For Seal Pups on Thin Ice, a Short Childhood

Stewart Cook/International Fund
for Animal WelfareA harp seal mother and pup on an unstable ice floe in the Gulf of St. Lawrence in Canada.

For a mother harp seal, timing is everything. Pups are born and nursed on seasonal sea ice during a brief window before the spring sun melts the ephemeral nursery. After only 12 days, the mother weans her pups and the family slides into the ocean together.

Should the ice melt early, however, the pups will meet an early end, either drowning in the sea or getting crushed by shifting floes. In 2010, nearly 100 percent of the eastern Canadian pups were estimated to have perished in this situation.

Stewart Cook/International Fund for Animal WelfareA harp seal pup on unstable ice in the Gulf of St. Lawrence.

“It’s unclear how populations will acclimate as ice forms farther and farther north,” said David Johnston, a marine scientist at Duke University. “We don’t know if animals will just head north or if they’re tied to a specific place and time.”

Dr. Johnston and colleagues found that warming in the North Atlantic had reduced harp seal breeding grounds by about 6 percent per decade over the last 32 years and that this had taken a drastic toll on seal pup survival. Their results, published in the journal PLoS ONE, call into question whether harp seals will be able to adapt to climate variability over the long run.

To successfully produce pups, harp seals must contend with both short- and long-term ice variations. Dr. Johnston and his colleagues assessed the cumulative effect of these factors. To measure the short-term impacts, they analyzed satellite images of winter ice from 1992 to 2010 in the Gulf of St. Lawrence, a major harp seal breeding region off Canada’s eastern coast, and compared the changes in ice cover to the number of dead pup strandings counted each year.

They also took into account the North Atlantic Oscillation, often referred to as the El Niño of the North Atlantic. This phenomenon, driven by atmospheric pressure differences that affect how storms cross the ocean, is the dominant mode of climate variability in the region. In years that the North Atlantic Oscillation resulted in lighter ice cover, seal pups had higher mortality rates.

In correspondence to patterns of the North Atlantic Oscillation, seal populations declined from 1950 to 1972 and then gradually recovered from 1973 to 2000.

“It’s interesting to see how the abundance of animals tends to fluctuate in sync with the North Atlantic Oscillation,” Dr. Johnston said. “This suggests that climate has been an important factor in shaping populations.”

Although these short-term changes would seem to indicate a natural cycle of decline and recovery, the retreat of ice further and further north over the long term is cause for concern. Since 1979, satellite records indicate that ice has steadily declined in all four harp seal breeding regions in the North Atlantic, the researchers note.

Even though harp seals have generally been on the rebound since 1973, extremely high neonatal mortality in recent years calls the population’s resilience into question. Virtually entire generations of pups were lost, with thousands of bodies washing ashore and dotting beaches.

As climate change progresses alongside the short-term variations of the North Atlantic Oscillation, Dr. Johnston said, “we’ll have good and bad years of ice, but over time the number of bad years are essentially outweighing the number of good years, meaning more and more seals are declining.”

Harp seals live in Canada, Greenland and parts of Russia and Norway. As one of the most abundant types of seals, they are not currently threatened, although Dr. Johnston and his colleagues plan to keep monitoring the populations to see how continuing changes in climate take their toll.

Some researchers report that harp seals are whelping farther north — a good sign. “We hope that’s the case over time,” Dr. Johnston said, “but the fact that we have high mortality in low ice years suggests that not all seals do acclimate well.” Breeding a bit earlier in the season or having the pups farther north could ensure that populations do not decline further, although the prospects for this are unclear.

In the future, Dr. Johnston hopes to gather more accurate harp seal population estimates and better data on pup survival. Because of poor ice conditions, researchers have not been able to go out and count the pups and instead rely on strandings as a proxy for estimating deaths.

“It’s really hard to estimate neonatal mortality for breeding seals because they’re giving birth out on ice, away from most scientists,” Dr. Johnston said. He and his colleagues hope to refine their methods for keeping tabs on populations.