Along with declining habitat for polar bears, harp seal populations in the far north are also taking a big hit from global warming, according to researchers who documented reduced winter sea ice cover in key harp seal breeding grounds.A new scientific study suggests harp seals in the North Atlantic are dying at high rates because of warming waters and a steady decline of sea ice in their traditional breeding grounds.
The research by scientists at Duke University in North Carolina tracked the decrease of sea ice due to global warming and the mortality of harp seals from 1992 to 2010.
David Johnston (right), a marine scientist who co-wrote the report, said it's the first study to show that seasonal ice cover in the four seal breeding areas of North America has receded by as much as six per cent per decade beginning in 1979 when satellite data began.
"There has been a string of light ice years recently and we're starting to be concerned that if ice continues to decline, this might have longer-term effects on the harp seal population," Johnston said from his office in Beaufort, N.C. "I'm concerned that these animals are in for a tough road with what we're seeing with climate change."
Harp seals rely on stable winter sea ice as safe places to give birth and nurse their young until the pups can swim and hunt on their own. Female seals typically seek out the thickest, oldest ice packs in sub-Arctic waters each February and March, and have adapted to the spring melt by developing unusually short, 12-day nursing periods.The authors warned that they could see the disappearance of a year's entire seal pup herd due to a lack of ice, where females traditionally go to give birth every February and March. Pups usually drown if born in the water or on thin, unstable ice. This has resulted in entire generations of newly born seal pups dying due to their disappearing habitat according to the study published in the open access science journal PLoS ONE. (click for original report).
"The kind of mortality we're seeing in eastern Canada is dramatic," said co-author David
Johnston, a research scientist at the Duke University Marine Lab. "Entire year-classes may be disappearing from the population in low ice years -- essentially all of the pups die," he said. "It calls into question the resilience of the population."For recent data, researchers looked at satellite images of winter ice from 1992 to 2010 in the Gulf of St. Lawrence, a prime breeding region off the east coast of Canada, and compared them to yearly reports of dead seal pup strandings in the region.
They compared stranding rates to records of a climate phenomenon known as the North Atlantic Oscillation (left), which controls the intensity and track of westerly winds and storms and exerts a major influence on sea ice formation. The research linked high seal death rates to a weaker North Atlantic Oscillation, or NAO, and the resulting poor ice conditions.They found in 1969 the NAO was weak and the ice cover in the Gulf of St. Lawrence was one of the lowest on record. Thousands of seals were crushed in moving ice or were prematurely forced to vacate whelping patches during ice breakup. The researchers found that years of weaker NAO and lighter ice cover showed higher death rates among seal pups.
While harp seals have adapted to the earlier spring melts in recent years by developing shorter
12-day nursing periods, it remains unclear if their population can sustain itself against sea ices losses over time."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.
It's estimated that up to 80 per cent of the seal pups died last year as a result of meager and thin ice, according to the department.
Johnston says the department has been aware of the phenomenon and its consequences for years, factoring it into assessments and the management plan for the annual seal hunts off northern Newfoundland and Nova Scotia.
He says it may be years before the true impact of bad ice is seen in the overall population, which has risen over the years and now sits at just below eight million. "The one safety that this
population has is that they're relatively long-living and they pup over a number of years," he said from St. John's, N.L. "So if you have one or two bad years, you still have lots of other years that are contributing to it."A key unanswered question, he added, is whether seals will be able to respond to the long-term trend by moving to other, more stable ice habitats.
Recent reports that some harp seals are whelping in new breeding grounds off East Greenland indicate some shifting may be taking place, but thousands
still return each year to traditional breeding grounds in the Gulf of St. Lawrence or along the Front, off Newfoundland, regardless of ice conditions.“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.”
Source:
Google News,"'Dramatic' loss of harp seals amid warming: study", accessed January 15, 2012
CTV News, "Lack of ice could be causing more seal deaths: study", accessed January 12, 2012
Summit County Citizens Voice, "Global warming: Harp seal habitat vanishing fast", by Bob Berwyn, accessed January 12, 2012
