Tens of thousands of dead fish have washed up on Sanya River beaches in China's southern-most island province of Hainan. Locals have been left wondering if something or someone has poisoned the water.

However, the Sanya Department of Marine Fisheries said the fish were likely killed by a lack of oxygen caused by weather changes, it was reported on Wednesday.

Officials collected water samples for analysis at a laboratory and found all indicators to be normal other than low dissolved oxygen levels.

Such levels could be the result of elevated temperatures and thus the inability of the water to hold the available oxygen, experts with the department said, adding that they would soon remove the fish to clean up the beach and continue to monitor water quality.

Locals first came across the dead fish and uploaded pictures of them creating a silvery blanket on the water's surface onto a forum on Tuesday.

Similar incidents have occurred in Sanya River before, local media reported.