Scientists Study Nurse Shark Mating Habits

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Nurse sharks are among the most common shark species. Many divers are likely to encounter one, Carrier explained, and when they do it's likely to be a very powerful and unforgettable experience. Because nurse sharks are not prone to attacks on humans, it's also likely to be a pleasant experience.

These characteristics make the sharks more easily available for study, and are quite conducive to the researchers' minimal impact approach. Pratt explained, "Nurse sharks are accessible, and we can interact with them more naturally. You can go one-on-one with an eight- to nine-foot [2.5 meter] nurse shark with a net. You're in their environment; it's a pure form of hunting."

The environment of the breeding grounds is also uniquely well suited to the study of these animals and their natural breeding behavior. Worldwide, not many areas exist where shark mating can be observed. Just locating the sites can be extremely difficult, and water visibility and depth create other problems for observers.

In the Keys, the shallow waters of the breeding grounds allow researchers to clearly study shark behavior in an area resembling a large, natural aquarium.

"We all try to be as unobtrusive as possible," Carrier explained. "If we can keep our impact to a minimum it works to the animals' benefit, and we also learn more about their natural behavior."

The sharks are observed, and then carefully netted. Researchers measure and tag each shark and take DNA samples before releasing them unharmed. "It's important for us to get them tagged, and important to get DNA samples so that we can better understand relatedness, social hierarchies, and interactions. We're finding out who hangs out with whom, who mates with whom."

DNA data will help create a clear picture of the area's nurse shark population and its dynamics. That's important for the nurse shark, and perhaps for the understanding of other shark species as well, because nurse sharks are not subject to the human pressures threatening other sharks.

"With the nurse shark, we have an animal to study that's not being commercially exploited," says Carrier. "If populations fluctuate it's probably for natural reasons, so it's almost like a control species in comparison with other types of sharks who are subject to commercial pressures."

Shark's-Eye View

Jeff and Wes have always attempted to observe the animals' natural behavior, as free as possible from human interaction. To push the boundaries of this type of observation, they've recently adopted a device called Crittercam, in collaboration with National Geographic's Remote Imaging Laboratory.

Crittercam is a video camcorder and data collection device that can be attached to wild animals. The video provides researchers, and the rest of us, with an amazing viewpoint—that of the animal itself as it ranges freely in the ocean. Crittercam has been used on 28 different species including whales, seals, sea lions, turtles, and penguins. The scientific data collection protocols monitor depth, speed, light levels, and other factors, adding valuable data to support the behavior observed on video.

Crittercam will be used to examine breeding habits outside of the breeding grounds. The breeding ground site is in the shallows, where it is difficult for male sharks to initiate mating. Researchers speculate that females use this environment to prevent unsuitable males from mating with them, and to select more desirable partners.

Crittercam will test this theory by observing whether deep-water mating occurs and if it is different than that in the shallows, and if so, how. It may also be able to suggest if the presence of the scientists themselves is having any impact on breeding habits within the grounds, by recording similar behavior in the deep ocean for later comparison to shallow-water mating events captured by conventional video.

Though they have learned much over the years, the new season finds the scientists hard at work once again, faced with a seemingly endless array of new challenges. "There are lots of questions, Pratt explained. "It's a real frontier here…It's hard to capture in numbers what's so significant about this study."

To explain the significance of their work, Jeff and Wes often go beyond the numbers to communicate to people just how important these fish are. Sharks are the apex predators of the ocean ecosystems, the lions and tigers of the sea. It's impossible to conserve, or understand, these vast ecosystems without knowing more about the sharks that sit atop their food chains.

That's why, while in the field, Carrier and Pratt strive to take any opportunity to educate the public about the importance of sharks and our responsibility to them. "We're not shy and we're not quiet," Carrier said. "If we have a live animal at the dock we're going to be talking to fishermen, kids, tourists, all the people there trying to get them involved and interested."

In the long run, public interest may be the most effective tool for helping sharks and other marine animals, and Carrier feels it's best created by engaged participation which creates a sense of ownership. "If I have a kid or a parent on the dock with a live shark that they helped put a tag on, their attitudes change drastically," he said. "I've seen it happen, and the transformation is miraculous."

Follow the progress of this National Geographic expedition to the Florida Keys:
Expedition Report Two: Researchers Tag Sharks to Study Breeding Habits
Expedition Report Three: Crittercams Provide Insights into Nurse Shark Behavior

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