The EPA has released the results of an investigation into the fish kill incident in the Scamander River in late June, finding no evidence of pollution as the cause of the fish death.
The EPA inspected the fish kill incident with the assistance of Break O’Day Council on 26 June and observed thousands of dead fish, mostly bream, along approximately 5 km of the river and riverbank, upstream of the river mouth.
EPA Direct, Mr Wes Ford said that no signs of pollution were observed at the time of inspection, and that water quality measurements, water samples, dead fish and a sample of the white residue observed on substrate along the river edge, were taken and subsequently analysed.
"Results of this testing have failed to provide conclusive evidence of the cause of death of the fish, or of any pollution event linked to the fish deaths," Mr Ford said.
"The fish pathology report found nothing to indicate the cause of death. No pesticides were detected in the water samples provided, and the levels of metals and nutrients were no higher than expected. The composition of the white residue was found to be predominantly silica-based, naturally occurring and non-toxic," he said.
"The water sampling for temperature, salinity and dissolved oxygen indicated that the waters in the sampled area were stratified, with a layer of cold fresh water lying on top of warm salty water, which contained low levels of dissolved oxygen.
"The most plausible theory is that the fish deaths resulted from natural causes.
"A possible reason is that the fish became trapped in the deeper more saline waters and died from lack of dissolved oxygen.
"Thermal stress may have contributed to the fish kill as ice sheets were observed on the water surface around the time," he said.
Mr Ford explained that stratification of the waters in the lower reaches of the Scamander River was a natural response to seasonal weather, including high rainfall and cold temperatures, and decomposition of naturally sourced materials in the deeper saline waters may result in hypoxic conditions.
At this stage, the EPA does not plan to undertake further testing.