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By Peter Rejcek
Just what does a humpback whale in the Southern Ocean
do all day?
Well, eat, that’s for certain. A lot. But how much
of the shrimplike krill can one of these baleen whales
consume in the frigid waters that surround Antarctica
before migrating north for the other half of the year?
It’s a question that a team of scientists, led by
Douglas Nowacek from Duke University , will address with
some high-tech tagging instruments — and steady hands
and sharp eyes — during a science cruise around the
Antarctic Peninsula beginning in late April.
Krill is the preferred meal for many of the apex, or
top, predators in the region, such as seals, penguins
and whales like the humpback, which can grow up to 16
meters long. But little is known about the ecology of
one of the Southern Ocean’s largest, most voracious
feeders.
“There’s already quite a bit of information
on diving patterns, feeding patterns, feeding rates and
energetic rates for the other krill predators,” explained
Ari Friedlaender , a research scientist at the Duke University
Marine Lab and a member of the science team that will
spend more than a month at sea aboard the ARSV Laurence
M. Gould .
“What we want to do with whales is to understand
how much they eat in a given day, the threshold of prey
necessary for them to be able to forage, and how they
make decisions about feeding,” he added.
Quantifying the consumption rates is important for understanding
the bigger ecological picture of the Antarctic Peninsula,
parts of which are becoming more subantarctic as climate
changes. Scientists have noted a decline in krill biomass,
which they say is linked to the declining duration of
winter sea ice, a key habitat for juvenile krill. Changes
in the climate, particularly in the winter, are affecting
the sea ice.
Meanwhile, humpback whale populations have rebounded
over the last two decades since a moratorium on whaling
went into effect in 1986. The International Whaling Commission
estimates there are now between 34,000 to 52,000 humpbacks
in the Southern Ocean. “They certainly have made
a comeback. Humpbacks, in particular, seem pretty healthy,”
Nowacek noted.
And commercial fishing of krill, while relatively low
and below peak numbers in the 1980s, could increase as
industries look for additional sources of fish farm food
and additives for omega-3 supplements.
These mounting pressures on the krill population make
it necessary to understand the humpback whale’s role
in the equation by putting some hard numbers on consumption
rates, according to Nowacek.
“It’s a fairly lofty goal, but it’s something
that we have the pieces to put together,” he said.
A technology he helped develop as a graduate student,
the acoustic recording tags are a noninvasive way of not
just tracking the animals but recording their acceleration,
movement and heading — information the researchers
can use to help characterize feeding behavior. For example,
there is a certain signal from that tag that links to
what Nowacek calls a “feeding lunge.”
“Getting the feeding lunges is a key, so that every
time they feed you assume they fill up their mouths,”
he said.
The tag, about the size of a paperback novel, contains
sensors and a hard drive, and attaches to the whale by
suction cups. It can also record the whale’s vocalizations,
allowing scientists to listen as whales feed and interact
underwater, as well as eavesdrop on seals and other biological
drums that may be beating nearby in the area.
The technology is unique. The technique to get close
enough to one of these 35-ton behemoths, with its unusually
long pectoral fins and knobby head, requires a soft touch.
“It’s not routine. You definitely have to know
what you’re doing,” Nowacek said. “Some
whales are not interested in being tagged, and they just
take off — and you just leave those alone.”
Tagging techniques vary by species, said Friedlaender,
who has tagged killer, sperm, pilot and humpback whales,
among others. Maneuvering by a small boat, like the inflatable
Zodiacs used by the U.S. Antarctic Program (USAP) , he
uses a 7-meter-long carbon fiber pole, same as a windsurfer
mast, to keep a safe distance.
“In a small boat we try to approach the whales as
stealthily as possible, and then literally we just slap
the tag on the whale,” Friedlaender said. The suction
cups then activate and the tag remains on the animal until
it drops off as programmed.
Friedlaender and colleagues have worked with humpbacks
off the New England coast for the last five years, so
the team is familiar with how the whales move and surface.
Still, their southern cousins are just different enough
to require a bit of a learning curve, he said. “In
the Antarctic, it will take us a little while to get in
a rhythm of how the animals are feeding.
The Gould follows a humpback whale as it navigates the
water around the Antarctic Peninsula.“When it works,
it looks easy. It looks like a baseball swing: When you
see a guy hit a homerun, it looks simple,” he said.
“It’s pretty challenging.”
The team riding in the tagging boat is only one of four
to the Antarctic operation. One team on the Gould is on
point to spot whales, while a second crew aboard ship
uses instruments, such as an echo sounder, to measure
the mass of krill in the whale’s hunting ground.
A fourth team rides in a second Zodiac that follows behind
the whale, also to measure the krill mass left in the
creature’s wake.
If the scientists can determine the amount of krill pre-
and post-gulp by the whale, they can pretty accurately
measure each mouthful, because they already have a good
idea of the size of its gular, or “throat,”
pouch.
The project is something of a follow-on from Friedlaender’s
PhD work on the Southern Ocean Global Ocean Ecosystems
Dynamics (SO GLOBEC) program, one of several regional
initiatives of an international GLOBEC project to understand
the world’s ocean ecosystem. He participated on five
science cruises during 2001-02 aboard the Gould and the
RVIB Nathaniel B. Palmer to study the spatial distribution
of whales — what characteristics would predict the
presence of different species.
“We found some very different things between humpbacks
and minke whales,” he said. For instance, it appears
the humpbacks hunt in more shallow waters while minkes
dive deeper in the ocean to feed, implying that they don’t
conflict over resources. While the upcoming project focuses
on humpbacks, Friedlaender said the scientists also hope
to learn more about minkes, a smaller, more elusive baleen
whale that lives in the Southern Ocean year-round.
“We had the idea that we wanted to do some very
focused studies of the diving behavior and the feeding
behavior of these two whale species, and see if in fact
there is this separation between these two species, [and]
how exactly are they doing it,” he explained.
This April will be Nowacek’s first foray into Antarctic
waters.
“It was a challenging area to go. It was an animal
that we had some expertise on already. We felt we could
bring something new to the table,” he said of the
impetus to leave the familiarity of the northern hemisphere.
“The attraction was working on a cool species and
interesting problem, using a technology that nobody else
could bring to bear.”
Added Friedlaender, “We’re going to have the
prettiest office in the world. There’s not a lot
of places like the Antarctic Peninsula to be able to do
your work. We’re really pretty fortunate to work
in such a cool place.”
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