Warming in the North Atlantic Ocean has decreased winter sea ice and increased the death rate for baby harp seals over the past three decades, according to a new study.
Winter sea ice cover in harp seal breeding grounds has decreased up to 6 percent per decade since 1979, when satellite observations of sea ice began, found the research, published yesterday in the journal PLoS Biology.
That's made it tougher for harp seals to breed and rear their young to adulthood, said the lead author, Duke University marine ecologist Daniel Johnston.
"These animals give birth to pups and nurse all within a very short period of time," he said, noting that the average harp seal nursing period is just 12 days. "These animals have evolved to take advantage of this ephemeral ice cover."
But his research suggests they're having a hard time dealing with the changing climate, which is accelerating the start of the spring ice melt, reducing the area covered by ice during February and March -- months when female seals use thick sea ice as a platform to birth and nurse their young.
"We expect there will be good ice years and bad ice years, but if things go the way they are going, the number of good ice years will continue to decline," Johnston said.
The scientist and colleagues at Duke and the International Fund for Animal Welfare based their findings on satellite observations of sea ice cover, recorded strandings of dead baby harp seals, weather records and computer models.
They found that in years when a natural climate cycle called the North Atlantic Oscillation (NAO) was in its negative phase during February and March, sea ice cover was lighter and the death rate for baby harp seals was higher. In years when the NAO was positive, ice cover was heavier and the death rate for baby seals was lower.
Long-term warming among the suspects
The scientists believe the NAO is a strong influence on year-to-year variability in sea ice cover and seal death -- and that their hypothesis could help explain why seal populations dropped on Canada's eastern coast between 1950 and 1972, and why those populations rebounded between 1973 and 2000.
But their analysis suggests that a longer-term warming trend is also playing a role, decreasing sea ice cover up to 6 percent per decade since 1979 in each of four harp seal breeding grounds in the North Atlantic Ocean.
With that warming trend expected to continue in coming decades, "the question now is whether there is enough flexibility in the reproductive capability of these animals to find and use habitats that are resilient to these changes," Johnston said.
Johnston also said scientists aren't sure how to explain reports last spring of harp seals venturing well beyond their normal habitat. The National Oceanic and Atmospheric Administration reported in March that harp seals had been observed as far south as North Carolina -- not far from Johnston's lab at Duke.
"Last year, a number of harp seals showed up here looking kind of uncomfortable," he said.
It's not the first time seals have been spotted far south of their traditional spring breeding grounds in Newfoundland and the Gulf of St. Lawrence. But NOAA reported last spring that harp seals do seem to be venturing far south of their usual habitat more frequently.
Johnston said the seals might be searching for more favorable breeding grounds, if conditions in their usual habitat aren't to their liking. Another possibility is that sea ice is melting faster or earlier in the season than the animals expect, depriving them of the environmental cues that guide their northward migration.
The scientist believes that the study of harp seals and their changing habitat offers an opportunity for biologists to study the relationship between the animals and the ice -- and act to help keep seal populations healthy as the climate changes.
"Harp seals aren't endangered," Johnston said. "So often, we wait until a population is depleted to do anything, and it's often too late. Here, we have a possibility of getting in front of it and predicting what might happen."
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500.