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Perfect weather for a fish kill

North Coast Local Land Services is working closely with local councils, other state government agencies and willing floodplain landowners to attempt to improve coastal floodplain health and reduce the frequency and size of the fish kills across the region.

Fish kills are caused due to either a lack of oxygen in the water, or from acidity, or a combination of them.

Max Osborne, Senior Land Services Officer said, “We know how to mitigate these problems and we need to identify farmers willing to work with government to make the changes needed.

“We will never be able to stop fish kill events, as they do actually occur in even the most pristine waterways and we acknowledge that the region’s economy depends on the valuable floodplain cattle grazing land.

“In acknowledging those two facts, however, we also note that improvements to current landuse practices need to be made.”

NSW coastal floodplains have been subdivided and modified over several generations making it very difficult to undo some of the drainage work that has led to the current situation. Some parts of the floodplains and in particularly the really low elevation areas require a total change in land use to address the issues that cause major fish kills.

“One thing that happens, particularly during our summer and autumn months is our lower floodplain grazing country becomes flooded by warm water for several days.

“The grass dies and bacteria use up dissolved oxygen in the water column while consuming the rotting grass.

“The water becomes black and is termed “black-water” and this deoxygenated water drains into the estuary where it impacts on fish,” Max said.

Furthermore, acid sulfate soils, or “the worst soils on earth” as they are known, naturally occur across eastern Australian coastal floodplains, but they cause no problem at all if they are not drained.

Prior to the various floodplain drainage projects which have occurred since European settlement most low elevation coastal floodplain areas were freshwater wetlands, tidal saltmarshes or mangrove forests.

Using levees, drains and “one way valve” tidal floodgates, much of the coastal floodplain groundwater table is now drained to low tide heights or lower during dry seasons. When the acid sulfate soil layer which is underlying the alluvial topsoil gets exposed to oxygen, it frees up naturally occurring sulphur and makes it water-soluble. When rainwater and sulphur combines it forms sulfuric acid which can have a pH of 3 which is more acidic than vinegar. Any water with a pH less than 6 can harm or kill aquatic organisms. This mixture also strips dissolved oxygen from the water column.

When black-water, sulfuric acid plus a cocktail of toxic dissolved metals including iron, aluminium and manganese and others (which are leached out of the soils by the sulfuric acid) mix in our tidal estuaries, it impacts everything from worms in the mud through to gigantic Estuary Cod.

North Coast Local Land Services is currently helping farmers develop and implement coastal floodplain rehabilitation projects in an attempt to reduce the amount of fish kills and improve estuarine ecosystem health. If you need help to improve the management on your farm and the health of our estuaries please contact North Coast Local Land Services on 1300 795 299 or email

This project is supported by North Coast Local Land Services, through funding from the Australian Government’s National Landcare Programme and Catchment Action New South Wales.

Media contact:           Max Osborne, Senior Land Services Officer, Phone 6563 6710

Photo caption:           Estuarine worms killed during the March 2017 Clybucca Creek fish kill event.
  Image credit: Scott Anderson DPI Fisheries