Island Sea Lions Offer Clues to Mysteriously Missing Mammals

Just off the southernmost tip of South America, a cluster of small islands presents a huge puzzle for marine mammal experts.

The Falkland Islands, isolated in the far reaches of the southwest Atlantic Ocean, once boasted one of the world’s largest populations of southern sea lions. Now, they have one of the smallest. Hunting is the main reason for historical declines of seals and sea lions worldwide, but the Falklands population never recovered even though commercial hunts ended more than half a century ago.

That’s especially odd, because elsewhere around the globe, other populations of sea lions and seals that were hunted to near extinction have bounced back. For example, the neighboring Antarctic fur seals, with pelts so prized that hunters had nearly wiped them out by the early 1900s, have rebounded to about three million individuals today.

For years, scientists blamed hunts off Argentina for the severe population decline in the Falklands. If the sea lions traveled from their island breeding grounds to the mainland while hunts were still happening, big commercial operations around the Argentine coast may have made too large a dent in their numbers.

But a new study based on sea lion genetics paints a much more complicated picture. For starters, it appears that the Falklands sea lions may not have been travelling to Argentina. Instead, a combination of changes to their habitat, including warmer waters due to climate change, has had a devastating impact that continues to ripple through the population.

“This realization is important, because it influences how we interpret threats to sea lion population persistence,” says Alastair Baylis, a marine ecologist with the Icelandic Seal Center and the South Atlantic Environmental Research Institute in the Falklands. “If we are correct, it suggests sea lions are more vulnerable than previously thought.”

Mystery Migration

About 33 species of seals and sea lions exist worldwide, and as top marine predators, they are considered sentinels of ocean health. Consequently, their fate has far-reaching implications not only for other marine species such as penguins, but also for humans.

“They are the canary in the coal mine,” says Robert DeLong, a marine scientist at NOAA’s Marine Mammal Laboratory in Seattle. “We consume the same squid they do.”

The Falklands sea lions are part of a larger population of southern sea lions that ranges from Peru to Uruguay, and they used to represent a significant portion of the breeding population. In the 1930s, about 80,550 pups were born annually to Falklands sea lions, according to counts taken then.

By contrast, just 4,443 pups were born in 2014. The population has made small gains since the 1990s, but overall represents 4 percent of what it once was.

Learning what happened to the sea lions may also provide insights into what happened to the Falklands elephant seal, which has suffered a 90 percent decline since 1982, and the rockhopper penguin, which has declined by more than a third.

Baylis, whose work was supported in part by a grant from the National Geographic Society Committee for Research and Exploration, made his first trip to the archipelago in 2008, after hiring on with the Falklands government as a fisheries observer on fishing boats. He had just earned his doctorate at the University of Adelaide in Australia by studying New Zealand fur seals, and he quickly became intrigued by the Falklands sea lion decline.

“The thing that really stood out was how little work had been done on this particular species,” he says, noting that most of it was limited to population counts. “The expertise in the Falklands didn’t exist. Lucky for me as a freshly minted Ph.D. student, this was an incredible opportunity.”

The notion that Falklands sea lions were being killed in Argentina first took hold in the 1930s, when a British scientist named James Hamilton observed that sea lions gradually disappeared from Falkland breeding beaches as winter approached. That suggested a winter migration, during which time the sea lions would have been especially vulnerable.

Between 1930 and the 1950s, commercial sealing in the Falklands killed 60,723 sea lions, while hunting in Argentina during the same period killed more than 500,000. But the wrinkle in this theory is that no one had ever tracked the Falklands sea lions or observed signs of a migration.

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“The emigration of sea lions to Argentina would have helped point the finger at commercial sealing, but then comes this question: Why would sea lions swim to Argentina, when there is a larger population there?” says Baylis. “They would have been competing for the same resources, and it didn’t make any sense to me.”

No Mixing

Baylis lived in the Falklands until 2012 and has returned almost yearly, motoring by Zodiac from one small island to the next to visit all 68 breeding colonies and count pup populations and collect skin samples, whiskers, and scat. The field work could be simultaneously thrilling and miserable, with stunningly beautiful landscapes but also bouts of foul weather that made affixing tracking tags to a 700-pound (317-kilogram), rain-soaked sea lion all but impossible.

The tracking tags told him what he already suspected—that Falklands sea lions don’t seem to travel far from the islands during local winter. Since that finding contradicted the earlier belief of a migration to Argentina, Baylis teamed up with Joe Hoffman, a molecular ecologist at Bielefeld University in Germany, to fill in more pieces of the puzzle.

“I wanted to know if the sea lions from the Falklands were genetically similar or different from those on the South American mainland,” Hoffman says. “Are the animals moving freely between them, or is there some geographic barrier that prevents the animals from moving between the two? We could address that with genetics.”

Described in July in the journal Royal Society Open Science, the genetic testing largely ruled out a migration to Argentina. The work also established that the Falklands sea lions population collapse was not prompted by a loss of genetic diversity, which could have diminished their ability to adapt to changing circumstances. That is important, says Hoffman, because it shifts the detective work back to the islands.

“This means the decline of the Falklands sea lions must have been due to local factors,” he says.

The genetics testing did not rule out the possibility that Falklands sea lions visited Argentina during winter months just long enough to be killed during hunts, but Baylis and Hoffman think that is a far-fetched scenario.

Building the Case

Plumbing historical archives has yielded one promising find: A review of sea-surface temperatures over 160 years revealed that a warm period in the Falklands between 1930 and 1950 coincided with a steep decline in sea lions. Warming water currents are believed to be reducing the abundance of krill, a shrimplike crustacean that is one of the sea lions’ primary foods.

That is strikingly similar to what is occurring on the South Georgia and South Sandwich Islands, about 960 miles (1,550 kilometers) away. Antarctic fur seals, that great post-seal-hunt success story, are on the decline again because of food shortages.

Hoffman, who has studied the seals for more than 20 years, used genetic tests to link warming waters there and a drop off in available krill to a 25 percent loss of breeding females over the last three decades. The study was published in Nature in 2014.

“We are now accumulating evidence to show these declining populations are linked to climate change,” he says. “The Falklands decline may reflect a more global decline.”

In the end, that may explain part of what happened. But for now, it remains only another lead, one of many to be run down.

Baylis is studying bone samples from sea lion skulls collected in the early 1900s, now stored at London’s Natural History Museum, to look for evidence of a change in diet over time—and, if that occurred, what that would reveal about ecosystem changes in the Falklands. And he’s also reviewing commercial fisheries records to look at competition between fishermen and seals for the same fish.

“We are slowly building a case, looking for clues, but we simply don’t have all the pieces yet—and I suppose we may never,” he says. “People think that sea lions are large, charismatic, pretty conspicuous, and well studied. But quite often, we know very little about them.”