A combination of low oxygen levels, high water temperature and changes to the salinity caused by recent rainfall has led to mass deaths of fish at Wallagoot Lake this week.
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We knew something was going on in the lake because recently it had gone from a rusty red algae colour to a khaki green.
- Resident Neil Fisher
Resident Neil Fisher said this week he witnessed as many as "thousands" of dead bream on Tuesday, following the deaths of a number of spined leatherjackets on Monday.
“We knew something was going on in the lake because recently it had gone from a rusty red algae colour to a khaki green,” Mr Fisher said.
Bega Valley Shire Council said comprehensive water quality monitoring across the lake on Monday found extremely low levels of dissolved oxygen and high water temperatures were likely contributors to the mass deaths.
A council spokesperson said low levels of oxygen formed in the lower layers of the lake were "turned over" causing the oxygen depleted water to come to the surface and impact aquatic life.
The NSW National Parks and Wildlife Service, NSW Department of Primary Industries Fisheries and council will monitor the situation over coming weeks, which "will include follow up testing of the dissolved oxygen levels to monitor ongoing changes", council said.
The DPI said while they do not have regular monitoring programs for coastal lakes, "during the course of normal fisheries patrols staff will keep an eye on known areas of concern and regional staff make regular contact with key stakeholders".
Council said it has "previously undertaken extensive monitoring programs across the shire which has provided important baseline information on water quality".
While council has received inquiries from residents requesting the lake be opened to the ocean, the spokesperson said it is not possible.
"The extremely low water levels in the lake, and the distance from the entrance to the main basin of the lake, means that attempting an opening is not practical," they said.
"The low dissolved oxygen levels could potentially be exacerbated if water was to drain from the lake, and the entrance closed on a low tide.
"The main basin of the lake is over 1km from the entrance bar and the area between is characterised by small isolated shallow pools of water.
"Any attempt to open would require extensive excavation with little chance of success."
Council said the wider ecology of the lake has not been impacted and due to the event's "location, access and scale", any wider public health concerns are "minimal".
