More than 150 mature Murray Cod were released into Valley Lake/Ketla Malpi by RecFish in May 2025 to create a recreational fishery and improve water quality. The introduction aimed to alter the lake’s food web, reduce algae levels and enhance the natural environment.
Unfortunately, in October 2025, Council reported to PIRSA through Fishwatch that several Murray Cod had been found deceased or dying.
“The cause was unclear. There were no signs of disease, physical damage, pollution, or changes in water quality and the fish were too degraded for analysis,” City of Mount Gambier Environmental Sustainability Business Partner Aaron Izzard said.
In November 2025, contractors undertook scheduled carp removal using electrofishing, a method that temporarily stuns fish without harming them. During the process, both healthy and sick Murray Cod were brought to the surface, providing an opportunity for further investigation. Three of the obviously affected fish were sent to a specialist pathology lab for analysis.
“Tests revealed the cod had ‘skin barrier compromise’, leading to secondary bacterial infections. The exact reason for this remains unknown. It is speculated that as the fish were only introduced in May, they have not yet experienced a full seasonal cycle to assess how they adapt. Elevated pH and seasonal fluctuations could be contributing factors to the naturally occurring conditions and subsequent infection.”
The Valley Lake/Ketla Malpi’s limestone geology causes a naturally high pH of around 9, believed to be at the upper tolerance for Murray Cod.
“Despite this, the introduction was considered worthwhile for its potential benefits to recreation and water quality. Monitoring since September 2022 shows the lake’s water quality remains very good, and further testing will continue for at least two more years,” Mr Izzard said.
“On the positive side it is good that we can rule out disease. It may be that the high pH means that some cod will survive, but some will not. Fortunately, this will mean that whatever cod do survive will be tolerant to high pH. Council will continue to monitor the water quality and work with PIRSA as required.”
Media contact: City of Mount Gambier Media and Communications Coordinator Sharny McLean on 0413 798 327 smclean@mountgambier.sa.gov.au
