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Fish kill in Lake Mac a ‘natural event’

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Water temperatures are being blamed for killing thousands of fish at Lake Macquarie’s Mannering Park earlier this month.

Following extensive water sampling, toxicity testing and analysis of the site, both the EPA and NSW Fisheries have deemed the incident a natural event.

The NSW Department of Planning and Environment labs tested for a full suite of metals and pesticides and found levels below the Australian and New Zealand Environment Conservation Council (ANZECC) marine water quality guideline, with pH levels and electrical conductivity falling within normal ranges for a saline lake.

The sampling results for Total nitrogen and Total phosphorus were above the guideline values, which the findings team say may indicate that disruption of sediment has occurred.

EPA director regulatory operations David Gathercole said the results, along with analysis of weather conditions at the time, indicated the most likely cause of the deaths was from a combination of factors.

“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,” he stated.

“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 4 August.

“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 5 August, and potentially the release of sulphides.

“Our results suggest the combination of the temperature and sulphide stress likely overwhelmed fish, acclimatized to higher temperatures.

“We know the important role Lake Macquarie and the bay plays in the community and the region’s wider environment and have deployed a ‘smart buoy’ at Wyee Point to allow ongoing monitoring in the coastal lake.”

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