Bigeye Trevally

The Bigeye Trevally, scientifically known as Caranx sexfasciatus, is a prominent member of the Carangidae family, commonly referred to as jacks. This widespread marine fish is also known by other names such as bigeye jack, great trevally, six-banded trevally, and dusky jack.

Found throughout the tropical and subtropical waters of the Indian and Pacific Oceans, its extensive range stretches from South Africa in the west to California and Ecuador in the east, encompassing regions like Australia and Japan. Primarily an inshore species, adult Bigeye Trevally inhabit coral and rocky reef complexes, typically found at depths down to 100 m. Juveniles, however, often venture into shallower sandy bays, lagoons, and estuaries, and have even been observed in freshwater environments.

One of the most captivating behaviors of the Bigeye Trevally is its tendency to form immense, slow-moving, semi-stationary schools during the day. These aggregations can consist of thousands of individuals, often swirling into mesmerizing "tornado-like" formations, a signature visual at many popular dive sites. These large schools are thought to offer protection from predators like sharks and larger fish. At dusk, these impressive schools disperse as the fish become active, solitary, or small-group hunters, primarily feeding on smaller fish, squid, and crustaceans. Their diet shifts as they mature, moving from a more crustacean-dominated intake as juveniles to an almost exclusively fish-based diet as adults. The Bigeye Trevally plays a vital role in marine ecosystems as both a predator and a food source for larger animals. Its conservation status is currently listed as Least Concern.

This species is frequently observed by divers at numerous locations, including:

• SS Yongala (abundant)
• USAT Liberty Wreck (common)
• Manta Point Nusa Penida (common)
• Castle Rock (common)
• Twin Rocks (common)
• Manta Bowl (common)
• USS New York (ACR-2) (common)
• Verde Island Wall (abundant)
• Mapating (common)
• Kirby's Rock (occasional)

Identification

• Size: The Bigeye Trevally is one of the larger members of its genus, capable of reaching a maximum recorded size of 1.2 m (4 ft) in length and weighing up to 18 kg (40 lbs).
• Body Shape: It possesses a compressed, oblong, and streamlined body, with a dorsal profile that is slightly more convex than its ventral side, contributing to its fast-swimming capabilities.
• Eyes: As its name suggests, a key distinguishing feature is its disproportionately large eyes, which are particularly adapted for hunting in low-light conditions at dusk and night.
• Coloration: Adults typically display an iridescent silvery-blue to olive-green coloration dorsally, transitioning to a silvery-white on their ventral side. Juveniles are silvery-yellow to silvery-brown and exhibit five to six distinct dark vertical bands on their sides, which gradually fade and disappear as they mature.
• Fins: The second dorsal fin is dark with a prominent white tip on its lobe, a key identification marker. The anal and caudal fins in adults range from yellowish to black, and the caudal fin is distinctly forked. Their pectoral fins are notably longer than their head.
• Other Features: Look for a small, dark spot located on the upper margin of the operculum (gill cover). They also have strong, dark scutes along the straight portion of their lateral line.

The Bigeye Trevally (Caranx sexfasciatus) is a highly adaptable and widely distributed species, making it a common sight for divers across vast tropical and subtropical marine environments.

Geographic Distribution:

This species is found throughout the expansive Indo-Pacific region, spanning from the eastern coast of Africa, including the Red Sea and Persian Gulf, eastward across India, Southeast Asia, Indonesia, and numerous offshore Indian Ocean islands. Its range extends north to Japan and south to Australia. In the Pacific Ocean, the Bigeye Trevally inhabits most tropical island groups, including Hawaii, and reaches the western American coastline from California in the north down to Ecuador and the Galapagos Islands. Notably, it is not found in the Atlantic Ocean.

Adult Habitat and Ecosystems:

Adult Bigeye Trevally predominantly inhabit inshore coastal waters, though they can also be encountered in pelagic settings far offshore, around remote islands, and on deeper seamounts. They are strongly associated with structurally complex ecosystems, including:

• Coral Reefs: Often found schooling over and around vibrant coral reef complexes.
• Rocky Shores and Walls: Frequently observed along rocky shorelines and steep drop-offs.
• Underwater Structures: Wrecks and other man-made structures serve as important aggregation points, as seen at dive sites like the SS Yongala and USAT Liberty Wreck.

During the day, Bigeye Trevally are typically found in large, often slow-moving schools, aggregating near reefs or other structures. These impressive schools can be a highlight at many dive sites, such as Manta Point Nusa Penida and Verde Island Wall. At night, they disperse to become active, nocturnal predators.

Juvenile Habitat:

Juvenile Bigeye Trevally exhibit remarkable adaptability, preferring shallower, more sheltered inshore areas. They are known to venture into a variety of environments, demonstrating a broad salinity tolerance:

• Estuaries and Bays: Commonly found in estuaries and sandy bays.
• Lagoons and Mangroves: Utilize shallow lagoonal areas and mangrove zones.
• Freshwater Systems: Uniquely, juveniles have been reported to penetrate well into the upper reaches of rivers, indicating their ability to tolerate freshwater conditions.

Depth Range:

Bigeye Trevally are typically found from the surface down to depths of approximately 100 m. While most commonly observed by divers in shallower reef environments, they have been recorded at maximum depths reaching up to 146 m. They prefer clear coastal and oceanic waters, actively avoiding turbid conditions.

The Bigeye Trevally (Caranx sexfasciatus) is a captivating species for divers, renowned for its distinctive schooling behavior and predatory prowess.

Underwater Encounter Behavior

• During daylight hours, Bigeye Trevally are typically encountered in large, mesmerizing schools, often comprising over 1500 individuals. These impressive aggregations can be seen stationary or slowly cruising around coral and rocky reef complexes, as well as near passes and outside reef barriers.
• This schooling behavior is believed to offer protection from larger predators.
• While generally shy and prone to darting away if disturbed, these schools occasionally exhibit curious behavior, circling divers and creating a breathtaking "fish tornado" effect.
• Juveniles tend to inhabit shallower, inshore waters, including sandy bays, lagoons, and even estuaries, while adults are found in deeper waters, often down to 100 meters, around reefs and offshore seamounts.

Feeding Habits

• Bigeye Trevally are primarily nocturnal predators, becoming most active and hunting at dusk and throughout the night.
• The large daytime schools disperse as individuals or smaller groups venture out to forage.
• Their diet consists mainly of smaller fish, squid (cephalopods), and crustaceans.
• Juveniles primarily feed on crustaceans, but as they mature, their diet shifts to almost exclusively fish.
• Equipped with large eyes, they possess keen eyesight, which aids their hunting efficiency in low-light conditions.
• They are fast-swimming, voracious predators, often relying on their speed to ambush and overpower prey, and have been observed using coordinated movements to herd their targets.

Social Structure

• The Bigeye Trevally exhibits a highly social structure, forming vast aggregations during the day, which can number over 1500 fish.
• These schools serve as a collective defense mechanism against larger marine predators.
• At night, this cohesive social structure temporarily dissolves as individuals or small groups scatter to hunt.
• Beyond daily schooling, large aggregations also form for reproductive purposes.

Notable Seasonal Patterns Relevant to Divers

• Bigeye Trevally reach sexual maturity at approximately 42 cm in length.
• Spawning occurs in large aggregations, with specific periods varying across their wide distribution. For instance, spawning is known to take place between July and September in the East Pacific and from November to March in South Africa.
• There is evidence to suggest that spawning events may coincide with new moon periods.
• During spawning, pairs will break away from the main aggregation, with the male often changing color to black, and chase off other approaching individuals. Courtship and spawning have been observed at dusk.