The department released a public report of its findings early this week, which reinitiated a spike of a group of algal diatom species Chaetoceros spp. was the likely cause for the death of more than 2000 fish in the Sound.
Spatial modelling was undertaken as part of the investigation and results indicated the southern section of Cockburn Sound, near Mangles Bay, was where the bloom of Chaetoceros began.
The report also outlined other contributing factors such as excess nutrients, higher than normal water temperatures and reduced flushing conditions.
“(The southern area of Cockburn Sound) has been historically associated with poor water quality issues including low dissolved oxygen levels associated with poor flushing of the embayment,” the report said.
“Low dissolved oxygen levels are also associated with algal blooms, as algae are a net consumer of oxygen at night time and following their breakdown by bacteria following a bloom event.
“Whilst there was no direct evidence of involvement of low dissolved oxygen conditions, such conditions likely contributed to the fish kill event.”
Despite the initial 700 reported dead fishing washing up on Careening Bay at Garden Island, the report said the kill event also appeared to have been confined to the southern section of the sound.
“Dead fish reported from the ocean side of the Garden Island causeway and northern reaches of Cockburn Sound were determined to be due to prevailing wind and tidal conditions,” the report said.
Sixty-five water samples taken from throughout the Sound were studied over a two- week period after the first deaths were reported.
Testing of the water and tissue samples from dead fish revealed no evidence of contaminants, including hydrocarbons, pesticides, herbicides, organic fertilisers, nitrogenous products (such as ammonia or nitrates), shellfish toxins or heavy metals.
“Water and tissue samples were analysed for algae species that are known to produce toxins that can harm fish and humans via exposure to skin or through ingestion,” the report added.
“Testing revealed no significant harmful algae species or associated toxins were associated with this fish kill event.”
Disease was also ruled out as contributors to the fish kill because disease only impacts a single species or group and the wide range of finfish and invertebrates killed indicated it was not a disease-related event.
Other events, such as the severe lightning storms around the time of the fish kill were considered as causes or contributory factors to the incident but ruled out.
The report said the department would continue to monitor the levels of Chaetoceros spp. and other algal species at locations in the southern part of the Sound, with a multi-agency debrief planned for later in the month.