Thousands of dead fish washed up on a beach as a choking plankton bloom dyed the sea a mossy green in Thailand.

Scores of ponyfish, pufferfish, crabs, and tilapia littered the Bang Saen beach in Chonburi province on September 5, driving away visitors who had visited for a swim.

Pantara Salao, 36, a tourist from Uthai Thani province some 250 miles away to take a dip, but was frightened by the sea's murky appearance and stinky smell. Instead of paddling in the ocean, he collected the fish kill to feed to his pet cat at home.

Resident Sophon Klongarsa, 64, said the bloom had followed heavy monsoon rainfall in the area for several days.

Ason Meenapha, a researcher at Burapha University's Institute of Marine Science, said the fish kill was likely caused when the plankton died and depleted the oxygen of water. The tiny organisms are also responsible for the sickening green colour.

Locals in the eastern province were also worried over a recent oil spill that occurred off Si Racha town on September 3.

The phenomenon was expected to last for around a week before the sea turns back to normal, locals said.

Plankton, or microalgae, are tiny organisms that live in the water, both in the ocean and in freshwater bodies. They are unable to swim against currents, so they drift with the water.