Fish killed by algal bloom

More than 2000 dead fish were reported in Cockburn Sound last November.
More than 2000 dead fish were reported in Cockburn Sound last November.

The department released a public report of its findings which reiterated 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.

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 associated with poor water quality issues, including low dissolved oxygen levels associated with poor flushing of the embayment,” it 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.

“While 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 fish 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,” it 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 water and tissue samples from dead fish revealed no evidence of contaminants. Disease was also ruled out as a contributor to the fish kill.

Other events, such as the lightning storms around the time of the kill were considered as causes or contributory factors to the incident, but ruled out.

The department will continue to monitor Chaetoceros spp levels and other algal species in the southern part of the Sound, with a multi-agency debrief planned for later in the month.