Fish
Giant Trevally
Caranx ignobilis · also called giant trevally, GT, ulua, lowly trevally
The giant trevally is the bruiser of the reef — a metre and a half of blunt-headed silver muscle that runs down anything smaller than itself. And in a few places in the Seychelles, “anything” includes birds: GTs there leap clear of the shallows to pluck young terns out of the sky.
Overview
Giant trevally are apex predators of Indo-Pacific reefs, lagoons, atolls, and coastal flats, from East Africa to Hawaii. They are the largest trevally (jack), up to 1.7 m and 80 kg, deep-bodied and powerful. They eat fish, squid, and crustaceans, and big adults will take lobsters, small turtles, and seabirds, hunting in explosive bursts and often cooperatively or by shadowing sharks and octopuses.
Hunting birds
At St Joseph and Farquhar atolls in the Seychelles, giant trevally gather under sooty tern colonies when the chicks fledge and ambush the fledglings, sometimes launching clear of the surface to grab a bird in mid-air. It is a seasonal, local, learned specialisation, not something the whole species does — and it was famously filmed for Blue Planet II.
Prized and pressured
The GT is one of the world’s top game fish for its size and fighting power, and is also heavily fished commercially and by spearfishers, so large old individuals have become scarce near people.
Status
The giant trevally is Least Concern and still common in remote areas, but locally depleted and reduced in size wherever fishing pressure is high.
Giant Trevally: common questions
Do giant trevally really jump out of the water to catch birds?
Yes. The behaviour was captured on film for the BBC series Blue Planet II at St Joseph Atoll in the Seychelles, and it has been studied there and at nearby Farquhar Atoll. During the few weeks each year when sooty tern chicks are learning to fly, giant trevally gather in the shallow lagoon beneath the colony and pick off fledglings that flutter low over the water, ditch into it, or lose height on a first flight. The largest trevally will accelerate up through the surface and take a bird cleanly out of the air a short distance above the water, judging the bird's speed and trajectory to intercept it. Outside the tern season those same fish hunt normally on fish and invertebrates.
Is the giant trevally dangerous to swimmers or divers?
It is not considered dangerous in the way a large shark is, and there are no records of giant trevally seriously injuring people unprovoked. However, big GTs are bold, curious, and territorial, and divers and snorkellers regularly report large individuals approaching closely, following them, and behaving aggressively, especially around bait, spearfishing catches, or feeding activity. A fish of 60 or 70 kilograms charging in fast can be genuinely intimidating and could knock a person or cause a defensive injury if it collided or if someone was holding speared fish. Spearfishers treat large GTs with respect and keep catch away from their bodies.
Why is the giant trevally such a popular game fish?
A combination of size, power, aggression, and accessibility. GTs grow large, they hit lures and baits explosively, often in shallow, visible water where an angler can see the whole take, and once hooked they make blistering runs straight for the reef, using their deep bodies and broad tails to generate enormous pull for their weight, so landing one on rod and reel is a serious physical contest. They are found on flats and around atolls where anglers can cast to them by sight, and they respond well to surface poppers, which makes for dramatic fishing. Most modern GT angling for trophy fish is catch-and-release, because large old individuals are now scarce and slow to replace.
How is the giant trevally different from other trevallies and jacks?
It is the largest member of the trevally group and one of the largest of the whole jack family, Carangidae, which also includes amberjacks, pompano, and scad. Compared with other common trevallies such as the bluefin trevally or golden trevally, the giant trevally has a steeper, blunter head profile, a deeper and heavier body, and plainer silver colouring without bright markings, though large males turn very dark. Ecologically it takes the role of a reef apex predator that smaller trevallies do not, hunting bigger prey, ranging into very shallow and even brackish water, and dominating other fish at a carcass or a bait ball.