Rescuers move a stranded melon-headed whale before releasing it into the ocean in Taizhou, East China's Zhejiang province, on July 7, 2021. [Photo by Pan Kanjun/for chinadaily.com.cn]

Four melon-headed whales rescued after being stranded this week in shallow coastal waters in Linhai, Zhejiang province, were released into the ocean on Wednesday night, experts said.

That took the number released to six, but two others died before they could be set free.

At 8 am on Tuesday, police received a report from villagers saying a dozen "big fish" were stranded hundreds of meters from shore. Experts then identified them as melon-headed whales, a Level II protected species in China.

Three of the 12 stranded whales died before the rescue team arrived. About 500 rescuers battled high temperatures as they worked for about 10 hours to move all of them off the mud flat by around 6 pm on Tuesday.

Experts said two whales sent to Baishawan Park that day were in good physical condition and were released into ocean waters at 9 pm on Tuesday.

Five were transferred to a fish farm in Taizhou and the other two were sent to Taizhou Ocean World.

Apart from one at the fish farm, the rescue team decided to release all the whales after considering their health conditions. However, two whales died before they could be carried onto the ship.

"Though we made every effort to rescue the melon-headed whales, two died on the way to the wharf. The reason is being studied," said Zhu Yupeng, head of Linhai's marine and fisheries law enforcement team.

The bodies of the two whales will be frozen and dealt with according to suggestions from experts. The remaining whale at the fish farm, which is in poor health, is being observed and treated.

Zhu, who joined the rescue effort on the first day, said that to keep the whales alive, the rescue team tried many approaches, such as digging out water pits, installing sunshade devices, cooling them with ice cubes, and constantly pouring water on them. Some workers held the whales' heads out of the mud to allow them to breathe. They used cranes and big stretchers to move them.

Research into the species, commonly seen in coastal waters around the Philippines, Hawaii and Australia, is quite limited worldwide, Zhang Peijun, an associate research fellow at the Chinese Academy of Sciences' Institute of Deep-sea Science and Engineering, told a news conference on Tuesday.

Zhang said they mainly feed on creatures such as octopuses. The whales swim about 30 meters below the surface during the day and about 200 to 500 meters deep at night.

"Cetacean stranding is a common phenomenon with complicated and diverse causes, ranging from problems with the whales' navigation systems to changes in the earth's environment. The reason for the stranding of the 12 whales is uncertain and needs further research," he said, adding it is not common for such a big number of melon-headed whales to be stranded at the same time.

Xinhua contributed to this story.