Officials and fishermen in Hebei Province are clashing over why tons of fish were found dead in the space of a few days in a nearby lake.

The mass deaths happened in Baiyangdian Lake near the city of Baoding between August 12 and 14, causing losses of tens of millions of yuan.

The local government claimed the water was lacking in oxygen, but the fishermen point to pollution caused by enterprises upstream along the Xiaoyi River. They are demanding an independent investigation.

Local authorities said the chemical oxygen demand, an index measuring organic pollutants in the water, was above normal standards, indicating the lake was sorely lacking in oxygen.

It added that this was due to downpours during the July-August rainy season that flooded the river.

However, the local fishermen mentioned that the fish had jumped out of the water before dying, which they claimed was a symptom of poisoning.

Liu Yongjun, a fisherman in the affected county of Anxin, told the Beijing Times that he also ruled out an overcrowding of fish being the cause as he has years of experience farming thousands of fish inside pens. 

Calls from the Global Times to the city government of Baoding went unanswered as of Monday, while an official from the Anxin county government said she was not familiar with the matter.

Zhang Yuanxun, a professor of resources and environment at the Chinese Academy of Sciences, told the Global Times that a process called eutrophication leads to the depletion of oxygen in water.

"Besides heavy metal contamination, eutrophication is a widely seen factor in water pollution. Too much nutrients in the water create a state of hypoxia, reducing oxygen levels and killing off the fish," Zhang said, pointing out that the fish leaping out of the water was also due to breathing troubles. 

"It is also possible that trash was flushed into the river and some companies may have secretly emitted pollutants during the torrential rains," said Zhang.