The migration patterns of tiger sharks offer an insight into the attacks that took place in Hawaii last month, researchers have said.
So far this year, eight people have been attacked by sharks in Hawaii. Jana Lutteropp, from Germany, died a week after her arm was bitten off by a shark while snorkelling off Palauea Beach in Makena, Maui County.
Just days later, 16-year-old surfer Jimmy Napeahi was attacked and suffered from severe leg injuries.
Researchers at the University of Florida and the University of Hawaii analysed the tiger shark movements over seven years, examining migration patterns rather than human-shark interactions.
They found male and female tiger sharks have very different migration patterns, with males moving little between islands.
However, the females travelled much further, with a quarter making their way from the remote French Frigate Shoals atoll to the main Hawaiian Islands during late summer and early autumn.
Data shows that since 1926, most shark attacks in Hawaii take place during October, November and December.
Spike in shark bites
The tiger shark, which can reach up to five metres in length, is one of the most dangerous breeds to humans as they often swim in shallow reefs and harbours, increasing the chance of encountering people. They also have one of the broadest diets of all shark species.
Yannis Papastamatiou, a marine biologist from the University of Florida, said: "We have previously analysed data to see which sharks are hanging around shark tours with cage divers on Oahu, and one of the things we noticed was that you'd get a spike in how many tiger sharks are seen in October, which would match our predicted model that you're having an influx of big, pregnant females coming from the north-western Hawaiian Islands.
"There even tends to be a spike in the number of shark bites that occur during that season. We knew tiger sharks had fairly complicated movement patterns and it seemed to be sort of a free-for-all.
"Once we looked at data for the full seven years and used the right analysis, everything started to make sense. Now we have a much better understanding of the migration patterns of these sharks."
As well as pregnant females, they found individual sharks might also swim to other islands because of food availability and better thermal environments.
"When you think of migration patterns, you think of all the animals in a population getting up at a certain time and migrating somewhere else, and then they all come back together - so everybody does the same thing. But that's actually rarely how it happens," Papastamatiou said.