Villagers dump dead fish in front of a factory in Tambon Bang Sadet in Angthong’s Pa Moke district during a protest yesterday. Villagers called on authorities to check on the factory, which they believe released wastewater that has killed thousands of fis

Pollution Control officials were sent to check on four factories along the Chao Phya River yesterday to see if any were responsible for the death of tens of thousands of freshwater fish and prawns on Sunday night in Angthong and Ayutthaya.

The fish are believed to have died from a sudden lack of oxygen. But the cause of the drama has yet to be determined.

Head of the Pollution Control Department's Water Quality Management Bureau, Anuphan Ittharatana, said two teams had been sent to the affected areas.

The first team would focus on wastewater issues at four nearby factories - the Mitrpol Sugar plant, the Thai Rayon factory, the Ayinotakara plant and the Thai Carbon Black factory.

The second team would collect water from Pathum Thani up to Angthong's Pa Moke district for analysis. The test results should be ready by next week, he said.

Anuphan said the PCD did not rule out a vessel carrying sugar that sunk upstream on March 3, as the possible cause of the pollution.

The 650 tonnes of sugar dissolved slowly in the current - estimated at 80 cubic metres per second. The lack of oxygen in the river might have been caused by water-borne bacteria using up oxygen in the area to "digest" the sugar, Anuphan said.

Meanwhile, fish farmers from Tambon Bang Sadet in Angthong's Pa Moke district held a protest at 10am yesterday in which they dumped dead fish in front of a factory that produces monosodium glutamate just upriver from them.

The farmers offered to collect a water sample from the front of the factory to be checked by a private lab. If the factory was found to have polluted the river, it must face punishment, they said.

The farmers said they did not believe the sunken sugar vessel caused the pollution because fish farmers at Tambon Pa Moke Municipality would have been affected too - but they had not.

Angthong Governor Wiboon Sanguanphong declared Pa Moke district a "freshwater epidemic zone" yesterday to speed up relief to affected residents and make way for the investigation.

Some 200 soldiers were also sent to help Pa Moke farmers bury decomposing fish to prevent outbreaks of disease and a foul smell.

Fisheries Department chief Charanthada Kannasuta said the department would file criminal and civil lawsuits against the polluter to help the affected farmers get compensation. They would also release young fish and prawns into the river soon to make up for the losses.

The water quality had improved now and no more dead fish had been found as the low-oxygen section of water had been diluted by "fresh downflows", Charanthada said.

Officials were assessing damage to assist the farmers in the local fishing industry reported hit by record losses - initially said to be some Bt40 million for farmed fish and Bt100 million for the loss of freshwater prawns, he said.

The Royal Irrigation Department (RID) spokesman Boonsanong Suchatpong said RID officials would check the water flowing from Angthong to Nonthaburi every day to learn when the contaminated water would arrive and its dilution rate.

The Chao Phya Dam had now raised its water release to 80 cubic metres per second to quicken the dilution and to prevent further harm to river life.

Permanent secretary for Public Health Dr Prat Boonyawongvirot said an initial test found no trace of pesticide in either fish or water samples. Workers at Medical Science Department labs were testing all samples for pesticides and heavy metals such as lead, mercury, cadmium and arsenic. The results should be known this week.

Prat urged people not to eat fish until toxicity tests had been done. He said the ministry would dispatch social workers to assist over 200 affected farmers - many of whom have just recovered from the devastating floods of last year.