A ground-breaking, four-year research project has found killer whales using sophisticated and cooperative techniques to intercept, isolate and then kill young gray whales part-way through their annual 5,000-mile trip to summer feeding grounds in the Bering and Chukchi seas.
It was early May, a time when the epic gray whale migration to the Arctic had reached the Unimak Pass entrance to the Bering Sea. A group of killer whales swam excitedly back and forth in a patch of ocean off Akatan Peninsula in the Aleutian Islands, about 650 miles southwest of Anchorage.
There were four or five of them -- all members of the secretive type that hunts only marine mammals -- and they appeared highly agitated. Soon one of the 10-ton, black-and-white predators breached from the sea near a boat with human observers, riding the swell about two miles off the wild, uninhabited coast. What was going on?
The answer -- and the kill -- came fast.
An adult killer whale suddenly surfaced with its jaws clamped tight on the snout of a gray whale calf, an animal nearly as large as itself.
"The calf wrenched itself free, but another killer whale immediately grasped it again in the same manner," the scientists reported in a study published last week in the journal Marine Ecology Progress Series. "The (two) killer whales held the calf with its blowhole inverted for several minutes until it released a mass of bubbles, stopped moving, and slowly submerged, apparently dead."
The team of scientists from Alaska, Washington and British Columbia also watched killer whales actually transporting their dead prey for miles along the shore -- apparently with the goal of storing tons of blubber and muscle in shallow water so they could return later and continue to feast. The predators gathered at dozens of carcass sites, revealed at the surface by oil slicks and a ripe smell.
It was the first time any whales have been documented caching food, a behavior common among terrestrial predators as dissimilar as bears or alligators, but reported only once before for a species of marine mammals, Antarctica's Weddell seals.
The annual gathering of killer whales to ambush gray whales traveling along the Pacific Rim sends ripples through the local ecosystem -- forcing gray whales to hug the coast, concentrating birds and scavengers, attracting enormous sleeper sharks from the abyss and drawing brown bears to beaches when hunks of reeking flesh wash up high and dry.
It shows, the scientists said, how the ocean's top predator might play a "central role" in the structure of the North Pacific's marine community.
These "killer whales have developed a unique set of culturally transmitted social and foraging behaviors that appear to be shaped by the behavior and distribution of their marine mammal prey," the scientists wrote.
"The carcass-storing behavior described here relies on shallow depths around Unimak Island and provides a way for killer whales to utilize a greater proportion of prey carcasses than they would in a single feeding bout, increasing the benefit of preying on large and potentially dangerous species."
Tracking down what killer whales really eat
Scientists say that North Pacific killer whales always occur in three eco-types that never co-mingle or overlap in diet: the noisy and gregarious salmon-eaters known as "residents," the little-seen, deep-ocean fish-eaters known as "offshores" -- and the prowling, sneaky marine mammal hunters called "transients."
Sorting out which brand of killer whale dominates the remote Aleutian Islands and Bering Sea -- and what these highly intelligent predators regularly eat for dinner - has become a crucial element in one of the region's most difficult ecological mysteries.
Over the past decade, some scientists have argued that killer whales may have been forced to switch prey after commercial whaling decimated the North Pacific's great cetaceans: the right, sei, fin, humpback and sperm whales.
This change in killer whale diet -- so the argument goes -- would then help explain population crashes of Alaska's Steller sea lions and Aleutian sea otters, and the steep decline in northern fur seals, much better than other theories based on disease, contamination, changes in fish abundance, competition with commercial fishing or dramatic shifts in ocean climate.
But most killer whale biologists didn't buy the connection, arguing that no evidence supported the notion that killer whales switch prey in that fashion -- and almost no data existed about orca habits, population numbers and dining preferences anywhere in the region.
Over the course of four springs, the scientists spent 102 days at sea traversing more than 5,300 miles near Unimak Island in a quest to find killer whales at work. (In the late-summer aspect of the research, they collectively spent 511 days over four years traveling an incredible 26,600 miles throughout the eastern Aleutians.)
Along the southern coast of Unimak Island, they intercepted orcas on 90 occasions that lasted a total of 275 hours. They identified 154 individual whales, all of them transient marine mammal eaters that were cruising in groups that averaged about 10 whales in size.
About half of the time, the whales were traveling, socializing or resting. The other half? Chowing down -- or trying to.
And what did they eat? Gray whales.
Once, the scientists did see a group of killer whales briefly pursue five Pacific white-sided dolphins. On another occasion, whales chased a fur seal up onto the beach. They once saw a whale bite a single Stellar sea lion only to apparently release it alive.
In contrast, the scientists documented killer whales killing gray whales four times, and mounting attacks three times. The details offered unprecedented glimpses of the deadly interactions among some of the ocean's biggest and most intelligent creatures.
Gray whales are baleen cetaceans that can grow up to 50 feet in length and 40 tons on a diet of crustaceans and clams sifted from sea-bottom muck. Although nearly driven to extinction by 19th century whaling, the eastern Pacific population has since recovered and may number as many as 20,000 animals. Each year, these grays undertake an epic 10,000-mile round trip from Baja California to feeding grounds in Alaska's Arctic and back, the longest known migration by any mammal.
The species has only two natural enemies: humans and killer whales. During the Yankee whaling era, gray whale mothers were known for ferociously defending their calves and even attacking whale boats, earning the nickname "Devil Fish."
Off Unimak Island, that same aggressive stance saved at least one calf from death in May 2005. Six killer whales had trapped an adult gray whale and a calf in about 15 feet of water inside a cove. This behavior -- moving into very shallow water and even partially grounding on purpose -- appears to be one strategy used by the grays to escape being eaten, the scientists said.
"The gray whales appeared agitated and lifted their heads from the water for several minutes, at which point the killer whales rushed in and one slid upside down over the back of the calf gray whale. The adult gray whale lashed out with lateral movements of its tail, appearing to strike at least one of the killer whales," they wrote.
The killer whales retreated, and the grays waited an hour before slipping from the cove. Accompanied by two Steller sea lions, the grays made a break down the coast. The killer whales began to follow, but left them alone after the grays entered a thick kelp bed, where they rested.
"The mothers will often defend their calves and, if the mother is very aggressive, the killer whales will give up. The very pugnacious females seem to rule," Barrett-Leonard told the Vancouver Sun. "It's a real battle to the death and, if the female releases the calf, it's very one-sided and they kill it very quickly by grabbing it by the snout or the pectoral flipper and pulling it under until it drowns."
Once isolated, a gray whale calf and a juvenile did not fight back during seperate attacks, the scientists said. "Young whales simply rolled over onto their backs and were subsequently killed."
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