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Poor water quality is concern for native fish

Loddon River Appin South
27 Oct 2022

Widespread natural flooding across the north central catchment is impacting water quality with an increased risk of hypoxic (low oxygen) blackwater.

Dr Peter Rose, fish ecologist and North Central Catchment Management Authority (CMA) Project Manager, said while water that carries carbon is normally good for native fish, the recent floods are worrying.

“When there is natural, small-scale flooding or environmental water delivery, carbon-rich water coming off the floodplain is a vital food source for native fish, but during large-scale flooding events it can be a different story as the water can become hypoxic.

“Floodwaters have carried large quantities of organic material such as leaves, twigs and grass into our rivers and creeks which turns the water a dark brown or black colour. As the material is broken down it can reduce oxygen levels in the water which can kill fish and other aquatic fauna,” Dr Rose said.

“North Central CMA staff have been monitoring water quality at a number of sites in the north of the state and as a result of hypoxic blackwater, dissolved oxygen levels in some waterways and creeks have plummeted.

“Fish deaths have been reported in and around the lower Loddon floodplains and other species such as the Murray cray are struggling. Unfortunately, we expect to see more fish deaths and other water dependent species succumb to low oxygen levels in the coming days and weeks particularly as we head into warmer weather.”

Dr Rose said that while the scale of the flooding means it is difficult to improve water quality, the North Central CMA is investigating options to help native fish and other water dependent species survive.

"In some cases, water for the environment can be used to provide refuge for native fish or help dilute the poor-quality but this is only possible if good quality water is available and when flow in the rivers and creeks drop to normal operational levels.

“Despite the difficulties we are investigating other options. This may include aerating the water at some sites or relocating native fish species to sites with acceptable dissolved oxygen levels to improve their chance of survival,” Dr Rose said.

“There is no guarantee either of these options will work. (But) because native fish populations are already in decline across the Murray Darling Basin, and we’ve been successful in increasing their numbers in the Gunbower and lower Loddon systems through our Native Fish Recovery Plan, we are willing to give them a go.”

The community is encouraged to report fish deaths to the Environment Protection Authority on 1300 372 842.

To learn more about hypoxic blackwater visit https://youtu.be/9aNB9pUfj5E

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For further information please contact Sonia Robinson, Communications Coordinator, North Central CMA m: 0437 195 143: sonia.robinson@nccma.vic.gov.au

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