Bigeye Tuna, Thunnus obesus
Bigeye Tuna, Thunnus obesus. Fish courtesy of the commercial fishermen of San Diego California, August 2021. Length: 92 cm (3 feet 0 inches).
The Bigeye Tuna, Thunnus obesus, is a member of the Mackerel or Scombridae Family, that is known in Mexico as patudo and atún ojo grande. 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 Bigeye Tuna has a large deep round fusiform body with a depth that is greater than 25% of fork length, being deepest under the center of the first dorsal fin, that is slightly compressed from side to side with large heads and large eyes. Bigeye Tuna are a dark deep metallic blue dorsally transitioning to a silvery white ventrally with an iridescent blue band that runs along the sides in live fish. Their caudal peduncle is black; their first dorsal fin is deep yellow, the anal and second dorsal fins are light yellow, and the finlets are yellow with black margins. Their head is long and pointed with large eyes and a large mouth. Their anal fin has 2 spines and 10 to 15 rays followed by 7 to 10 finlets their caudal fin is lunate with a slender peduncle; their first dorsal fin has 13 or 14 spines; their second dorsal fin has 14 to 15 rays followed by 8 to 10 finlets; and, their pectoral fins are very long reaching past the second dorsal fin origin in juveniles and between the first and dorsal fins in adults. They have 23 to 31 gill rakers on their first arch. They possess a swim bladder and a straited liver. They are covered with very small scales.
The Bigeye Tuna is a highly migratory oceanic epipelagic and mesopelagic species that is found between 52oN and 48oS latitude and 180oW and 180oE longitude in all open temperature and tropical waters of the Atlantic, Indian, and Pacific Oceans between 3oC (37oF) and 29oC (84oF), but primarily between 17oC (63oF) and 22oC (72oF). They travel, on average 117 km (73 miles) per day. They normally migrate following warmer surface waters. They seldom go below depths of 500 m (1,640 feet) but have been documented at depths up to 1,900 m (6,240 feet). Younger fish are found inshore and remain near the surface and form large multispecies schools with Albacore, Bluefin, Frigate, and Yellowfin Tuna and with Bonito and Yellowtail, based on size. They have exceptional low light vision. Their blood has a high oxygen affinity which allows them to tolerate oxygen-poor waters. They also have the ability to maintain their body temperatures above ambient water temperatures which allows them to stay in cooler waters. Adult Bigeye Tuna undertake a daily Diel Migration descending at dawn to deeper, 300 to 500 meter waters. that are 20oC (68oF) cooler and returning to shallower, warmer waters at dusk. These migrations follow the Diel Migrations of their prey. They undertake thermoregulatory behavior whilst at depth needing to return to the surface periodically to re-warm. The Bigeyes must remain in constant motion to stay alive. They are fast growing with males being larger than females and reach a maximum (fork length) of 2.36 m (7 feet 9 inches) in length and 210 kg (462 lbs) in weight. As of January 1, 2023, the International Game Fish Association world record stood at 197 kg (435 lbs) with the fish caught in coastal waters off Peru in April 1957. They feed on a wide variety of epipelagic and mesopelagic fishes, cephalopods and crustaceans during the day and night. In turn they are preyed upon by billfish, larger tunas, and toothed whales. Reproduction is oviparious with batch spawning. They are multiple prolific spawners that may spawn every 1 or 2 days over several months throughout the year in tropical waters that are approximately 24oC (75oF). Each female releases 2.9 million to 6.3 million eggs, with external fertilization. The eggs and larvae are pelagic. They reach sexually mature in 2 to 4 years with 60% of all schools being males. They have life spans of up to sixteen years.
For fish from the Pacific Ocean the Bigeyes are easily confused with the Albacore, Thunnus alalunga (exceedingly long pectoral fins; body deepest under the origin of the second dorsal fin), the Pacific Bluefin Tuna, Thunnus orientalis (second dorsal fin lobe higher than the first; short pectoral fins), and the Yellowfin Tuna, Thunnus albacares (more aerodynamic body; smaller head, smaller eyes; elongated anal and dorsal fins; lack a striated liver). For fish from the Atlantic Ocean the Bigeyes can be confused with the Albacore and the Yellowfin as-well-as the Bluefin Tuna, Thnunnus thynnus (lower body with spots; pectoral fins end before the second dorsal fin; 34 to 43 gill rakers); and, the Blackfin Tuna, Thunnus atlanticus (anal and dorsal fins dusky).
The Bigeye 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. They are also present in all Mexican waters o the Pacific Ocean with the exception that they are absent from the Sea of Cortez.
From a conservation perspective the Bigeye Tuna is currently considered to be VULNERABLE, that is attributed to global overfishing with declines estimated to be over 40% over the past 15 years. Population demographics indicate that their populations are becoming smaller and younger. They are particularly vulnerable when the younger immature juveniles congregate in shallower waters. With large number of juvenile and young fish being removed from their native environments they are not old enough to reproduce significantly 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 caught commercially via deep set (150 feet to 1,000 feet) longlines, purse seine, pole-and-line at a level of 500,000 tons annually. They are a by-catch of Skipjack fishery. Smaller fish are found in shallower waters in close proximity to seamouths, floating objects and buoys. They are also being affected by the rapid warming of the ocean that has caused the reduction of marine phytoplankton. They are currently the focus of major conservation efforts. The commercial fishing of Bigeye Tuna is heavily regulated in some areas of the world with gear restrictions, bait restrictions, size restrictions, regional closures, monitoring and inspection programs and annual quotas, but as their range is vast the majority of which is in international waters monitoring, accurate reporting and enforcement has been difficult. The Recreational Fishery is also heavily regulated with permits and size limits. They are considered to be an excellent food fish including use as sashimi and sushi with meat that is deep red in color that has a high fat content with an extended shelf life. The Bigeye Tuna was added to the Greenpeace International Seafood Red List in 2010, indicative that each commercial fish is of very high risk of being sourced from an unsustainable fishery.