The not-so-little Murray cod that might: fish tracked swimming 900km alongside Australia’s largest river system | Murray-Darling Basin

A younger Murray cod has swum one of many longest ever recorded journeys for the species, travelling about 900km via the Murray River, its streams and backchannels.

Murray cod, Australia’s largest freshwater fish, develop as much as 1.5 metres and may reside for half a century. Research by Victoria’s Arthur Rylah Institute has proven the species, listed as susceptible below federal atmosphere legal guidelines, is able to protecting excessive distances.

Dr Zeb Tonkin, a freshwater ecologist with the institute, stated Murray cod had large cultural and conservation significance.

“They’re the apex predator of the Murray-Darling Basin,” he stated.

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In the years following a significant mortality occasion because of low oxygen ranges in 2016, researchers tagged about 70 juvenile cod utilizing small audio tags – a bit like a microchip on a cat or canine – enabling their actions to be tracked by way of “listening stations” alongside the river and adjoining streams.

Initially, the younger cod didn’t transfer far. “They basically hunkered down in their nursery habitat,” Tonkin stated. But as they approached maturity – concerning the age of 4, after they had been 50cm lengthy – a number of took off in quest of a brand new dwelling vary.

One champion swimmer, which the scientists dubbed “Arnie” in honour of Australia’s a number of Olympic gold medallist Ariarne Titmus, left your complete examine space and stored swimming, he says. It later popped up over the border into New South Wales.

Taking benefit of the removing of boundaries in the course of the 2022 floods, the cod swam into the Wakool then the Niemur rivers earlier than heading again in direction of dwelling. In all, the fish travelled practically 900km in lower than two years. It was most lately tracked in late 2024 at a piece of the mid-Murray, close to Belsar Island, simply upstream of Euston, and seems to have settled there in the intervening time.

“For this species, it’s the longest we’ve seen,” Tonkin stated. “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 are usually regarded as a sedentary species and may typically be discovered hanging out close to snags, or submerged logs. But because the analysis discovered, in addition they transfer – in some circumstances lots of of kilometres – to disperse and breed.

Another particular person, the “casanova cod”, was beforehand discovered to have made a 160km spherical journey 4 instances over 4 years.

By monitoring fish actions within the river for greater than a decade, ARI researchers have discovered that in dry years cod are likely to choose flowing anabranches of the river, whereas in moist years they’re extra doubtless to decide on the principle channel. The outcomes have helped inform the way in which regulators handle flows and gates within the river to help fish breeding and survival.

“It really highlights the importance of connectivity,” Tonkin stated. “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 stated monitoring the cod over such giant distances was solely doable because of collaboration with interstate businesses 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, stated the fish was a “keystone species” – each reliant on and demanding to the variety of animals and vegetation within the river floodplain – that had been closely fished since Australia’s gold rush within the 1850s.

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

Humphries, who was not concerned within the ARI analysis, stated motion – to feed, breed, disperse and migrate – was as biologically essential to fish as residing and respiration. But it was additionally essential for conservation, by permitting 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 stated, with thousands of dams, weirs and other structures all through 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.”