Minimum-Depth Requirement for Drift Gillnets Isn’t Protecting Sharks Seasonal Changes in Depth of Thermocline a Factor

August 14, 2007

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

La Jolla – Many people might be surprised to learn that drift gillnetting still exists in California. It does–in federal waters. They might also be surprised to learn that biologists are concerned about what this means for the future of sharks.

Thresher sharks, like this one, live where most gillnets are thrown. Credit: Dan Cartamil

To learn more about sharks, and to help protect them from overexploitation, California Sea Grant biologists have been tagging and tracking mako and thresher sharks in the Southern California Bight to see, among other things, what these great predators do at night. Understanding the animals’ nocturnal movements is especially important because that is when drift gillnetting occurs.

Gillnets are gargantuan nets, hung vertically like giant tennis nets drifting in the ocean currents. For any animal too big to squeeze through the mesh–swordfish, ocean sunfish, sea lions, dolphins, sea turtles and sharks–contact with the net means almost certain entanglement.

To create a safe corridor at the surface for air-breathing animals, nets must be submerged at least 11 meters.

“The regulations are in place primarily to protect marine mammals, seabirds and turtles to a lesser extent, certainly not sharks,” said Sea Grant Trainee Dan Cartamil, who conducted the research with lead investigator and thesis adviser professor Jeffrey Graham at SIO.

Cartamil has found that thresher sharks spend an annual average of 40% of their nocturnal hours in the

danger zone of the drifting nets. For mako sharks, the figure is 20%.

The sharks’ diving patterns seem to be determined by the depth of the thermocline, which is the zone that separates warm waters at the surface and cold waters in the deep. Because of this pattern, Cartamil speculates that sharks are at higher risk of encountering a gillnet in winter when the transition-zone, or thermocline, deepens.

Ongoing research will determine whether seasonal depth requirements for gillnets could protect sharks from overexploitation.

“Through these studies, we are finally figuring out the biology of an animal whose habitat and lifestyle have long been a mystery,” Cartamil said. “We want to be able to apply this information to management and conservation.”