Melting Sea Ice Causing Decline in Harp Seal Populations

Warming temperatures in the North Atlantic Ocean have lead to a decline in sea ice, which is leaving populations of young harp seals dead in the water, according to researchers at Duke University.

"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," said David Johnston, a research scientist at Duke's Nicholas School of the Environment, of the pups not even a year old.

"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," he said. "It appears genetic fitness has little effect on this."

Sizabile masses of sea ice play an important role in the lives of harp seals, which use the ice as platforms to birth and nurse young until they are big enough to swim, hunt and fend off predators for themselves. Johnson said that in years of extremely low ice cover, entire year-classes of seal pups may be wiped out.

Prior research on harp seal populations in the North Atlantic indicates that sea ice cover in key harp seal breeding regions has declined by as much as 6 percent since 1979, when satellite records of ice conditions in the region began.

Building upon the prior research, Johnson and colleagues compared images of winter ice from 1992 to 2010 in a major harp seal breeding regions off of Canada's east coast with instances of dead harp seals grouped by gender and age.

The comparison revealed a noteworthy find: in the years where ice cover was reduced, the number of dead seal pups rose sharply, even though stranding rates for adults remained relatively stable. DNA samples from 106 harbor seals dead on shore were compared with DNA from harbor seals accidentally caught by fishing boats in the region in the same period.

"We used measures of genetic diversity to determine if the dead seals that came ashore were less fit than the presumably healthy ones that had been caught by fishermen, but found no difference," said Thomas Schultz, director of Duke's Marine Conservation Molecular Facility. "The stranded animals appear to have come from a genetically diverse population, and we have no evidence to suggest that genetic fitness played a role in their deaths."

The study is reportedly the first of its kind to assess harp seal stranding events in terms of the relative roles of genetic, environmental and demographic factors such as age and gender. Results of the analysis revealed that male seals stranded more frequently than females during the study period, and that during light ice years, that relationship was strongest.

Researchers concluded that sea ice cover and demographic factors have a greater influence on harp seal stranding rates than genetic diversity.

Kristina Cammen, a Duke Ph.D. student who also co-led the study, said the findings "provide more context for what we're seeing in high-latitude species in general. The effects of climate change are acting on younger animals; it's affecting them during the crucial first part of their life.

The research is published in the journal PLOS One.