On the vast expanse of sea ice in Canada's Gulf of St. Lawrence, a hundred shades of white and gray blend together to the untrained eye. But female harp seals can spot a difference—they seek out the thickest, oldest ice packs on which to give birth, a new study finds. This choice habitat may become harder to find in a warming world, forcing seals to search for other options.
Harp seals (Pagophilus groenlandicus) depend on stable sea ice to keep their pups safe from predators and close to seafood. However, this ephemeral environment calls for a delicately timed duet between the seals and their maternity ward, starting in late January when the ocean surface begins to freeze. Slushy silver dollars of ice coalesce into larger pancakes, which thicken, grow, and change from gray to white. Hundreds of thousands of female seals congregate on the floating ice to give birth in late February and early March.
A mother's choice of an icy platform is crucial to her pup's survival. After just 12 days of nursing, mom leaves, and the weaned pup stays adrift for another month. At first too buoyant to dive, the blubbery youngster fasts for a few weeks, then gradually practices hunting while returning to rest on the ice.
Meanwhile, the moving ice starts to fizz and creak, breaking apart in early April as the water warms. The timing can spell life or death for young seals if they have not become strong swimmers and outgrown their dependence on the ice. Giant floes sloshing around can crush an animal. Seals can also drown from exhaustion if waves and wind sweep them off unstable ice floes one too many times.
Canadian government scientists who set quotas for the annual seal hunt wanted to know whether climate change might alter this narrow window of prime ice conditions for the seals, and if mothers cue in to particular ice features. They dug up yearly records dating to 1977 on the location of the whelping patch, a 5000-square-kilometer area of sea ice occupied by the birthing females. The team overlaid these observations with more than 40 years of data from the Canadian Ice Service on ice cover and thickness and on floe size. They also examined yearly variations in ice quality.
Female seals were clearly drawn to the thickest ice: gray-white ice (15 to 30 centimeters thick) and first-year ice (30 to 120 centimeters thick), which is the thickest the seasonal ice can get before it melts away each year. Quantity and quality of ice matter, says study author Mike Hammill, a biologist at Fisheries and Oceans Canada in Quebec, the government department that manages Canada's marine resources. Because more young seals are likely to drown on unstable, thin ice, monitoring the conditions of the thick ice can improve estimates of young seal deaths. This data will help the government set more conservative quotas for the seal hunt, he says.
Although many climate change projections focus on total ice cover, the team found that the availability of thick ice is not necessarily related to the total ice cover in the Gulf of St. Lawrence. In years of poor overall ice conditions, good tracts of thick ice can still exist, and vice versa. The study also confirms the long-term sea ice decline  other researchers have observed. Poor ice conditions were more common in the past decade than in the preceding 30 years, the team reports in this month's issue of the Canadian Journal of Zoology, including less extensive total ice cover and shorter seasons for the thick ice that seals prefer. "Over time we may see a degradation of ice for seals, but it's not going to be smooth," Hammill says. "It will be a series of good years, a series of bad years."
The findings will help scientists predict whether the seals can adapt to keep pace with their changing surroundings, says David Johnston, a conservation ecologist at the Duke University Marine Lab in Beaufort, North Carolina. "The seals are going to be making decisions," Johnston says. "They're going to want to find the place that's going to be the most stable for their pups." This might send them searching for ice in more northerly latitudes in the future, he says.