Mystery at Sea
Once-plentiful sea otters have a whale of a problem.
Newsday,
By Madeline Bodin
FIVE DAYS before Christmas in 1994, Tim Tinker stood on a
bluff on Amchitka Island, one of the western-most
spots in the United States. For Tinker, a researcher in a decades-long study of
sea otters in
Behind Tinker the
In front of Tinker were the calm waters of
This was not the kind of fluctuation that can be attributed to the natural up and down cycle of the wild.
Historically, sea otters have been in peril before. Centuries of hunting had reduced their numbers from 300,000 to just 1,000. Then, in 1911 with passage of a ban on hunting fur seals and sea otters, the pendulum swung back.
No longer hunted, and for the most part ignored by the
humans fighting World War II and the Cold War from Aleutian-based military
outposts, the Aleutian sea otter population grew. By the 1950s, sea otters had
spread out from
In the 1970s, Dr. James Estes, an ecologist with the U.S.
Geological Survey Biological Resources Division and the
The patchwork distribution of sea otters among otherwise similar islands created a living laboratory for studying the sea otter's role in the food chain. Estes found that the islands with sea otters also had underwater kelp forests, rich ecosystems that support not only sea otters but also a wealth of fish, snails, starfish and seabirds. The islands without sea otters had no kelp forests. The sea there supported few species. The link between the kelp and the sea otters is sea urchins. If not kept in check by sea-otter predation, sea urchins will mow down kelp forests.
Times were good for the sea otters when the study began. As
sea otters recolonized new areas, Estes and his researchers
watched the sea otters eat sea urchins and watched the kelp forests grow. The
detailed study, which continues today, keeps shifts of three to six researchers
in the
On that day in December, 1994, Tinker, one of those researchers, could see not only the otter he was studying just below the bluff, but another sea otter near the shore and a group of 15 to 20 recently weaned juvenile sea otters resting in a favorite spot in mid-harbor.
Like seals and sea lions, sea otters spend most of their lives in water. But unlike other marine mammals, sea otters do not have an insulating layer of blubber. Instead, sea otters have incredibly dense fur, with 10 times as many hairs in a square inch as humans have on their entire heads. These juvenile sea otters, like all sea otters, could float with ease and were insulated against the heat-robbing water by air trapped in their dense fur.
Suddenly, the two sea otters nearest Tinker pressed themselves up against the shore and looked out into the harbor. Something out there had caught their attention. Tinker refocused his spotting scope to see what it was.
Three killer whales, two adult females and a calf, had entered the harbor. Tinker had often seen killer whales cruise through the harbor, then leave, so he thought nothing of them. The juvenile sea otters continued to rest, floating on their backs in their favorite spot.
This time, however, the killer whales didn't quickly leave the harbor. They swam directly through the group of juvenile sea otters, which scattered as the whales passed. The three whales surrounded a single sea otter that was separated from the others. The whales swam within 10 yards of the otter, their dorsal fins slicing the surface of the water as they closed in.
The trapped sea otter raised its head and upper body out of the water, a behavior scientists call "periscoping." It made several short dives and continued to periscope when it came to the surface.
Abruptly, one of the adult killer whales leapt with a third of its body out of the water and landed with a splash directly on top of the otter. Four thousand pounds of whale had slammed less than 70 pounds of sea otter. Both animals disappeared under the water, but only the killer whale returned to the surface.
"You get pretty good at finding sea otters when they surface," Tinker said. This time he watched with his spotting scope and scanned with his binoculars but never saw the otter again.
The attack was only the second observed by the
"I remember thinking that it was a very bizarre, freaky event," Tinker said. Killer whales aren't known to dine on otter, preferring sea lions and seals.
Tinker could deduce what happened to this sea otter. But was
it a fate shared by the thousands and thousands of sea otters disappearing
throughout the
These mysterious disappearances left no corpses floating in the waves, as would have been the case if the sea otters were dying from disease or starvation. Nor were populations increasing anywhere along the 1,200 mile island chain, as would have been expected if the sea otters were merely moving to more fruitful hunting grounds.
"We had this mind-set based on what had happened up
until the 1990s - that the population would continue to increase," Estes
said. " It was hard to come to grips with the
realization that the population was going down. On
Once he had accepted the population decline, Estes did not immediately connect it with the rising number of bizarre killer whale attacks on sea otters. First he and the other researchers eliminated more likely possibilities. But it wasn't a decrease in the birth rate. It wasn't a disease. It wasn't starvation. It wasn't poaching.
With other causes eliminated, a link between the killer whale attacks and the sea otter decline, which once seemed so unlikely, now seemed possible. The evidence grew when a study showed that five times more sea otters disappeared from an open bay than disappeared in the same time from a lagoon protected from killer whales.
Still, the scientists doubted their findings. Killer whales
are found everywhere that sea otters are found, and reports of their attacks on
sea otters are rare. But when the
Estes, Tinker and two other scientists involved in the study published a paper on their findings in an October, 1998, issue of the journal Science. The paper included their theory that killer whales have turned to eating sea otters because of the collapse in the populations of their normal food - sea lions and harbor seals - in the western North Pacific. It is believed that the seal and sea lion populations have collapsed because of human overfishing in the area.
"When I read the paper, I thought, `Ooh, this could
really be the case,' " said Dr. Marilyn Dahlheim,
a killer whale specialist with the National Mammal Laboratory in
Studying sea otters in the remote and sparsely populated
Because of this difficulty, little is known about these
killer whales. Dahlheim has identified 400 killer
whales in the
Has a single group of killer whales discovered a new food
source, or is the phenomenon more widespread? Have a few whales turned to sea
otters for most of their diet, or are many whales occasionally grabbing a sea
otter or two? Are the killer whales responsible for the attacks in the
Dahlheim knows that killer whales
in the area have shown several new behaviors. In the '80s, killer whales in the
Dahlheim has no definitive answers
for the questions raised by the findings of Estes and his colleagues. She is
thrilled, however, to have other scientists interested in the killer whales of
the
Dahlheim hopes the sea otter researchers also will be able to take biopsy darts of the killer whales that visit their research sites. These biopsies will not only identify individuals, but will tell Dahlheim what the whales have been eating.
These findings should help the scientists begin to answer
some of the questions surrounding the sea otter's decline and the killer whale' s behavior change. It will be one point of
information, the first of many needed to tell the tale of the complex
relationship between sea otters, killer whales and their environment in the
North Pacific. The mystery of the
SIDEBAR: What Kind of Killers Eat Sea Otters?
THERE ARE two types of killer whales, residents and transients. The residents eat fish, form large social groups (called pods) and have a complex language. Transients eat marine mammals as well as fish, travel in small groups and have a simple language. It appears that transients are the ones attacking the sea otters both in the Aleutians and in Prince William Sound. "From three miles away, I could tell they were different," says Scot Anderson, an independent researcher who was one of the first scientists to observe a killer whale attack on a sea otter, on July 18, 1992, in Prince William Sound. "They were slamming these salmon out of the water. They came on like bad boys, squealing and thrashing the shoreline," he says. Anderson recognized them as transients from their behavior, the shape of their dorsal fins and their skin, scarred from fights with sea lions and other prey. Eventually the whales rushed some sea otters resting on rocks, then grabbed an otter resting in the water. It is the only time a dead sea otter has been observed in the mouth of a killer whale. - Bodin
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