Some 300 metres (950 feet) of beach on Thailand's Naka Yai island was littered with dead ponyfish

Hundreds of dead fish washed ashore on a luxury tourist resort on Thailand's south coast, officials said Saturday, blanketing a long stretch of pristine white sands in a rotting stench.

Authorities said they believed the catch had been dumped by fishing trawlers

Some 300 metres (950 feet) of beach on Naka Yai island was littered with dead ponyfish, Phuket provincial fisheries chief Siripong Panasonthi told AFP, adding authorities were working to determine the cause of the deaths but had ruled out pollution in the water.

"We have checked the water quality... It cannot cause the fish to die en masse," Siripong said.

He said he believed the catch of ponyfish -- which, when caught alive are sold to make animal feed but are worthless when dead -- had been dumped by local fishing trawlers.