Trevally: Australia's Schooling Predators

Trevally are the fish that make a dive site feel alive. They appear at 39 of the dive sites we cover, more than any other fish group in our records, and they are the silver wall that materialises out of the blue on a Sydney bommie or a Great Barrier Reef mooring. Divers who dismiss them as "just trevally" are missing one of the most reliably spectacular encounters in Australian water.

## What they are

Trevally belong to the family Carangidae, the jacks, a group built entirely around speed. The body plan is a giveaway: deep and laterally compressed, tapering to a narrow tail base carrying a stiff, deeply forked tail. That is the shape of an animal that accelerates hard and turns fast.

Most Australian trevally also carry a row of hardened, keeled scales called scutes along the rear of the lateral line. Running a hand along one, which you should not do, feels like a bread knife. They are a structural reinforcement for the tail base at speed.

Several species matter to Australian divers:

- **Bigeye trevally** (*Caranx sexfasciatus*), the classic schooling species, silver with a dark spot on the gill cover, forming the enormous slow-rotating tornadoes seen on tropical reefs - **Giant trevally** (*Caranx ignobilis*), the GT, reaching well over a metre and behaving like a completely different animal: solitary or in small groups, blunt-headed, and openly aggressive toward baitfish. In the Seychelles they have been filmed launching clear of the water to take fledgling terns in flight, the sequence that made them famous in the BBC's Blue Planet II - **Silver trevally** (*Pseudocaranx georgianus*), the temperate workhorse, common right through southern Australia and the one most Sydney and Melbourne divers actually see - **Golden trevally** (*Gnathanodon speciosus*), yellow with dark bars, whose juveniles famously shadow large sharks, rays and even divers

## Behaviour worth watching

The schooling is the obvious drawcard, but the interesting behaviour is hunting.

Trevally are ambush predators that use structure and light. They hold in the current on the up-current side of a bommie or wreck, wait for baitfish to be pushed past, then accelerate through the school in a coordinated rush. Dawn and dusk are when this happens most, which is why an early boat is worth the alarm.

They also use divers. A rising column of exhaust bubbles disorients baitfish, and trevally learn this quickly. If a school starts holding station just behind your shoulder, you are being used as a hunting tool, and staying still is the polite response.

Golden trevally juveniles take the strategy further by riding alongside larger animals, using them as moving cover. At sites like [Gold Coast Seaway](https://www.scubadownunder.com/dive-sites/gold-coast-seaway) they will happily adopt a diver as a substitute.

## Where to see them in Australia

**Tropical schooling.** [Cod Hole](https://www.scubadownunder.com/dive-sites/cod-hole) on the Ribbon Reefs and [Wigton Reef](https://www.scubadownunder.com/dive-sites/wigton-reef) on the outer Great Barrier Reef both deliver the big bigeye aggregations. [Turquoise Bay](https://www.scubadownunder.com/dive-sites/turquoise-bay) at Ningaloo adds them to a Western Australian trip.

**NSW.** [Pimpernel Rock](https://www.scubadownunder.com/dive-sites/pimpernel-rock) off Coffs Harbour is an offshore pinnacle that concentrates pelagics, and [North Solitary Island](https://www.scubadownunder.com/dive-sites/north-solitary-island-anemone-bay) is similarly productive. Closer to Sydney, [North Bondi Bommie](https://www.scubadownunder.com/dive-sites/north-bondi-bommie) holds silver trevally through the warmer months.

**Southern Australia.** [Pope's Eye](https://www.scubadownunder.com/dive-sites/popes-eye) in Port Phillip Bay is the standout, a small artificial structure that concentrates fish life dramatically. The South Australian jetties, particularly [Port Giles Jetty](https://www.scubadownunder.com/dive-sites/port-giles-jetty), reliably hold silver trevally around the pylons.

**Cool temperate.** [Montague Island](https://www.scubadownunder.com/dive-sites/montague-island) off Narooma combines trevally schools with seals, which is a very good day out.

## Getting the encounter right

Trevally reward stillness and punish pursuit. A school will tolerate a motionless diver at remarkably close range and will dissolve the instant somebody swims at it.

Approach obliquely rather than head-on, since a direct approach reads as a predator's approach. Get below the school and shoot upward if you can, because silver flanks against surface light is the shot, and a school photographed from above against dark reef is not. Control your breathing: a burst of bubbles at the wrong moment scatters everything. And hold position on the up-current side of structure and let the fish come to you, because they are already there for the same reason you should be.

## A note on numbers

Silver trevally and several other Carangidae are commercially and recreationally fished across Australia. Stocks are generally managed, but aggregating species are inherently vulnerable because a school that predictably forms at a known pinnacle is easy to target. If you dive somewhere that consistently holds a big aggregation, it is worth being thoughtful about how publicly you share the exact location.

## Sources

- [Australian Museum, Fishes](https://australian.museum/learn/animals/fishes/) - [Atlas of Living Australia](https://www.ala.org.au/)