FISH KILL: The recent fish deaths in the Richmond River, at Moore Park, Old Grevillia is explained by scu'S Dr Damien Maher.
FISH KILL: The recent fish deaths in the Richmond River, at Moore Park, Old Grevillia is explained by scu'S Dr Damien Maher. Dailan Pugh

Why fish die in the Richmond River

AS INVESTIGATIONS into recent fish deaths across the state continue, a river expert has compared this week's Richmond River event to major fish kills of the catchment's past.

On Monday, NSW Department of Primary Industries Fisheries officers confirmed approximately 70 fish died in the fish kill in the Richmond River at Moore Park, Old Grevillia due to "low dissolved oxygen levels ... due to minimal river flows at the site and high temperatures."

A DPI spokeswoman further confirmed the species of fish affected consisted "mainly of mullet."

But the event has caught the attention of those behind the health and future of the catchment, such as Ecohydrologist, Eco Hydrologist, river expert and SCU Associate Professor in the School of Environment, Science and Engineering, Professor Dr Damien Maher.

Dr Maher yesterday visited the site to get a scope on how serious the situation was.

He confirmed the dry and hot weather conditions causing low dissolved oxygen levels was the major reasons behind the deaths, but shed some further light by revealing extensive algal blooms played a significant role.

"The river levels are low and this has essentially led to stagnation of the water, but where I visited there was a lot of algae in the water, along the banks and water's edge. It's the break down of the algae that consumes all of that oxygen ... temperature plays a big role here too," Dr Maher said.

"Algae is a natural phenomenon - essentially its always there in our rivers systems - but the flow is so low it's given the perfect conditions for the algae to bloom and take off .

"Because it's not being flushed out, what we've got there is a series of small ponds or pools in the upper river that have disconnected from the river flow ... so until we get some rain to flush that algae and the nutrients are supplying that algae, the algal bloom is just going to hang around."

He likened the "small scale" kill to the major 2001 and 2008 fish kills in the Richmond River.

February 2001: Following major flooding in the upper reaches of the Richmond catchment, a major fish kill occurs in the Richmond River due to extremely low dissolved oxygen levels in the rivers. As a response, NSW Fisheries closed the Richmond River and near shore areas to all forms of fishing for three weeks initially and then extend this closure for a further three months.

January 2008: A second major fish kill occurs after prolonged rainfall and extensive flooding throughout the Richmond River catchment. The quality of the river water deteriorated due to an increased load of sediment, debris and nutrients. Black water (when oxygen is stripped from the water) soon developed. The result is the death of millions of aquatic animals in the lower Richmond, including at least 2 million fish. Over 30 tonnes of dead fish are removed and a ban on fishing is put in place until fish populations recover.

"The main reason the fish are dying now is similar to those flood events and that's because of the lack of oxygen in the water," Dr Maher said.

"But the processes causing the low oxygen are different as I earlier explained."

He said, weather conditions aside, the fish deaths were just another reflection of the poor state of the Richmond River.

"There is no doubt we have modified the hydrology of the Richmond catchment and that's been essential to develop agriculture in the area, but as a consequence, the river and estuary down stream has really suffered," he said.

"The water quality is far worse than it once was.

"Getting the River back to a healthy state is something that will need a long term vision and strategy to turn things around. It will take all levels of government and the community to work together to improve things."

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