Shark Nursery Yields Secrets of Breeding

Brian Handwerk
for National Geographic News
June 5, 2002

Sharks Photo Essay: Go >>

Each year, as spring turns into summer, hundreds of juvenile lemon sharks fill a mangrove-fringed lagoon in Bimini, Bahamas. Wading among them, you'll find intrepid shark researchers Samuel "Sonny" Gruber and his team.

These shark specialists return annually to the shallow-water nursery to conduct a unique long-term study of the area's relatively undisturbed shark population. It is perhaps the most extensive study of any shark species ever undertaken, and has uncovered important information about the ocean's great predators and how they reproduce.

Despite the growing interest in the importance of sharks to marine ecosystems, surprisingly little is known about their life habits. That's why Gruber, considered to be one of the world's top shark researchers, and his team return to the lagoon nursery each year.

Gruber (sometimes known as "Doc") is a marine biologist at the University of Miami and has studied Bimini's lemon sharks for decades. The particular lagoon in Bimini, part of a relatively uninhabited island about 50 miles (80 kilometers) east of Miami, is a sanctuary for baby lemon sharks.

Lemon sharks are large coastal sharks found in the western Atlantic from New Jersey to Brazil. They are also found along the West African coast, and in the Pacific from Baja California to Colombia. They are members of the family Carcharhinidae, which includes over 50 species of large, active sharks.

While it's not possible to generalize across species, research on lemons may be useful for understanding a much broader variety of sharks. Information on breeding and populations is crucial to the creation of management and protection plans for these threatened ocean animals.

Lagoon Yields Hundreds of Sharks for Study

At Bimini, the sheltered, shallow water lagoon environment provides predator protection and abundant food for juvenile lemon sharks, which may remain in the lagoon for several years. It also allows the research team to capture them at an extremely high rate, possibly over 95 percent of the nursery's population. "It's a small island and they attach close to the mangroves, which they don't seem to do elsewhere," says Gruber. "So we can get high rates of return for those who survive, whereas elsewhere 5 percent might be a good number."

By fishing with nets strung about the lagoon, Gruber and his team carefully measure, weigh, and tag each shark. They also take DNA samples before returning all the young sharks to the lagoon. By the end of the expedition they manage to sample nearly all sharks under two years old and some of the three-year-olds—about 200 sharks in all.

Such an extensive population sample has allowed for some significant research breakthroughs on reproductive habits, which is particularly important information for threatened and slow-breeding species like sharks.

Continued on Next Page >>


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