Monday | December 11, 2017
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Study: Female tiger sharks migrate to main islands for pupping

A recent University of Hawaii and University of Florida study backs up what Hawaiians knew for centuries about sharks.

Hawaiian oral tradition held that the number of sharks in waters around the main Hawaiian Islands increased in late summer and early fall. The study, which will be published in the November issue of Ecological Society of America’s journal Ecology, tracked tiger sharks for seven years, noting where the animals moved between the Northwestern Hawaiian Islands and the main islands.

“It’s not surprising to me when we find overlap between our scientific research” and what Hawaiians recorded through their oral traditions, University of Hawaii at Manoa’s Hawaii Institute of Marine Biology professor Carl Meyer said.

The oral tradition to which the study refers is a fairly commonly repeated one regarding sharks, Meyer said. Tradition told Hawaiians that when the wiliwili tree bloomed, they were entering a period of increased risk of shark bites.

Tracking exact population counts for tiger sharks has proved, thus far, to be virtually impossible. Meyer attributed that to the sharks’ transient nature, moving from island to island, or just circulating from one area of an island to another. What researchers were able to do, using tags that provided signals and data for about three years each, was see when a particular shark swam close to an acoustic station, or listening station. That gave researchers data about that particular shark and where it swam over a long-term period.

“This study has shown that a quarter or so (of female tiger sharks) are making this run to the main Hawaiian Islands,” Meyer said.

Scientists already knew that about a third of mature female tiger sharks give birth each year, and noted the female sharks were making the trip during months scientists know coincide with pupping season.

“What we don’t know is whether that represents in the main Hawaiian Islands a doubling of female tiger sharks or just a drop in the bucket,” Meyer added.

Scientists hypothesized in their paper about some of the reasons the female sharks could be coming here. The Northwest Hawaiian Islands are primarily low atolls, very dry, very exposed to wave energy, Meyer said. The main islands are much larger, wetter, cooler and the surrounding waters are very different, he added.

The draw may not even be something attracting the female sharks to the main islands, either, he said. it could be that pregnant tiger sharks are attracted to the Northwestern Hawaiian Islands for some reason that appeals to the animals while gestating.

“We don’t know specifically what it is,” he said.

Yannis Papastamatiou of the University of Florida, the first author on the paper, said in a press release the study also shows that tiger sharks are transient, despite some popular beliefs that the animals become residents of a particular cove or coastal area. He referenced efforts from the 1960s and 1970s, in which officials attempted to curtail shark attacks by killing sharks thought to have established a territory.

“I don’t think it works,” Papastamatiou said. “There is no measurable reduction in attacks after a cull.”

Meyer is in charge of another shark study, this one commissioned last by the Department of Land and Natural Resources to investigate tiger shark movements around the islands. The state has recorded an increase in shark attacks in recent months. Meyer said while late summer and early fall may present a slightly higher risk for shark attacks, the risk is still very low overall.

“People get bitten at all times of year, but throughout the risk is extremely low,” he said. “What do we do to mitigate an already low risk of shark bites?”