"Muzza", Sydney Aquarium's resident one-meter long Murray Cod fish, waits to be fed on November 7, 2011.
GREG WOOD/AFP via Getty Images

An Australian freshwater Murray cod has surprised scientists by swimming a marathon 530 miles along a major river system, believed to be a record for the species.

The fish, named Arnie after Australia's recently retired Olympic champion Ariarne Titmus, was first tagged in early 2022 in Mullaroo Creek, about a 13-hour drive west of Sydney, said Arthur Rylah Institute researcher Zeb Tonkin.

Four years old and weighing in at 3.7 pounds at the time, it "took off" when floods hit the area in the spring of that year, Tonkin said, traveling an initial 470 miles upstream in fewer than two months.

The scaly river dweller, an apex water predator, was able to cover longer distances because barriers had been removed along the Murray River to let flood waters pass through.

"It basically provided free passage for fish," said Tonkin.

Arnie then turned around at some point in the past 12 months, swimming another 60 miles downstream toward home.

Researchers only discovered the extent of the fish's travels when sharing data with colleagues a couple of weeks ago.

"We've been working on these species for decades ... and we haven't come across that sort of scale of movement beforehand," Tonkin said. "Probably the best we have seen a Murray cod do is around 160 kilometers (100 miles)."

Understanding Murray cod allows researchers to adapt water flows to protect their environment, said Tonkin, whose institute is part of the state of Victoria's department of energy, environment and climate action.

The fish — not related to ocean cod species — can live more than 48 years, grow to about 6 feet and weigh more than 180 pounds, according to the government's survey of Australian fish stocks.

"If we see this species breeding, growing well, migrating well, it's a pretty good indication that the ecosystem in general is pretty healthy," Tonkin said.

The decade-long fish tagging project works closely with the Mallee Catchment Management Authority. It is funded by the state-financed Living Murray program, managed by the Murray-Darling Basin Authority.

According to the Australian Museum, the Murray cod has a concave snout with a large mouth and small eyes. It preys on fish, mollusks, turtles and some small terrestrial animals, including birds, mammals and snakes.

Murray cod are listed as "vulnerable" under Australia's Environmental Protection and Biodiversity Conservation Act. The species' populations have declined dramatically, according to the Australian Museum, initially because of commercial fishing.

"More recently, overfishing, river degradation and human modification of the environment have caused major population declines," the museum says. "Murray Cod are now relatively uncommon in most areas."

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