Tuna are remarkable inhabitants of the ocean, recognized globally for their speed, migratory habits, and distinctive physiology. These powerful fish are warm-blooded, a rare trait among fish, allowing them to maintain a body temperature higher than the surrounding water. This adaptation enables them to thrive across diverse marine environments and pursue prey with efficiency. As a commercially important group, tuna support major fisheries worldwide, supplying a significant portion of the seafood market.
Counting the Species
True tunas belong to the tribe Thunnini, a subgroup within the mackerel family (Scombridae). This tribe encompasses 15 distinct species, organized into five genera: Thunnus, Katsuwonus, Euthynnus, Auxis, and Allothunnus. This classification helps scientists understand the evolutionary relationships and unique characteristics within this diverse group.
Key Tuna Species and Their Defining Traits
Several tuna species are well-known for their commercial significance and distinct biological features. The Thunnus genus includes eight species, often divided into bluefin and yellowfin groups.
Bluefin tunas, including Atlantic, Pacific, and Southern species, are among the largest and most sought-after. Atlantic bluefin can reach 15 feet and weigh over 1,500 pounds, characterized by a torpedo-shaped body, short pectoral fins, and a dark blue upper body with a silvery-white underside. They are known for extensive migrations and diving to depths exceeding 3,000 feet. Pacific bluefin are slightly smaller, while Southern bluefin also attain large sizes, inhabiting the southern hemispheres of the Atlantic, Indian, and Pacific oceans.
Yellowfin tuna (Thunnus albacares) are identifiable by their bright yellow dorsal and anal fins, and a yellow stripe down their sides. They have a streamlined, torpedo-shaped body that is metallic dark blue or greenish on top and silvery white below, often with vertical lines. They are highly migratory and inhabit tropical and subtropical waters, schooling near the surface or around floating objects. Yellowfin can grow up to 6 feet long and weigh around 400 pounds, with a lifespan of about 7 years.
Skipjack tuna (Katsuwonus pelamis) are smaller and more abundant, reaching up to 3 feet and 75 pounds. They are distinguished by dark purplish-blue backs, silvery bellies, and four to six prominent dark longitudinal stripes on their lower sides. Skipjack lack scales except for a band around their head and along the lateral line. These fast-swimming fish are found globally in tropical and warm temperate waters, forming large schools that can include other tuna species.
Albacore tuna (Thunnus alalunga) are identifiable by their exceptionally long pectoral fins, which extend beyond the second dorsal fin. They have a torpedo-shaped body with a dark blue back and lighter blue-gray sides and belly. Albacore are smaller than bluefin or yellowfin, weighing up to 80 pounds and reaching about 4 feet. They are found in temperate and tropical waters across all oceans and the Mediterranean Sea.
Bigeye tuna (Thunnus obesus) share similarities with yellowfin but are identified by their notably larger eyes, which give them their common name. They have a robust, streamlined body that is dark metallic blue on the back and upper sides, transitioning to nearly white on the lower sides and belly. Bigeye tuna can reach 7.5 feet and weigh over 400 pounds. They inhabit subtropical and tropical areas of the Atlantic, Indian, and Pacific Oceans, found in deeper, cooler waters during the day and moving to shallower waters at night.
Other tuna species include the frigate tuna (Auxis thazard) and the kawakawa (Euthynnus affinis). Frigate tuna are smaller, with a robust, elongated body, bluish-black backs, and wavy lines above the lateral line. Kawakawa, also known as little tuna, are medium-sized with a typical tuna body shape, dark blue backs, and often one to three dark spots between their pectoral and pelvic fins.
Conservation Status of Tuna Species
The conservation status of tuna species varies, reflecting diverse global pressures. While some populations remain stable, others face significant threats primarily due to overfishing. Organizations like the International Union for Conservation of Nature (IUCN) assess these statuses, providing a framework for understanding their vulnerability.
Bluefin tuna populations, such as the Atlantic bluefin, are vulnerable. Their high commercial value, especially for the sashimi market, has historically driven intensive fishing efforts. This pressure can lead to population declines if not carefully managed.
Other tuna species, such as skipjack, are considered more resilient and classified as “Least Concern” due to their abundance and faster reproductive rates. However, even abundant species can face indirect threats, such as the bycatch of juvenile yellowfin and bigeye tuna that school with adult skipjack. This unintended capture of younger fish before they can reproduce can impact the long-term health of those populations. The conservation outlook for each tuna species is distinct, requiring tailored management strategies to ensure their sustainability.
Understanding Tuna Classification
Scientific classification provides a structured way to understand tuna relationships within the animal kingdom. All tuna species belong to the Phylum Chordata, indicating they possess a notochord at some stage of development. They are categorized into the Class Actinopterygii (ray-finned fish) and are part of the Order Scombriformes, a diverse group that also contains mackerels.
Tuna are placed within the Family Scombridae, which includes a wide array of fast-swimming marine fish. Within this family, the Tribe Thunnini groups all tuna. The concept of genus and species further refines this classification.
Different genera, such as Thunnus and Katsuwonus, group distinct types of tuna with shared characteristics. For example, Thunnus includes bluefin and yellowfin, while Katsuwonus contains skipjack tuna. This hierarchical system helps biologists track evolutionary relationships and manage distinct populations.