A study comparing the nutritional demands of killer whales with the caloric content of their prey has startling implications for the potential impact of these large predators on populations of other marine mammals. The study also raises issues relevant to the establishment of marine reserves and efforts to reintroduce large predators into terrestrial ecosystems, said Terrie Williams, professor of ecology and evolutionary biology at the University of California, Santa Cruz.
Williams and her coauthors at UCSC and the University of Alaska published their findings in the December issue of the journal Ecology. Their analysis suggests that as few as 40 killer whales preying on Steller sea lions along the coast of the Aleutian Islands could have caused the fivefold decline that has been recorded in that population since the early 1980s. Similarly, the more recent tenfold drop in sea otter numbers in the same area could be the feat of just one pod of four or five killer whales specializing in sea otter hunting.
"Our main finding is the magnitude of the appetite of the killer whales," Williams said. "Predators with such a large body size and high metabolic rate have high nutritional demands."
This study bolsters a hypothesis put forth by the authors in earlier papers that linked declining marine mammal populations in the Pacific Northeast to the decimation of the great whales by industrial whaling.
The story begins more than a decade ago when Williams's coauthor James Estes, adjunct professor of ecology and evolutionary biology at UCSC, was trying to understand the rapid disappearance of sea otters in parts of the Aleutian Islands, a 1,200-mile archipelago west of the Alaska Peninsula. The first eyewitness reports of killer whale attacks on sea otters came in the 1990s, a time when the Aleutian sea otter population was declining by 25 percent each year. But in protected bays that were inaccessible to killer whales, Estes found the sea otter populations to be stable. He concluded that killer whale predation was driving the disappearance of the sea otters.
At the same time, researchers had been puzzling over the precipitous decline of Steller sea lions and harbor seals in the same region. They had first blamed overfishing by commercial fisheries, which would deprive seals and sea lions of their staple food. But they couldn't find evidence that the animals were dying of starvation, nor for that matter of any illness.
"After having seen what killer whales did to sea otters, it wasn't hard to imagine that they could do the same thing to seals and sea lions," Estes said.
Looking at records of marine mammal counts dating back to the 1950s, coauthor Alan Springer of the University of Alaska in Fairbanks observed successive waves of population collapses. The sea otters' nosedive that began in the early 1990s came after similar trends had affected Steller sea lions in the late 1980s and harbor seals in the late 1970s. Before these declines, the 1960s had seen the decimation of the North Pacific great whales by commercial whaling.
The team proposed that killer whales had once fed primarily on great whales, until those large prey became too scarce to provide a steady food supply. The killer whales then redirected their hunting to progressively smaller animals.
But did the numbers add up? That is the question the researchers set out to answer in their latest paper.
It is difficult to evaluate the nutritional requirements of large predators, Williams said. Scientists know how much killer whales eat in captivity, but not when they are swimming and hunting in the open ocean. Direct measurements of killer whale diets in the wild were not feasible, but Williams and others have done extensive research on the physiology and nutritional needs of smaller marine mammals, such as dolphins and seals. Williams extrapolated from this work to estimate the caloric intake of the much larger killer whales. She calculated that an adult female, weighing approximately two tons, would need more than 190,000 kilocalories per day; a 4.5-ton male would need 290,000 kilocalories per day.
Adult sea otters weigh between 50 and 75 pounds, which represents, on average, 50,000 kilocalories. Depending on its size and the size of its prey, an adult killer whale could satisfy its appetite by gobbling up between 3 and 7 sea otters a day--or 1,095 to 2,555 otters per year. Williams estimated that the decline of sea otter numbers in the 1990s in the Aleutian Islands corresponded to the loss of approximately 10,000 adults per year. An average pod of killer whales (one adult male and four to five adult females) with a single-minded appetite could easily eliminate that many otters.
Biologists recognize at least two different types of killer whales in the northeastern Pacific Ocean. They differ in behavior, social structure, genetics, and diet. Most killer whales eat fish, but a small proportion--perhaps 10 to 17 percent--feeds primarily on marine mammals. A conservative estimate places the number of marine mammal-eating killer whales at around 170 in the Aleutian Islands waters, according to Williams and her colleagues. While no killer whale may care for a diet consisting exclusively of sea otters, each one of those 170 orcas would only have to eat one or two sea otters a week to cause the observed decline.
Compared to the lean sea otters, blubber-padded seals and sea lions have a high caloric content, so fewer are needed to feed a killer whale. Williams estimated that an average adult killer whale would be satiated on two to three Steller sea lion pups every day or one adult every two to three days. A predator population of 170 killer whales bent on a sea lion diet would remove 40,000 individuals from circulation each year, three times the number needed to drive the yearly rate of Steller sea lion decline observed since the 1980s.
By comparison, one great whale represents an enormous supply of calories. "You could feed 17 killer whales for a day on a single grey whale carcass," Williams said.
Williams noted that she probably underestimated the potential impact of killer whales, because she didn't factor in the greater caloric requirements of pregnant females and growing juveniles. Her calculations do not prove that killer whales are responsible for the disappearance of smaller marine mammals, but they show that it is a definite possibility.
"It's really a feasibility study," Estes said.
Over years of field observations in the Aleutian archipelago, Estes has found no evidence that sea lions or otters were starving, a common hypothesis to explain large population drops. On the other hand, there is a lot of historical and current evidence that killer whales attack and feed on large whales, he said. After the great whales declined, it would make sense for killer whales to turn to other kinds of prey.
In the Monterey Bay, reports of killer whale attacks on grey whales are common during the spring migration. The killer whales prey on juvenile grey whales as they migrate with their mothers from calving grounds in Baja California to feeding grounds in Alaska.
"What is remarkable is how quickly we could lose the sea otter population in the Monterey Bay if killer whales decided to change from hunting grey whale calves to feeding on otters. Theoretically, the otters would be gone in two months," Williams said.
Switches in predatory behaviors are well documented among land animals, Williams said. Lions, for instance, may prefer wildebeests, but they will hunt small antelopes in the absence of larger prey, she said.
"Many carnivores chose prey that make energetic sense," she said. "Rather than starve when preferred foods are gone, they may turn to the prey equivalent of rice cakes and popcorn."
Killer whales are resourceful, Estes said. Different groups of marine mammal-eating killer whales have different diets and hunting strategies, he said. Some specialize in seals, others in porpoises.
"A few animals learning to do something clever could have a lot of local impact," Estes said. "The implication for nature conservation is that whale populations should be allowed to recover. Grey whales may have completely recovered, but they are a very small part of the whole whale community."
The methods Williams and her colleagues used--demographic analyses and measurements of caloric value--have been around since the 1940s, she said. What's new is putting together physiology and demographics.
"It is amazing that we haven't made more use of these simple analyses," she said.
Knowing the nutritional requirements of killer whales has important implications for the design of marine reserves. Williams drew a parallel with the reintroduction of wolves in Yellowstone. "We would like to re-create a natural ecosystem, but we need to do it with a realistic idea of what predators are going to need to survive," she said.
She is currently applying her approach to estimate the ecological impact of terrestrial predators, such as coyotes and mountain lions, on local ecosystems.
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Note to reporters: You may contact Williams at (831) 459-5123 or williams@biology.ucsc.edu and Estes at (831) 459-2820 or jestes@ucsc.edu.
