Three decades of warming in the North Atlantic has led to thinning ice cover in harp seal breeding grounds and drastically higher death rates among seal pups, according to a new study.

"There’s only so much ice out there and declines in the quantity and quality of it across the region, coupled with the earlier arrival of spring ice breakup, is literally leaving these populations on thin ice," said David W. Johnston of the Duke University Marine Lab, who led the study, published Wednesday in the scientific journal PLoS ONE.

"The kind of mortality we’re seeing in Eastern Canada is dramatic. Entire year-classes may be disappearing from the population in low ice years – essentially all of the pups die. It calls into question the resilience of the population."

Satellite records of ice conditions in the region, which go back to 1979, show sea-ice cover declining by as much as six per cent per decade in all four harp seal breeding grounds.

Female harp seals seek out the thickest, oldest ice packs in sub-Arctic waters every February and March to give birth and nurse their pups until they can swim and hunt on their own. They have developed unusually short 12-day nursing periods to adapt to the spring melt.

"As a species, they’re well-suited to deal with natural short-term shifts in climate, but our research suggests they may not be well adapted to absorb the effects of short-term variability, combined with longer-term climate change and other human influences such as hunting," Johnson said.

For the study, researchers reviewed satellite images of winter ice from 1992 to 2010 in the Gulf of St. Lawrence – a major breeding ground off Canada’s East Coast – and compared them with yearly reports of dead seal pup strandings in the region.

They also compared the stranding rates to recorded measurements of the relative strength of the North Atlantic Oscillation (NAO), a climate phenomenon that controls the intensity and track of westerly winds and storms that greatly affects winter weather and sea ice formation in the region.

Their analysis revealed that higher pup mortalities occurred in the Northwest Atlantic harp seal herd in years with lighter ice cover and when the NAO was weaker.

It’s unclear whether seals will be able to adapt by moving to more stable ice habitats. While some harp seals have moved to new breeding grounds off Greenland's east coast, thousands return each year to traditional breeding grounds in the Gulf of St. Lawrence or along the Front, off northeast Newfoundland, regardless of ice conditions.

The International Fund for Animal Welfare funded the study.