Great white sharks, long considered the sea's most
bloodthirsty killers, are actually cream puffs compared
with killer whales. A study in Nature Scientific Reports took the first comprehensive look at the rarely seen,
sometimes violent, encounters between two of the
world's largest ocean carnivores and found that great
whites run like scaredy-cats whenever killer whales
show up at their feeding grounds at the Farallon
Islands, near San Francisco.
"What we saw was that when orcas came close to
the island during shark season, all of the sharks would
take off," said Salvador Jorgensen, a Monterey Bay
Aquarium researcher and the study's lead author. "As a
predator that has been successful for millions of years,
that may be the card white sharks know how to play
that has kept them alive so long -- knowing when to
fold."
The report, which used acoustic tracking data from
165 white sharks between 2006 and 2013 and observations
on Southeast Farallon Island dating to 1987, found
that the sharks would all vanish around the islands,
even if orcas were just passing through. The fear is
so deep rooted that the sharks stayed away from the
islands for the rest of the feeding season even after the
killer whales left, sometimes as long as a year.
The data confirms what many scientists suspected
after a famous attack in October 1997 near
the Farallones by orcas on a great white in full view
of a whale-watching boat (the orcas ripped out and
devoured the shark's liver) -- that killer whales are the
undisputed champions of the food chain. Researchers
believe pods of killer whales were also responsible
for the deaths of at least five great whites that washed
ashore in South Africa with their livers missing over
the past few years. It's not known whether killer
whales attack because they consider great whites a
food source or competition, but shark livers are a
major source of calories, weighing up to 25 percent of
their body weight.
Offshore orcas eat schooling fish and Pacific sleeper
sharks, and roaming pods that scientists call "transients"
prey mostly on marine mammals. Some of the
140 or so transients identified as existing off California's
shores were present at the Farallon Islands every time
tagged sharks were observed bolting en masse between
1992 and 2013. The sharks skedaddled even though
the whales attacked only pinnipeds and smaller prey
during the two hours they were in the vicinity. One
shark returned a week later and a few tagged sharks
that weren't present during the original orca sighting
later visited, but they all left within minutes or, at
most, hours. Each time, the fleeing sharks reconnoitered
near Año Nuevo Island farther south or Point Reyes
National Seashore up north, but they avoided the
Farallones.
It is still a mystery how the sharks --solitary hunters
and usually not close enough to see the orcas -- knew
the predators were around. Orcas communicate with
one another at a higher frequency than white sharks
can hear, but they are known to remain silent when
hunting prey that have the ability to detect their higher
tones. "My gut feeling, and this is the topic of future
work, is that sharks are able to detect orcas using their
sense of smell," Jorgensen said.
To him, the most interesting thing about the clashes
between these seagoing titans is how they impact the
ocean ecosystem. Elephant seal predation decreased
fourfold in the years orcas visited the Farallones.
Sharks, meanwhile, were forced to compete for food
elsewhere, which may have reduced the energy
reserves needed to fuel their long ocean migrations.
"We don't typically think about how fear and risk
aversion might play a role in shaping where large predators
hunt and how that influences ocean ecosystems,"
Jorgensen said. "It turns out these risk effects are very
strong, even for large predators like white sharks."
-- Condensed from an article by Peter Fimrite published in the San Francisco Chronicle