A freshwater fish named Arnie just swam a record 530 miles, mostly upstream, Australian scientists say

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

The fish, named Arnie after the recently retired Australian Olympic champion Ariarne Titmuswas first marked in early 2022 at Mullaroo Creek, about 13 hours’ drive west of Sydney, said Zeb Tonkin, a researcher at the Arthur Rylah Institute.

Four years old and weighing 3.7 pounds at the time, he “took off” when floods hit the area in the spring of that year, Tonkin said, initially traveling 470 miles upriver in less than two months.

The scaly river dweller, a top water predator, was able to cover longer distances because barriers along the Murray River had been removed to allow floodwaters to pass through.

“It basically provided free passage for the fish,” Tonkin said.

Arnie then turned around sometime in the last 12 months and swam another 60 miles downriver toward his home.

The researchers discovered the magnitude of the fish’s journeys when they shared data with colleagues a couple of weeks ago.

“We’ve been working on these species for decades… and we hadn’t encountered that kind of scale of movement beforehand,” Tonkin said. “Probably the best we’ve seen a Murray cod do is about 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, which is 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 study of Australian fish populations.

“If we see this species reproducing, growing well and migrating well, it’s a good indication that the overall ecosystem 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-funded Living Murray programme, 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 feeds 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. According to the Australian Museum, populations of the species have declined dramatically, initially due to commercial fishing.

“More recently, overfishing, river degradation, and human modification of the environment have led to significant population declines,” the museum states. “Murray cod are now relatively uncommon in most areas.”