Potato Cod: Meet the Giants of the Cod Hole

You notice the shadow before the fish. A grey-brown shape as long as a diver drifts in from the coral, turns broadside and hangs there, studying you with a calm, slightly baffled expression. Then a second one arrives, close enough that you can count the dark blotches along its flank. This is the Cod Hole on the far northern Great Barrier Reef, and these are potato cod: curious fish that can reach 2 m long and weigh around 100 kg.

For decades, divers have travelled to Ribbon Reef No. 10 to meet them. The story of how these fish came to be so approachable, how their numbers fell, and how they came back after cyclones and coral bleaching says a lot about how the Great Barrier Reef is managed.

Identifying the potato cod

The potato cod (*Epinephelus tukula*), also called the potato rockcod, potato grouper or black-spotted rock-cod, is a large, robust grouper. It is pale brownish-grey with large, dark, widely spaced blotches, the "potatoes" that give it its name, plus dark streaks radiating from the eyes and small dark spots on the fins. Very large adults can be almost black.

Most adults are around 1.5 m long and 90 kg, with the biggest reaching about 2 m and 100 kg or more.

Don't confuse it with the Queensland groper (*Epinephelus lanceolatus*), an even bigger relative and the largest bony reef-dwelling fish in the world. Adult Queensland gropers are a mottled greyish-brown with yellowish or darker fins, rather than pale with the potato cod's bold, widely spaced blotches.

Biology and behaviour

Potato cod live on coral reefs, typically in channels and around seamounts exposed to currents. Adults are found from about 10 m down to 150 m, while juveniles turn up in shallow tide pools. They are ambush predators that wait behind coral before lunging at prey, which includes small rays, crabs, fish, squid, octopus and lobsters.

Large potato cod are solitary and strongly territorial. Western Australia's Department of Biodiversity, Conservation and Attractions (DBCA) notes that most big individuals live alone in the same patch of reef, often for years, which helps explain why divers can meet the same fish at the same site.

Like other members of the grouper family, potato cod are protogynous hermaphrodites: they mature as females at about 90 cm and later change sex to become males.

The Cod Hole: history of a famous dive

The Cod Hole is on Ribbon Reef No. 10, one of the remote northern Ribbon Reefs of the Great Barrier Reef Marine Park, near Lizard Island. Its protection owes much to Valerie Taylor, the filmmaker and conservationist who worked alongside her husband Ron. After watching potato cod there being shot for fun, she ensured the cod's habitat was protected, which the Australian National Maritime Museum describes as the first gazetted protection of any part of the Great Barrier Reef. She also campaigned to make the grey nurse shark the world's first protected shark, declared by the NSW Government in 1984.

When the Great Barrier Reef was World Heritage listed in 1981, the Cod Hole was singled out as a site of natural phenomena and exceptional natural beauty.

The feeding years

For years, divemasters fed the cod by hand, and the fish learned to approach divers boldly. The attention was not without problems. Monitoring by the dive industry from April 1992, written up for the Great Barrier Reef Marine Park Authority (GBRMPA), found that the average number of potato cod seen per dive fell from 11.9 in 1992-93 to between 6.5 and 7.3 in later years. The cause was unknown, but the report recorded injuries to potato cod and other large fish, most probably from fish fighting during divers' feeds. In October 1995 the Cod Hole was declared a Special Management Area, with anchoring banned and visiting boats required to use moorings.

Today, GBRMPA's fish feeding guidelines require a Marine Parks permit for any tourism fish feeding, allow only staff to feed, and ban feeding fish directly by hand. The guidelines exist so fish are not overfed, do not come to rely on handouts, stay at natural numbers, and so visitors stay safe.

Decline and comeback

Then the site took a run of hits: Cyclone Ita in 2014, Cyclone Nathan in 2015 and the 2016 bleaching event, which stripped the ridge of almost all its coral and temporarily displaced the cod.

The recovery has been remarkable. In 2025 the Reef Authority reported that fast-growing plate and staghorn corals had returned and the potato cod were back, with up to five seen on a single dive. The Cod Hole is now one of 26 stewardship sites in the Tourism Reef Protection Initiative, under which operators such as Mike Ball Dive Expeditions carry out reef health surveys and monitoring. Guides on these trips occasionally see dwarf minke whales, manta rays and even whale sharks in the Cod Hole passage.

Where else potato cod live in Australia

Potato cod are found from the Shark Bay area of Western Australia around the north to Moreton Bay in Queensland, and more widely across the tropical Indo-West Pacific. In Western Australia, DBCA lists Ningaloo, the Montebello Islands and the Rowley Shoals among the marine parks where they occur.

Are potato cod dangerous?

Generally not, but they deserve respect. Fishes of Australia describes the potato cod as harmless but inquisitive, adding that on rare occasions large individuals may be aggressive. DBCA says there are no records of potato cod being aggressive towards people, but warns that their powerful jaws could cause serious injury. FishBase notes that although the species is hand-fed by divers in some places, it is potentially dangerous to the inexperienced, and GBRMPA's guidelines require feeding operators to warn participants about the dangers of fish feeding.

Most of the risk is linked to food. Dive travel guides describe cod used to handouts nudging divers in search of a hidden snack, so never carry food, keep your hands to yourself, and give a cod room if it gets pushy.

Protection and diving etiquette

Potato cod are protected across the waters where Australians usually dive them:

- Great Barrier Reef Marine Park: a protected species, and intentionally taking one requires permission from the Reef Authority. - Queensland: potato rockcod is a no-take species in tidal waters. - Western Australia: totally protected and may not be taken.

Globally, the IUCN Red List assesses the potato cod as Least Concern.

At the Cod Hole and anywhere else you meet one, let the cod set the distance, avoid touching it, don't feed it, and follow your operator's briefing. These are long-lived fish that stay on the same reef for years, so the way each group of divers behaves shapes the experience for everyone who follows.

Sources

- The Cod Hole is back, Great Barrier Reef Marine Park Authority - The Cod Hole: long-term monitoring of human usage, fish populations and injuries, GBRMPA Reef Research (1998, archived) - Guidelines for Fish Feeding in the Great Barrier Reef Marine Park, GBRMPA - Protected species of the Great Barrier Reef, Reef Authority - Size and possession limits in tidal waters, Queensland Government - Potato Rockcod, Epinephelus tukula, Fishes of Australia - Potato Cod, Department of Biodiversity, Conservation and Attractions (WA) - Ocean changemakers: Valerie Taylor, Australian National Maritime Museum

Header photo: A potato cod (Epinephelus tukula) photographed in the Lizard Island area of the northern Great Barrier Reef, the region that includes the Cod Hole, July 1990 by desertnaturalist (iNaturalist), CC BY 4.0, via Wikimedia Commons.