Fish pathology and water sample tests have been completed in response to the fish deaths in the Collie River in February.
The Department of Water South West Region Manager Adam Maskew said analysis of all information indicated that the fish died as a result of extremely low oxygen levels in the water.
“Samples taken during the response showed oxygen at levels known to cause fish deaths,” Mr Maskew said.
“The low dissolved oxygen in the water was most likely caused by mobilisation of anoxic sediments and water during a river flow event.”
Mr Maskew said a significant increase in river flow was recorded in the 48 hours prior to the fish kill incident report being received, and analysis of rainfall and stream flow data reveals the increase in flow was likely due to a rainfall event in the catchment of the Bingham River, a tributary of the Collie River East Branch.
“There was no evidence to support some publicly expressed concerns that the increase in flow was due to discharges from nearby industry, or that the deaths were caused by a flush of hot water, as records show the water temperature reduced at the time of the increased river flow.”
Mr Maskew said pathology tests undertaken by the Department of Fisheries identified kidney and liver damage in specimens, which is consistent with a response to acute stress. These results could be explained by low oxygen supply to tissues.
“High levels of hydrogen sulphide in water samples and field observations of black sediment support the conclusion that anoxic material was stirred up and carried down river during the flow event, stressing the fish.”
“Analysis of algae samples revealed no potentially harmful species at concentrations likely to have contributed to the fish and marron deaths.”
Mr Maskew said tests for a wide range of nutrients and other contaminants including herbicides were conducted, however results did not suggest that these were responsible for the fish kill.
Approximately 3000 freshwater cobbler, night fish, marron and redfin perch species were estimated to have died over a four day period from January 30 to February 3 this year. The deaths occurred in the east branch of the Collie River off Boys Home Road, about 7kms east-north east of Collie town site.
