Hawai'i's tiger sharks roam large expanses and make brief, infrequent visits to shallow coastal sites used by swimmers and surfers, according to a new study.

Their wide-ranging movements and long absences between quick visits to a location may be a hunting strategy that prevents prey from anticipating when tiger sharks will appear, said Carl Meyer, a biologist with the Hawai'i Institute for Marine Biology.

"After arriving at a reef site, tiger sharks probably have only a short window in which to successfully ambush prey because potential quarry soon detect the shark and evade capture. Ranging over wide areas and avoiding predictable patterns of behavior may help tiger sharks to retain the element of surprise while hunting," he said.

For years, tiger sharks were thought to be territorial, prowling limited areas. Previous studies have shown they swim long distances, even across oceans, but Meyer's nearly four-year study is the first long-term examination of their movement patterns.

"There are still a lot of people who think that tiger sharks live in fairly small areas, perhaps along one reef. We have quite clearly shown this isn't the case," he said. "These sharks are wide-ranging animals, and their pattern of behavior is such that they move continually."

The research, funded by the state Department of Land and Natural Resources's Division of Aquatic Resources, also calls into question the oft-repeated advice that staying out of the ocean at dusk and dawn can reduce the risk of shark attack. Meyer's study found little evidence linking the risk of encountering a tiger shark to the time of day. The transmitter-equipped tiger sharks in his study were detected inshore at all times of the day and night.

In fact, 60 percent of shark attacks in Hawai'i occur between 10 a.m. and 4 p.m., when more people are in the water.

"The evidence suggests that it doesn't make any difference. When you look at when people are getting bitten by sharks, there's no reason to be particularly concerned about (the dawn and dusk hours) based on what we've seen of tiger shark movements," he said.

George Burgess, director of the International Shark Attack File at the Florida Museum of Natural History, said whether it's prudent to avoid the ocean at dawn and dusk "depends on the shark and on the circumstances. That advice is given as general advice for sharks across the board, and it should be tailored to a particular area."

In any event, it may be wise to avoid swimming at times when visibility is low, he said.

"In most areas of the world, shark fishermen fish at night. That's the best time because the sharks are more active," he said. "Tiger sharks feed by day and are happy to feed by night, when they have a competitive advantage. They're professional predators, and we're amateur invaders."

OFTEN IMPLICATED

Tiger sharks, which can grow to 18 feet long and have distinctive blunt snouts and vertical bars on their sides, are most frequently implicated in shark-bite incidents in Hawai'i. They have been responsible for at least 20 of the 44 shark attacks in the state over the past 10 years, according to the Division of Aquatic Resources.

Meyer and Burgess emphasize the risk of attack remains extremely low despite the occasional presence of large sharks at popular beaches and bays.

"We periodically detected large tiger sharks in shallow waters at swimming beaches during the day. These sharks were apparently unseen by ocean users and there was no interaction with any people present. People who regularly swim, surf or dive in Hawaiian waters have probably been close to a large tiger shark without knowing it," Meyer said.

The shark movement patterns also reinforce the conclusions of previous studies that culling programs are unlikely to be effective in either catching sharks responsible for attacks or reducing an already low attack risk.

Meyer and his team tracked 15 sharks using an array of 61 acoustic receivers installed on the ocean floor around the islands of O'ahu, Maui, Kaho'olawe and Hawai'i. The sharks were hooked and brought up alongside the team's vessel, where they were turned on their backs, putting them into a trancelike state. Small ultrasound transmitters were surgically implanted through a small incision in the abdominal wall before the sharks were released. The entire process took less than 20 minutes, Meyer said.

The listening devices recorded the dates and times of visits by the tagged tiger sharks and were retrieved by divers at six-month intervals from December 2003 to June 2007. The information they contained was used to plot shark movements.

Meyer said shark visits to specific acoustic receiver sites were typically brief — averaging about three minutes in duration — unpredictable and interspersed by absences of weeks, months or even years.

Most coral reef predators, including Galapagos and gray reef sharks and ulua, have very predictable patterns of behavior and often use the same day and night habitats for years, he said. Tiger sharks may move on soon after arriving in an area because reef fish, turtles and other prey become wary and difficult to catch.

COASTAL PATROLLING

Most of the sharks in the study exhibited periods of coastal patrolling behavior, swimming back and forth along 10 to 70 miles of West Hawai'i coastline, interspersed with absences from the listening array.

However, three of the sharks spent up to several days in the vicinity of Honokohau Harbor in Kona on repeat visits. Meyer said that's probably because the dumping of fish carcasses into the harbor switched the animals from wider-ranging movement patterns to scavenging behavior associated with a predictable source of food.

He advised that harbor users might want to avoid rewarding tiger sharks for visiting sites used for ocean recreation.

The predators also made rare, brief appearances at other popular ocean recreation spots, including Kealakekua Bay and the Kona Coast State Park on the Big Island, Honolua Bay on Maui, and Kane'ohe Bay on O'ahu.

The study notes that tiger sharks are known to switch foraging strategies to take advantage of seasonally abundant resources or inexperienced prey. Each summer in the Northwestern Hawaiian Islands, for example, tiger sharks temporarily cease their wide-ranging behavior to congregate around sandy islets to gorge on fledgling albatrosses.

Although the study provided new insight into the long-term movements of tiger sharks, Meyer said, much remains to be discovered about these elusive predators.

"Although we now have a much better understanding of tiger shark movements in Hawaiian waters, we still have many unanswered questions about their basic biology. We still need to figure out how frequently they capture prey, where they go to breed and how often the tiger sharks found in Hawai'i travel to other areas of the Pacific," he said.

Other researchers involved in the study, published April 17 in the journal Marine Ecology Progress Series, are Timothy Clark, Yannis Papastamatiou, Nicholas Whitney and Kim Holland.