Bigeye tuna (Thunnus obesus) are native to tropical and temperate waters in the Atlantic, Pacific, and Indian Oceans. They inhabit oceanic waters at latitudes between 60 °N and 50 °S. Juveniles and breeding adults are most common in tropical waters. Bigeye tuna are absent from Pacific coastal waters along the southern half of South America. (Collette, et al., 2021)
Bigeye tuna inhabit open-water oceanic environments, in waters with temperatures between 13 and 29 °C, although the optimal temperature range for bigeye tuna is 17 to 22 °C. They demonstrate flexibility in the depth and temperature of water on daily and seasonal scales. Bigeye tuna spend the daytime at depths of 200 to 300m below the surface, where water temperatures average 13 to 14 °C. They move to warmer, shallower waters at night, spending time at depths less than 50 m below the surface. They typically move up and down through the water column around dusk and dawn, respectively. Bigeye tuna occasionally exhibit deep-diving behavior and have been reported at depths between 0 and 1,902 m below the surface.
Studies on human-made floating mats where marine vegetation grows and attracts groups of fish, called Fish Aggregating Devices (FADs), reported schools of bigeye tuna coexisting with skipjack (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) in areas where their geographic ranges overlap. When associated with FADs, bigeye tuna usually stay at depths outside of mixed layer regions of the ocean. They likely also use naturally occurring floating objects (e.g. driftwood, algal mats) similarly to FADs. (Collette, et al., 2021; Cornic, et al., 2018; Evans, et al., 2008; Schaefer and Fuller, 2002)
Bigeye tuna have elongated bodies with 2 dorsal fins, 2 ventral fins, 2 pectoral fins, a caudal fin, and an anal fin. They also have a series of non-retractable finlets on their dorsal and ventral sides, posterior to their rear dorsal and ventral fins, respectively. The function of these finlets are suspected to interrupt water flow over the posterior side of the body, potentially improving the efficiency of long-distance swimming.
Bigeye tuna have dark, metallic blue coloration on their dorsal sides and whitish coloration on their ventral sides, which provides them countershading camouflage in marine environments. They also have shimmery-blue bands that extend laterally on each side of their bodies. Their anterior dorsal fins are dark yellow and their posterior dorsal fins and anal fins are light yellow. The finlets are bright yellow with dark margins. Bigeye tuna get their common name from the size of their eyes, which is larger than the eyes of closely related species, such as yellowfin tuna (Thunnus albacares).
A study in 2018 found that the size and shape of bigeye tuna scales varied between individuals. Their scales, also called scutes, are small and some are partially embedded in their skin and thus less visibly apparent. Their most apparent scales are located on their corselets (around their pectoral fins), on their tails, and the lateral sides of their heads.
Adult bigeye tuna range in length from 81 to 206 cm, as measured from their snouts to the fork of their tails (fork length; FL). Adult females at half of their maximum estimated lifespan reach an average FL of 102.4 cm, whereas males reach 86.6 cm. However, a study from 2010 reported that male specimens were predominantly larger than females. Of the length classes measured in this study, all specimens with a FL greater than 192 cm were males. The FL of individual bigeye tuna is directly proportional to their mass, which ranges from 11 to 95 kg. Bigeye tuna eggs are typically light pink and range in length from 0.4 to 0.5 mm. Newborn bigeye tuna are transparent and less than 3 mm long, but within one year they develop coloration similar to adults and grow to be as large as 80 cm.
Yellowfin tuna and bigeye tuna are somewhat similar in appearance, but differ in fin color and body size. Bigeye tuna have wider bodies with large eyes, whereas yellowfin tuna are thinner with relatively small eyes. Also, bigeye tuna have black margins on their finlets, a feature which yellowfin tuna lack. (Ahmad, et al., 2019; Brill, et al., 2004; Collette, et al., 2021; Hino, et al., 2021; Wainwright, et al., 2018; Wang, et al., 2020; Zhu, et al., 2010a; Zhu, et al., 2010b)
Bigeye tuna hatch from eggs and measure an average of 2.65 mm at birth. The amounts of time between hatching, maturity, and spawning remains unknown. However, they reach 30 to 80 cm in total length within the first year of life. Mature adults reach 81 to 170 cm in length, as measured from snout to tail fork, and spawning adults are often over 100 cm.
Tuna exhibit indeterminate growth; although growth rates slow significantly after maturity, they grow throughout the entirety of their life. A study from 2006 found that the fastest rates of growth occur in the first several years, but begins to plateau after 9 to 10 years. (Farley, et al., 2006; Matsumoto and Miyabe, 2002)
Bigeye tuna reproduce year-round via external fertilization. Spawning females remain stationary until a male approaches them, after which females release their eggs and males release their sperm into the water column. Bigeye tuna are polygynandrous and mate every 1 or 2 days during a reproduction period.
There are no data regarding mate selection or behaviors, except that timing of mating events tends to coincide with daily vertical movements close to the surface of the water. Bigeye tuna in both the Pacific and Atlantic Oceans spawn most often between 1900 h and 2400 h. (Matsumoto and Miyabe, 2002; Nikaido, et al., 1991; Sun, et al., 2013; Zhu, et al., 2010a; Zhu, et al., 2010b)
Bigeye tuna spawn year-round, though some sources state that spawning periods are limited to February through September. Females release eggs every 1 or 2 days when actively spawning. Females release 0.84 million to 8.56 million eggs per spawning effort. They release their eggs into the water column to be externally fertilized by males. Newly laid eggs are typically light pink and range from 0.4 to 0.5 mm long. There are no known reports on the embryo development time, from fertilization to hatching.
Juvenile bigeye tuna are immediately independent upon hatching. They reach sexual maturity at 2 to 4 years of age, regardless of sex. However, sexual maturity is also dependent on body size; most mature adults measure more than 100 cm in total length. (Sun, et al., 2013; Farley, et al., 2006; Nikaido, et al., 1991; Sun, et al., 2013; Zhu, et al., 2010a; Zhu, et al., 2010b)
Bigeye tuna exhibit no parental care beyond the act of spawning. Eggs are fertilized externally and newly hatched young are immediately independent.
There is limited information regarding longevity in bigeye tuna. However, a 2020 study estimated ages by radiocarbon dating otoliths and reported that individuals from the Atlantic Ocean lived up to 18 years. Another study from 2022 similarly estimated the ages of bigeye tuna from the Pacific Ocean using otoliths and reported the oldest individual they measured to be 14 years old.
A closely related species, yellowfin tuna (Thunnus albacares), can live more than 20 years in captivity. It is likely that bigeye tuna would have similar captive lifespans, but they are not kept in captivity. (Andrews, et al., 2020; Estess, et al., 2017; Norwood, 2022)
Bigeye tuna juveniles and small adults live in schools with multiple species of fish that are similar in size. They school with several species, both in shallow and deep waters. Bigeye tuna migrate vertically each day, spending time between 45 to 74 m below the surface at night and moving to deeper waters during the day, reaching depths between 196 m and 1,902 m.
Bigeye tuna reach lengths of 30 to 80 cm at around one year old, at which point they often live in mixed schools with skipjack tuna (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) of similar sizes. During this time, bigeye tuna school and feed in shallow water during the day. As they get older and larger, bigeye tuna become more solitary and disperse throughout the ocean. As sexually mature adults, they occupy neither pure nor mixed schools. Adults travel long distances across the open ocean, with movements determined by seasonal changes in water temperature and food availability. Bigeye tuna may aggregate at feeding and spawning zones. They are opportunistic predators, eating a variety of species whenever and wherever possible.
Radio tracking studies have found that only around 5% of tagged bigeye tuna make long-distance movements between major populations. The long-distance movements that individuals do make can be over 1,500 km and are not seasonal, but instead permanent. The other 95% of tagged individuals are rather sedentary, staying in the same general area in which they were originally tagged, but making daily vertical movements between deeper waters and waters closer to the surface.
During reproduction periods, male bigeye tuna seek out ovulating females. Females remain stationary while waiting for males and release their eggs into the water column when males are present. Mating may occur between solitary individuals or within schools of adults. Spawning typically occurs between 1900 h and 2400 h. There are no apparent mating behaviors associated with mate choice. (Andrews, et al., 2020; Collette, et al., 2021; Evans, et al., 2008; Lam, et al., 2014; Matsumoto and Miyabe, 2002; Stoltz and Neff, 2006)
Bigeye tuna do not have defined home ranges and do not defend specific territories. They make daily movements between shallow and deep water, and occasionally migrate more than 1,500 km to new populations.
Bigeye tuna have various sensory systems that allow them to travel through water with varying levels of visibility. They rely on visual cues when they are in well-lit water near the surface. However, their eyes are also adapted to low-light situations. Bigeye tuna have rod cells and only one cone pigment, which allows them to detect blue and green wavelengths of light. They use visual cues to navigate their surroundings and to detect food, predators, and mates.
Tuna also rely on chemical cues to find sources of prey. Chemical signals are generally only useful at short distances due to their diffusion in water and slow travel. However, chemical signals may also play a role in detecting large aggregations of prey species farther away.
Bigeye tuna frequently communicate with with auditory signals. They use calcium carbonate structures in their heads, called otoliths, to sense vibrations propagating through water. They also have a lateral line system along each side of their bodies, which helps them detect movements or pressure gradients in the water. Juvenile and small adult bigeye tuna travel in schools of fish with multiple species, including skipjack tuna (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares). They likely respond to stimuli from other individuals within their schools to react to potential predators. (Atema, et al., 1980; Bleckmann and Zelick, 2009; Dagorn, et al., 2000; Fritsches and Warrant, 2004)
Bigeye tuna are opportunistic, feeding on a variety of other fishes, cephalopods, and crustaceans. Large juveniles and adults consume mesopelagic fishes, cephalopods, and crustaceans. A study from 2012 examined bigeye tuna diets in the western tropical Atlantic Ocean. They found over 80 prey items, including 46 fish species, 20 cephalopods, and 13 crustacean species. Caribbean pomfrets (Brama caribbea) were the most abundant fish species, being found in 50.8% of stomachs examined, and Atlantic bird squids (Ornithoteuthis antillarum) were the most commonly-documented cephalopods, in 17.4% of stomachs examined. Other frequently occurring prey species included longnose lancetfish (Alepisaurus ferox), constellationfish (Valenciennellus tripunctulatus), sabertooth fishes (genus Evermannella), dwarf dories (Zenion), and various unidentified ray-finned fishes. Diets of bigeye tuna vary depending on their size and depending on geographic location, depth, time of day. In general, larger bigeye tuna eat larger prey items.
A study from 2000 found bigeye tuna in the Pacific Ocean foraged at greater depths at night, coming closer to the surface during the day to regulate their body temperature and have greater access to oxygen. In Hawaii, during daylight hours, larger bigeye tuna traveled closer to the surface every 2.5 hours, whereas smaller individuals would come back towards the surface every hour. (Dagorn, et al., 2000; Lin, et al., 2020; Ohshimo, et al., 2018; Vaske Júnior, et al., 2012)
Bigeye tuna are apex predators in their ecosystems when they reach mature adult size. However, juveniles and small adults likely serve as prey for toothed whales (suborder Odontoceti), sharks, and large fishes, such as blue marlins (Makaira nigricans) and other tunas (genus Thunnus). Bigeye tuna are opportunistic predators, and likely cannibalize smaller conspecifics.
Humans (Homo sapiens) are the only known predator of large, mature bigeye tuna. Tunas are popular in sport fishing and commercial fishing practices. (NOAA, 2022)
Bigeye tuna are opportunistic predators that eat a variety of mesopelagic fish, cephalopods, and crustaceans. As juveniles and small adults, they likely serve as prey for large fish and toothed whales.
Bigeye tuna are hosts for a large number of parasites. They are hosts for many trematodes, including Brachyphallus parvus, Didymocystis alalongue, Didymocystis lamotheargumedoi, Didymocystis bifurcata, Didymozoon longicolle, Tetrochetus coryphaenae, Lecithochirium microstomum, Nephrodidymotrema ahi, and species in the genus Koellikeria. They are also hosts for cestodes, including Sphyriocephalus dollfusi and species in the genus Trypanorhyncha. Bigeye tuna have also been reported to host the monogenean parasite species Sibitrema poonui and acanthocephalan parasites in the genus Rhadinorhynchus. (Bussieras and Aldrin, 2017; Kohn, et al., 2003; Moore, et al., 2019; Moreira-Silva, et al., 2019; Nascimento-Justo and Kohn, 2012; Nascimento Justo and Kohn, 2015)
Bigeye tuna are a popular tropical fish in fishing industries throughout the Atlantic, Pacific, and Indian Oceans. They are considered to be one of the most valuable tuna species, as measured by the number of people that rely on them as a source of food or income. In the 1960s, humans fished an estimated 100,000 metric tons of bigeye tuna annually. Since then, the amount of bigeye tuna captured has quadrupled, to around 400,000 metric tons annually. The global income from bigeye tuna fishing was estimated at $4.3 billion in 2018.
There are two main markets for tuna meat: canned products and sashimi/sushi products. A study from 2012 found that over 50% of captured bigeye tuna in the western central Pacific Ocean were caught accidentally by purse seine fisheries and were canned and sold for less than $2/kg. The other 50% of captured bigeye tuna were fished using longlines and sold whole for greater than $10/kg. Tuna head protein is a popular product in sashimi and is known to produce antioxidants. (Artetxe-Arrate, et al., 2021; Collette, et al., 2021; Sibert, et al., 2012; "The State of World Fisheries and Aquaculture: Contributing to food security and nutrition for all", 2016; Wu, et al., 2014; Yang, et al., 2011)
Bigeye tuna have no reported negative economic impacts on humans.
Bigeye tuna are listed as "Vulnerable" on the IUCN Red List. They have no special status on CITES, the U.S. Federal List, or the State of Michigan list.
Across oceanic populations, overfishing seems to be the largest threat. The threat of overfishing is ongoing in the Atlantic Ocean, but may have subsided in the Pacific Ocean, although recent reports call into question the stability of Pacific populations. Many smaller bigeye tuna are caught in purse seines as part of multiple-species schools. Because they are captured in large numbers with similar-looking species, it can be difficult to determine the exact number of bigeye tuna caught using purse seine methods and thus the magnitude of impact these fishing practices have on bigeye tuna populations. Furthermore, the capture of large numbers of immature bigeye tuna negatively impacts the recruitment of reproductively active adults.
Current conservation efforts in place for bigeye tuna include efforts to monitor the catch-per-unit-effort in different global populations and better regulate purse seining activities. There were brief moratoriums on the use of Fish Aggregating Devices (FADs) in sections of the Atlantic Ocean in 2020 and 2021, which may have alleviated pressure on bigeye tuna populations. In the Pacific Ocean, FADs are also regulated and there are maximum harvest limits on fishing boats. However, FADs are still allowed throughout much of the geographic range of bigeye tuna and populations may be at risk in areas without regulations surrounding FADs. Although bigeye tuna are not currently considered to be overfished in the Indian Ocean, there are trends that suggest they may be in the future. (Collette, et al., 2021; Gong, et al., 2017; Lennert-Cody, et al., 2008; Pérez Bielsa, et al., 2021)
Kyia Carter (author), Radford University, Sierra Felty (editor), Radford University, Karen Powers (editor), Radford University, Galen Burrell (editor), Special Projects.
the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.
body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.
uses sound to communicate
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
an animal that mainly eats meat
uses smells or other chemicals to communicate
active at dawn and dusk
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
fertilization takes place outside the female's body
union of egg and spermatozoan
A substance that provides both nutrients and energy to a living thing.
Animals with indeterminate growth continue to grow throughout their lives.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
eats mollusks, members of Phylum Mollusca
having the capacity to move from one place to another.
specialized for swimming
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).
an animal that mainly eats fish
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
mainly lives in oceans, seas, or other bodies of salt water.
remains in the same area
reproduction that includes combining the genetic contribution of two individuals, a male and a female
associates with others of its species; forms social groups.
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
movements of a hard surface that are produced by animals as signals to others
uses sight to communicate
breeding takes place throughout the year
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