Hundreds of dead fish were found floating on the surface of the sea in southern Thailand.

Footage shows the marine animals drifting off the coast of Koh Samui Island in Surat Thani province on September 25.

The area lies in the Gulf of Thailand, and locals believe the fish kill was caused by reduced salinity due to higher freshwater levels flowing from the Mekong River.

Tour guide Oat said: 'The increase in freshwater levels in the Mekong River can significantly reduce salinity, leading to lower salt concentrations in the seawater.

'This sudden change in salinity can cause fish to go into shock, as many aquatic species are sensitive to rapid environmental changes. As a result, some fish may become stressed or 'knocked out,' and in severe cases, this could lead to mass fish deaths.'