A DEADLY parasite and a freshwater fungus have been blamed for killing about 8000 Brisbane River catfish.
Piles of bloated fork-tailed catfish began washing up on the banks of the river between Lowood and Kholo mid-way through September, prompting an investigation by scientists from the Department of Environment and Heritage Protection.
Seqwater, Somerset Regional Council and EHP staff spent several days collecting dead fish from the river banks.
Despite the enormity of the kill, only fork-tailed catfish seemed to be affected.
Although the majority of dead fish washed up near Lowood, a Kholo resident reported seeing some of the victims floating in the river near the Kholo Bridge, only a few kilometres north of Ipswich.
EHP executive director Andrew Connor said extensive field inspections and laboratory testing of the catfish and water quality in the area was carried out immediately after the fish kill.
Scientists found the most likely causes of the catfish deaths were infections caused by the parasite Ichthyophthirius multifiliis, commonly known as white spot disease, and a freshwater fungus.
"Fish samples taken from the river showed damage to the gills and skin of the fish which is consistent with symptoms of white spot disease," he said. "Catfish was the only affected species and white spot disease is known to cause high mortality in catfish.
"Laboratory testing also excluded pesticides and toxic algae from involvement in the fish kill."
The investigation was conducted by EHP in consultation with the Department of Science Information Technology, Innovation and Arts and Department of Agriculture, Fisheries and Forestry.
There have been no other reports of dead fish in the river since September.
Somerset mayor Graeme Lehmann said the test results were good news."
"It's pleasing to know it's not a water quality issue or a chemical spill that caused the death of all these catfish," Cr Lehmann said.
"People can now enjoy the river as normal without being concerned."