Steller sea lions, iconic Alaska marine mammals known for their loud roars and endangered status, continued their decadeslong downward spiral in Alaska's westernmost islands even as their relatives to the east are staging a comeback, said a federal report issued Tuesday.
New population counts from the National Oceanic and Atmospheric Administration suggest that decades of protective measures for Steller sea lions are paying off in the central Gulf of Alaska region, but not yet in the western Aleutian Islands.
The Steller population is just a faint shadow of what it used to be in the remote western Aleutians, the region where the animals have been most vulnerable. The new counts, made this year, found a continued decline of nearly 7 percent a year since 2003.
That put the total western Aleutian population at only about 5 percent of what it was 30 years ago, according to NOAA.
"That's a real troublesome trend out there," said Lowell Fritz, a research biologist with NOAA's Alaska Fisheries Science Center.
Sea lions in the western Aleutians, either adults or pups, do not appear to be starving or sick, Fritz said. There are just far fewer in number, he said. "Clearly, the ecosystem doesn't seem to be as able to support as many as it used to," he said.
Low pup production may be plaguing the western Aleutian sea lions, Fritz said. Females might not be able to bring new pups to term, or the newborns might be dying right after birth, he said.
This year on Attu, the westernmost Alaska Aleutian island, he said, cameras installed at a rookery recorded six or seven pups who died just after they were born, a heavy toll in an area that normally nurtures only 35 or 40 new pups a year.
The Steller sea lion story in the east Aleutians is far happier.
Faring best are the species' eastern stock, which ranges from Cape Suckling, a spot just east of Prince William Sound, down to the California coast. Those sea lions have rebounded from earlier problems, and are so healthy their threatened status under the Endangered Species Act was lifted in 2013.
Even Steller western stock sea lions in the eastern portion of their habitat — from Prince William Sound to just west of Unalaska — have been showing improvement in recent years, according to NOAA. That part of the population is up by more than 50 percent since 2003, and is showing steady improvement.
"Half of the western stock in Alaska is doing pretty well," Fritz said. "It's still way, way, way below what it used to be, but it's on the right track."
Physical characteristics of the western stock animals are likely making a population rebound slower than the eastern stock, Fritz said. Western Stellers are bigger and reproduce more slowly, so the population increase, since it hit bottom in 2003, might be 15 or 20 years behind the eastern stock, he said.
The Steller sea lion crash has been one of the big wildlife sagas of the northern Pacific Ocean. In Western Alaska, the population lost about three-fourths of its size between the 1970s and 1990.
NOAA in 1990 listed Steller sea lions as threatened throughout their range. In 1997, NOAA split the population into two groups and classified the more-ailing western group as endangered.
Exactly what caused the crash, and why the westernmost population is not recovering, are questions attracting much study and debate.
Since Steller sea lions share territory with the industrial-scale North Pacific fisheries that scoop up pollock, cod and other seafood, many theories have cited fishing as a culprit in the big decline.
Several studies have addressed the problem of nutritional stress in Steller sea lions, either from lack of forage fish, or diminished quality of the fish available.
Federal fishery managers over the past decades have imposed a suite of Steller sea lion protections such as no-fishing buffers, seasonal restrictions and new rules limiting gear types.
Such protections appear to have worked well in the Gulf and Bering Sea, where there is a lot of space to separate vessels and sea lion gathering sites, Fritz said. But the Aleutians, with both fishing and sea lion activities concentrated in a narrow zone near the islands, do not offer the same flexibility, he said.
"There aren't that many places to move fisheries around," he said.
Seafood harvests in federal waters off Alaska have fluctuated over the years, with quotas set based on stock abundance.
Harvests of Alaskan pollock, the top-volume commercial seafood species, have generally been between about 1.1 million and 1.5 million tons a year. The North Pacific Fishery Management Council, which oversees commercial fishing in federal waters off Alaska, is currently meeting in Anchorage to set 2017 harvests for pollock and other species.
Other possible culprits in the western Alaska decline are sweeping changes in the marine ecosystem, contaminants, disease, predation by killer whales or sharks or killing by humans, with varying evidence for each.
Some mercury hot spots, for example, have been discovered in the past few years, Fritz said, though they are difficult to interpret.
"It's hard to imagine any other way than though mom eating low levels of mercury in fish, then transferring it to pups through milk," he said. But mercury findings are highly variable, he said, with elevated levels found in one area and absent in others nearby.
Evidence about predation is ambiguous, he said. While sea lion-eating transient killer whales are present in the western Aleutians, they also hunt amid the thriving eastern population, he said.
A big help to Steller sea lions, Fritz said, has been reduction in what NOAA calls "direct mortality" — actions by humans who kill fish, sometimes deliberately.
Changes in fisheries gear and practices off Alaska greatly reduced risks that sea lions would be tangled in nets and drowned, according to NOAA.
Far fewer sea lions are being shot and killed than in the past, Fritz said. The practice, a violation of the Marine Mammal Protection Act, was once common among fishermen and other people who viewed sea lions as competitors for fish. However, such incidents still happen; last year several of the animals were shot and killed near Cordova.
Steller sea lions are not the only western Alaska marine mammals in trouble.
Sea otters, which declined in the Aleutians by 70 percent from 1992 to 2000, are now classified as threatened in western Alaska.
Northern fur seals in the Pribilof Islands in the Bering are classified as "depleted" under the Marine Mammal Protection Act. The Pribilofs hold about half the world's northern fur seals, and pup production there declined by 45 percent from 2008 to 2014, according to NOAA.