Hakai Magazine

Coastal science and societies

Scalloped hammerhead sharks are highly migratory, so identifying nursery sites is particularly important to efforts to protect this endangered species. Photo by Stephen Frink Collection/Alamy Stock Photo
Scalloped hammerhead sharks are highly migratory, so identifying nursery sites is particularly important to efforts to protect this endangered species. Photo by Stephen Frink Collection/Alamy Stock Photo

Scientists Discover a Likely Hammerhead Shark Nursery

Endangered scalloped hammerhead sharks are thriving in Fiji’s Rewa River.

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by Nik Hubbard

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Nursery sites in coastal waters are a haven for young sharks, a place of relatively abundant food and safety away from larger predators of the open seas. Fiji’s Rewa River could be one such haven. According to new research, the lower reaches of the Rewa may be a vital nursery for endangered scalloped hammerhead sharks.

Fishers living along the length of the Rewa were the first to bring the presence of young hammerheads to the attention of Kelly Brown, a marine conservationist at the University of the South Pacific in Suva, Fiji.

“I started thinking about the estuary potentially being a nursery for hammerhead sharks, and I was hooked,” says Brown. “I wanted to research this.”

Scalloped hammerhead sharks live throughout the world’s oceans, with subpopulations everywhere from the Gulf of Mexico to the Indian Ocean and the North Atlantic to the South Pacific. They are a rare example of a shark species that spends a great deal of time in large schools. Why hammerheads adopt a social lifestyle remains something of a mystery, though hypotheses are plentiful: it could have to do with adolescent socializing, a possible prelude to mating; or simply grouping up near rich food sources. Or, it could be safety in numbers.

This schooling behavior also means the sharks are particularly vulnerable to fishing. By being in such close proximity to each other, large groups of hammerheads can become ensnared by the same fishing gear, or can be more easily targeted en masse. Accidental by-catch and the fin trade are the two largest threats they face.

Like all sharks, scalloped hammerheads do not care for their young—the sharks are born with everything they need for a predatory life. But to give their young something of a head start, hammerheads lay large litters of up to 30 shark pups in particularly habitable coastal areas. These locations, known as nurseries, are consistently and faithfully used as natal sites, and they are an essential place for young scalloped hammerheads to develop and mature.

If hammerheads are indeed using Rewa as a nursery, confirming it would be fantastic news for those working to protect the embattled shark, and so Brown and his team set to work.

Often working in strong winds after sundown, Brown and his colleagues captured, examined, and subsequently released more than 80 nocturnal baby hammerheads, many less than two weeks old. Their findings indicated a healthy population of young sharks living and feeding in the estuary. While they couldn’t confirm that the area is an established nursery—confirmation would also require the observation of pregnant females visiting the area to give birth—the discovery of so many juveniles is strong evidence that the lower Rewa is likely a nursery for scalloped hammerheads, and the first known in Fijian waters.

“There is potential for more research, not only in the Rewa River estuary but in other major river estuaries in Fiji to find out if these are nursery areas,” says Brown. “I ultimately hope that we can protect these areas to allow young scalloped hammerhead sharks to have a fighting chance to make it to adulthood.”