A freshwater fish named Arnie just swam a record 530 miles, much of it 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 Australia’s recently retired Olympic champion Ariarne Titmuswas first tagged in Mullaroo Creek, about a 13-hour drive west of Sydney, in early 2022, said Arthur Rylah Institute researcher Zeb Tonkin.

It was four years old and weighed 3.7 pounds at the time, but “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 travel longer distances because barriers had been removed along the Murray River to allow floodwaters to pass through.

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

Arnie turned around at some point in the last twelve months and swam another 60 miles downstream towards home.

Researchers only discovered the extent of the fish’s travels when they shared data with colleagues a few weeks ago.

“We’ve been working on these species for decades… and we haven’t encountered this scale of movement before,” Tonkin said. “Probably the best we’ve seen a Murray cod do is about 160 kilometers (100 miles).”

Understanding Murray cod can help researchers adjust water flows to protect their environment, said Tonkin, whose institute is part of Victoria state’s Department of Energy, Environment and Climate Action.

The fish – which is not related to the ocean cod species – can live for more than 48 years, grow to about 6 feet long and weigh more than 180 kilograms, according to the government’s survey of Australian fish stocks.

“If we see this species reproducing, growing well and migrating well, that’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-funded Living Murray programme, administered by the Murray-Darling Basin Authority.

According to the Australian MuseumThe Murray cod has a hollow snout with a large mouth and small eyes. It preys on fish, molluscs, turtles and some small land animals, including birds, mammals and snakes.

Murray cod is listed as “vulnerableunder Australia’s Environmental Protection and Biodiversity Conservation Act. Populations of the species have declined dramatically, initially due to commercial fishing, according to the Australian Museum.

“More recently, overfishing, river degradation and human modification of the environment have caused major population declines,” the museum says. “Murray Cod is now relatively rare in most areas.”