carries the dart tag while Don Nelson operates the telemetry system.
Part 3 (back to Part 2)
NOVA: How fast do they swim?
Klimley: About a yard per second when they're just cruising. That's kind of a
magic number for most large fish. However, they can move quickly when they want
to. I was once at the surface playing low-frequency sounds over a speaker to
attract sharks. A hammerhead shot up from beneath me, rushing at me at perhaps
30 feet a second before he suddenly turned around. A very fast burst of speed.
Many fish do that sort of thing—they're near the bottom, and they make a
dash to the surface because it minimizes the profile prey have of them. But
that's uncommon behavior among hammerheads. They're not a species that chases
down prey such as white sharks do with seals or makos do with mackerel. Rather,
they're feeding on schools of bioluminescing squid at night, perhaps making
little dashes here and there but not prolonged pursuits.
NOVA: Why do hammerheads school?
Schooling may aid males in choosing a
Klimley: It may have something to do with mating. Hammerhead schools are
primarily formed of females. They range in size from sub-adult to adult, and
there's a dominance hierarchy among them. The largest, physically fit, more
fecund females force the smaller ones out of the center of the school, into
which males dash to find a mate. It's a form of intra-sexual selection that
ensures that males pair with the biggest and baddest females—namely, those
that are going to have the most young. It's amazing to me how structured and
predictable behavior is at times.
NOVA: What do you most want to know about hammerheads that you don't already
Where do hammerheads go when they leave Cocos Island
Klimley: Three things. First, where do they go when they leave the seamount for
good—not just for the night—as they do every year. When do they leave?
How far do they go? Do they have a chronological set of way points at which
they stop? I suspect that they don't just go anywhere. They go to a specific
place at a specific time and then go to the next place at a specific time and
then the next place, and they do this together. I'd be very interested to learn
more about that.
Second, do all those fish that congregate at the seamount with hammerheads
constitute a community? Do they come and go as a group? If they don't, what
determines when each species comes? We need to be better at predicting when
these species come and go, which could be important to fisheries and to
questions such as, "Is the Gulf of California a dying sea?" Last summer, it
looked like a dying sea. But you need to know how abundances of species
fluctuate from year to year based on events like in El Niño before you
can answer such questions.
Though sometimes seen alone, hammerheads
prefer the company of others.
The final thing I'd really like to know is whether hammerheads indeed detect
the seamount by the magnetic field. Years ago I proposed winding a huge wire
coil around the Espiritu Santo seamount. The hammerheads are found just on the
north side of the mount, not on the south side; this is true, in fact, of the
whole pelagic assemblage. I wanted to see if, by reversing the mount's
polarity, the sharks would move to the southern side of the seamount. I never
did it, but if that was so, it would have told me that they're using the
seamount as a landmark, and that they're orienting to a specific part of it—the positive pull or the negative pull of this dipole. Perhaps these fish, and
maybe even birds, are orienting along the magnetic reversal lineations on the
ocean floor. And the way they get to the Arctic and back again is just to get
on that road and go along it.
A pony bottle at the ready, Pete
Klimley readies himself for another plunge.
Photos: (1,3-6,8-12) ©Howard Hall;
(2) ©Flip McKlin; (7) ©Michele Hall; (13) ©Brad Kurtz.
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