Shark sense

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3 researchers tracking the predators find they have favorite spots to fill their jaws

Tiger sharks memorize where they've scored consistently good meals and find their way back there across vast expanses of ocean, Hawaii researchers have found.

Unlike bears, which can teach each other where to find food, sharks apparently don't share that information, the scientists said. Instead, each shark learns on its own where to find food and incorporates that information into a "cognitive map," a kind of personal GPS navigator.

Beginning in May 2006, University of Hawaii marine biologists Carl Meyer, Kim Holland and Yannis Papastamatiou tracked five tiger sharks and three Galapagos sharks around in the Northwestern Hawaiian Islands using acoustic and satellite transmitters.

The results, published online May 5 by the journal Marine Biology, showed that one popular dining site is tiny islands at French Frigate Shoals where albatross eggs hatch in late spring and early summer.

"The precise, seasonal arrival of certain tiger sharks at (French Frigate) in time for albatross fledgling indicates these sharks may also use internal clocks to guide their movements," the research paper says.

On voyages aboard the Hi'ialakai, a ship operated by the National Oceanic and Atmospheric Administration, the scientists caught sharks using hooks baited with tuna heads. From a 20-foot skiff, they roped each shark's tail and flipped it belly up, which places the predator into a docile mode known as "tonic immobility."

They then installed three types of trackers:

» An acoustic transmitter surgically implanted in the shark's belly that emits periodic pings to identify each shark.

» A position-indicating radio transmitter, bolted to the dorsal fin, that communicates with a satellite when the fin breaks the surface.

» A tag attached to the hide that records temperature, depth and light intensity. It is programmed to later detach and pop to the surface and upload its data to a satellite.

The installation process took between 30 and 45 minutes, after which the hook was removed and the shark released.

At French Frigate Shoals, 560 miles west-northwest of Honolulu, the sharks were attracted not only to fledgling albatross but also to green sea turtles and monk seals, species under federal protection, the study found.

The satellite tracks showed that several tiger sharks navigated over great distances between rich sources of food. Two sharks in particular made "highly directional movements"—beelines—between submerged banks and seamounts, the study found.

"This behavior indicates these sharks knew the locations of the bathymetric features from previous experience and were navigating between them."

In contrast, Galapagos sharks found at French Frigate Shoals never strayed far from home.