Locals found thousands of dead fish washed ashore on Lake Wyangan in New South Wales’ (NSW) Riverina region on May 23.
Among the thousands of fish kills were the native murray cod, yellow belly perch, and bony bream.
Griffith City Council promptly alerted the NSW Environmental Protection Agency and Department of Primary Industries (DPI) for an investigation.
“Samples have been taken of each fish species and will be analysed. Several water samples have also been taken and are being sent to an independent laboratory for testing,” Griffith City Council General Manager Brett Stonestreet said in a media release on Monday.
Murray MP Helen Dalton visited the lake on Monday and found it to be “absolutely putrid, dark in colour, full of foam and sludge with dead native fish floating up on the shore.”
Griffith Mayor Councillor John Dal Broi said the council recognised the severity of the situation.
“The fish fatalities are obviously very distressing and the council is making every endeavour to find out why this occurred,” Dal Broi said.
Stonestreet said it was too early to speculate on what caused the fish fatalities but Dalton said she suspected contaminated overspill could be to blame.
According to a recent WaterNSW’s regional water availability report (pdf), native fish are at short-term risk of being harmed in NSW rivers and lakes due to increased rainfall, which is resuspending decomposed litter in dry riverbeds that “can cause short term impacts on water quality.”
“Members of the public can be on the lookout for signs of stressed or dead fish and report them through the Fisher Watch Hotline on 1800 043 536,” the report said.
As Lake Wyangan is a man-made lake, the system needs to be engineered to maintain the ecological balance over time.
“The Lake is actually an old quarry and over the decades has accumulated large quantities of sediment loaded with nutrients,” Dal Broi said.
The council is currently finalising the selection of a contractor to construct sedimentation ponds at Lake Wyangan, which will help capture coarse sediment and litter carried by water.
“The strategy to improve the water quality is also dependent on improving circulation of the water and discouraging the growth of blue-green algae,” Dal Broi added.