Giant Grouper

The giant grouper is the largest bony fish known from reefs, a long-lived, solitary ambush predator that reaches 270cm and 400kg and swallows even small sharks and young sea turtles whole. Despite its size it is curious and often comes close to divers.

It lives singly in shallow water around caves, wrecks and coral reefs, in estuaries and harbours, and over silt or mud in turbid water. Adults keep territories in lagoons and along the outer reef, and big fish have even been caught from the shore.

Appearance

A robust, heavy-bodied fish, usually about 180cm long, with a convex head profile, a large mouth and a slightly rounded tail. Adults are grey-brown with a mottled pattern and darker fins. Small juveniles look quite different, yellow with wide, irregular dark bars.

Behaviour

It normally keeps to itself and holds its patch of reef, yet shows real interest in people in the water. Most of its hunting is done near the bottom, usually at dusk.

Feeding

It is an opportunistic hunter that waits in ambush. Fish, small sharks and rays, young sea turtles, crustaceans and molluscs are all on the menu, and food is drawn in by powerful suction as the large mouth opens, then swallowed whole. What it eats depends on where it lives: fish around reefs and rocks favour spiny lobsters, while those in estuaries lean heavily on crabs.

Life cycle

Like most groupers it can change sex, with males developing either from functioning females or directly, without ever breeding as a female. Spawning is seasonal and follows the moon, with fish gathering, usually several females to each male, and releasing eggs and sperm into the water over about seven days. Studies of captive groups suggest the dominant male and female do nearly all the spawning for the first day or two. Others then fertilise more of the eggs as the event goes on, and even newly changed males father young. Juveniles are secretive and rarely seen on reefs until they start changing into adults.

Dangers

It is not generally seen as dangerous to people, but large fish have been reported attacking humans. Divers are advised to treat big ones with caution and never to hand-feed them.

Threats

Giant groupers are protected from all fishing in Queensland, and any caught by accident should be carefully returned to the water.

Relationship with people

Prawn fishermen have caught it in murky water over silt and mud, and one fish of 177cm was taken off Maui in Hawaii.

Away from the sea, it can be seen on show at the Georgia Aquarium, while the American Museum of Natural History holds a skeleton.

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