Yabbies, cod and crayfish are escaping on to river banks to avoid poor water conditions.
The Murray-Darling Basin Authority warned against contaminated water conditions such as low-oxygen blackwater and blue-green algae that could lead to fish deaths.
MDBA environmental management senior director Janet Pritchard said hypoxic blackwater events occurred as leaf litter and other carbon-based debris was swept into waterways.
“As this material is consumed and broken down by bacteria, oxygen can be sucked out of the water, making it difficult for fish and other aquatic animals to survive,” Dr Pritchard said.
“With the onset of warmer weather in the coming weeks, water temperatures will start to climb and the possibility of hypoxic blackwater will increase, and unfortunately we could see fish deaths like we did during the 2011 and 2016 floods.
“The high flows and widespread flooding will also add nutrients to waterways, which will favour the growth of blue-green algae with warmer conditions. As floodwaters recede back to normal levels, we will also be on the lookout for elevated salinity levels in some locations.”
NSW Department of Planning and Environment water planning implementation director Allan Raine said agencies were closely monitoring the steady decline of oxygen in the river.
“The scale and magnitude of this flooding on public and private property means mitigation methods to get oxygen back into the water are extremely limited,” Mr Raine said.
“Methods used in drought situations, such as artificial aerators in specific river sections, won’t be viable for combating large volume and widespread blackwater events.
“We haven’t had any reports of large-scale fish deaths so far, which is good news, but as large amounts of organic material enter rivers from the floodplain, we know that this situation could change, which is why we are watching it closely.”
Commonwealth Environmental Water Holder Simon Banks said state and federal environmental water managers were working with industry and communities to divert small volumes of good-quality water from the main river channel, via irrigation channels, and then diverting it back into creeks further downstream where there was poor water quality.
“These small flows can create localised pockets of better water quality for fish and crayfish to move into,” Dr Banks said.
“This worked well last year and is being used again in the Edward Kolety Wakool region, where natural flooding has already resulted in low-oxygen blackwater.
“We will avoid delivering water where it will exacerbate the impact of floods.”
To report potential blackwater events in NSW, call the NSW fisheries hotline on 1800 043 536.
Community members in Victoria can report fish deaths to the EPA’s pollution hotline on 1300 372 842.