In June 1999, the National Marine Fisheries Service (NMFS) began a satellite-tagging project to help determine movement patterns and post release survival of sub-adult common thresher sharks tagged in southern California. The common thresher is thought to move from off the coast of Baja, California into southern California waters in early spring and summer. This movement follows the warming of coastal waters and increased quantities of small pelagic prey. Large adult threshers usually enter the Southern California Bight in April and May and continue north, arriving offoregon and Washington in July. Pups, born offshore in the early spring, move into shallow inshore waters where they remain for two to three years. Sub-adults apparently move into southern California in June and continue moving north along the coast to at least San Francisco. As coastal waters cool in the fall and winter months, adults and sub-adults return to Baja.

Temperature archiving satellite pop-up tags were deployed on eight common thresher sharks released near Laguna Beach and Santa Monica. The tags were designed to release from the fish and pop-up to the surface after 60, 120, 210, and 300-day intervals. The tags were then designed to transmit archived temperature and location data to Argos satellites where the data would be retrieved by fishery biologists. Five of the eight pop-up tags released and reported their data as scheduled.

The first 60-day tag to pop-up and report data released from a 240 cm female thresher. She traveled west of Point Conception, a net traveling distance of 225 nm. The second 60-day tag released from a 246 cm male thresher. It reported at the appropriate time, but the shark was caught by a fisherman and taken to his home in Oceanside after only 29 days. The first 120-day tag released on a 260 cm male. It reported on October 18 near Oceanside, only 20 nm from its tagging location. The second 120-day tag was deployed on a 237 cm female thresher. She traveled south offMagdalena Bay, BCS, Mexico, a net distance of nearly 240 nm. The final tag to transmit data released from a 255 cm female thresher after 210 days. She moved 895 nm south to just west of Clarion Island off Mexico.

Recently, another thresher tagged with a satellite tag was recaptured. That pop-up tag was scheduled to report after 300 days and was one of the three pop-ups that did not report. The shark was at liberty for 538days and was caught by a gill net vessel in the same area it was tagged. An observer on the vessel did not see any evidence of the satellite tag ever being attached which indicates that the wound was completely healed. Either the tag fell off, or it was pulled from the fish.

The first 60-and first 120-day tags confirm limited movement of sub-adult threshers within the Southern California Bight in summer and early fall. "The last two tags to pop-up confirm the southward movement back into Mexican waters in the winter. The data also indicates thresher shark movements between Mexico and the U.S. EEZ in a more complex movement pattern than previously thought.

Six of the eight satellite tagged threshers survived from at least 29 to 538 days, resulting in a 75 percent survival rate. This rate is encouraging because threshers had once been considered too fragile to survive the catch, tag, and release process. If you would like more information about this study, contact NMFS biologists at (858) 546-7186.