Reports of several hundred fish deaths following recent minor to moderate flooding in the Richmond and Clarence Rivers are being investigated by NSW Department of Primary Industries.
“As floodwaters have subsided, water quality has deteriorated and fish kills have occurred along the middle reaches of both river systems,” said NSW DPI Senior Fisheries Conservation Manager, Marcus Riches, based at Wollongbar.
“While it is difficult to determine exact numbers, observed fish deaths from recent boat patrols undertaken by DPI are in the order of several hundred,” said Mr Riches.
Mr Riches said NSW DPI, Richmond River County Council and Clarence Valley Council will continue to monitor oxygen levels in the Richmond and Clarence Rivers and DPI Fisheries will be inspecting and monitoring any escalation in fish kills.
“Floods in summer can have a significant impact on dissolved oxygen levels in rivers,” he said.
“When floods cover floodplain vegetation for several days, it starts to die and rot and this process strips oxygen from the surrounding water. Warmer water temperatures exacerbate the situation.
“The low dissolved oxygen water then drains back into the river and can cause fish kills.
“Other factors that contribute to the deoxygenation of river water during summer floods include the heavy sediment load in the river, decaying organic matter, the mobilisation of drain mud (known as monosulfidic black ooze) and chemical processes associated with the wetting and drainage of acid sulphate soils,” he said.
“We are expecting that critically low oxygen levels in the Richmond and Clarence Rivers will continue as flood waters continue to discharge from the floodplain and this may cause further fish deaths.”
The Department is requesting that members of the community report any observed fish kills using the Fishers Watch Hotline on 1800 043 536 or their local Fisheries office.
In recent years, NSW DPI has been working with catchment managers, landholders and commercial and recreational fishers to minimise the impacts of flood events on the fish and crustaceans that provide important biodiversity, recreational and economic benefits to the region. The introduction of water retention devices in some drains to hold floodwaters in backswamp areas has worked successfully in areas of the Richmond and Clarence floodplains.
These devices help minimise the impact on aquatic biodiversity by restoring the flooding regime in former backswamp areas and allowing regeneration of native wetland, flood tolerant vegetation.
State and local agencies are working together to continue these efforts with other stakeholders.
Erosion control in the upper catchment is also very important – especially on agricultural lands, stream banks, and gravel roads.
Other measures include increasing the number of actively managed floodgated systems to improve water quality within drains.
