Iraqi officials and fishermen are at a loss to explain how hundreds of tons of carp have suddenly died in fish farms in the Euphrates River, fueling anxieties about soaring water pollution.

Local authorities used excavators to skim dead fish from the river surface near the town of Hindiyah, 80 kilometers (50 miles) south of Baghdad, where residents and local farmers have long complained about substandard water management.

The fish were being farmed in cages for sale in domestic markets, where grilled carp is considered a national dish, called masgouf.

Ayad Talibi, head of Iraq's Fish Producers' Syndicate, called it "annihilation" and a blow to the country's "strategic fish reserve."

Water pollution and scarcity have been on the forefront of Iraqi discourse after matters reached crisis levels last summer.

Health officials said some 100,000 people were taken to hospital for stomach illnesses in the southern Basra province, where sludge and yellow water was recorded flowing out of the taps. Demonstrators rioted, demanding better services.

Iraqi officials could not say for certain what caused the fish die out. Wissam Muslani, deputy governor for Babil province, which includes Hindiyah, said initial tests suggested it was the result of a virus that infected the gills.

But scientists speculated in media that the annihilation may have caused by low oxygen levels, agricultural runoff or wastewater pollution.

Ali Akbar, a medical officer for the World Health Organization, called it a "man-made disaster," and advised Iraqis to stop eating fish.

"My first message is that make sure nobody eats any fish. And my second message is make sure that no fisherman does fishing these days until all this is cleaned," he said.

Photos and video of the carp-clogged Euphrates have had an immediate impact on fish sales, according to fishmongers from Hindiyah to Baghdad.

Ali Ibrahim, a vendor by the side of the highway outside Hindiyah, said he was closing his stall and returning to his family.

"People are no longer eating fish," said Ibrahim. "It's all over."