Tracking a Great White
Data from an advanced electronic tag reveal a great white shark's movements and behavior
Ever since the 1975 summer blockbuster Jaws, the specter of a great white shark lurking under sparkling blue waves has haunted beachgoers. Part of what makes great white sharks so frightening is that we simply don't know all that much about them.
But researchers at the Monterey Bay Aquarium, in California, this past spring took a big step forward in penetrating the secrets of the great white shark, thanks to the capture of a young female shark last August and a fourth-generation electronic tracking tag.
This adventure started when a commercial halibut fisherman in southern California inadvertently caught the young great white. Monterey Bay Aquarium marine biologists trucked her north and put her in the aquarium's 3.8-million-liter ocean-water tank, where she made history as the first great white shark to survive for an extended period in captivity. By March she had grown to 1.93 meters and was wearing out her welcome, snacking on a few of the smaller sharks that shared her tank. It was time to release her--and find out what she would do after that.
It isn't easy to track a great white. To obtain basic information from large, oceangoing fish like tuna, researchers use tags with microprocessors and flash memory that record various parameters, such as water pressure and light levels. These implanted tags can give some information about fish behavior over a period of time, as long as someone catches the fish and finds and turns in the tag.
But many such tags are never turned in, so information acquired in this way is sketchy at best. And such implanted tags make little or no sense for a fish that is not expected to be recaptured and kept, like a great white shark.
Enter the PAT--for pop-up archival transmitting--tag, developed over the past five or six years by researchers initially interested in more accurate data about the travels of bluefin tuna. PAT tags do not have to be recovered. Rather, after a predetermined time, they break away from the fish and float to the surface, and then transmit data to receivers that are part of the Argos satellite system, an internationally operated satellite location and data collection system dedicated to monitoring the environment.
PAT tags have become successively more sophisticated, and the one that researchers from the Monterey Bay Aquarium and California's Stanford University attached to the great white shark's dorsal fin represents the current state of the art. About the size of a handheld microphone, it has twice as much onboard flash memory as its predecessors (2 megabytes) and incorporates fast-response thermistors to provide new temperature readings every 10 seconds [see photograph, ]. Its maker is Wildlife Computers, in Redmond, Wash.
The shark was released with the tag on 31 March, and on 30 April an electric current passed through the metal pin that attached the tag to a thin plastic line fixed under her skin by a surgical dart. The current caused the pin to quickly corrode and break loose. A bulbous float carried the tag to the surface and oriented the antenna upward; at a scheduled time, it began transmitting to an Argos satellite.
The amount of data that can be transmitted is limited--10 000 packets of 32 bytes each over 7 days--so the microprocessor on board the tag summarized the data and then selected individual days of data to transmit in a random order. (That way, in case of premature battery failure, data transmitted would still be representative of the whole time recorded.)
Because so little information is available about great white sharks, says Randy Kochevar, a marine biologist at the Monterey Bay Aquarium, researchers wanted to have as much data from the tag as possible--more than could be transmitted before the batteries failed. They were able to recover the tag and all the data, including water temperature readings, by homing in on the radio signal it was sending.
From this information, researchers could put together a complex picture of the shark's behavior. Computers at Stanford's Hopkins Marine Station, in Pacific Grove, Calif., working from sunrise and sunset times derived from the light sensor, could estimate the shark's location each day and map her travels--an odd journey that took the shark far out to sea before she turned to head south to her home waters. Researchers theorized that she was choosing warmer waters.
The experiment was, in the eyes of the researchers, a resounding success. In the first place, says Kochevar, "since this was the first time anyone kept a white shark in captivity for so long, the mere fact that she went back into the ocean and thrived was a great relief to us all."
Especially intriguing is the prospect of using PAT tags not only to gather information about fish behavior but also to penetrate the mysteries of the ocean itself. "One of the things we began realizing through this project was that marine animals can obtain data that could otherwise cost millions of dollars to get," says Kochevar.
Take the elephant seal. "One seal dives to 600 or more meters 60 times a day throughout the North Pacific," he says. "If you have a tag on this animal, you can get a temperature profile of the ocean. Measure salinity, too, and you would have a full-featured oceanographic profile."
The Monterey Bay Aquarium is working with the Sea Mammal Research Unit of the University of St. Andrews, in Scotland, to develop a temperature and salinity tag. Meanwhile, the great white shark, free of her PAT tag, wanders the Pacific in secret, bearing only a thin plastic strip with a number to identify her in the unlikely event that she is ever recaptured. Monterey Bay Aquarium marine biologists are out there roaming the ocean, too, in search of a second great white to recruit for their research tracking program.
--Tekla S. Perry