Rainwater runoff blamed for mass fish death at Nai Harn lake

The mass fish death was first reported to The Phuket News by local residents on Monday (May 8). “There are dead fish floating everywhere in Naiharn lake today. Never seen anything like it,” one resident said.

“There are thousands of fish found dead and floating on the surface of Nai Harn Lake today,” another wrote.

Rawai Mayor Aroon Solos explained to The Phuket News today (May 10) that officers at the Rawai Municipality public health division were informed of the mass fish death on Monday, and began investigating the cause.

“We contacted the public health office to check the water condition to see whether the water is polluted or not and find out what killed the fish,” he said.

Officers tested the water and found no evidence of pollution. However, they did find that the PH level of the water had changed dramatically, Mr Aroon explained.

Heavy rains in the area in recent days had resulted in a large influx of freshwater into the lake, which is usually brackish through the small channel across Nai Harn Beach that connects the lake to the sea.

The fish that had died were saltwater fish, he added.

The influx of water had altered the salinity of the water. “The volume of runoff from the after heavy rain caused an imbalance of the water PH level [in the water],” Mr Aroon said.

He added that the surface water had been affected much more than water at deeper levels in the lake, which had maintained some of its salinity.

Officers were dispatched to remove the dead fish to prevent the water being further fouled, he said.

The event was natural, but not normal, Mr Aroon admitted.

“This has never happened before when it comes to the beginning of the rainy season. This year, the tide was very low and it was very dry before the rain came,” he said.

“There was no rain for a while, and the water at deep levels had been hot and salty, but rain from recent days made the water much less salty, causing an imbalance in the water PH,  resulting in the death of the fish,” he explained.

Mayor Aroon said he has ordered municipality workers to excavate the channel connecting the lake to the sea in the hope of restoring the lake water to its normal salinity by allowing more saltwater to enter the lake at high tide.

“I inspected the lake yesterday and today, and the  water in the lake is getting better already,” he added.

“There are no more dead fish floating in the water, and the fish I could see were swimming about normally,” he said.