Bluefin Tuna, Thunnus thynnus
Bluefin Tuna, Thunnus thynnus. Fish courtesy of the commercial fishermen of San Diego California, August 2021. Length: 64 cm (2 feet 1 inch).
Bluefin Tuna, Thunnus thynnus. Fish caught from coastal waters off Chatham, Massachusetts, June 2019. Length: 1.60 m (5 feet 3 inches). Weight: 71 kg (156 lbs). Catch, photograph and identification courtesy of Marc Eberlein, Grand Rapids, Michigan.
The Bluefin Tuna, Thunnus thynnus, is a member of the Mackerel or Scombridae Family, that is also known as the Atlantic Bluefin Tuna and the Common Tunny and in Mexico as atún aleta azul or simply ATÚN! Globally, there are eight species in the genus Thunnus, of which six are found in Mexican waters, two in the Atlantic, one in the Pacific, and three in both the Atlantic and the Pacific Oceans.
The Bluefin Tuna has a large torpedo-shaped robust body with a circular cross-section that is deepest at the insertion point of the pectoral fins and taper to the caudal peduncle and are aerodynamically designed for speed. Bluefin Tuna are a deep metallic blue dorsally transitioning to a silvery white ventrally. They have colorless line and rows of dots along the lower sides. Their anal fin and finlets are yellow with black margins; their caudal keel is black; their first dorsal fin is yellow or blue; their second dorsal fin is brown or red. Their head is long and pointed with small eyes and a large mouth. Their anal fin has 0 or 1 spine and 13 to 16 rays followed by 7 to 10 finlets and is inserted well behind the second dorsal fin; their caudal fin is lunate with 3 keels on the peduncle; their first dorsal fin has 12 to 14 spines; their second dorsal fin is taller than the first and has 13 or 16 rays followed by 7 to 10 finlets; and, their pectoral fins are relatively short but do not reach the gap between the first and second dorsal fins. They have 34 to 43 gill rakers on their first arch. They possess a swim bladder. They are covered with small scales. They have a complete lateral line.
The Bluefin Tuna is a highly migratory pelagic species that is found between 69oN and 58oS latitude and 99oW and 42oE longitude that is capable of making trans-oceanic runs in less than 60 days that can tolerate wide changes of water temperatures (3oC or 37oF to 30oC or 86oF) and can be found both above and below the thermocline. They seldom go below depths of 100 m (330 feet) but have been documented at depths up to 1,006 m (3,300 feet). They are fast swimmers capable of reaching speeds of 50 mph. They normally move northward during the warm water summer months and return southward in the fall. Younger fish up to 80 kg (130 lbs) will form large multispecies schools with Albacore, Bigeye, Frigate, and Yellowfin Tuna and with Bonito and yellowtail, based on size. They are known to come close to shore. They are warm-blooded with an ability to thermoregulate allowing them to access the chilly waters of the North Atlantic. The Bluefins must remain in constant motion to stay alive. They have life spans of up to 50-years. They are the largest tuna species reaching a maximum (fork length) of 4.58 m (15 feet 0 inches) in length and 680 kg (1,500 lbs) in weight making them one of the largest oceanic fish. As of January 1, 2023, the International Game Fish Association world record stood at 679 kg (1,498 lbs) with the fish caught in coastal waters off Nova Scotia, Canada in October 1979. They are known to regulate their body temperatures to levels above that of their environment. They are opportunistic top apex predators preying on fish (anchovies, hakes, herring, mackerel, sardines), pelagic crustaceans, red crabs and squid. They compete heavily with other scombrids and billfishes for food. The juveniles prey on zooplankton and small pelagic fishes; the mid-sized fish prey on medium sized pelagic fishes, shrimps and cephalopods. In turn they are preyed upon by numerous marine mammals including Killer Whales, Pilot Whales, various Sharks and other predatory fishes. Their juveniles are preyed upon by Bluefish and various sea birds. They have the ability to escape many predators due to their swimming speeds and their strong endurance. Reproduction is oviparious with batch spawning that occurs in know natal areas one of which is in the Gulf of Mexico with water temperature between 24.9oC (76.8oF) and 29.5oC (85.1oF). Each female will produce up to 30,000,000 pelagic eggs, which are fertilized externally, that hatch within 48 hours and the larvae are also pelagic. They grow rapidly in the first year but then growth rates decrease as they require 10 years to reach two-thirds of their maximum length. They are slow to sexually mature reaching maturing in 6 to 8 years. They have life spans of up to 50 years.
The Bluefin Tuna is similar in appearance to several other tuna family member but are the only tuna with short pectoral fins and striated livers.
The Bluefin Tuna is a resident of all Mexican waters of the Atlantic Ocean including the Gulf of Mexico and the east coast of the Yucatán Peninsula in the Caribbean.
From a conservation perspective the Bluefin Tuna is currently considered to be ENDANGERED. They have been caught for centuries via artisanal fishermen and by commercial fishermen utilizing nets, long-lines, drift nets, seine nets and traps. They also have been a recent target of recreational sportfishermen being prized for their speed, strength, and stamina as well as food value. It has been reported that it took six men 62 hours to boat an 800 pound fish via hook and line in 1934. Due to overfishing the global populations have declined by approximately 50% and the Atlantic populations by nearly 90% over the last 40 years. Population demographics indicate that their populations are becoming smaller and younger. They are particularly vulnerable when the congregate in specific well known well documented areas of spawning. They are current a major focus of aquaculture where vast quantities of small fish are caught and raised in pens which now comprise about 50% of the commercial sales of Bluefin Tunas today. Farming removes large populations of juvenile fish from their native environment that never reproduce significantly contributing to the demise of the global populations. With large populations of juvenile and young fish being removed from their native environments they are not old enough to reproduce significant contributing to the global declines in populations. Of late due to the overabundance of fishing pressure the global price for Tuna putting additional pressure on catch levels as the commercial fishermen must catch more fish to maintain revenue levels. They are presently harvested at a level of 50,000 tons per year with about 50% originating from farmed fish. They are a favorite fish of Japanese culture with 80% of the world’s commercial catch being consumed in Japan. One fish sold in Japan for $247,000 in 2008 which is pale in comparison to the prices paid for the Pacific Bluefin Tuna. They are currently the focus of major conservation plans targeted at rebuilding global populations and also included in multiple seafood watch lists. They have been heavily regulated with fish quotas, fishing area closures, and seasonal closures, however, as their range is vast the majority of which are in international waters monitoring, accurate reporting and enforcement has been difficult.