Scientists investigating the deaths of more than 1,000 fish at Cockburn Sound, south of Perth, say early tests suggest environmental factors are most likely to blame.
The dead fish first washed up in late November.
This week there were more reports of large numbers of dead blowfish and snapper near the Garden Island Causeway and Point Peron boat ramp.
Department of Fisheries researcher Dr Michael Snow said disease had been ruled out as a possible cause.
Tests on the fish for more than 120 chemicals were still underway, but had so far not produced any results.
Dr Snow said the focus of the investigation was now on water quality.
“Growing evidence is indicating that it may well be a natural event,” he said.
“Algal blooms, anything leading to dissolved oxygen, or it could be an increase in turbidity.
“Anything that influences the gills of the fish as it swims through the water.”
Dr Snow said he was certain it was safe to swim in the area and also safe to eat fish caught in the affected waters.
Local fishermen are keen to get a definitive answer about what caused the deaths and say they are concerned about the long-term implications on overall fish stocks.
Justin Smith from the Mangles Bay Fishing Club said the deaths had occurred in the middle of the pink snapper breeding season.
“It couldn’t have happened at a worse time,” Mr Smith said.
“What happens in our sound here affects the whole coast because the snapper migrate to different parts of the coast.
“We just want to get to the bottom of it, really.”
West Australia’s Opposition Leader Mark McGowan said regular monitoring was needed to ensure events of this type did not happen again.
The Government-funded Cockburn Sound Management Council had provided that monitoring role until it was closed two years ago.
Mr McGowan said the Government needed to take responsibility for the fish deaths.
“If there was regular, ongoing monitoring of the sound as was the case during the Labor years, then perhaps we would have known what was happening and we would have seen it coming,” Mr McGowan said.
Premier Colin Barnett said the deaths had been distressing but was confident scientists would determine the cause soon.
Topics: environment, marine-biology, fish, rockingham-6168