a biological hydraulic system involving the lymphatic system. The behavior of abruptly diving to deeper levels may be a tactic to escape predators. Evidence from trackers even includes a case in which a diving yellowfin tuna may have been swallowed at a depth of . Commercial fishery Modern commercial fisheries catch yellowfin tuna with encircling nets (purse seines), and by industrial longlines. In 2010, 558,761 metric tons of yellowfin tuna were caught in the western and central Pacific Ocean. Pole and line Formerly, much of the commercial catch was made by pole-and-line fishing, using live bait such as anchovy to attract schools of tuna close to the fishing vessel that were then taken with baited jigs on sturdy bamboo or fiberglass poles or on handlines. This fishery, which targeted skipjack and occasionally albacore, as well as yellowfin, for canning, reached its heyday between World War I and the 1950s before declining. The most well-known fleet of pole-and-line boats sailed from San Diego in California and exploited abundant stocks in Mexican waters, as well as further south to Panama, Costa Rica, and the Galapagos Islands. Pole-and-line fishing is still carried out today in the Maldives, Ghana, the Canary Islands, Madeira, and the Azores. Few pole-and-line boats now specifically target yellowfin, an incidental take compared to the total commercial catch. In the Maldives, the catch is a mix of skipjack tuna and small yellowfins that often associate with them. Purse seining Purse seining largely took over commercial tuna fisheries in the 1960s and 1970s. Today, purse seines account for more of the commercial catch than any other method. The purse-seine fishery primarily operates in the Pacific Ocean, in the historic tuna grounds of the San Diego tuna fleet in the eastern Pacific, and in the islands of the western Pacific, where many U.S. tuna canneries relocated in the 1980s, but significant purse-seine catches are also made in the Indian Ocean and in the tropical Atlantic Ocean, especially in the Gulf of Guinea by French and Spanish vessels. Purse-seine vessels locate tuna using onboard lookouts, as was done in the pole-and-line fishery, but they also employ sophisticated onboard electronics, sea-surface temperature and other satellite data, and helicopters overhead. Once a school is located, the net is set around it. A single set may yield . Modern tuna seiners have a capacity up to , reach speeds of over , and carry multiple spotting