May 9, 2005
Lacking a decent meal, killer whales reach
for the popcorn
By Françoise Chanut
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 skull of a killer whale in relation to a sea
otter skull emphasizes the size difference. Recent studies
indicate that killer whales have been preying on sea otters
in areas of the Aleutian Islands where the sea otter population
Photo: T. M. Williams
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.
Williams and her coauthors at UCSC and the University of Alaska
published their findings in the December issue of the journal
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
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
Compared with 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 gray 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 gray
whales are common during the spring migration. The killer whales
prey on juvenile gray whales as they migrate with their mothers
from calving grounds in Baja California to feeding grounds in
"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 gray whale calves to feeding on otters.
Theoretically, the otters would be gone in two months,"
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
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
"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. Gray 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
"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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