Jackfish

Jackfish

Few sights in diving are as exhilarating as being enveloped by a swirling vortex of silver fish. This is the world of the jacks and trevallies, collectively known as 'jackfish'. Belonging to the family Carangidae, and particularly the genus Caranx and its close relatives, these are the wolves of the reef: powerful, intelligent, and built for speed. Their classic fish-shape is a masterclass in hydrodynamic design, featuring a sleek, laterally compressed body, a deeply forked tail for powerful propulsion, and large eyes for spotting prey in all conditions. A defining feature is the row of sharp, bony scales known as scutes located along the lateral line just before the tail, providing stability and a sharp defence. Divers will encounter several key species. The undisputed king is the giant trevally (Caranx ignobilis), a brutish, powerful predator that can reach up to 80 kg and often patrols the reef alone or in small gangs as a mature adult. Its smaller, schooling cousin, the bigeye jack (Caranx sexfasciatus), is famous for forming the mesmerising 'tornadoes' of fish seen at sites like Cabo Pulmo or Cocos Island. The bluefin trevally (Caranx melampygus) adds a splash of incredible colour, with electric blue fins and speckles over a brassy-gold body. In the Atlantic, divers will meet the horse-eye jack (Caranx latus), with its namesake large eye and often yellowish tail, while the enigmatic black jack (Caranx lugubris) is a darker, deeper-bodied fish often found in smaller groups on deeper oceanic drop-offs. The schooling behaviour of jacks is legendary. While larger individuals like giant trevally become more solitary, many species, especially as sub-adults, form vast aggregations numbering in the hundreds or thousands. This is primarily a defensive strategy, confusing predators and making it difficult to single out an individual. For bigeye jacks, this behaviour elevates into a true natural wonder: the 'jacknado'. The school coalesces into a vast, swirling vortex of fish, a constantly moving column that can be breathtaking to witness from within. This is not just for defence; it is also thought to be a form of communication and a hydrodynamic aid during rest periods in high-current areas. Jacks are highly effective mesopredators, playing a crucial role in a healthy reef ecosystem by controlling populations of smaller fish. Most are pursuit predators, using blistering speed to run down prey in open water. They are often most active during the low-light hours of dawn and dusk, using the dim conditions to their advantage. Some species have developed remarkable cooperative hunting strategies, working together to herd schools of baitfish against the reef wall before striking. The giant trevally is famously opportunistic and has even been observed launching itself from the water to snatch fledgling seabirds from the surface, a testament to its power and intelligence. Reproduction for many jack species is a spectacular, large-scale event. They are broadcast spawners, gathering in huge aggregations at specific times of the year, often cued by lunar cycles and water temperature. At sites like Fakarava Atoll in French Polynesia, thousands of jacks congregate in channels on the outgoing tide to release their eggs and sperm into the open ocean. The resulting larvae are pelagic, drifting in the ocean currents for weeks or months before settling. Juveniles typically seek refuge in the calm, protected waters of mangrove forests, seagrass beds, and estuaries, graduating to the main reef and its high-energy currents as they grow larger and stronger.