Scientists may have discovered a pattern that could explain recent shark attacks in Hawaii: The migration of mature female tiger sharks.
According to the International Shark Attack File, in 2012 there were 10 shark attacks near Hawaii (out of a worldwide total of 80). In 2013, eight shark attacks have been reported in Hawaiian waters, including one fatality -- the first human killed by a shark in the Aloha State since 2004. Data going back to 1926 show that the months with the highest numbers of reported attacks are October, November and December.
Now, a new study of tiger shark migration suggests that September and October are high-traffic months for tiger sharks around Hawaii. For the past seven years, University of Florida and University of Hawaii scientists have been closely following the movements of the sharks through the Hawaiian Islands. In a new paper forthcoming in the journal Ecology, the scientists describe the sharks’ curious migration behavior.
“When we think of animal migrations, we tend to think of all individuals in a populations getting up and leaving at the same time, but it’s not as simple as that,” lead author and UF researcher Yannis Papastamatiou said in a statement. With tiger sharks, “some are resident and some are transient.”
Papastamatiou and his colleagues have tagged more than 100 tiger sharks since 2004. Transmitters affixed to the sharks emitted high-frequency noises that were picked up by any one of 143 underwater “listening stations” whenever the animal swam by.
Their tracking data shows that many mature female tiger sharks prefer to head to Hawaii in late summer and fall to give birth (baby tiger sharks develop inside eggs, but the eggs remain inside the mother, and young sharks, called “pups,” emerge from the mother shark after hatching). About a quarter of mature females that normally swim around in the coral atolls of the Northwestern Hawaiian Islands will make journeys of up to 1,500 miles to reach the waters around the more populated Hawaiian islands, according to the researchers.
“Both the timing of this migration and tiger shark [birthing] season coincide with Hawaiian oral traditions suggesting that late summer and fall, when the wiliwili tree blooms, are a period of increased risk of shark bites,” co-author Carl Meyer of the University of Hawaii said in a statement.
But the researchers caution that migration patterns are not the lone explanation for clusters of shark attacks in fall, and that their study focused more on tracking sharks rather than monitoring their interactions with humans. They are also strongly against any official response that to attacks involves killing sharks in areas near incidents, with the goal of removing the "problem" sharks. Their data shows that tiger sharks do not stick to strict territories, the researchers found. A shark that attacks a person won’t necessarily be in the same place a week later.
“The one thing I hope they don’t do is try to initiate a cull as was done in the '60s and '70s. I don’t think it works. There is no measurable reduction in attacks after a cull,” Papastamatiou says.
SOURCE: Papastamatiou et al. “Telemetry and random walk models reveal complex patterns of partial migration in a large marine predator.” Ecology, November 2013.