This additional heat, when transmitted to oxygen-rich blood, gives an extra boost to already powerful muscles, permitting the bluefin tuna, for instance, to reach speeds of over 40 miles per hour for short distances. The tuna's steady, powerful swimming sustains a uniquely high metabolic rate, which permits its extraordinary growth rate. This also places a large oxygen demand on the fish, requiring tuna to swim continuously in order to meet that demand. Tuna must swim at a rate of at least one body length per second to pass enough oxygen over their gills. Tuna normally travel in small schools ranging from six to as many as 40 fish, all of the same size but often including several species.
Tunas are agile predators, often feeding on smaller, particularly schooling, fishes.
The species of prey depends on what is locally available at the time, but common species include herring, menhaden, hake, cod, bluefish, whiting, and mackerel, as well as squid and crustaceans such as shrimp. Because of their size, large tuna have few predators besides billfish, some sharks, toothed whales such as orcas and pilot whales, and humans. Fishery Among the world's most valuable commercial species, tuna are fished in over 70 countries worldwide, and marketed in fresh, frozen, or canned form. The most important commercial species caught in the Northeast are yellowfin, bigeye and bluefin tunas. The method used varies locally and among species.