A river "burp" is being blamed for the death of approximately 300 black bream on the Murray River near Ravenswood last week.
An investigation into the catchment was sparked after a number of local anglers reported hundreds of dead fish floating in the water from Murray Bend to Rodoreda Crescent on June 9.
Department of Water scientists determined the dead fish found at the site were not suitable for autopsy, which restricted their investigation to the water quality in the area.
A department spokesperson a "burp" was determined to be the cause of death.
"The water testing results and monitoring this week indicate that the fish died as a result of low oxygen due to cooled surface waters," they said.
"In this case, as the surface waters have cooled, the temperature change caused the movement of very low oxygen waters and sediments at the bottom of the system.
"This led to bubbles of hydrogen sulphide gas being released, which in turn reduced the oxygen available to fish. This process is colloquially known as a 'burp'."
The investigation comes after another mass fish death on the Murray River in the South Yunderup area last month, which confirmed nearly 30,000 fish had died in the catchment as a result of 'poor water quality'.
University of Western Australia aquatic ecology expert Anas Ghadouani urged the public and the government to "ask harder questions" in the wake of May's "catastrophic" fish kill.
"It's a bit like saying what's the cause of death? Well, their heart stopped," he said.
"Different species of fish don't all hang out in the same place. They are not sensitive to the same level of oxygen. So I don't buy the one explanation. It is simplistic.
"And why are these systems at this level that a rain could wash things into the river and cause this much damage? This is serious damage and a serious incident; multiple species, large numbers.
"Thirty thousand fish dying is not an 'incident'. It is not business as usual. It is a reminder that the system is fragile."
Professor Ghadouani, executive director of the Cooperative Research Centre for Water Sensitive Cities, said he was not criticising the government, but gave the analogy of the need to go to a specialist when a blood test at the GP showed a major abnormality.
"The official line is we are monitoring, we are measuring," he said.
"Monitoring doesn't cut it. A better understanding of the systems is needed. They are doing it by the book but we need more than a textbook reaction. We need serious attention."