A young Murray cod has swum one of the longest ever recorded journeys for the species, travelling about 900km through the Murray River, its streams and backchannels.
Murray cod, Australia’s largest freshwater fish, grow up to 1.5 metres and can live for half a century. Research by Victoria’s Arthur Rylah Institute has shown the species, listed as vulnerable under federal environment laws, is capable of covering extreme distances.
Dr Zeb Tonkin, a freshwater ecologist with the institute, said Murray cod had huge cultural and conservation significance.
“They’re the apex predator of 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 – a bit like a microchip on a cat or dog – enabling their movements to be tracked via “listening stations” along the river and adjoining streams.
Initially, the young cod didn’t move far. “They basically hunkered down in their nursery habitat,” Tonkin said. But as they approached maturity – about the age of four, when they were 50cm long – several took off in search of a new home range.
One champion swimmer, which the scientists dubbed “Arnie” in honour of Australia’s multiple Olympic gold medallist Ariarne Titmus, left the entire study area and kept swimming, he says. It later popped up over the border into New South Wales.
Taking advantage of the removal of barriers during the 2022 floods, the cod swam into the Wakool then the Niemur rivers before heading back towards home. In all, the fish travelled nearly 900km in less than two years. It was most recently tracked in late 2024 at a section of the mid-Murray, near Belsar Island, just upstream of Euston, and appears to have settled there for the time being.
“For this species, it’s the longest we’ve seen,” Tonkin said. “There’s a couple of other species that we know do this as part of their life cycle regularly, species like golden perch and silver perch.”
Murray cod tend to be thought of as a sedentary species and can often be found hanging out near snags, or submerged logs. But as the research found, they also move – in some cases hundreds of kilometres – to disperse and breed.
Another individual, the “casanova cod”, was previously found to have made a 160km round trip four times over four years.
By tracking fish movements in the river for more than a decade, ARI researchers have found that in dry years cod tend to prefer flowing anabranches of the river, while in wet years they are more likely to choose the main channel. The results have helped inform the way regulators manage flows and gates in the river to support fish breeding and survival.
“It really highlights the importance of connectivity,” Tonkin said. “With that big flood event and those barriers out of the way, they’ve got capacity to really disperse and distribute, which is quite important in terms of species recovery.”
Tonkin said tracking the cod over such large distances was only possible thanks to collaboration with interstate agencies and with funding from the Murray-Darling Basin Authority.
Associate Prof Paul Humphries, a fish and river ecologist at Charles Sturt University, who has written a book on Murray cod, said the fish was a “keystone species” – both reliant on and critical to the diversity of animals and plants in the river floodplain – that had been heavily 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 – to feed, breed, disperse and migrate – was as biologically important to fish as living and breathing. But it was also important for conservation, by allowing them to recolonise areas, swap genes and breed.
“One of those things that we’ve managed to be very successful at as humans is to put barriers in the way of fish movement,” he said, with thousands of dams, weirs and other structures throughout the Murray-Darling basin.
“If we are to maintain our populations of fish in a healthy condition, we do need ultimately to allow them to have unfettered movement – to go where they want to go, rather than confining them by the way that we manage our rivers.”
