Alaska's killer whales chow down
April 12, 2007
Courtesy of Far North Science
The elusive killer whales that roam the eastern North Pacific in search of marine mammals aren’t fussy eaters after all.
They clamp down on gray whale snouts and drown them under the waves. They munch calves. They snatch northern fur seals. They scarf down Minke and humpback whales. They chase Dall’s porpoise and seals. They even, occasionally, eat Steller sea lions.
If it’s got flippers, fur and flesh, it may be on the menu du jour, according to four recent scientific papers that compiled hundreds of field observations, video monitoring of a sea lion rookery, and sophisticated analysis of fatty acids and stable isotopes in killer whale tissue.
But these killer whales, the secretive ecotype known as “transients,” aren’t very numerous and don’t eat fish. So far, they’re keeping some secrets to themselves, like where they spend the off-season.
“The most important finding is that, in the Eastern Aleutians, Steller sea lions are not the primary prey of killer whales,” said Homer biologist Craig Matkin, the region’s leading killer whale researcher and the only contributor to all four papers. “Fur seals and gray whales are very important, (but) we don’t know what the transient killer whales … that feed on gray whales in the spring do in the summer.”
That particular insight undercuts a controversial theory that fingered transients — what Alaska Sen. Ted Stevens once called “rogue” killer whales — for the 80 percent population crash that landed Steller sea lions on the Endangered species list.
The findings are part of a vast field investigation by a dozen scientists into the ecology of the North Pacific’s most intelligent and wide-ranging predators, the three distinct types of iconic black-and-white killer whales that prowl Alaska’s coasts.
The results combined hard-fought field observations with biochemical wizardry, video surveillance with submarine recordings of orca chit-chat. A summary of Matkin’s Aleutian observations was published in NOAA’s Fishery Bulletin.
Several groups of scientists gathered blubber and skin samples the size of pencil erasers by shooting whales with a special crossbow, and then analyzed the samples for clues into what the whales had been eating for dinner and what contaminants they’d ingested. (Because among whales, as humans, you are what you eat.) Their results were published in Marine Ecology Progress Series and Marine Environmental Science.
Another group, coordinated by John Maniscalco at the Alaska SeaLife Center, used remote video cameras to monitor action at the Chiswell Island sea lion rookery. After observing killer whales on 118 days over six seasons and recording 25 different attacks, they estimated that at least 59 sea lions had been consumed between 2002 and 2005.
One crafty old transient killer whale female nicknamed Matushka often prowled offshore, and once returned with a whale that might have been her daughter, and a young male that might have been her grandson. In an extraordinary moment in 2003, the crew at the SeaLife center watched on camera as the two females snatched sea lion pups near the rocks, ferried them offshore in their mouths, and released them in front of the adolescent.
They were training the calf to hunt.
“All three whales would then lunge at the pups, make occasional tail slaps, and repeatedly pull them under water and set them free,” Maniscalco, Matkin and three co-authors wrote in Assessing Killer What Predation On Steller Sea Lions From Field Observations in Kenai Fjords, Alaska, published in Marine Mammal Science. “These apparent training bouts lasted approximately 10 min. before the pups were consumed.”
Over three years, 2002 to 2004, Matkin and his team counted an estimated 901 resident whales in the eastern Aleutians. Blubber samples and hundreds of observations continue to confirm that these whales target fish exclusively, usually salmon, and never eat marine mammals at all, Matkin said.
Matkin and his team also caught glimpses of seldom-seen Offshore killer whales, estimated at 54, including several animals that had been identified in California and Gulf of Alaska waters. Blubber samples suggested these whales may be snatching the biggest fish of all in a diet much different than that of resident pods.
“There is no evidence that offshores eat marine mammals,” Matkin said. “But there are some fatty acids they have that can also be found in some pinnipeds. From the stomachs we do have, and our limited field observations, I would think it very unlikely they are eating marine mammals. But they are getting some unique fatty acids from eating things like sharks.”
The Transient Predation Puzzle
Rarest of all, appear to be the transients, the whales that stalk, thrash and chew up the ocean’s marine mammals. Matkin wanted to find out how many existed, what they ate, how they behaved. It meant searching some of the roughest and most remote waters on Earth.
The research had a direct bearing on one of the most heated controversies in marine mammal science: did roving pods of transients drive the population crashes of Alaska’s Steller sea lions and Aleutian sea otters, and possibly help trigger the steep decline now underway in northern fur seals?
Here’s some background:
Nationally ranked ecologist Jim Estes jolted the marine science world in 1998 with a paper that argued killer whales had been targeting sea otters, triggering an ecology cascade along certain Aleutian Islands that devastated nearshore kelp forests.
Five years later, University of Alaska Fairbanks oceanographer Alan Springer and Estes took the hypothesis one step further. The removal of most of the North Pacific’s great cetaceans — right, sei, fin, humpback and sperm whales — by Japanese and Soviet industrial whaling in the 1960s forced killer whales to switch prey to sea otters, sea lions and various seals.
The hypothesis, once debated in front of an international audience of marine mammal scientists, argued that this change in killer whale consumption explained the population crashes of sea lions and sea otters better than other theories based on disease, contamination, changes in food, competition with commercial fishing or dramatic shifts in ocean climate.
But Matkin and other biologists who specialize in killer whale studies disagreed. They argued field evidence didn’t support the hypothesis, and that there was no direct evidence that killer whales had ever changed their preferred prey. The very few observations of actual kills by transients suggested they rarely bothered with sea otters, and didn’t often attack sea lions.
But most data about transients came from the Gulf of Alaska and Prince William Sound areas. Almost no one had studied or counted killer whales in the Aleutians, where sea lions and sea otters declines were steepest.
Some Transient Answers
So, beginning in 2002, Matkin led a team of scientists that ventured out in small boats and scanned for glimpses of the Aleutian killer whales at work in a grueling effort that continues to this day. During the first three seasons, the scientists motored about 22,000 miles over 421 field days. Their reward?
They encounted killer whales on 250 occasions — identifying 901 residents, 165 transients and 54 offshores. They sorted the animals by their underwater calls, visible behavior, photos of their left-side dorsal fin, and biopsy samples of their blubber.
Over that time, they watched transients harass marine mammals only 31 times, and kill 26 times. The prey included 19 gray whales harassed and 18 killed. The whales chased three Dall’s porpoise, five northern fur seals and two Minke whales. They were seen to kill four fur seals, two Menke whales, one humpback whale and one Steller sea lion.
“Steller sea lions were not a primary prey,” Matkin and four co-authors wrote in Ecotypic Variation and predatory behavior among killer whales (Orcinus orca) off the eastern Aleutian Islands.
“Our work underscores the importance of determining lineages and ecotypes of killer whales before making assumptions regarding feeding habits and potential impact of killer whales on prey populations.”
Fur seals fill their bellies during some periods, and gray whales during others. But it’s not always the same killer whales, Matkin said. And lots of mysteries remain.
“We need to know where (gray whale eating) transients go in the summer, and we need to do tagging,” he added. “Also, we need to look at feeding habits and transients around the Pribilofs, where there is also substantial killer whale activity.”
Most of Far North Science is written and edited by Doug O'Harra, a writer and journalist based in Anchorage, Alaska.