Bering Sea ice seals see decline as ice melts

Last Friday, two scientists with the National Oceanic and Atmospheric Administration gave a Strait Science talk on their past, present and future research on ice seals in the Bering Sea. Peter Boveng’s and Michael Cameron’s work shows that the health of ribbon seal and spotted seal populations has been declining in the last decade, a pattern correlated with the loss of sea ice.

“I think it’s probably easy for most people to imagine that a lack of ice on the breeding grounds for icedependent seals species like ribbon and spotted seals could directly impact production or pup survival, just because there isn’t enough ice,” Boveng said.

He showed now-familiar data about the loss of winter ice in the Bering Sea over the last ten years. Ribbon and spotted seals normally spend April raising their pups in small dens on the ice around St. Matthew Island, but during April 2018 that area was entirely ice-free, forcing the seals to breed hundreds of miles to the north.

He added that larger ecosystem changes brought about by warming waters may also be negatively impacting the seals, like the loss of the cold pool and corresponding influx of large fish that may compete with the seals for smaller forage fish.

To monitor how the seals may be responding to this rapid environmental change, Boveng and his team have been going to the ice edge roughly every other year since 2007. They capture seals with nets, making measurements and taking samples of hair and tissue before letting them go unharmed.

Over the years, they’ve documented 94 different spotted seals and 98 different ribbon seals, including both adults and pups. The study also involved 80 harbor seals captured in the Aleutians, but they occupy a different ecosystem from the ice seals of the Bering Sea.

The primary metric Boveng is interested in is “body condition,” a measurement of how heavy a seal is for its length. Seals with a greater body condition are “fatter” and generally considered healthier, since they rely on fat reserves to make it through the winter. He then compared trends in body condition to other variables like age, sex and year to identify patterns.

Some trends were immediately apparent. Body condition tended to increase with age class, with subadults being fatter than pups and adults being fatter still. “And that kind of makes sense,” Boveng said. “Adults have had time to build muscle and gain fat, subadults would be somewhere in between and pups tend to be relatively thin while they’re trying to learn how to feed. So that order seemed sensible.”

Another clear trend was that body condition declined later in the spring, which also made sense to Boveng. Adults tend to lose fat as the winter wears on and they raise their pups, and then gain it again over the summer. Males also tended to be heavier than females, which Boveng said was typical for both of the species.

The most interesting comparison was average body condition over the multiple years. In both species, body condition significantly declined in the period from 2007 to 2018.

Those declines were not uniform. Pup body condition declined the most in both species. In spotted seals, adult body condition actually increased slightly, although pup condition still decreased.

“That might seem a little bit puzzling, but actually it’s consistent with a prediction we made some years ago,” Boveng said. “Ribbon seals seem to prefer deeper water for foraging. So if the ice edge is receding north, ribbon seals may be getting further and further displaced from their preferred habitat.” Spotted seals, on the other hand, can forage in shallower water closer to the new ice edge, and may be seeing a slight increase in available food because they’re no longer competing with ribbon seals.

Boveng emphasized, though, that any explanations like that are only theories. The decline in seal condition is correlated with receding ice and warming waters, but there’s no conclusive data to show why exactly that is. “It’s tempting to say one is causing the other, but that’s always hard to do,” he said.

Still, the trend is concerning, and shows no sign of letting up. The team was planning on coming to the region last summer, but the trip was cancelled because of the pandemic. The measurements would have been crucial, Boveng said, after the record low ice year of 2018 and the unusual mortality event of 2018 and 2019 which saw large numbers of seals washed up dead on regional beaches.

Michael Cameron, another NOAA scientist, presented research plans for the next few years. In the summer of 2021 they won’t travel to the region, but to aquariums in California and Seward to test new research drones.

The plan is to use unmanned aircraft to take pictures of seal haul outs when they travel for their fieldwork and use those pictures to get a better idea of the total populations.

Then in the summer of 2022, they’ll return to the ice edge in late spring, both to measure seals’ body condition as in previous years and to attach satellite-linked tracking tags to some individuals, which will record their movements and behavior for years to come.

Cameron said he wasn’t totally sure where their research would take place in 2022 – the receding spring ice edge may mean that they need to operate close to St. Lawrence Island or Norton Sound communities. But he said their research vessel is specially designed to run quietly, and that they’ll stay at least 12 nautical miles from shore and 30 nautical miles from any communities.

“And of course, we’ll be keeping Bering Strait communities well informed of our planned activities,” Cameron said. “We believe that this critical monitoring can be done without impacting the hunting success of coastal communities.”