updated 09/13/2017 01:12 EDT

Fasting polar bears lose as much weight as ones eating food from the land: study

CHURCHILL, Man. — A study suggests food from the land is not going to save polar bears.

The study, published in the journal Physiological and Biochemical Zoology, was undertaken by the University of Alberta and the Manitoba and federal governments.

Researcher Andrew Derocher, a biological sciences professor at the university, says 142 nuisance polar bears were weighed during their stays at the so-called polar bear jail in Churchill, Man., between 2009 and 2014.

The bears, most of them adult males, were given water but no food, because conservation officers did not want to entice the animals to stay in town once they were released back into the wild.

The bears lost about one kilogram of body mass a day — the same weight loss experienced by free-ranging bears able to forage on land-based food such as berries, kelp and goose eggs during the ice-free season along the coast of Hudson Bay.

"It's not exactly a good news story for the bears," Derocher said Monday.

"It's in some respects confirmatory of what we've measured from free-ranging bears. But there's always been the question of what could those bears be getting from feeding out there. And, it turns out, not very much."

The population of polar bears near Churchill has declined by more than 30 per cent since the 1980s because of declining sea ice, said Derocher. Ice is melting sooner and forming later in the year, creating a shorter seal-eating season for the bears, which use the ice as a hunting platform. Polar bears have traditionally relied on an energy-rich diet of seal blubber.

Derocher said projections show that bears can go about 180 days without food before they start dying of starvation.

The Hudson Bay bear population is currently stable because the ice-free season has shown some recent, short-term stability, he said, but scientists wonder what will happen if the sea ice declines again.

Other studies had suggested polar bears might adapt to land-based food to survive.

This study "suggests there's not much in the terrestrial environment for these bears to feed on," said Derocher.

"There's no saving polar bears by hoping they're going to be more terrestrial."

— By Chris Purdy in Edmonton