Some Alaska seals are getting skinnier — probably because of retreating sea ice
NOAA scientists documented the declines between 2007 and 2018.
As sea ice off Alaska continues its long-term vanishing trend, two seal species that depend on ice may be showing the effects in their bodies.
Ribbon seals, distinctive for their black-and-white striped patterns on their fur, and spotted seals, known for their speckled coats, became thinner over time, according to measurement taken during six Bering Sea research cruises conducted between 2007 and 2018.
The results, detailed in a study by National Oceanic and Atmospheric Administration scientists, described declining body condition for adults, subadults and pups.
In plain terms, the seals have gotten skinnier, said lead author Peter Boveng, who is with the Marine Mammal Laboratory at NOAA’s Alaska Fisheries Science Center in Seattle.
“We just used a very simple index of body condition, which is weight divided by length,” he said.
The methods for measuring body condition were straightforward, as described in the study. Seals resting on the ice were captured in nets, sedated and weighed in a body sling. An ultrasonic scanner measured blubber thickness. Additional measurements and work, including tagging, were conducted for related seal studies; seals were allowed to recover and were released back into the sea.
The trend toward skinnier bodies happened as Bering Sea ice diminished, he said. And it appears that marine heat waves that struck in the latter part of the study period exacerbated the trend — and could continue to exacerbate it.
“Those kinds of events are going to be more and more frequent,” he said.
Body condition declines for ribbon seals were more pronounced than for spotted seals. The study noted that ribbon seals, which feed at the edge of the continental shelf, dive deeper for their food and are expected to be more vulnerable as the ice retreats northward and away from the best foraging grounds. Spotted seals get their food over the shelf and have more flexibility in foraging locations, the study pointed out.
Another climate change warning sign is the imprint in seals’ bodies of exposure to wildlife diseases, a subject being studied by several of the same NOAA scientists. They are analyzing results from blood samples taken from seals during research cruises dating back to 2007 in the Bering Sea, as well as blood samples taken from bearded seals during research cruises conducted from 2009 to 2012 in Kotzebue Sound in the southern Chukchi Sea.
The analysis of those blood samples is still pending.
At the time the samples were taken, there was no outward sign that seals were actually ailing from any infections, said NOAA researcher Heather Ziel, also with the Marine Mammal Laboratory at the Alaska Fisheries Science Center. The question being studied is whether the seals have antibodies showing past exposure to the diseases.
Among the disease pathogens of concern is the phocine distemper virus. Exposure to that virus is noteworthy; it had not been recorded in Alaska marine mammals until the early 2000s.
Scientists believe that reduced sea ice was the catalyst that allowed marine mammals to spread phocine distemper from the North Atlantic, where it had killed large numbers of gray seals, to the North Pacific. For Alaska marine mammals, such spreads may be part of a trend. There are concerns that the warming climate and reduction in ice “will result in some new pathogen or pathogens getting distributed to new areas.” Boveng said.
Ribbon and spotted seal populations off Alaska have been considered in the past for Endangered Species Act protections. Both populations were the subject of listing petitions — and ribbon seals were considered twice — but NMFS at the time concluded that listing was not warranted, largely because of a lack of information.
That is a contrast to the situation for Alaska’s ice-dependent ringed and bearded seals, both of which were listed as threatened in 2012. Those listings resulted from the same general review that included spotted and ribbon seals.
NOAA in January announced that it is launching its five-year status review of the ringed and bearded seal populations, a periodic process required under the Endangered Species Act. NOAA in January also announced a proposal for designated critical habitat, areas where additional protection under the Endangered Species Act would be required for federally permitted activity. The public comment period for that critical habitat proposal was extended into April.
Ribbon and spotted seals, though they are not listed under the Endangered Species Act, still merit some special attention, Boveng said.
“I think all of the ice-associated or ice-dependent seals are facing major changes in their habitat,” he said.
Analysis of a different population of Bering Sea seals — harbor seals in the Aleutian Islands, which are not dependent on sea ice — demonstrates how collecting information can be difficult.
The Boveng-led study on seals’ body condition included data on harbor seals that was gathered during a shorter period, during seasonal research cruises conducted from 2014 to 2016. It estimated a loss of about 6 kilograms a year for an average harbor seal, which would normally weigh about 64 kilograms.
The Aleutian harbor seal has been in sharp decline since the 1970s. The population suffered a two-thirds reduction from the late 1970s to the late 1990s across the entire island chain, and it declined by 86 percent during that period in the western Aleutians. The population has shown no sign of recovery since then, but also no sign of continued decline, according to NOAA.
Boveng said the 2014-to-2016 decline in those harbor seals’ body conditions appears to be related to marine heat waves that made high-quality food less available. “When you get environmental changes that are significant enough to impact the forage fish, then you impact a lot of things,” he said.
But gathering information on harbor seals in the Aleutians is difficult, something that is reflected in the smaller dataset. Those seals do not gather in large groups or in easily predicted spots, and they can be elusive, Boveng said. “They’re really skittish,” he said.
Scientists also have to contend with the fierce Aleutian conditions. During the 2016 research cruise, for example, storm winds were so strong that the ship’s anchor line broke, he said. “We went three weeks without an anchor,” he said.
A previous version of this story referred to findings in a scientific paper about seals’ disease exposure. The paper, published online in the journal Marine Mammal Science, is being revised, with findings re-analyzed using more advanced technology. The updated story refers to the disease-exposure analysis as pending.