Global Warming and Shrinking Ice Threaten Baby Harp Seals

As temperatures warm and ice melts, northern marine mammals are facing some difficult and harsh consequences. Sea ice is shrinking, which means that there is less solid surface for these creatures to rest and recuperate during their searchers for food. Now, scientists have discovered that young harp seals off the eastern coast of Canada are at much higher risk of getting stranded because of disappearing ice.

Harp seals are known for their white fur when they're first born, allowing them to camouflage themselves amidst the snow and ice. When they mature, this white fur greys as they become better suited to duller-colored ocean waters. Yet harp seals, both young and old, rely on stable winter sea ice. There, mothers give birth to pups and nurse their young until they can swim, hunt and fend off predators for themselves. In years of extremely light ice cover, though, entire year-classes may be disappearing from the population of seals.

In order to examine how sea ice might be affecting harp seal populations, the researchers compared images of winter ice from 1992 to 2010 in a major whelping region off Canada's east coast with yearly reports of dead harp seal strandings along the U.S. northeast coast that were grouped by gender and the estimated age of the seal. In the end, they found significant differences. In years when ice cover was reduced, strandings for younger seals rose sharply. Strandings for adult seals, in contrast, remained relatively stable.

The scientists weren't done yet, though. The researchers also compared DNA samples from 106 harp seals that had been stranded ashore with DNA from seals that had accidentally been caught by fishing boats in the region during the same time. This allowed them to determine whether the seals on shore were less fit than the presumably healthy ones that had been caught by fishermen. In the end, though, the researchers found no difference between the two. This meant that genetic fitness didn't play a role in the seal deaths.

"Stranding rates for the region's adult seals have generally not gone up as sea ice cover has declined; it's the young-of-the-year animals who are stranding (those less than one year old)," said David Johnston, one of the researchers, in a news release. "And it's not just the weakest pups--those with low genetic diversity and presumably lower ability to adapt to environmental changes--that are stranding. It appears genetic fitness has little effect on this."

The findings reveal that ice cover could drastically impact harp seal populations in the future. It shows how climate change can affect younger animals, which could spell bad news in the future.

The findings are published in the journal PLOS One.