An alarming fish kill on the Darling River at Menindee in outback NSW shows what is believed to be as many as a million dead fish.
Menindee, a town of about 500 people, is located an hour’s drive from Broken Hill.
It has been the site of several mass fish kill events in recent years, with a 40-kilometre algal bloom blamed for the death of hundreds of thousands of fish in 2019.
A DPI spokesman told news.com.au that an estimated “one million fish, predominantly Bony Herring (Bony Bream) have been affected, as well as smaller numbers of other large-bodied species such as Murray Cod, Golden Perch, Silver Perth and Carp”.
“This event is ongoing as a heatwave across western NSW continues to put further stress on a system that has experienced extreme conditions from wide-scale flooding,” the spokesman said.
The Bony Herring species typically booms and busts over time, according to the DPI.
It “booms” in population numbers during flood times and can then experience significant mortalities or “busts” when flows return to more normal levels.
They can also be more susceptible to environmental stresses like low oxygen levels especially during extreme conditions such as increased temperatures currently being experienced in the area.
“These fish deaths are related to low oxygen levels in the water (hypoxia) as flood waters recede,” the DPI spokesman said.
“Significant volumes of fish including Carp and Bony Herring, nutrients and organic matter from the flood plain are being concentrated back into the river channel.
“The current hot weather in the region is also exacerbating hypoxia, as warmer water holds less oxygen than cold water, and fish have higher oxygen needs at warmer temperatures.”
Multiple agencies across NSW, including Water NSW and the local council are continuing to work together on the response.
Claims of water mismanagement and poor river health of the Darling River in the Murray Darling Basin have been raised by locals as a potential cause of the mass deaths, but the DPI has denied this in the past.
Residents criticised a decision by the Murray Darling Basin Authority to drain the Menindee Lakes in 2014 and 2017, to meet water demands downstream.
Fish kills are defined by the NSW Department of Primary Industries as a sudden mass mortality of wild fish.
“Fish kills can occur at any time although data indicates fish kills are more likely to occur in summer or following sudden changes in temperature,” the department said on its website.