Thunnus obesus and Thunnus albacares
These two species of tuna make up the bulk of the supply of fresh and frozen tuna sold in the U.S. market. Of the two, yellowfin is a much bigger resource, but bigeye is considered to be a more desirable tuna because of its higher oil content. Both species are often marketed simply as ahi, the Hawaiian name for tuna.
Bigeye and yellowfin are found in warm and temperate waters around the world, however, the populations are much larger in the Pacific and Indian oceans. Yellowfin is fished by purse seine, longline and handline and annual landings usually exceed 1 million metric tons. The purse seine fishery is by far the largest. Almost all the seine catch is brine frozen and used for canning. Many longline boats freeze their catch at sea for the Japanese sashimi market. This fish has to be held at very low temperatures (usually -60F) to preserve its attractive red meat color. Longliners are also the primary source of fish for the fresh market.
Bigeye, which are found in much deeper water, are primarily caught by longline. Annual worldwide landings are about 400,000 metric tons.
The U.S. is not a major producer of either yellowfin or bigeye. Off the East Coast and Gulf of Mexico, U.S. longliners land about 1,500 metric tons of yellowfin and about 500 metric tons of bigeye a year. The largest U.S. longline tuna fleet is based in Hawaii, where boats land about 6,000 metric tons of bigeye and about 1,100 metric tons of yellowfin a year.
Most of the fresh bigeye and yellowfin consumed in the U.S. is imported from a wide number of countries around the world. In recent years, the U.S. has imported about 4,000 metric tons of fresh bigeye and about 15,000 metric tons of fresh yellowfin. Major sources of fresh imported tuna include Costa Rica, Ecuador, Fiji, Mexico, the Philippines, Trinidad and Vietnam.
The U.S. also imports about 12,000 metric tons of “treated” frozen tuna steaks and loins a year, most of which is produced from yellowfin. This is tuna that is exposed to carbon monoxide before it is frozen. This procedure, which binds in the tuna’s red color as the CO reacts with the myoglobin in the meat, allows it to be held at normal frozen temperatures without turning brown. About 80 percent of this treated tuna comes from Indonesia.
Buying tuna can be tricky due to the complex grading. First, most of the true No. 1 sashimi-quality tuna is exported to Japan, where the market pays a premium price for tuna with the best fat content and color. Most of the fresh tuna sold in the U.S. would be considered a No. 2 tuna by Japanese standards. U.S. suppliers have come up with a variety of confusing grades like No. 1- or A—or B++. Make sure you and your supplier are on the same page when it comes to grading standards.
The ideal tuna is bright red and almost translucent. Yellowfin tends to be somewhat lighter than bigeye. Be careful when buying tuna caught by handline. These fish can literally cook themselves as they struggle on the line. Their flesh can be almost brown and they will have a very short shelf life.