NOVA Online | Island of the Sharks

Klimley carries the dart tag while Don Nelson operates the telemetry system.

Dr. Hammerhead
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.

Schooling may aid males in choosing a mate.

NOVA: Why do hammerheads school?

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.

Where do hammerheads go when they leave Cocos Island for good?

NOVA: What do you most want to know about hammerheads that you don't already know?

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.

Though sometimes seen alone, hammerheads prefer the company of others.

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.

A pony bottle at the ready, Pete Klimley readies himself for another plunge.

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.

Photos: (1,3-6,8-12) ©Howard Hall; (2) ©Flip McKlin; (7) ©Michele Hall; (13) ©Brad Kurtz.

Cocos Island | Sharkmasters | World of Sharks | Dispatches
E-mail | Resources | Site Map | Sharks Home