Tuna are powerful, highly migratory fish that roam the world’s oceans. They are among the most commercially valuable fish species, supporting massive global fisheries. Because the term “tuna” is often used loosely in commercial markets, confusion exists about which species are being purchased. Understanding the scientific distinctions clarifies the true number of species and defines the major types that dominate global commerce.
Defining True Tuna: The Genus Thunnus
The classification of “true tuna” is based on the scientific genus Thunnus. This genus contains the largest and most specialized members of the tuna tribe, Thunnini. There are eight recognized species within the Thunnus genus.
These eight species share a unique biological trait: regional endothermy. Unlike most cold-blooded fish, true tunas possess a specialized vascular network called the rete mirabile (“wonderful net”). This system acts as a countercurrent heat exchanger, allowing them to conserve metabolic heat and maintain core muscle temperatures warmer than the surrounding water. This internal warmth provides the physiological engine necessary for their speed and sustained migrations across vast oceanic distances.
Characteristics of the Major Commercial Species
The four most commercially significant members of the Thunnus genus are Bluefin, Yellowfin, Albacore, and Bigeye tunas.
Bluefin Tuna
The Bluefin group represents the apex of the tuna family, comprising three distinct species: Atlantic (Thunnus thynnus), Pacific (Thunnus orientalis), and Southern (Thunnus maccoyii). The Atlantic Bluefin is the largest, capable of reaching lengths exceeding 10 feet and weights over 1,000 pounds. All three Bluefin species are highly prized in the sashimi market for their fatty, rich red meat.
Yellowfin Tuna
Yellowfin tuna (Thunnus albacares) is identifiable by its bright yellow dorsal and anal finlets, which earn the species its common name. Mature Yellowfin also develop long, scythe-like second dorsal and anal fins that can nearly reach the tail. These fish are found in tropical and subtropical waters globally, growing up to 6.5 feet in length and weighing over 400 pounds. Yellowfin is widely consumed as fresh steaks and is a major component of canned tuna labeled “chunk light.”
Albacore Tuna
Albacore tuna (Thunnus alalunga) is the only species permitted to be marketed as “white meat” tuna in the United States due to its lighter flesh when cooked. A distinguishing characteristic is its long pectoral fins, which extend past the beginning of the second dorsal fin. Albacore is generally smaller than Yellowfin, with an average market weight between 10 and 30 pounds. This species is a staple of the canning industry.
Bigeye Tuna
Bigeye tuna (Thunnus obesus) gets its name from its larger eyes, an adaptation for hunting in deeper water layers. Although similar to Yellowfin tuna, Bigeye possesses a more robust, deeper body shape. It typically dives to depths of 800 to 1,600 feet during the day to feed in cooler waters. This species is highly valued in the sashimi market and is distinguishable from Yellowfin by its black-edged finlets.
Species Often Marketed as Tuna
The tuna market includes several species that do not belong to the true Thunnus genus but are widely sold as tuna. Skipjack tuna (Katsuwonus pelamis) is the most significant, accounting for the largest volume caught worldwide. Skipjack is recognizable by the four to six dark, horizontal stripes running along its belly.
Unlike true tunas, Skipjack is smaller, rarely exceeding 3 feet in length, and has a shorter lifespan, which contributes to its population resilience. Skipjack belongs to the broader Thunnini tribe but is in its own genus. Its meat is typically dark and strongly flavored, forming the basis of most canned “chunk light” products. Another group often confused with true tuna is Bonito, primarily belonging to the genus Sarda. These fish are classified outside of the true tuna tribe, featuring a more elongated body and distinct oblique dark stripes on their backs.
Conservation Status of Key Species
High commercial demand has led to intense fishing pressure, requiring monitoring through assessments like the IUCN Red List. Skipjack tuna is currently classified as Least Concern, due to its high reproductive rate and short lifespan, which allows populations to recover quickly. Albacore and Yellowfin tuna populations have shown signs of recovery, with both species now listed as Least Concern globally.
The large, high-value species face greater conservation challenges due to their long lifespans and slow reproductive maturity. Bigeye tuna is currently listed as Vulnerable worldwide, reflecting concerns about overfishing in certain ocean regions. The Bluefin species historically faced severe pressure, though recent management efforts have yielded mixed results; Atlantic Bluefin tuna has improved to Least Concern, but the Southern Bluefin tuna remains classified as Endangered.