Researchers Tag Sharks to Study Breeding Habits

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"It's a waiting game, but just observing while you're there is critical," says Carrier. "We do things such as water chemistry analysis and try to figure out from that standpoint what might be attracting the animals."

Once the sharks begin to arrive, the crew springs into action. The female sharks arrive first, entering the lagoon at a languid pace, and then wandering off alone into the shallows. The researchers carefully note where the females rest, and what they do in the cove.

The male sharks enter the cove with considerably more speed, driven by their single-minded drive to mate.

The male's first mating move is to bite and grab the female's pectoral fin, providing a cue to the research team that it's time for action. The researchers jump into their kayaks and race over to the site for an up-close and personal look at shark mating. The sharks aren't shy, and don't seem to mind the audience.

The exertions of mating leave both sharks listless and exhausted, providing the researchers their best opportunity for capturing their quarry.

Grabbing a Handful of Shark

Capturing live sharks is not easy under any circumstances. Doing so without bait and hooks is even more difficult. The strategy Pratt and Carrier employ is both simple and daunting. They go into the water and net the animals by hand. Pratt developed the hoop nets—think glorified butterfly net—based on a model he saw used at SeaWorld.

Even in their satiated state, the large sharks can put up a pretty good fight. It takes a team effort to entice the larger specimens into a hoop net.

"We back each other up," Carrier explained. "The bottom line is that we're studying sharks, so with an eight- or nine-foot [2.5-meter] animal, you want to have someone watching your back."

Even then, sometimes there are complications. Occasionally sharks will bite a kayak and not let go; they could just as easily latch onto a researcher.

"One of the things we've learned from the sharks is that turnabout is fair play," said Carrier, "and sometimes you get bit."

Shark Out of Water

Once a shark has been netted, the data collection process begins.

Nurse sharks are hardy animals and the smaller ones can be safely kept out of the water for about 15 minutes, as long as someone occasionally irrigates their gills with water and helps to support their body weight.

Still, the team moves deliberately. First, they check to see whether the shark already has an identification tag. The 12-year study has been so successful that most of the sharks have been captured at least once before. Many are captured nearly every year. The ability to collect annual data on the sharks is especially important to the study.

Next, some sharks are outfitted with a "pinger," a transponder that emits a signal once a minute. Data-retrieving monitors placed around the lagoon record the signals, allowing the researchers to track the sharks and see who is going where, and with whom. The pingers transmit signals year-round, allowing the scientists to collect data even after they have returned home.

If a shark doesn't already have a "PIT" tag—a pet identification tag that is often used on dogs and cats—it will soon receive one. The tags are about the size of a grain of rice, and are placed under the skin with a hypodermic needle. The information contained in the PIT tag can be read using a device much like the bar code scanner at your local grocery store.

After a quick mouth-check for fishing hooks and parasites, the sharks are measured. They range in size from 1.5 feet (0.5 meters) to up to nearly 10 feet (3 meters) long. The researchers weigh each shark by hoisting it in a net. The growth data collected from these measurements, often gathered from the same sharks year after year, is one of the uniquely valuable results of the long-term study.

Ed Heist then collects a small piece of the dorsal fin. Back in the lab, he analyzes the sharks' DNA to determine patterns of shark paternity and population genetics.

A few sharks are fitted with a Crittercam, a video recording device that allows the researchers to watch the sharks in their natural environment. Mike Heithaus can attach the camera to the animal's dorsal fin in about 20 seconds. It's designed to release at a designated time, and can be retrieved using radio transmitter tracking.

Finally, the sharks are released, unharmed, to roam the tropical waters of the Keys until their next annual appointment with the researchers.

The tremendous amount of data Pratt and Carrier gather each year is of vital importance to furthering our understanding of sharks, and by extension, the vast ocean ecosystems they inhabit. But while Carrier is tireless in his pursuit of information, there is one line he's unwilling to cross—killing a shark for research purposes.

"I'm a physiologist, so I study the processes that keep animals alive and healthy," he explained. "While I know there are data to be gained from sacrificing animals, these animals in our study site area are part of the family. Once they are sacrificed for a study, we'll never see them again.

"Anyway," he added, "There is enough killing of sharks."

That is something of an understatement. Peter Benchley, in his new book Shark Trouble, noted one of the grimmer shark statistics: For every human being killed by a shark, roughly ten million sharks are killed by humans.

Follow the progress of this National Geographic expedition to the Florida Keys:
Expedition Report One: Scientists Study Nurse Shark Mating Habits
Expedition Report Three: Crittercams Provide Insights into Nurse Shark Behavior

National Geographic Shark Resources

News Stories:

Jaws Author Peter Benchley Talks Sharks
Do Hammerheads Follow Magnetic Highways in Migration?
Shark Nursery Yields Secrets of Breeding
South Africa Rethinks Use of Shark Nets
Sharks Falling Prey to Humans' Appetites
Satellites Clear Up White Shark Mysteries
Are People Eating Sharks Out of Existence?

Shark Sites on

Creature Feature: Great White Sharks
Ten Cool Things That You Didn't Know About Great White Sharks
Print 'N' Go Coloring Book: Great White Sharks
Shark Surfari: Online Quiz

Related Lesson Plans:

Use this National Geographic News article in your classroom with these Xpeditions lesson plans:

Lesson Plan: A Trip to the Beach
Lesson Plan: Are Sharks As Dangerous As We Think They Are?
Lesson Plan: Does the Hammer Help?
Lesson Plan: Sharks—Setting the Record Straight
Lesson Plan: Sharks—Should They Be Afraid of Us?
Lesson Plan: What's the Hammer For?

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