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Scientists Study Nurse Shark Mating Habits |
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Brian Handwerk and Mark R. Holmes for National Geographic News |
| July 4, 2002 |
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Sharks Photo Gallery: Go >> In the remote islands of the Florida Keys, Jeff Carrier and Harold "Wes" Pratt are studying sharks. Again. They've spent much of the past 30 years delving into their underwater world, to gain a better understanding of what Pratt calls "the ultimate form of marine life." It's a never-ending learning process, one Carrier undertakes in the spirit of John Steinbeck, who mused in The Log from the Sea of Cortez: "The true biologist deals with life, with teeming boisterous life, and learns from it. Learns that the first law of life is living." "When we observe nature," Carrier explained, "and all the wondrous aspects of life, the survival strategies that work the best are those that give animals a survival edge. Everything that they do is geared toward giving them the best chance to survive in their habitats, and it's a wondrous thing to see." In the Keys, the ongoing cycle of survival can be seen each year at the site that serves as the breeding grounds of the nurse shark. The sharks return annually to these same grounds, and the researchers return with them. It's Carrier and Pratt's twelfth straight year on the site, where Carrier estimates that he has caught, studied and tagged some 1,500 nurse sharks over the past 30 years. Ancient Breeding Grounds The hard work of these marine biologists is part of a unique, long-term study of the breeding and social habits of these animals. Such lengthy and detailed studies are a rarity in the world of shark research, in which the subjects are not always easy to track down. The Keys-area nurse sharks, however, are easy to locate. Unlike other species, they don't migrate and they prefer shallow coastal waters. They return to the same breeding ground each year, and the scientists follow to study their mating behaviors. According to historical reports, however, the sharks have been around the area much longer than modern science. Breeding nurse sharks were reported in the same Keys mating grounds as early as 1860, and may have been there long before that time. This kind of patterned behavior provides a clue to the challenge of understanding the sharks' mating processes, and the relationship that they have with the habitat itself. "What is it about that site that's so important to the species?" asked Carrier. "It's not just nurse sharks returning to this site, it's the same specific animals, year after year. So there is something enormously important here about site fidelity that we're trying to understand." 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 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 Nationalgeographic.com: 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? Join the National Geographic Society Join the world's largest nonprofit scientific and educational organization, and help further our mission to increase and diffuse knowledge of the world and all that is in it. Membership dues are used to fund exploration and educational projects and members also receive 12 annual issues of the Society's official journal, National Geographic. 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