The NSW Department of Primary Industries said the likely cause behind the death of hundreds of fish in a private dam adjacent to Dawson River was related to critically low dissolved oxygen levels caused by overland floodwater from the river.
A spokesperson for the department said NSW DPI staff investigated the site following reports of a large number of dead birds.
"On inspection it was discovered reports of dead birds were incorrect, and that a fish death event had occurred," said the spokesperson.
"The main species impacted was mullet and numbers were assessed as in the hundreds."
The spokesperson said localised fish kills in impoundments after rainfall or flood events are not uncommon.
"It is likely that these floodwaters created poor water quality in the dam that affected the fish.
MidCoast Council was also involved in investigating the situation after receiving similar reports, immediately notifying the matter to the NSW Department of Primary Industries in accordance with their protocol.
A spokesperson for council said NSW Department of Primary Industries (DPI Fisheries) are responsible for the investigation of fish kills in non-metropolitan areas while EPA officers will investigate fish kills in Sydney, Newcastle and Wollongong metropolitan areas.
"In many cases a joint inspection will be appropriate.
"Regardless of the location, EPA officers will be responsible for detailed investigation of kills which appear to be related to pollution events, hazardous chemical incidents or discharges from commercial or industrial premises."
MidCoast Council undertook two separate visual inspections and reported its findings to the NSW DPI.
"All DPI Fisheries Officers in NSW have fish kill response kits available with required fish and water sampling and water quality testing equipment.
"There was no evidence of pollution (i.e source, odour, visual) and schools of smaller sized fish were observed within the water."
Council believes that somewhere between 200 and 1000 fish may have been affected and advised the DPI that the mullet died due to a black water event following the recent flood (not a pollution event).
"Under low oxygen conditions larger fish suffer the most due to higher oxygen demand," said council's spokesperson.
"Schools of smaller mullet have been observed swimming in the dam and it is not unusual for larger fish to be impacted most by oxygen-poor water."
Council referred to the NSW Department of Primary Industries about what would happen to the fish, noting that the dam is privately owned, within a private allotment and the owner has also been notified.
"If the fish aren't removed, they will decompose and nutrients will be used by other aquatic life and plants."
The spokesperson for the NSW DPI said the NSW Government's $10 million commitment to support native fish during the current drought and bushfire season will be critical in assisting the recovery of populations when conditions improve.
"Community members are encouraged to report any fish deaths or observations through the Fishers Watch hotline on 1800 043 536."
For more information on fish deaths, visit www.dpi.nsw.gov.au/fish-deaths
For more information on the NSW Government's Native Fish Drought Response, visit www.dpi.nsw.gov.au/nativefish
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