Mary River Cod

Interesting Info

• The Mary River Cod is one of Australia’s most endangered freshwater fish, found only in the Mary River system of southeast Queensland.
• They are a cultural icon of Queensland’s Mary Valley, valued by both Indigenous communities and modern conservation groups.
• They are chunky, deep-bodied fish with a cream to olive-green colour, covered in darker blotches and marbled markings, similar in look to Murray Cod but genetically distinct.
• Historically widespread in the Mary River, their numbers collapsed in the 20th century due to overfishing, habitat destruction, weirs, and water extraction.
• Today, only a handful of small, wild breeding populations remain, mostly in upper river reaches and tributaries.
• Young Mary River Cod feed mainly on insects, shrimps, and small crustaceans, while adults are ambush predators, eating yabbies, fish, frogs, and occasionally small waterbirds or reptiles.
• They are highly territorial, with adults often returning to the same snag or log year after year.
• Breeding happens in spring to early summer (September–December) when water temperatures rise. Like Murray Cod, they are nest spawners, with males guarding eggs in hollow logs or rocky crevices until they hatch.
• Females can lay 10,000–90,000 eggs, but survival is low unless habitats are clean and undisturbed.
• Unlike Murray Cod, Mary River Cod are fully protected and cannot be targeted by anglers. Restocking has also created “conservation populations” in nearby rivers to act as insurance against extinction.
• Like other cod species, they have a physoclistous swim bladder, which means they regulate buoyancy through blood gas exchange instead of gulping or burping air. This adaptation lets them sit almost motionless in deep pools, waiting to ambush prey — but it also makes them highly vulnerable to barotrauma if pulled up quickly from depth.
• Lifespan is long — they can live 20–30 years, which makes them highly vulnerable if adult breeding fish are removed from the population.