Bigeye Tuna: Characteristics, Habitat, Threats and FAQ

The bigeye tuna (Thunnus obesus) is an impressive apex predator that roams the world’s tropical and temperate oceans hunting smaller fish, squid, and crustaceans.

Reaching lengths over 8 feet and weights approaching 400 pounds, their large size, speed, and global distribution make them highly sought-after by commercial fisheries.

However, despite being one of the most economically valuable tuna species, surprisingly little is known about the biology and behavior of these mysterious wanderers of the open ocean.

Bigeye Tuna Quick Overview

Attribute	Information
Scientific name	Thunnus obesus
Kingdom	Animalia
Phylum	Chordata
Class	Actinopterygii
Order	Perciformes
Family	Scombridae
Genus	Thunnus
Species	obesus
Common names	Bigeye tuna
Description	Dark metallic blue on the back and upper sides, white on the lower sides and belly. Large heads and eyes, long pectoral fins. Can grow up to 2.5 m (98 in) in length and weigh over 180 kg (400 lb) .
Appearance	Dark metallic blue on the back and upper sides, white on the lower sides and belly. Large heads and eyes, long pectoral fins. 13 or 14 dorsal spines.
Size	Can grow up to 2.5 m (98 in) in length and weigh over 180 kg (400 lb) .
Habitat and distribution	Highly migratory, favor water temperatures between 55° and 84° F. Found in tropical and warm temperate waters of the Atlantic, Pacific, and Indian oceans. Schools at the surface, near floating objects, and seamounts. Western and Central Pacific Ocean and the Indian Ocean bigeye are each considered single and separate stocks.
Diet	Prey on fish, crustaceans, and squid.
Ecological role and predators	Opportunistic predators, preyed upon by sharks, billfish, and toothed whales.
Conservation status and economic importance	Important in commercial fisheries, accounting for nearly 10% of the world’s catch of major tunas. Overfishing has caused populations to fall below sustainable levels. Fisheries managers should modernize regulations and improve oversight to protect tuna and marine ecosystems.
Human interaction and research	Principal target of tropical longline fisheries. Recovery can be relatively fast, but overfishing has caused populations to fall below sustainable levels. Bigeye recovery is a “key, immediate management priority”.

Physical Appearance and Characteristics

Bigeye tuna have a distinctive physical appearance fitting of a powerful pelagic predator designed to chase down fast-moving prey.

Their bodies are robust, deep, and streamlined for reducing drag in the water during rapid acceleration bursts. The pectoral fins are also exceptionally long and slender, reaching back to or beyond the second dorsal fin. This expanded surface area improves maneuverability and balance during high-speed turns and attacks.

In addition to their hydrodynamic contours, bigeye tuna exhibit countershading coloration on the exterior of their bodies.

The dorsal surface and sides are a dark, metallic blue that fades into silvery-white on the underbelly. By being darker on top and lighter below, bigeye tuna blend in when viewed from above and below to avoid detection by prey and predators.

But perhaps the most striking feature that gives the bigeye tuna its name is the disproportionately large, bulbous eyes positioned on either side of the head.

In fact, their eyes are so well-developed and contain specialized pigments that may enable enhanced vision in low-light conditions. This superior eyesight likely provides a key evolutionary advantage when hunting in the ocean’s dimly lit depths.

Global Distribution and Habitat Preferences

Bigeye tuna inhabit the world’s tropical and temperate oceans, absent only from the Mediterranean Sea.

More specifically, valid stocks for fishery management purposes exist in three major regions: the Atlantic Ocean, Western and Central Pacific Ocean, and Indian Ocean. Despite some mixing, the populations show enough separation across ocean regions to be assessed individually.

Within their expansive range, electronic tagging studies show bigeye tuna typically spend the majority of their time in surface waters, particularly at night to capitalize on vertically migrating prey.

However, they frequently dive to remarkable depths during the day, up to at least 1500 feet to potentially avoid predators, reduce metabolic expenditures, or track deep scattering layers of organisms. Regardless of the exact motivations behind these pronounced vertical movements, such broad physiological tolerances expand the niches bigeye can occupy.

In terms of geographic preferences, bigeye tuna migration patterns are driven largely by water temperatures between 55° to 84° Fahrenheit and association with certain features. For instance, floating objects, seamounts, and meandering currents all tend to attract increased densities, either due to organism accumulation or structural shade and protection.

Bigeye also associate closely with whale sharks, manta rays, dolphins, and marine mammals potentially to exploit the food resources or hydrodynamic advantages of swimming alongside these large animals.

Ecological Role and Significance

As apex pelagic predators, bigeye tuna play an integral role in ocean ecosystem dynamics. Capable of reaching swimming speeds over 50 miles per hour in short bursts, they are impressive hunters feeding on a diverse assortment of smaller bony fishes, squids, and crustaceans. In many ways, they help structure marine food webs through top-down forcing.

Yet, bigeye tuna must also avoid becoming prey themselves throughout their long-distance travels across the sea. Open ocean sharks, marlins, sailfish, and marine mammals all represent potential predators to varying degrees. But the combination of schooling behavior, high-speed maneuverability, acute vision, and distinct countershading collectively minimize risks.

Beyond influencing ecosystem structure through feeding connections, bigeye tuna also transport significant nutrients horizontally and vertically through their daily movements and migration patterns.

In effect, they physically connect distant ecoregions and depth zones that may otherwise remain largely independent. The degree to which bigeye tuna migrations and behavior match shifting environmental conditions brought on by climate change remains unknown at this time.

Threats From Overexploitation

As valiant ocean roamers maintaining balance in pelagic ecosystems, bigeye tuna now face intensifying threats from industrialized overfishing. In fact, global catch rates now hover nearly 30% above maximum sustainable yields each year.

Consequently, bigeye stocks show declining population trends in the Eastern and Western Pacific, while Atlantic stocks seem comparatively healthier for now. But persistent overexploitation across much of their range damages reproductive potential and leads to fewer large individuals over time.

Compounding this issue of growth overfishing, nearly 20 percent of the total catch consists of juvenile fish measuring under 3 feet in length. By excessively harvesting individuals before reaching maturity, recruitment overfishing further jeopardizes population stability moving forward.

In summary, unsustainable harvest intensities coupled with indiscriminate gear types that capture juveniles warrant legitimate near-term concerns over bigeye conservation if left unchecked. Although inherently productive with high fecundity levels, their complex migratory circuits depend on international cooperation and responsible, adaptive decision-making.

Management Efforts and Future Priorities

Encouragingly, coordinated management efforts to rebuild bigeye tuna stocks have expanded greatly since the mid 20th century rise of industrial-scale fisheries.

The Western and Central Pacific Fisheries Commission implemented a series of harvest control rules with specific effort and catch limits in the last decade.

In the Atlantic and Indian Ocean, the International Commission for the Conservation of Atlantic Tunas serves as the regulatory authority working to curtail fleet capacities and quotas. They also oversee extensive data collection programs recording catch records by species and gear type.

Integrating these monitoring initiatives with ecological research on distribution patterns and population structure remains a key priority looking ahead.

Electronic tags capable of tracking horizontal movements and vertical habitat preferences now play pivotal roles in mapping stock boundaries. Similarly, genetic analyses help delineate the level of immigration and emigration between management jurisdictions.

Armed with this knowledge, agencies can designate quotas and protected areas more judiciously based on exploitation histories and recovery timeframes across districts.

If reforms also restrict access to fish aggregating devices and nets capturing juveniles, a combination of spatial planning, effort restrictions, and size limits applied internationally offer promise for rebuilding bigeye stocks to sustainable levels in harmony with commercial fisheries.

Looking Ahead at the Future of Bigeye Tuna

Despite facing mounting threats from industrialized fishing fleets spanning every ocean, bigeye tuna as a species appear resilient thus far thanks to their exceptional growth capacity and high fecundity.

However, warning signs clearly demonstrate overexploitation occurring broadly across stocks. And the reality is scientists still understand little regarding their reproduction, movement ecology, feeding habits, and responses to environmental shifts on global scales.

Bigeye tuna serve as a valuable litmus test for humankind’s willingness to restrain short-sighted impulses overpowering the natural world, sacrificing sustainability for profits without knowledge of unintended consequences.

As mysterious wanderers of tropical seas, their fate rests precariously hinged upon an increasingly connected society centered around commerce choosing cooperation over competition at junctions determining conservation outcomes. Perhaps only then can the world’s most magnificent pelagic predators persist roaming far beyond horizons into future generations.

Frequently Asked Questions About Bigeye Tuna