A HUGE deluge of putrid floodwater run-off has sparked the largest fish kill on record in the Hunter.
Tens of thousands of fish and marine life have been killed after dead or ‘‘blackwater’’ flooded the lower reaches of the Hunter, Williams and Paterson rivers.
The water has created a pungent odour throughout the Hunter that is expected to take weeks to dissipate.
The filthy water, caused by dead and rotting vegetation, strips oxygen levels from the rivers and releases tannins that turn the water black.
The environmental disaster is expected to kill all fish and marine life in the affected area downstream from Morpeth and Irrawang.
Environment Minister Robyn Parker last night ordered additional testing to determine the cause of the fish kill.
A spokeswoman from the Department of Primary Industries said yesterday the first dead fish were located at Hexham on Saturday.
She said it could take months for the rivers to recover and fish stocks to re-enter the estuary from the coast.
The chief environmental regulator for the Environment Protection Authority, Mark Gifford, said the problem was caused by water levels in the Williams, Paterson and Hunter rivers peaking at the same time, increasing the duration of the flooding.
Mr Gifford said that as the floods moved over low-lying areas surrounding the rivers, they picked up large quantities of organic matter, including decaying vegetation, leaves, dirt and sand.
This process stripped oxygen from the surrounding water.
‘‘EPA tests for dissolved oxygen levels taken at various points on Monday all clearly showed that the fish were impacted by low oxygen levels,’’ Mr Gifford said.
Professor of marine ecology at the University of Technology in Sydney David Booth confirmed the large numbers of dead eels and catfish suggested that anoxia (a lack of oxygen), possibly caused by significant amounts of sediment-laden run-off, may have contributed to the fish kill.
‘‘Those fish are pretty tough species, they are usually the last fish to go,’’ he said. ‘‘The fact that eels have been coming up to the surface gasping for air suggests anoxia. You can imagine that fish gills don’t work very well when they are all clogged up.’’
Record numbers of pelicans, about 450, flocked to Kooragang Dykes yesterday to feast on the dead fish.
Liz Crawford, of the Hunter Bird Observers Club, said records indicated there were only 35pelicans at the dykes this time last year. The largest number recorded last year was 192in April.
Hunter New England Health has issued a warning to residents not to drink or swim in the rivers.
Hexham-based commercial fisherman Jason Hewitt said the mass kill was a ‘‘complete disaster’’ for the 26prawn trawlers and 10netters and crabbers who worked the rivers.
Mr Hewitt said everything from eels and stingrays to prawns and catfish were dead.
‘‘We want further investigation to find out exactly what has happened here, we don’t believe this is only due to lack of oxygen ... We’ve been watching prawns crawling out of the water and dying along with everything else.’’
Opposition environment spokesman Luke Foley said he was concerned that the government was unable to manage waterways such as the Hunter River.
Greens environment spokeswoman Cate Faehrmann said the government needed to rule out the possibility that upstream discharges of saline mine water had not contributed to the fish kill, and the EPA should release all the water monitoring data.
Environment Minister Robyn Parker said she had been advised that the saline mine water had not contributed to the fish kill.
Similar large-scale kills have occurred in the Macleay, Richmond and Clarence rivers in 2001 and 2008.
According to the Department of Primary Industries, water quality readings taken this week downstream of Morpeth measured between 0.1 and 0.2milligrams of gas per litre of water, indicating it was almost devoid of oxygen.