Over the last 32 years, warming in the North Atlantic has dramatically reduced winter sea ice cover in harp seal breeding grounds. According to a new study from Duke University, this has led to a sharp rise in death rates among seal pups.
“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,” said David W. Johnston, research scientist at the Duke University Marine Lab. “It calls into question the resilience of the population.”
Harp seals depend on stable winter ice to give birth and nurse their young until the pups are old and strong enough to swim and hunt on their own. This study is the first to show that, since 1979 when satellite records of ice conditions began, seasonal sea ice cover in all four harp seal breeding areas in the North Atlantic has declined by up to six percent a decade.
“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 and by-catch,” Johnston said.
The research team analyzed satellite images of winter ice from 1992 to 2010 in the Gulf of St. Lawrence and compared them to yearly reports of dead seal pup strandings in the regions. They also compared stranding rates with the strength of the North Atlantic Oscillation (NAO). The NAO is a climate phenomenon that controls the track and strength of westerly winds and storms and greatly affects winter weather and sea ice formation. Their analysis showed that pup mortality rates were higher when ice cover was lighter and the NAO was weaker.
“This clearly shows that harp seal populations across the Atlantic fluctuate pretty much in sync with NAO trends and associated winter ice conditions,” Johnston said. “But there’s a caveat. Regardless of NAO conditions, our models show that sea ice cover in all harp seal breeding regions in the North Atlantic have been declining by as much as 6 percent a decade over the study period. The losses in bad years outweigh the gains in good years.”
One question still remains: Will the seals be able to respond to the long-term trend by moving to more stable ice habitats?
“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,” Johnston said. “It may take years of good ice and steady population gains to make up for the heavy losses sustained during the recent string of bad ice years in eastern Canada.”
The study was published in the January 4 issue of the journal PLoS ONE. Co-authors of the study are doctoral student Matthew T. Bowers and research scientist Ari S. Friedlaender, both of Duke, and David M. Lavigne, science advisor at the International Fund for Animal Welfare, which funded the study.
Copyright © 2012 by Marine Science Today, a publication of OceanLines LLC.