A streak of reddish sea water along the shoreline of a beach in the north-central province of Quang Binh has disappeared one day after emerging on Wednesday, leaving authorities in the dark over why the sea had turned red.

On Thursday, the sea water along the beach in Nhan Trach Commune was no longer red like it had been only one day earlier, Phan Xuan Hien, the commune’s chairman, told Tuoi Tre (Youth) newspaper.

“The red color started to fade away on Wednesday afternoon,” he added.

As observed by Tuoi Tre reporters, the streak of red sea water has gone, but the water was quite murky. On Thursday morning, dead fish continued washing ashore, but only in modest numbers.

But the cause behind the red sea water remains an unanswered question, the local environmental agency admitted.

“Relevant agencies in Hanoi took samples of the red sea water and we have to wait for their test results,” said Phan Xuan Hao, head of the environmental agency under the provincial Department of Natural Resources and Environment.

The department has warned local residents against swimming at the beach and using the red sea water for cooking, cleaning, and other daily activities.

Vietnamese scientists have ruled out ‘red tide,’ a result of algae blooming at an abnormal rate and producing toxins, as a cause of the red sea water.

The red sea water was seen on May 4, 2016. Photo: Tuoi Tre

It is also unclear whether the streak of red sea water is linked with the mysterious mass fish deaths that have been hitting Ha Tinh, Quang Binh, Quang Tri and Thua Thien-Hue, all provinces in central Vietnam, since April 6.

One month after the phenomenon occurred, severely affecting fishermen, fish farmers, and consumers, it remains a mystery as to what caused fish to die en masse.

A Ha Tinh-based steelmaking plant, operated by a Vietnamese unit of Taiwan’s Formosa Plastics, underwent a complete inspection on Thursday after its waste discharge system was linked as an alleged cause of the fish deaths.

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