Do Hammerheads Follow Magnetic Highways in Migration?

Brian Handwerk
for National Geographic News
June 6, 2002
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As a young researcher, Peter Klimley decided that it was not only safe but also important to dive freely among hammerhead sharks to acquire a better understanding of the magnificent animals. "I believe you can learn about 80 percent of what's going on in the natural world by observation," he said in a recent interview.

Some people feared disastrous consequences of such a venture. Instead, it put Klimley on the path to becoming a leading shark expert.

Klimley, of the Department of Wildlife, Fish, and Conservation Biology at the University of California–Davis, has been studying hammerheads and their behavior for more than 20 years. His observations of their elaborate social rituals and communication have considerably expanded knowledge of one of the most distinctive sharks.

There are nine species of hammerheads, which range from three to 20 feet (one to six meters) in length. They are found in the tropical and subtropical waters of oceans around the world. Klimley's current research project focuses on hammerhead sharks' little-known migration habits—knowledge that's important for the conservation of the sharks, and perhaps other marine species.

He is looking closely at the curious tendency of hammerheads, as well as other organisms, to school in large groups around underwater mountains. "Since the mid-80s," said Klimley, "I've been arguing that there is a whole assemblage of species that move north and south via stepping stones," or seamounts.

Underwater Schooling

For more than 20 years Klimley has been studying the behavior of scalloped hammerheads at underwater mountains, or seamounts, such as El Baho Espiritu Santo in the Gulf of California. There, he has observed breathtaking numbers of the sharks.

Why they create such a striking spectacle is not completely understood.

One thing that seems clear is that the sharks are not gathering at such locations because seamounts are a source of abundant food. In fact, the hammerheads gather at the seamount during the day but feed elsewhere at night. They leave the area at night—alone or in small groups—and spread out through the ocean for miles to feed on fish and squid.

Using ultrasonic telemetry, Klimley has tracked this feeding behavior. His research showed that at a certain time in the early morning, the sharks return to the seamount, generally following the same paths with remarkable regularity. They seem to use the underwater mountain as a kind of base.

Klimley thinks that the gathering of hammerheads around a seamount and the sharks' movements in the waters beyond may be related to their response to magnetic fields, made possible by the presence of electro-receptors at the bottom of their uniquely shaped heads.

If the idea proves to be valid, it could provide a big piece of the puzzle in the mystery of hammerhead shark migration.

Seamounts as "Stepping Stones"?

When basalt, secreted from Earth's crust, hardens on the ocean floor, it creates magnetic bands of varying intensity. Underwater mountains, which are usually volcanic, often have magnetic dipoles.

Years ago Klimley observed that hammerheads congregating in large numbers at a seamount always stayed on one side of the formation. Although they traveled up to 10 miles (16 kilometers) away at night in search of food, they did not go to the other side of the mountain.

Klimley wondered whether the behavior was related to the magnetic polarity of a seamount.

To test the idea, "I even proposed one time to take a 12-gauge wire and coil it around the mount and attach it to two 24-volt truck batteries and reverse the polarity of the seamount," he recalled. That experiment was never conducted, but Klimley's interest in exploring magnetism and marine life has continued.

He speculates that when hammerheads leave each night to feed, they follow the magnetic patterns, using them almost like roads to guide their travel. Thus, a seamount might serve as a kind of magnetic landmark for marine organisms in what appears to be a featureless deep ocean.

Similarly, when it comes time for the sharks to continue their migration and leave their schooling sites near seamounts each year, the animals may follow magnetic "highways" across the ocean floor—swimming from landmark to landmark using the seamounts as "stepping stones."

Toward Marine Protection

Although Klimley's research focuses on hammerhead sharks, other marine species may exhibit similar patterns of behavior.

"We can use the hammerhead to answer a bigger question, relevant to lots of species," said Klimley. "The hammerhead is like a proxy for a whole group. This one species, the hammerhead, I've associated with many of these other species based on some past evidence."

Over the next two years, Klimley and his team plan to tag hammerhead sharks with a variety of electronic tags to get a clearer picture of the animals' movements. "We hope to find out over a period of two years when they're there and when they're not there," he said.

Adult sharks at the seamount, for example, will be tagged with "pop-up" archival tags and coded ultrasonic beacons. The tags release from the sharks after a year, providing information about the sharks' daily positions.

Another type of tag—a beacon with a unique signalling tone that exceeds the hearing range of both humans and sharks—will be attached to hammerheads and recorded by listening devices moored at each end of the seamount ridge.

Both these types of tags will be applied by diving down into the groups of sharks and applying the tags from the end of a pole spear.

A third type of tag, a data-storage or archival tag, will be applied to juvenile hammerhead sharks caught in nets close to shore. These tags, which can record data for as long as five to seven years, provide not only a record of a sharks daily positions but a continuous record of the sharks depth and surrounding water temperatures—thus, more accurate positions.

The tracking information, Klimley explained, "can be superimposed on a chart of the eastern Pacific to determine whether the sharks are moving from seamount to seamount." If that is shown to happen, he added, "then we can argue that some parks might be made in those areas."

The findings could be crucial to the establishment of effective protective zones for hammerheads and other species.

The sharks face population pressures, and Klimley has seen the results firsthand. "All these islands in the Gulf of California," he recalled, "back in the early 1980s I studied hammerheads and there was a great abundance of them there. Since then I've done many studies there of social behavior, migration, and other topics. In the last few years I've seen fewer and fewer sharks."

"Like many of the other species, they're disappearing," he said, "so there's concern about how we manage these species."

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