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).”