Native fish in Australia are hardy, having adapted over millennia to live in one of the driest continents on Earth. However, research shows that even the hardiest of native fish species aren’t immune to the effects of human changes to natural environments. River regulation, in particular, alters aquatic ecosystems significantly, changing the natural environment and causing negative physiological and behavioural impacts in native fish.

Australia’s Murray-Darling Basin is one of the most regulated river systems in the world, with river regulation often in the form of dams. The release of water from those dams often involves releasing cold water from the bottom of the dam – called the hypolimnetic zone – where the release valves are usually located. Cold water pollution changes the natural environmental conditions and causes problems for native fish and other aquatic life. This is because fish life processes are often reliant on natural changes in water temperatures to trigger processes such as spawning, recruitment, growth and migration.

Scientists based at the Arthur Rylah Institute have been researching the effect of cold water pollution in the Goulburn River on larvae for two species of native fish – Murray cod (Maccullochella peelii) and Macquarie perch (Macquaria australasica – commonly known as Maccas).

A Macquarie Perch is held gently out of the water by human hands.

Macquarie perch. Image credit: Renae Ayres & Joanne Sharley

Murray cod larvae cover the fingers of a researcher in this close-up photograph.

Murray cod larvae collected in a river. Image credit: Arthur Rylah Institute.

Globally, the bulk of research on cold water pollution is based in the Northern Hemisphere and concentrates on adult fish, so this new research on native Australian larvae provides critical information for future water management.

Cold water pollution from Eildon Dam, on the Goulburn River, is known to extend more than 100km downstream and can be 7oC lower than temperatures the river would have experienced before the dam was built. The timing of colder water releases coincides with early life stages of Murray Cod and Macquarie Perch.

The study looked at the impact of different temperatures on Murray cod and Macquarie perch larvae of different ages – >24 hours old, 7 days old and 14 days old, with temperatures ranging from 8°C-20°C. Overall, Murray cod larvae were more sensitive to lower water temperatures than Macquarie perch larvae. They found that Murray cod larvae were most sensitive at 7-days old, whilst Macquarie perch were most sensitive when the larvae were younger than 24 hours.

Hundreds of Murray cod larvae are seen in a laboratory environment, photographed up-close in a container.

Murray cod larvae. Image credit: Arthur Rylah Institute.

An indoor laboratory setup with water tanks, tubes and other equipment.

Laboratory trials to test the sensitivity of larvae to cold water pulses. Image credit: Arthur Rylah Institute.

Combining the laboratory data and modelling thermal profiles of the Goulburn River below the Lake Eildon, helped identify how vulnerable the two species and different age classes were to cold water pollution. Murray Cod were predicted to be absent from the first 26 km of river downstream of the Eildon Dam on the Goulburn River, compared with no recorded impact on the distribution of Macquarie Perch. The greater sensitivity of Murray Cod larvae to cold water may be because this species more commonly occurs in warmer lowland habitats. Macquarie Perch tend to prefer cooler mid to upland river reaches.

The native Murray cod. Image credit: SARDI

How science informs flow releases

These findings help us understand the impacts of cold water pollution for our native fish species like the Murray cod and Macquarie perch. Ultimately, native fish larvae have the best chance of survival if cold water is timed to not be released during their key early life stages and if structural changes to dams can reduce the amount of cold water being released from the bottom of the dam.


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Banner image: Aerial view of the Eildon Dam wall spillway. Image credit: Allistair (Adobe Stock).

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