Every host knows when you run out of ice, the party's over. For young seals surviving on ice floes, the festivities are breaking up sooner than they used to. That sends vulnerable youngsters into the ocean before they're ready—maybe to end up stranded on a beach near you.
To keep their young from becoming drifting bait in a predator-filled ocean, female harp seals give birth on top of winter sea ice. The pups stay on the ice, undercover in a coat of white fur, until they're old enough to survive in the ocean. Then they shed their white coats, dive in, and begin migrating with the rest of their population.
Harp seals live in two main populations, one on either side of the northern Atlantic. From the population on this side of the pond, increasing numbers of seals have been showing up stranded along the U.S. coast, from Maine all the way down to North Carolina. Researchers at Duke University looked for patterns in these strandings—more than 3,000 over the past two decades.
Not too surprisingly, there was a clear relationship between strandings and the amount of sea ice. Years with more ice had fewer strandings. In years with less ice, when melting floes might force pups into the water before they're ready, strandings went up.
The same trend didn't apply to adults, however. Strandings of adult harp seals weren't linked to the amount of sea ice in that year. But in all years, the majority of stranded seals were pups. That means fluctuations in sea ice have the strongest effect on young harp seals.
The researchers also saw that males were more likely to strand than females. This may be due to what they call a "tendency to wander" among males. Brianne Soulen, one of the paper's lead authors, adds that because adult females need to spend more energy on things like pregnancy, they're less likely to stray from safe feeding grounds.
Genetic factors may also be at work. Soulen says they found slightly less genetic diversity among males, which in theory could make them more susceptible to disease or other factors. No matter the reason, if a harp seal does wind up on the shore of your local beach, it's likely to be a baby boy. Get blue balloons.
In the most recent years included in the study, 2009 and 2010, the usual pattern didn't hold up. The year 2010 was bad for ice, but didn't have a lot of strandings. However, Soulen doesn't see this as reason for optimism. An earlier study saw harp seals changing their migratory behavior in response to shifting ice cover; they may simply be stranding someplace else now, where they're not counted. Or the population as a whole may have dropped dramatically.
It matters, of course, because the ice is running out everywhere. Despite year-to-year fluctuation, the authors write, ice cover in the North Atlantic is disappearing at up to 6% per decade. And Soulen says what's happening with the harp seals provides a big hint about the state of other marine mammals. "Harp seals are a good representative species of the effects of ice changes." The hooded seal population in the western North Atlantic, for example, has declined by 90% since the 1940s, as ice there has steadily disappeared.
When sea ice is gone, there's no one who can dash out for more. The party may not be over quite yet, but it's getting pretty lame.
Image: courtesy of the International Fund for Animal Welfare
Brianne K. Soulen, Kristina Cammen, Thomas F. Schultz, & David W. Johnston (2013). Factors Affecting Harp Seal (Pagophilus groenlandicus) Strandings in the Northwest Atlantic PLOS ONE DOI: 10.1371/journal.pone.0068779