A little bit of natural science
news here today that is worth noting.
The offshore population of killer whales just happens to prey on the
shark population. This is unexpected
because the other better studied populations work on either salmon or marine
mammals. In fact it had been thought
that large sharks lacked significant predators and this shows just the
opposite.
It is not what anyone would have
first guessed, but the present study underway has provided the evidence. Thus our outer Orcas do target sharks and
possibly that is the mainstay of their diet.
I am always surprised how small
the predator population actually is, particularly when neither the Orca nor their
prey are on any human dinner list. Perhaps
the prey population is naturally small for other reasons.
Orcas, not sharks, the dominant predator off Vancouver
Island coast
By Keven Drews, The Canadian Press | The Canadian Press – Sun,
4 Sep, 2011
Ford, a research scientist with the Department of Fisheries and Oceans,
was aboard a boat north of Haida Gwaii but just south of Alaska, studying the
feeding habits of a little-known group of offshore killer whales.
The mammals were hyperventilating, arching their backs and diving deep.
On the hydrophone, Ford could hear their excited songs.
Minutes passed and then a chunk of tissue -- about 250 grams in size
and later proven to be part of a liver -- floated to the surface, coming to
rest in a slick of oil.
More and more tissue and oil soon appeared, covering an area of ocean
in a sheen hundreds of metres in size and flattening the water's ripples.
Ford and a colleague collected samples, which were later analyzed at
DFO's Pacific Biological Station in Nanaimo, British Columbia
The tests confirmed Ford's long-held hypothesis: the offshore orcas
weren't eating salmon or sea lions. They were chowing down on sharks, specifically
sleeper sharks.
The sleeper shark is one of 14 shark species found in B.C. waters.
Blackish brown or slate green in colour, the shark can grow to 4.3 metres in
length.
"It was one of the top days in my 30-plus-year career studying
wild killer whales," said Ford, of watching the feeding frenzy. "It
was the culmination of many years of speculation, debate, you know, pondering
about what it is that these animals feed on.
"It was really gratifying to see that this piece of the jig-saw
puzzle finally fell into place."
The May 2008 incident was published in a recent issue of Aquatic
Biology and included as an exhibit at the Cohen Commission, the inquiry
examining the causes of the 2009 collapse of the Fraser River
The incident and the subsequent paper have provided researchers with a
few more details about offshore orcas, a population almost entirely unknown to
scientists.
Three groups — residents, transients and offshore orcas — make up
B.C.'s killer whale population, but the last of the three was first identified
off the B.C. coast only in the late 1980s.
Ford said about 300 to 500 whales make up the population, which travels
from California to the Aleutian
Island in Alaska
The orcas, he added, look different than their resident and transient
cousins: their fins' shapes appear to be different and their body sizes,
smaller.
He said the mammals travel in groups of as many as 100.
Congregating mainly on the edge of the continental shelf, the orcas
rarely and unpredictably venture into the waters between Vancouver
Island and the mainland, so little is known about them and
encounters are opportunistic, added Ford.
While resident killer whales feed on salmon and other fish and
transient orcas focus almost exclusively on marine mammals, the diets of
offshore killer whales have stumped researchers.
They began to hypothesize that the whales were targeting sharks after
they observed worn-down teeth on some stranded offshore orcas, said Ford.
A photograph published in a newspaper in the early 1940s also showed
worn-down teeth on the mammals.
Ford thinks orcas are targeting sharks because their livers are rich in
fatty oils and energy.
"We believe that for killer whales generally that they are going
after the most profitable prey, which tend to be the bigger kinds of body sizes
and the highest oil content," he said.
One year after the May 2008 incident, a colleague of Ford's observed
another, similar feeding frenzy in Prince
William Sound , Alaska
On that occasion, offshore killer whales fed on about seven sharks over
some three hours.
Just in the past few weeks, a researcher contracted by DFO witnessed
orcas eating sharks about 80 kilometres off the southwest coast of Vancouver Island .
Brian Gisborne, a retired commercial fisherman who also runs a
water-taxi business, said he watched between 17 and 19 orcas attack and eat
what's believed to be blue sharks.
Blue sharks are about three metres in length and are a indigo blue on
their backs.
Gisborne said at least one of the sharks was nearly two metres long.
"They'd (the orcas) go on over ... swimming around through lots of
sharks, and then every so often I guess they'd finally get hungry.
"Somebody would grab one and the whole family would come together
and they'd tear it all up and different ones would get pieces of it. As best as
I could tell that what was going on."
Gisborne said he took samples so Ford should soon be able to tell what
type of sharks became dinner.
With genetic work still to be done, there's little chance research work
on the offshore orcas will come to an end any time soon.
Several remote, acoustic-monitoring sites are moored below the ocean's
surface off B.C.'s coast, and Ford said researchers hope to get a better
understanding of where the mammals congregate.
Once they know that, he added, researchers will have a better chance of
finding them and learning about their diets.
"We feel like we've still got to learn much more about their diet
throughout the year.
"It's going to take a long time. And we're just hoping that we'll
have more opportunities as we get to better understand where these whales tend
to be.
"These are long-lived animals, so is our study," said Ford.
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