The mysterious case of the sinking seals
When Joel James, a Yup’ik hunter with over 50 years of experience, lost his first seal this year, he thought it was a fluke.
Every year, a certain number of seals are lost to hunters after they are shot for one reason or another - a bad shot, the hunter can’t find the carcass, or - for a reason not entirely understood - the seal sinks to the bottom of the ocean.
This year, that latter exception became the rule for hunters of the two St. Lawrence Island communities of Gambell and St. Lawrence, where the Yup’ik hunters, isolated from the mainland, rely heavily on the harvest of seals, walruses, and sea birds for food.
Rather than hunting seals that have hauled out on solid sea ice, hunters there prefer to hunt in open water in the fall, when seals are at the height of their weight before the long winter darkness. But this year, James said that it took nearly a dozen trips before he came home with his first seal of the season, which usually begins in August.
“It's been like I think the third year of the occurrence of sinking. It seems like it's happened right after the sick seals from four or five years ago,” he said, referring to an unusual mortality event from 2011 to 2016.
That meant that he didn’t get a fat-rich animal until mid-November, a heavy cost for someone who relies primarily on marine mammal meat for calories.
So what was the cause? The first answer that came to Joel James’ mind was body fat percentage.
“I guess they don't have enough blubber for the seals to be floating,” he said.
The relative density of any object will determine whether something will float or sink: the Greek scientist Archemides’ great revelation when he was tasked with finding a way to determine the authenticity of a piece of gold. Since fat - ie. blubber - is less dense than muscle or bone, a less fatty seal means a denser seal, and one that is more likely to sink. Previously, James said that he would regularly find bearded seals - one of the four types of ice seals that live in the Bering Sea - with up to four inches of calorie-rich blubber.
Now, it’s rare to find a seal with enough fat to float, a thickness James estimates at at least two inches.
The change in body fat is a tough one to measure with Western scientific methods. To measure body fat across an ecosystem requires either killing animals at a massive scale in order to test each seal, or somehow capturing and weighing live animals. Either would be difficult, expensive and disruptive to wildlife, so scientists have to rely on a sort of intermediate method: measuring seal body fat using samples from subsistence hunters in places around the Bering Sea region.
Lori Quackenbush is a biologist with Fish & Game’s Arctic Marine Mammal Program and has been taking samples from subsistence harvested seals since the early 2000s.
She said that between the three of the ice seal species hunted around St. Lawrence, body fat measurements have been variable.
“So we did see a dip in blubber thickness in recent years for ringed and spotted seals, but not lower than has been seen in the past,” she wrote in an email.
She said that unpublished numbers showed bearded seals had a higher than average body fat percentage in 2017, but in 2018, not enough samples were collected to be statistically significant. That’s not to say that seals aren’t getting skinnier around St. Lawrence, which could be susceptible to its own climatic and ecosystemic idiosyncrasies.
Perhaps more importantly, it fails to account for a glaring hole in the data: if seals are shot and not recovered, then they don’t become part of the dataset.
Hunters around the Bering Sea area, for the most part, take the fact that seals have been less healthy the last decade or so with a fair amount of certainty. Colleen Swan of Kivalina says that changes began about ten years ago, but has really accelerated in the past five.
“In the last five years, it's not gradual anymore,” she said. “It's just a new normal.”
Previously, she said, getting a bearded seal with four inches of blubber was a regular occurrence. Now, she said, it’s rare.
“It's always a happy occasion and a surprise when you get one that has normal blubber,” she said of this year’s hunting.
And it’s not just changes in body fat.
“The seal has been showing signs of trauma, they're not as plump and lively as they used to be. A lot of people are finding seals that are sickly,” she said. This year,
after they were found washed up on beaches at rates five times above normal.
For Swan, the causes are relatively straightforward, though interconnected.
“The fact that ocean is warming up means that a lot of the little microorganisms are dying, they're suffering. It's changing so it's very concerning when you start to see the effects, it starts to affect you directly as a hunter,” she said.
It is now no secret that the Northern Bering Sea is undergoing a dramatic ecosystem shift. The Bering Sea can roughly be divided into two parts: the southern part is deeper and, traditionally, the home to some of the most productive commercial species like cod and pollock. The shallower northern portion, nourished by phytoplankton growing on the underside of sea ice, is even richer by biomass, but without the same commercially important fish.
With less and less sea ice in the Northern Bering Sea, phytoplankton no longer have a surface on which to attach during the majority of the year, and thus cannot form the carpets that once blanketed the underside of the sea ice. Previously, those carpets would fall to the ocean floor, nourishing an abundance of bottom-feeding species like crabs and clams, as well as the mammals - walruses and seals - that fed on them.
Now that the ice is gone and the and a cold pool of water that once covered the Northern Bering has disappeared, the overall productivity of those species is declining, new species have poured in, most notably the commercially important cod and pollock, which have seen thousands-of-percentage increases in the Northern Bering Sea in just the past few years. Essentially, the Northern and the Southern Bering Sea ecosystems are becoming one.
“You have two ecosystems merging, this doesn't happen very often in the world,” said Gay Sheffield, a marine mammal biologist who has worked with subsistence communities in the Bering Sea region for decades.
Sheffield, who works for UAF’s Alaska Sea Grant program said that federal scientists haven’t been able to keep up with the magnitude of the changes underway, despite designating an Unusual Mortality Event.
That’s because they’re dealing with eleven other UMEs across the country, from Florida to the Pacific Seaboard.
“In Alaska, these seals are not just watchable wildlife or an interesting academic question, or a conservation question, there is food security and human public health concerns,” she said about the lack of federal funds to study seal mortality in the Bering.
It’s left state researchers with the task of trying to grasp the changes, from harmful algal blooms that could sicken subsistence users to declining body conditions of marine mammals. While current numbers are inconclusive about the latter, Sheffield says that in the early stages of the Unusual Mortality Event investigation, indications are the deaths are related to body condition.
Even if scientists are unable to pin down a biological explanation for sinking seals, another explanation for the phenomenon could be at least partially found in physical oceanographic changes.
Along with the change in species distribution resulting from a warming climate, there are a slew of changes with sea ice and the ocean currents that it affects, and with one component in particular: salt.
James may have discovered that when he finally captured his first seal of the season during a cold stretch in mid-November when the sea ice almost formed around the island.
“When I shot that seal two days ago it was in the light slushy part and it floated real nicely, so that might be,” he said around Thanksgiving time.
By ‘slush,’ James is referring to what scientists call frazil ice - tiny shards of ice that develop as the ice cover begins to form. This slush, forming in the salty seawater, has a unique property of expelling highly saline water that, just like the Dead Sea, makes objects within more likely to float.
That property gave James just the floating seal he needed, but the formation of frazil ice was short-lived. The next day, the water was open again.
“The wind blew away the slush, so it's back to open water,” he said. “We had a teleconference with someone last month about the sea ice. And they're saying we aren't gonna see any significant sea ice until February, March.”
It’s now late December, and James still hasn’t seen any solid ice form - or even slush.
But is the salinity changing earlier in the season as well and thus making it more likely seals sink?
Again, a relative dearth of information makes it a difficult thing to measure, but there are consistent measurements for about a half a century taken by research vessels that show a slow but detectable decrease in salinity in the Bering Sea over the years. Dr. Seth Danielson, now a professor of physical oceanography at UAF, says the freshening of the water could be related to two factors.
The first is related to James’ observation about the seal floating as ice forms. As ice formation is happening later and is also happening less, that means that less salt is being expelled as the ice forms and develops during the fall and winter.
The second related factor is the increased melting of ice during the summer. As the ice edge moves farther north, expelling freshwater as it moves north, salinity continues to decrease, especially at the top of the water column. Usually, this forms what is called the freshwater lens - a layer of low-salinity water that rides over the deeper saltwater below - until time or waves mix up and homogenize the entire water column.
“I do not know if this is sufficient to change the overall nature of ice seal buoyancy from positive to negative, but if the freshwater lens scenario is a common occurrence it would suggest that the ice seal buoyancy is probably always close to being negatively buoyant and maybe the recent "extra" freshenings (and warmings) we are seeing could be playing a role,” he wrote.
The freshwater lens has interesting implications for seal hunters. The low-density freshwater lens at the top of the water lets a dead animal sink - but only until it reaches the barrier where the freshwater meets the denser saltwater, where the seal will then “float” albeit underwater.
That usually can allow a hunter to recover a seal that is underwater with a seal hook. If water is shaken up by, say, a late fall storm, then the freshwater lens gets mixed in with the salty water and homogenizes the salinity. If the overall concentration of water is low enough, seals will sink.
While freshening of the Bering Sea waters across the board has been slight, Danielson noted that near St. Lawrence Island it is more pronounced. Though the reason isn’t clear, the marginal changes could be enough to tip the scales from floating to sinking.
While the trend is worrisome for hunters in St. Lawrence, there are silver linings. For one, the later freeze-up means that the season is longer than it once was. Seal hunters can now work well into December - or even January and February - before the water freezes up.
“We have an extended chance of harvesting the seals cause of no ice, you just gotta catch the right ones,” said James.
Perhaps even more than that, hunters in St. Lawrence are adaptable. James, optimistically, calls it making a “correction.”
“I did make a correction, I got some sea ducks. Some people are doing that, make a little adjustment. Besides the seal, they'll get the ducks. The ducks seem nice and healthy,” he said.
But as the larger sea mammals struggle to keep up with the rapid changes in the Northern Bering Sea, the calorie-rich food the Yup’ik people there once counted on is becoming more and more of a luxury.
“Usually we like a balance, like seals, walruses, ducks, and fish, but it seems like we're getting limited no choice now,” he said.