Authority director of client services Wayne Robins said a fisherman reported dead fish and sick fish downstream from the Horsham Weir pool on Christmas Eve.

He said Department of Primary Industries and EPA staff saw about 150 dead native fish in the river on Christmas Day. Species included murray cod, golden and silver perch and catfish.

"The river water contained low dissolved oxygen levels and the native fish were significantly affected," he said.

"The EPA required Horsham Rural City Council to discharge a significant volume of water into the river system to flush the river to mitigate against further fish deaths.

"This release of water has returned the dissolved oxygen levels to normal and while no other fish appear to be distressed, the river will continue to be monitored over the coming days."

Mr Robins said possible causes for low dissolved oxygen levels included high water temperature, lack of flow, stagnant water and algal blooms.

He said only native fish were affected and no sick or dead carp were found.

The dead fish have been removed from the river.

Tony Lawes was one of the fishermen who found the sick and dead fish.

Mr Lawes said he had never seen anything like it.

Mr Lawes said he walked among fish in the shallow river.

"There were fish at the top of the water gasping for air; the yellowbelly were very pale and the fish were lethargic," he said.

"You couldn't spook them, you could tap them with the tip of the rod. There was a cod in the water and I was able to walk out and lift it up and try to get it back to deeper water.

"It was just sad because the fishing had been so good lately and the fish were breeding."

Mr Robins said people should report sick or dead fish or eels to the EPA immediately.

Johnson Asahi has responded to rumours fire-fighting activities to suppress a major hay fire at its Plumpton Road site had polluted the river.

Johnson Asahi general manager John Barber said stormwater run- off to the river was blocked.

"We are in contact with the Environment Protection Authority all the time," he said.

"Water is being carted away and a spillage pond is in place."

A Department of Human Services spokesman advised people to be cautious when catching fish for human consumption in the Wimmera River.

"If someone catches a living fish in the area where fish have been reported dead it should be fine, but if you have any doubts about the health of the fish be cautious," the spokesman said.