Southern bluefin tuna (Thunnus maccoyii) is a critically endangered species of tuna that is overfished because its flesh is considered a delicacy, particularly in Japan. The tuna, which generally resides in the Southern Atlantic, Pacific, and Indian Oceans, is a relative of the marlin and swordfish. Dark blue on its top and silver on its bottom, the southern bluefin tuna possesses a yellow band on its side. Fast swimmers, the tuna can reach speeds of more than 55 miles per hour (90 kilometers per hour) and can grow to lengths approximately six feet (two meters) in length and tip the scales at more than 300 pounds (135 kilograms).
Part of the Scombridae family of tuna, the muscular southern bluefin tuna has a curved tail, two dorsal fins, and retractable fins that lower resistance as the fish travel in the ocean. Migratory in nature, the southern bluefin tuna possess a unique circulatory and respiratory system that help it travel great distances. The tuna is able to keep a constant body temperature that is warmer than the water the tuna is traveling in. The creature also possesses a heart larger than most fish, allowing the tuna to expand a great deal of energy oxygen on its long journeys.
Around the age of nine, the southern bluefin is sexually mature. The tuna breeds between September and April and generally heads to the Indian Ocean to spawn in the temperate waters near Java, Indonesia. Typically, the tuna spawns in water with temperatures of 68° to 86° Fahrenheit (20° to 30° Celsius).
During spawning, a female southern bluefin tuna may release millions of eggs. The eggs take only a few days to hatch, releasing tuna of less than an inch (2.5 millimeters) each. The juvenile tuna spends about the first five years of life near the coast of Australia before departing for deeper waters that provide more feeding options.
The bluefin tuna can live up to 40 years and uses its heightened vision and hearing to catch prey such as krill, squid, octopus, and crustaceans. Predators of the bluefin tuna include sharks, birds, killer whales, and even other tuna. The tuna is heavily fished as the tuna is coveted for its fatty flesh, which is often served as gourmet food including fillets or sushi. Countries such as Japan, Australia, and New Zealand have limited the amount of southern bluefin tuna allowed to be caught.