Shark Nursery Yields Secrets of Breeding

Brian Handwerk
for National Geographic News
June 5, 2002
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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.

Shark Litters Have Multiple Fathers

"The most interesting and important work [at Bimini] is on the breeding biology of the animals we've caught—young sharks and some adults, including pregnant females," Gruber says. "By using genetic sampling, we've demonstrated unequivocally that a female that carries a litter could have up to four different sires, so ten or 12 different babies might have four different fathers, or two different fathers. So we have multiple paternity, and we suspect that this happens with a lot of other big sharks."

Kevin Feldheim, of the University of Illinois at Chicago, is in charge of the team's DNA sampling. "We use a leather punch," he says, "all I need for the genetic work is a little hole from the fin."

The tiny DNA samples have proven to be a valuable tool for lemon shark research, but initially the genetic code was a tough one to crack. "Sharks' DNA structure is pretty resistant to mutation," Gruber explained, "more so than fishes or humans or plants. Maybe this is why they're cancer-resistant, or why species last for millions of years. We don't know. That's speculation."

What they were able to determine, after several years of painstaking research, was a unique genetic fingerprint for each shark. Feldheim used a genetic marker, called microsatellites, to solve the DNA puzzle.

Female Sharks Return to the Lagoon on a Two-Year Cycle

The DNA breakthrough allowed Gruber's team to determine the parents of each juvenile shark. With this information, they confirmed several hypotheses that were based on Gruber's long experience of tagging, recapturing, and observing the sharks. One is that female lemon sharks return to the same nursery to give birth. The other was that female lemon sharks breed on a two-year cycle, giving birth every other year.

Although genetic work provided hard proof of several of Gruber's hypotheses, it also raised a sticky problem: "With a fish like salmon there are enough numbers that return to the same place to keep a diverse genetic pool," he said. "But with the sharks you don't have the numbers, so with these returning, long-living animals in small populations there should be inbreeding in this population like you couldn't believe—yet the negative results of inbreeding weren't there."

The answer was once again confirmed in the DNA lab, by a genetic examination of the parents of some 900 young sharks. "Usually for males, we see them as parents only once, and not again, although the females we see returning to Bimini every other year." So while the mother sharks are homing to the same nursing grounds, roving males ensure that the population remains genetically diverse.

Although the Bimini population is genetically varied, it's dependent on a surprisingly small number of female sharks. The team was able to reliably estimate the year of birth for 735 young sharks in the sample. Of those, nearly all—708—could be assigned to one of 45 adult females. "When we realized just how few females contribute to this population, we become even more careful with them when we do capture them for study," says Feldheim.

As the study of Bimini's lemon sharks continues, more important discoveries lie on the horizon. "What we're really interested in seeing now through long-term tagging," said Feldheim, "is whether or not individuals born at Bimini actually return to their own birthplace to give birth like salmon do." Such a discovery could have large repercussions for shark management in the area (and elsewhere). The data that solves that puzzle should become available in the next few years, as the young sharks sampled in the mid-1990s begin to reach reproductive age and continue the sharks' cycle of life.

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