The not-so-little Murray cod that could: fish tracked swimming 900km along Australia’s biggest river system | Murray-Darling Basin

A young Murray cod completed one of the longest journeys ever recorded for the species, covering around 900km across the Murray River, its streams and side channels.

Murray cod, Australia’s largest freshwater fish, measure up to 1.5 meters and can live for half a century. Research by Victoria’s Arthur Rylah Institute showed the species, classified as vulnerable under federal environmental laws, is capable of traveling extreme distances.

Dr Zeb Tonkin, a freshwater ecologist at the institute, said Murray cod had huge cultural and conservation importance.

“They are the main predator in the Murray-Darling Basin,” he said.

In the years following a major mortality event due to low oxygen levels in 2016, researchers tagged about 70 juvenile cod using small audio tags – much like a microchip on a cat or dog – allowing their movements to be tracked via “listening stations” along the river and adjacent waterways.

At first, the young cod did not go far. “They have basically retreated into their breeding habitat,” Tonkin said. But as they approached maturity – around the age of four, when they were 50 cm long – many left in search of a new home range.

A swimming champion, whom scientists nicknamed “Arnie” after Australia’s multiple Olympic gold medalist Ariarne Titmus, left the entire study area and continued swimming, he said. It then appeared across the border with New South Wales.

Taking advantage of the removal of barriers during the 2022 floods, the cod swam in the Wakool then Niemur rivers before returning home. In total, the fish traveled nearly 900 km in less than two years. It was last spotted in late 2024 in a section of the mid Murray, near Belsar Island, just upstream from Euston, and seems to have settled there for the moment.

“For this species, it’s the longest we’ve seen,” Tonkin said. “We know that a few other species do this regularly as part of their life cycle, species like golden perch and silver perch.”

Murray cod tend to be considered a sedentary species and can often be found near snags or submerged logs. But as research has shown, they also move – in some cases hundreds of kilometers – to disperse and breed.

Another individual, the “casanova cod”, had already made a 160 km round trip four times in four years.

By tracking fish movements in the river for more than a decade, ARI researchers found that in dry years, cod tend to prefer the flowing anabranches of the river, while in wet years they are more likely to choose the main channel. The findings helped shed light on how regulators manage flows and gates in the river to support fish reproduction and survival.

“It really highlights the importance of connectivity,” Tonkin said. “With this big flood and these barriers up, they have the ability to really disperse and distribute, which is very important in terms of species recovery.”

Tonkin said tracking cod over such great distances was only possible through collaboration with interstate agencies and funding from the Murray-Darling Basin Authority.

Associate Professor Paul Humphries, a fish and river ecologist at Charles Sturt University, who wrote a book on Murray cod, said the fish was a “keystone species” – both dependent on and essential to the diversity of animals and plants in the river’s floodplain – which had been intensively fished since Australia’s gold rush in the 1850s.

“Murray cod are like the lions and tigers of our rivers,” he said.

Humphries, who was not involved in the ARI research, said movement – ​​feeding, breeding, dispersing and migrating – was as biologically important to fish as life and breathing. But it was also important for conservation, allowing them to recolonize areas, exchange genes and reproduce.

“One of the things we’ve been very successful at as humans is putting barriers in the way of fish movement,” he said, with thousands of dams, weirs and other structures across the Murray-Darling Basin.

“If we want to maintain our fish populations in a healthy state, we ultimately need to allow them to move freely – to go where they want, rather than confining them to how we manage our rivers. »