Australian researchers document a Murray cod swimming 860km in an unprecedented migration event

An Australian freshwater Murray cod surprised scientists by running an 860-kilometer (530-mile) marathon along a major river system, likely a record for the species.

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

Four years old and weighing 1.7 kilograms (3.7 pounds) at the time, he “took off” when floods hit the area in the spring of that year, he said, initially traveling 760 kilometers upstream in less than two months.

This scaly river inhabitant, an aquatic apex predator, was able to travel longer distances because barriers had been removed along the Murray River to let floodwaters through.

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

Arnie then turned around at some point in the last 12 months, swimming another 100 kilometers downstream toward 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’ve never encountered movements of this magnitude before,” Tonkin said.

“The best we’ve seen a Murray cod do is probably about 100 miles.”

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

This fish, unrelated to the oceanic cod species, can live more than 48 years, measure about 1.8 meters and weigh more than 83 kilograms (183 pounds), according to the government’s survey of Australian fish stocks.

“If we see this species reproducing, growing well, migrating well, that’s a pretty good indication that the ecosystem in general is pretty healthy.”

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

© 2025 AFP