Seven species of tunas are of major commercial importance on a global scale: three species of bluefin tuna, albacore, bigeye, yellowfin, and skipjack. Due to differences in their distributions and the different fisheries that exploit them, the species are classified as "temperate" or "tropical." The temperate tunas are the bluefins, plus albacore; they are found in waters as cold as 10° C, but can also be found in tropical waters. Skipjack and yellowfin are classified as tropical and are found in waters with temperatures greater than 18° C (although they can dive in colder waters). Bigeye could be classified as intermediate, but is often treated as a tropical species in fishery statistics. For the purposes of this review, further discussion is limited to skipjack, yellowfin, and bigeye because these are the three tuna species encountered during tropical tuna purse seine fishing.
Skipjack (Katsuwonus pelamis) are found mainly in the tropical areas of the Atlantic, Indian, and Pacific Oceans. Their geographic limits are 55° – 60° N and 45° – 50° S, with their greatest abundance in equatorial waters. Skipjack form both free schools and schools associated with floating objects. They are the principal species associated with FADs and are caught in conjunction with juvenile yellowfin and bigeye tunas. Skipjack are the smallest of the major commercial tuna species, with a common size range of 40 to 80 cm, except when occasionally caught in association with dolphin schools, in the eastern Pacific Ocean, where they reach larger sizes.
Bigeye (Thunnus obesus) are found in the subtropical and tropical areas of the Atlantic (but not the Mediterranean), Indian, and Pacific Oceans. Their geographical limits range from 55° – 60° N to 45° – 50° S. Juveniles and reproductively active adults are found in equatorial waters as well as at higher latitudes. Bigeye rarely form free schools, and usually are caught in the purse seine fishery while associated with floating objects. Small bigeye will form schools with juvenile yellowfin and skipjack tunas while associated at FADs. Bigeye reach similar maximum sizes to those of yellowfin (over 200 cm). Individuals as large as 150 cm occur in some fisheries, but it is common to find the size range down to 40 cm. Bigeye does not associate with dolphin schools in the eastern Pacific Ocean.
Yellowfin (Thunnus albacares) are found in the subtropical and tropical areas of the Atlantic, Indian, and Pacific Oceans. The yellowfin’s geographic limits are from 45°– 50° N and S, although in the Pacific they occur mainly within 20° of the equator. Yellowfin form both free and associated schools, with adults generally forming schools of similarly sized individuals. The free-swimming schools tend to contain large individuals and comprise yellowfin only. In the eastern Pacific, schools are often associated with dolphin pods, an association not common elsewhere. Yellowfin tuna generally reach sizes approaching those of bigeye, with a common range of 40 to 150 cm (or even larger when found in association with dolphins).