Tuesday, September 11, 2007

Great white sharks: more here than we knew

Great white sharks are reported in the Hawaiian islands, though rarely.

But new research suggests they may be regular visitors that spend most of their time in deep water around the Islands so they're not noticed. And they've done so for centuries, at least.

Satellite tag data, white shark attacks and sightings all place the sharks in the Hawaiian Islands generally during the first half of the year—no earlier than December, and no later than August.

“It is likely that white sharks have been making these movements for a long time, and there are ancient records of white sharks from Hawaii in native Hawaiian artifacts and history,” wrote researcher Kevin C. Weng, of the University of Hawai'i's School of Ocean and Earth Science and Technology, in an email.

They appear to be white sharks that spend much of their team feeding on the seal and sea lions of the California coast, and which take annual forays into the mid-eastern Pacific—some to Hawai'i and most to other mid-oceanic gathering spots.

Their knowledge of the location of Hawai'i is so clear that satellite tags show that some migrations are fast and straight, from the Baja, California, coast, right to Hawai'i.

The just-published work, “Migration and habitat of white sharks (Carchardodon carcharias) in the eastern Pacific Ocean,” was printed in the journal Marine Biology. Its authors are Weng, along with Andre Boustany, Peter Pyle, Scot Anderson, Adam Brown and Barbara Block.

They report on the satellite tagging of 20 white sharks in the seal rookeries of the Baja Peninsula. Four of those sharks swam to Hawai'i during winter. Others went to other offshore aggregation sites presumably for feeding. The departures coincide with reduced seal and sea lion numbers off California.

Why Hawai'i? The authors in their paper discuss some of the local feeding choices.

“White sharks have been observed near aggregation sites for spinner dolphins on the west side of Oahu, as well as near Hawaiian monk seal colonies on Niihau, and their presence corresponds to the timing of birth for humpback whales, allowing for the possibility of feeding on placentas.

“Sharks may also forage on fishes, sharks and squids while near the islands.”

The sharks that come to Hawai'i don't appear to be of a certain class. They included both male and female, adult and subadult animals.

Why don't see see more of them? Largely because they seem to do their feeding in the deep. The satellite tags suggest that they spend only about 7 percent of their time within 16 feet of the surface.

Based on the limited samples available (4 satellite taggings) the Hawai'i sharks make their moves from the California coast at the same time most of the other white sharks do. They leave when pickings get slim in December or January. And they stay several months—a maximum recorded stay from satellite tagging data of 122 days, although there are reports of white sharks being spotted as late as August..

Anyone who does whale-watching will note that the shark season overlaps the peak of humpback whale season.

Most of the known sightings of great whites in Hawaiian waters have been during the early part of the year, although individuals have reported seeing what they believed were white sharks as late as July and August.

Clearly, the animals have been traveling to Hawai'i for hundreds of years. Some early Hawaiian tools have used the teeth of white sharks for cutting edges.

Two known shark attacks on humans occurred in May 1926, when the remains of William Goins were found inside a 12.5-foot white caught off Kahuku. In March 1969, a white shark bit surfer Licius Lee off Makaha. The animal was identified by its characteristic tooth marks in his surfboard. The area is known for nearby spinner dolphin populations, and there was a dead whale on the beach at about that time.

Hawai'i residents have reported great whites during summer in recent years off Makua, Keawa'ula and Ma'ili. Witnesses said they believe a shark that bit a woman off Ka'anapali in 1999 was a white. And the species has been seen in the waters off northern Ni'ihau repeatedly during the 1990s.

The satellite tagging results confirm the presence of the animals in those regions.

Weng said that there are still several mysteries about the behavior of white sharks, including a suggestion that some females may only return to the California coast every other year.

Does that mean that in places like Hawai'i there might be a year-round great white shark presence? Weng said he can't say, and researchers may have to wait for technology to catch up with the research needs.

Currently available satellite tags don't seem to last more than about 9 months, he said. What's needed is a way to electronically track the sharks for a period of years.

As in much of science, it's a field that demands more research.

© 2007 Jan W. TenBruggencate

1 comment:

TravisHiroshima said...

Fascinating post, Jan. Thanks for keeping us informed about this interesting topic. I remember hearing about a great white sighting maybe about seven years back and thinking, "this stuff doesn't happen at random." Great whites are animals who se very similar ancestors have beaten the odds for a hundred million years or more. They HAVE a migration pattern and probably a highly advanced instinct for opportunistic hunting, which people watching from Hawaii might only get to see once in a long time.