An Australian freshwater Murray cod has surprised scientists by swimming a marathon 860 kilometres (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 13 hours' drive west of Sydney, said Arthur Rylah Institute researcher Zeb Tonkin.

Four years old and weighing in at 1.7 kilograms (3.7 pounds) at the time, it "took off" when floods hit the area in the spring of that year, he said, travelling an initial 760 kilometres upstream in less 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

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