Authorities in Yichang, Hubei province, said on Monday that they are still investigating the source of a number of dead pigs spotted in the Yangtze River last week.

Local authorities have been removing carcasses from the river since Saturday, but have not been able to identify which farm they came from or why they were dumped in the river.

So far, four pigs have been removed from the water and their bodies burned to prevent the spread of disease, said Dan Yazi, a spokeswoman for the Yichang publicity office.

She said that the stretch of the river concerned is under control and local authorities have arranged patrols of the river to look for signs of contamination.

A spokesman from the local water management bureau, who declined to give his name, said that since the first four carcasses were removed from the water, another two had been found, and that they had rotted badly due to the heat. "No one wants to salvage them as the smell is so bad and they are worried that they might be infected by diseases from the dead pigs," said the spokesman.

He said that no tests for disease had been conducted due to the difficulty in transporting the rotted carcasses.

A local animal husbandry official named Long said it would be hard to find where the animals had come from.

"Backyard farmers have the habit of throwing away dead pigs as they believe that it is the most convenient way to dispose of them," he said.

According to the Ministry of Agriculture, pig farmers can get an 80 yuan ($12) subsidy for each pig that dies of a disease. Regulations require that animals that die of a disease are disposed of in a sanitary way, such as cremation or burial at least 1.5 meters deep.

However, very few farmers follow these rules, Long said. "They believe that when they bury the dead pigs, their remaining live pigs will not grow," he said.

"What's even more important is that the subsidy is not enough to encourage farmers to dispose of dead animals properly, and the application process for subsidies is complicated," Long added.

Farmers who dispose of dead animals incorrectly, such as by throwing them in a river, are liable for a fine.

The incident in Yichang has triggered widespread media coverage, reminding people of a similar incident earlier in the year, in which a large number of dead pigs were found in the Huangpu River in Shanghai.

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