Scientists Track Giant Sunfish by Satellite

By David Braun
National Geographic News
February 2, 2003
"The world's largest bony fish looks like a massive swimming head and is
extremely gentle and curious," says Tierney Thys of the giant ocean
sunfish, a sea creature that can grow to more than 4,000 pounds (1,800
kilograms). "Yet it is an animal we know very little about."

A marine biologist with the California-based Sea Studios Foundation, Thys and her team including Heidi Dewar, Steve Karl, Anna Bass, and Todd Streelman, are on a mission to discover the secrets of the Mola mola.

Found in all tropical and temperate oceans, the Mola mola, or giant ocean sunfish, eats mainly jellyfish. "One of the favorite prey of the sunfish is the moon jelly, so there is a kind of cosmic convergence," says Thys.

At hatching, a sunfish larva is no larger than one-tenth of an inch (2.5 millimeters), but it grows to as much as 14 feet (4.2 meters) from dorsal fin tip to anal fin tip and ten feet (3 meters) in length.

Titanic Fish

If it lives to adulthood, a mola can gain over 60 million times its starting weight, says Thys.

"That's the equivalent of a healthy, bouncing human baby growing to a weight equal to six Titanics," says Thys. The R.M.S. Titanic was the largest moving object ever built before it sank on its maiden voyage in 1912.

Thys is concerned that molas comprise an "alarmingly" large portion of the eastern Pacific drift net fishery by-catch, and that population numbers in the western Pacific are declining.

In a project supported by the National Geographic Society's Committee for Research and Exploration, Monterey Bay Aquarium, the Lindbergh Foundation, and the Taiwan Fisheries Research Institute, Thys and her team are tracking molas using satellite tags. Over a period of several months, each animal's tag will record in two-minute intervals the location, temperature, and depth of the water the animal is traveling through.

Upload to Satellite

At a predetermined time the tags detach, rise to the surface, and upload the accumulated data via satellite directly to the team's computer in California.

From this information, Thys and her team are able to record where each tagged mola travels during the recording period.

The team has tagged three molas in California and two in Taiwan. It plans to tag two more in Taiwan, two in Japan, and one each in South Africa and Australia or New Zealand.

Thys hopes the data will provide the first-ever baseline biological data on molas, offering insights into how they migrate, whether there are interactions between regional populations, and where they are spawning.

"With increasing threat to the molas, it is important to get this data before it is too late," says Thys.

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