Thousands of fish are among the casualties of the scorching heat wave in the city of Shanghai, officials said yesterday, raising fears of river water pollution where some of the dead fish are being dumped.

On an average about 1 ton of fish are dying every day since the beginning of July at the Dafeng Fish Farm in Qingpu District, said Liu Kelin, director of the farm which has over 30 ponds spread over 530,000 square meters.

"The hot weather raises the water temperature which in turn decreases the oxygen content in the ponds that kills the fish," Liu said. He said the number of the dead fish in the pond has been alarmingly high this year — over 60 percent. In previous summers, it usually varied between 5 and 10 percent.

Qingpu police also reported that an exceptionally large amount of stinking dead fish, mostly herring, were found floating in the rivers this week.

A local resident surnamed Jiang said most of the fish piled up on the banks of the river.

"Small and big fish, lying dead, can be seen everywhere in the river and in fishing ponds near my home," Jiang said. He reported his sighting to the police.

The Jinze County police claimed yesterday that many fish farmers chuck the dead fish into rivers and creeks which creates the stink and floats in the river. Fishing ponds spreading over some 20,000 square meters near Jiang's residence have several hundred dead herrings every day, police said.

The local water authority has dispatched boats to salvage the huge amount of dead fish in the district's major rivers. Many of them that link to the city's tap water sources remain unpolluted.

"Officials will patrol the fishing ponds in the district every day to ensure all the dead fishes are treated properly," an official with the water authority said.

The pond owners have been asked to bury the dead or feed them to the soft shelled turtles and avoid polluting the rivers, the police said.

The high number of dead fish is mainly due to the sizzling weather, said Shi Jianhua, director of the Shanghai Aquatic Product Promotion Station.

Shi said poor management of fishing farms is also the reason behind such big numbers of dead fish. It can be avoided if the farmers keep the water clean and change the water frequently.

Other fishing ponds in the city have not been severely affected.