THE conditions that killed a number of fish in Lake Macquarie earlier this month were likely natural, an investigation into the Mannering Park phenomenon has concluded.
Officials from NSW Environment Protection Authority, Central Coast Council and NSW Fisheries collected fish and water samples for laboratory analysis after the August incident.
It is at least the second Hunter fish kill examined by authorities this year.
"Through extensive water sampling, toxicity testing and analysis there are no significant findings of pesticides, metals or toxicity," the authorities said in a statement.
"Levels of pH and electrical conductivity were within normal ranges for a saline lake."
Samples returned nitrogen and phosphorus levels above guidelines, which the NSW EPA said "may indicate that disruption of sediment has occurred".
Director regulatory operations David Gathercole said the results and weather around the time had likely created a perfect storm.
"The bay at Mannering Park has elevated temperatures compared to the rest of Lake Macquarie due to the cooling water discharges from Vales Point power station," Mr Gathercole said.
"A range of fish, including tropical fish, are attracted to the bay due to these higher temperatures.
"The fish kill was preceded by cool nights in the first week of August and a shift to northerly winds, with the strongest winds on August 4.
"This may have caused cold lake water to enter the bay and resulted in a turnover of the water column.
"If this was rapid enough it would have caused a disturbance of the sediments, which explains the high ammonium concentrations observed on August 5, and potentially the release of sulfides.
"Our results suggest the combination of the temperature and sulfide stress likely overwhelmed fish, acclimatized to higher temperatures."
A smart buoy will remain at Wyee Point to continue monitoring conditions in the lake.
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