Scientists Enlist Fishers to Return Tags on Thresher Sharks

March 16, 2006

Contact: Christina S. Johnson, csjohnson@ucsd.edu, 858-822-5334

To help them gather the information they need, shark biologists are asking fishers to return tags on thresher sharks.

The tags look like a tiny piece of computer hardware [see photo] and are placed on a shark’s fin. Directions to call the shark lab leading the tagging study are inscribed on the tag’s back side.

Tag put on thresher sharks. Photo: Scripps Shark Lab

The tagging data are important because they make it possible for biologists to track shark movements over long periods of time and to correlate their movements with environmental conditions such as ocean water temperature. The only way to retrieve the data, however, is to recapture the tags and download their information to a computer. This is where fishers come in. Since threshers are targeted by commercial and recreational fishers, they are basically the only people who will ever see, or be in a position to remove, the tags.

California Sea Grant is funding the tagging project in the hopes of learning more about the factors controlling thresher sharks' movement patterns. The ultimate goal of the project is to help fisheries biologists have access to the information they need to manage sharks in the Southern California Bight, to reduce accidental takes of juvenile sharks and to learn more about the habitats that support sharks through different stages of their lives. So far, about 70 thresher sharks have been tagged off La Jolla, Calif.

Mike Garret is a sport fisher in Dana Point who recently returned a tag and received the $100 reward for his efforts. "We are as interested as the scientists in learning about sharks but for a different reason," Garrett said, explaining why he returned the tag. "We want to target them more efficiently."

Garret believes better science can help fishers and the resource. Personally, he would like to know more about what sharks eat and how this changes seasonally, to better lure sharks with appropriate bait. "We already know threshers feed on bait fish, but I've also caught them on squid," he said.

Thresher sharks are believed to migrate seasonally, but only part of their migration route is known. Nobody is sure where adults breed. The habitat requirements of juveniles are a mystery, too. There are, in short, many gaps in basic information on sharks.

Tag on a thresher shark's fin. Photo: Scripps Shark Lab

Scripps Institution of Oceanography professor Jeffrey Graham, the lead investigator of the project, and his graduate student Sea Grant Trainee Dan Cartamil, are filling some of these information gaps. They have learned, for instance, that thresher sharks spend about 95 percent of their nocturnal life in waters shallower than 36 feet.

The 36-foot mark is significant because it is also the minimum-depth requirement for drift gill nets. The regulation, designed to create a safe corridor for marine mammals and sea turtles, also offers some protection for thresher sharks, the scientists say. Fishers set their drift gill nets at night, which also reduces shark takes.

If you find a tag, please call Dan Cartamil at the Scripps Shark Lab in La Jolla at (858) 534-8044. More information about the project can be found at http://elasmoworld.org/thresher/