by Todd McLeish
Now that sea otters are back in a big way in Southeast Alaska, the vocal opposition to them from Natives and non-Natives alike is making Williams cringe. While he may be one of the few Native Alaskans benefitting from the abundance of sea otters in the region, he is nostalgic about picking abalone and other traditional foods from the beach as a child. But he is also bothered by what he calls the “sea otter hate” that spews forth from too many people. The growing sea otter population in the region is directly attributable to the overexploitation of them centuries ago, he said, as was the unnatural abundance of shellfish and other marine invertebrates when sea otters were absent for so long.
“That imbalance is due to us, to human beings,” he said. “So I’m hesitant to blame an animal for living, for thriving and expanding. That’s life…and life is programmed to explore and expand. So I’m cautious about saying that’s a problem because it seems like that’s the natural order.”
Chapter 10: Crash
ALEUTIAN ISLANDS, ALASKA
JIM ESTES had studied sea otters in the Aleutian Islands for nearly twenty years when he traveled once again to Amchitka Island in 1990. He went to see what he could learn about sea otter behavior in a population that was so large there was barely enough prey to sustain them. At the time, it was the only place in the world where sea otters had reached carrying capacity. When he arrived, he thought he knew most of what there was to know about the ecology of the resident otters that were thriving in the 1,200-mile arcing chain of islands in western Alaska. But things were about to change dramatically.
Sea otter numbers recovered rather quickly in the Aleutians after the end of the fur trade, but they did so in what Estes called “a spatially asynchronous manner.” By that he meant that when legal sea otter hunting ended in 1911, the few otters that were left were found in tiny pockets scattered throughout the islands rather than evenly spread across the region. No one knows how many remained around Amchitka, but Estes described it as “just a handful, a very very few.” There may have been one or two other sites in the Aleutians where otters survived, but even that is uncertain. Their recovery from such tiny numbers was initially quite slow, because the animals produce just one pup each year and perhaps because it was difficult to find mates when so few animals were left. But when otter numbers finally started to build, they grew rapidly thanks to a healthy ecosystem with plenty of available prey.
Yet because sea otters are so dependent on the shallow coastal environment and females disperse only short distances, it took decades for the animals to spread from one island to the next. For example, when the otter population at Amchitka was completely recovered and filling almost every available corner of habitat, they had yet to make it to Shemya Island, just 225 miles to the west. By the late 1980s, however, most of their historic range across the Aleutian Islands had been recolonized; there were about six to seven thousand sea otters at Amchitka Island, and well over one hundred thousand in all of the Aleutians.
A number of surprises were awaiting Estes on his return to Amchitka, however. Up until then, everything had appeared to be happening as expected—the otters were keeping the sea urchins in check, and the kelp beds had recovered to support an abundance of fish and other marine life. His research plan for the next few years was to compare the behavior of the sea otters at Amchitka with those at Adak Island, 180 miles to the east, where the population was still recovering. He sought to compare parameters like growth rates and body conditions, birth and mortality rates, diets, time spent foraging, and other factors. He hoped to capture one hundred sea otters, place tracking devices on them, and study them in the same way he had conducted other research projects—by following each animal’s activities from day to day.
The first surprise was how difficult it was to capture the animals. He expected to capture all one hundred sea otters in one week, but it took more than three weeks to capture just ninety. “At the time I reconciled it as just the way it was,” Estes told me. “Maybe the weather was just too good and the animals were harder to capture for that reason. But that was the beginning of it all. Over the next two years as we studied those animals, every one of our hypotheses about what we expected in the sea otters of Amchitka Island just did not pan out.”
Previously, when Estes began studying the Amchitka otters in the 1970s, it wasn’t uncommon to find large numbers of dead otters as he walked the beaches. He collected data about the health, age, and gender of each one of them. Some days he found twenty or thirty otter carcasses, but he wasn’t alarmed by them. When there are large numbers of animals in a population, it is expected that some will die and, in the case of sea otters, be washed onto the shore by the tides and waves. But during his two-year study of sea otters around Amchitka in 1990 and 1991, not one dead otter was found on any of the beaches.
“That was a huge surprise,” he said. “I had no idea what was going on. All I knew was that I didn’t know that ecosystem like I thought I did. I had gone out there expecting it was going to be a question of just documenting the things we already were fairly sure were true. Instead, I was really very humbled because I just didn’t know what was happening.”
When Estes followed up his Amchitka study with an analysis of sea otters at Adak Island—with similar results—he became convinced that sea otter populations in the region were declining rapidly. And the culprits, he believed, were killer whales.
* * *
THE 1995–1996 STUDY on Adak was primarily conducted by a young Tim Tinker working on his second sea otter research project. He and his wife, Julie, lived on Adak for a year, capturing and tracking sixty sea otters. During his time on the island, Tinker observed several killer whales attacking and eating sea otters. “I was sitting there on a cliff collecting foraging observations, and one of our tagged animals was really near shore, and all of a sudden she started periscoping. Something was going on,” he said animatedly. “So I got my binoculars and I see these killer whale fins circling around this group of otters, and all of a sudden they came up and sucked down an otter. That was new to me. Then they ate a couple more and circled around and all of the otters fled. Most of the adult otters that had been offshore all suddenly were against the shore, craning their eyes and obviously very perturbed.”
At the time, Tinker thought it was just an interesting natural-history observation. He didn’t link the killer whale attacks to what he believed was a decline of the otter population around the island. When told about Tinker’s hypothesis that otter numbers were slumping, Estes was skeptical until he returned to Adak and Tinker walked him through his reasoning. Estes found the explanation compelling. Still, he initially thought the decline was a local event rather than a region-wide population crash.
A key part of Tinker’s evidence came from Clam Lagoon, a three-mile-long tidal pond with a sandbar across its mouth that prevents large marine mammals from entering. When the research project began, there were about one hundred sea otters living inside the lagoon and a similar number outside the lagoon. Four years later, 90 percent of the otters living outside the lagoon had disappeared, while numbers in the lagoon remained constant. Today there remains a healthy population of sea otters who spend nearly their entire lives inside Clam Lagoon, and none live outside it.
In 1997, Tinker and Estes surveyed several other islands in the Aleutians and found dramatic population declines everywhere they turned. They even conducted an underwater survey of the kelp beds around Adak Island and found massive numbers of sea urchins consuming the kelp, a sign that sea otters were virtually absent. An aerial survey of the entire Aleutian chain in 2000 turned up similar results. The scientists estimated that the Aleutian population of sea otters had sunk to less than 10 percent of its peak levels.
Their evidence of killer whale predation on sea otters included some unique otter behaviors that occur nowhere else. Tinker said that otters in the Aleutians now spend most of their time entirely hauled out of the water or resting on submerged rocks where the water
is so shallow that killer whales can’t reach them. “We’ve seen a complete behavioral shift across the entire Aleutians to avoid predators,” he said. “It’s so obvious what’s going on out there when you see it.”
But it was difficult for Tinker and Estes to convince their colleagues that killer whales were responsible for the sea otter decline. Their first research paper about it was rejected by a research journal. So they went back to work examining other possible explanations. Could the population decline be caused by contaminants from the military facilities on some of the Aleutian Islands? No, because sea otters experienced similar declines at pristine islands as well as those located near military bases. Was there a limitation on the availability of food? No, adult otters and their pups were observed to be robust and healthy, and as otter populations fell, more and more food was available. What about a disease? No dead otters were found washing up on coastal beaches, and when the researchers captured otters to test for diseases, they found no evidence of any illness. Which brought them back to the idea of predation by killer whales.
Little is known about the killer whales that live in the Aleutian Islands, because the area is difficult to get to and the animals are hard to study for any length of time. Like elsewhere in their range, there are two types of killer whales—transient animals (sometimes called Bigg’s killer whales) that feed exclusively on marine mammals, and resident whales that eat only fish. According to Craig Matkin, a killer whale biologist with the North Gulf Oceanic Society in Homer, there are about 450 transient killer whales living in the Aleutian Islands and southern Bering Sea area, including the Pribilof Islands to the north of the Aleutian chain. In addition, Matkin claims there are several thousand resident killer whales that spend much of their time harassing the fishing fleet in the region. The mammal-eating transients are spread out thinly across the North Pacific, wandering back and forth in small groups across this massive body of water and feeding opportunistically on a mix of Steller sea lions, harbor seals, Dall’s porpoises, northern fur seals, and juvenile whales, especially minke, gray, and humpback whales.
A sea otter, however, is a tiny piece of popcorn to a killer whale that typically consumes prey five or six times larger than the largest otter. To a killer whale, an otter is a little fur ball containing none of the precious blubber it prefers; it’s barely a meal. And yet Tinker and Estes say that at least a few killer whales that traverse the Aleutian Islands somehow developed a taste for sea otters. In fact, all it would take is a few otter-eating killer whales to wipe out the region’s entire sea otter population over a twenty-five-year period.
“The most plausible thing we came up with is that one matrilineal line of killer whales, just five or six related killer whales eating a few sea otters each per day, could be responsible for the entire Aleutian sea otter decline,” said Tinker. “That’s the mechanism that makes the most sense. And we watched it happen. We could see the same killer whales over and over again swimming through the kelp beds.”
The development of novel behaviors among killer whales is not uncommon. Usually, the dominant female in a small group of whales may start to do something new—in this case, hunting and eating sea otters. That behavior then spreads to others in her pod through teaching and observation. “The idea that it started through cultural innovation among one group of killer whales is speculative,” Tinker admits, but the idea “that killer whales are eating otters” is not.
Not everyone agrees.
Matkin is adamant that killer whales are not responsible for the dramatic decline in sea otter numbers in the Aleutians. He has observed killer whales killing sea otters in Kachemak Bay and Prince William Sound, but he says the otters usually aren’t consumed. Instead, the whales use the otters as “training tools” to teach their offspring how to hunt and kill. And the numbers of otters killed are insignificant when compared to the total population.
“The only time I could imagine a killer whale taking sea otters as regular prey is if it was in a desperate situation,” Matkin said. “Maybe it’s lost most of its group and can’t hunt like they usually do….But switching to sea otters as their primary prey without a reason? That just doesn’t make sense to me.”
Estes and Tinker say there is a reason for the switch, though.
Prior to the sea otter population crash, the Aleutian population of Steller sea lions declined by about 80 percent in the 1970s and 1980s, and at least in the western Aleutians, the decline continues at about 4 to 6 percent annually. And the northern fur seal population has declined from about two million animals in the 1950s to about six hundred thousand today; though this was triggered initially by a culling of large numbers of females to protect commercial fisheries, the population continues to decline today for unknown reasons. Harbor seal numbers in the Aleutians are down about 80 percent, too, from their peak in the 1960s and no one can explain that, either.
This massive loss of marine mammal biomass in the Aleutian region over the last fifty years—all of which are species commonly eaten by killer whales—seems to Estes and Tinker like a logical reason for killer whales to go looking for new sources of food.
* * *
THE STELLER SEA LION population decline, which immediately preceded the beginning of the sea otter decline, seems to have been the last straw for the killer whales, at least if you believe Tinker and Estes. That apparently triggered the switch to a diet of sea otters, they say.
According to Doug Demaster, director of the Alaska Fisheries Science Center, which conducts extensive research on marine mammals in Alaska waters, Steller sea lions began declining rapidly in the 1980s. For most of that decade, 15 percent of the population died each year, which is a huge loss for any marine mammal population. Initially researchers speculated that the primary driver of the decline was fishermen, who could legally protect their catch by shooting any sea lion that was stealing their harvest or damaging their nets. Many fishermen also went to the sea lion rookeries and haul-out sites and illegally shot as many animals as they could find. When, in 1989, it finally became illegal to shoot Steller sea lions under any circumstances, the population decline eased to just 4 or 5 percent. The institution in 2001 of a fishing ban within a ten-mile buffer zone around sea lion rookeries in the eastern Aleutians put a halt to the decline, and the population there began to recover. But in the central and western Aleutians, the fishing industry claimed that a buffer would put them out of business, so they proposed instead to reduce their fishing effort by half. That doesn’t seem to have worked, since Steller sea lion numbers continue to decline at about 5 percent per year in the region. No additional steps have been taken to reduce the decline, even though more than 450 sea lions were killed by human activities—mostly unintentional fishing-gear entanglement and hooking, though several were shot—between 2010 and 2014.
Demaster does not dispute the notion that killer whales are responsible for the dramatic decline of the sea otter population in the Aleutian Islands. But he’s not convinced, as Estes and Tinker are, that the decline of Steller sea lions had anything to do with it. And he is even less convinced of the sea otter biologists’ later hypothesis that explains the decline of the other marine mammals in the region.
According to Estes, killer whales started eating sea otters in the Aleutians right at the time that the Steller sea lions were at their lowest point, and the killer whales must have done so because there were so few sea lions around to eat. But as he thought about it more, he was led to another conclusion, one that became known as the sequential megafaunal collapse hypothesis. And it sparked significant controversy.
Estes looked back to the years prior to World War II when large whales were somewhat common in the North Pacific, before industrial-scale whaling caused a significant drop in whale numbers in the region. He said that those whales and their young were a key food resource for killer whales at the time. And when large numbers of whales were killed by commercial whalers in the 1950s and ’60s, it caused a chain reaction th
at led killer whales to expand their diet and start feeding on smaller species. And one by one those smaller species declined as well—first fur seals, then Steller sea lions, and then harbor seals—until the killer whales got down to the smallest marine mammal of them all, the sea otter.
In the whale and marine mammal research community, this proposal didn’t go over well. “There was a lot of emotion and a lot of pushback from those folks,” Estes said. Tinker thinks that the individual pieces of the puzzle are less controversial than Estes’s claim that all are connected and that one link in the chain caused the next. Everyone agrees, however, that there is little direct evidence to support the hypothesis. And yet there are so many unexplained population declines of marine mammals in the region that I found it almost comforting to have one theory explain them all. Tinker calls it “a great hypothesis that’s hard to prove definitively.”
One of the leading skeptics is Craig Matkin, who coauthored a scientific paper responding to Estes’s ideas. He told me that there is no evidence suggesting that killer whales will approach a healthy population of animals and prey upon it until there are none left. He thinks other factors like environmental conditions, disease, or food availability caused the initial population declines, and killer whale predation may then be keeping the populations too low to recover. A shift in the Pacific Decadal Oscillation, a cycle of changing oceanographic conditions that warmed the North Pacific in the 1970s and may have caused many marine mammals to switch their preferred prey species, is one change many people suggest may have initiated the decline of seals and sea lions.
In light of the limited evidence for the sequential megafaunal collapse hypothesis, Demaster took the high road. “I don’t think there is any argument that killer whales are an important factor in the dynamics of all the marine mammals in the Bering Sea and Gulf of Alaska and Aleutian Islands,” he told me. “But I think the jury is still out in terms of how all these factors fit together. There are holes in all of the current hypotheses as to what explains the dynamics of these five marine mammal species over the last thirty years.”