Trevally: Australia's Schooling Predators

Trevally are the silver schools that wheel around reef pinnacles from the Ribbon Reefs to Sydney, and the big lone hunters that patrol reef edges as the light fades. Four species turn up most often on Australian dives, and they look and behave quite differently.

What they are

Trevally belong to the family Carangidae, which Fishes of Australia calls the trevallies and which is known elsewhere as the jacks. The family lives in tropical and temperate seas around the world, some species enter brackish or even fresh water, and many are valued food and game fish.

The body plan is consistent: a deep, compressed body, a very slender tail base and a deeply forked tail. Most also carry a row of enlarged, bony scales called scutes along the rear, straight part of the lateral line; a bigeye trevally has 28 to 37 of them.

Four species worth knowing:

- Bigeye trevally (*Caranx sexfasciatus*) ranges across northern Australia from Cape Cuvier in Western Australia to Sydney Harbour. Adults live on coral reefs to about 100 metres and usually sit in large, slow-moving schools by day. They are dark bluish-green to gold above with a dark tail, and FishBase notes a small blackish spot at the upper corner of the gill cover and a white tip on the second dorsal fin. Juveniles have four to seven broad dark bars. - Giant trevally (*Caranx ignobilis*), the GT, is found from Perth around the tropical north to Wattamolla in New South Wales. FishBase gives a maximum length of 170 cm and a common length of about a metre, and notes that adults usually occur singly. Fishes of Australia describes a steep head profile, robust scutes and no dark spot on the rear of the gill cover. GTs often cruise reef drop-offs looking for prey, feed mostly on fish, and gather to spawn in summer on shallow seaward reefs. The BBC Earth clip with this article, filmed for Blue Planet II on an Indian Ocean atoll, shows GTs leaping clear of the water to take fledgling terns. - Golden trevally (*Gnathanodon speciosus*) juveniles are bright yellow with narrow blackish bars, the first running through the eye; adults are silvery grey with scattered black spots or blotches and reach about 120 cm. Juveniles protect themselves by piloting sharks, other large fish and turtles, and the smallest shelter among jellyfish tentacles. FishBase notes that adults also school around sharks and follow divers. - Silver trevally (*Pseudocaranx georgianus*), which the Australian Museum lists as the araara, lives in southern Australia from north of Sydney to just north of Perth. It forms large schools over reefs and nearby sand, enters estuaries and schools over artificial reefs. It is greenish-blue above and silvery below, with a large, diffuse black spot on the upper edge of the gill cover, and juveniles carry a yellow stripe along the side. Until 2006 it was confused with the widespread *Pseudocaranx dentex*.

Behaviour worth watching

Bigeye trevally are a study in timing. FishBase says they are often seen in large daytime schools, milling in stationary aggregations, but are solitary at night when they feed, and Fishes of Australia says adults are most active at night or twilight. A daytime diver usually meets a resting school; the hunting happens later.

Night dives on the Ribbon Reefs show that hunting at close range. Mike Ball Dive Expeditions says the night dive at Gotham City (Challenger Bay) can be very active, as trevally, red bass and sharks hunt in divers' torch light and come very close. Dive The World describes giant trevally and red bass there snapping up any small fish the moment a torch lights it, and suggests keeping the beam off small fish so you do not put them on the menu.

Golden trevally juveniles are the ones to watch around big animals: Fishes of Australia links footage of a juvenile sheltering beside a turtle.

Where to see them in Australia

Ribbon Reefs and the Coral Sea. Mike Ball lists schools of bigeye trevally at Steve's Bommie near Ribbon Reef No. 3, with giant and bluespotted trevally often seen hunting, and large schools of giant trevally at Gotham City, between Ribbon Reefs 9 and 10. At Osprey Reef in the Coral Sea it describes resident barracuda and schooling bigeye trevally at False Entrance. Near Lizard Island, scientists filmed a huge school of giant trevally on Day Reef in November 2018.

Gold Coast. Ian Banks's footage, linked from Fishes of Australia, shows bigeye trevally schooling at the sand bypass pipe at the Gold Coast Seaway, reached as a shore dive. In 2012 he described it as a daily gathering that could exceed 1,000 fish; ask local divers how the school is behaving now.

Ningaloo. The Reef Life Survey photo on Fishes of Australia's golden trevally page was taken at Ningaloo Reef.

Coffs Harbour. Jetty Dive lists schooling silver trevally at Grey Nurse Gutters and along the Boulder Wall at South Solitary Island, and Reef Life Survey's photos of both bigeye and silver trevally on Fishes of Australia were taken in the Solitary Islands.

Sydney. Michael McFadyen lists silver trevally among the fish at North Bondi, a shore dive off the Ben Buckler rock platform.

Getting the encounter right

Mike Ball's photo notes for Lighthouse Bommie, near the bottom of Ribbon Reef No. 10, sum up the approach. Schools of trevally and barracuda there are easily approached, which makes it a good dive for a shot of your buddy inside a school. Keep strobe power down so the bright silver fish do not blow out, and try to keep the sun behind you, because shooting up towards the surface often over-exposes it.

At night, follow Dive The World's advice and keep your torch off the small fish.

A note on numbers

Fishes of Australia says silver trevally are targeted by recreational and commercial fishers throughout their range, that declines have been detected in New South Wales, and that the fishery is managed with catch, size and bag limits. The species is slow-growing and long-lived, with a recorded age of more than 45 years. FishBase rates the giant trevally's vulnerability to fishing as high to very high. If you dive a site that reliably holds a big school, think about how publicly you share the exact spot.

Site Access and Logistics

- Ribbon Reefs and Osprey Reef: these are liveaboard sites. Mike Ball's Ribbon Reef guide covers sites from Cod Hole to Steve's Bommie, and the company runs separate Osprey Reef liveaboard trips. - South Solitary Island: Jetty Dive says the island is about 8 km off the coast and 30 to 40 minutes by boat from Coffs Harbour. Currents can be strong at the Grey Nurse Gutters mooring, so the operator plans drifts along the Boulder Wall rather than fighting the flow. - North Bondi: McFadyen describes parking in the lower car park off Ramsgate Avenue, walking down the stairs at its north-western end to the rock platform, and choosing an entry to suit the sea. The bottom is about 5 to 6 metres at the start of the dive. - Gold Coast Seaway: Ian Banks reached the bypass pipe school as a shore dive. - Night dives: keep torch beams off small fish.

Sources

- Fishes of Australia: Trevallies, Carangidae - Fishes of Australia: Bigeye Trevally, Caranx sexfasciatus - Fishes of Australia: Giant Trevally, Caranx ignobilis - Fishes of Australia: Golden Trevally, Gnathanodon speciosus - Fishes of Australia: Silver Trevally, Pseudocaranx georgianus - Australian Museum: Araara, Pseudocaranx georgianus - FishBase: Caranx ignobilis summary - FishBase: Caranx sexfasciatus summary - FishBase: Gnathanodon speciosus summary - FishBase: Pseudocaranx georgianus summary - BBC Earth on YouTube: Bird vs Fish, Blue Planet II - Discover Wildlife: Watch giant trevally hunt tern - Mike Ball Dive Expeditions: Ribbon Reef dive sites - Mike Ball Dive Expeditions: Osprey Reef liveaboard - Dive The World: Ribbon Reefs and Cod Hole dive sites - Lizard Island on YouTube: Giant Trevally near Lizard Island - Ian Banks on YouTube: Bigeye Trevally of the Gold Coast Seaway Sand Pipe, 8 August 2012 - Jetty Dive: Dive sites of the Solitary Islands Marine Park - Michael McFadyen's Scuba Diving: North Bondi

Header photo: A school of bigeye trevally in the Coral Sea off Queensland, by Mantas Vidutis, CC BY 3.0, via Wikimedia Commons.