People are advised not to swim at Stanley Main Beach after a suspected red tide was sighted in Southern District, Hong Kong Island, on Monday, the Leisure and Cultural Services Department said.

As a red flag has been hoisted at Stanley Main Beach, and people are advised not to swim there until further notice, the department said in a statement on the government’s website.

A red tide, or algal bloom, is usually seen in ocean areas near Hong Kong and beaches during spring, and usually lasts three to four days.

Red tides occur when colonies of algae — simple plants that live in the sea and freshwater — grow out of control while producing toxic or harmful effects on people, fish, shellfish, marine mammals, and birds. The human illnesses caused by these harmful algal blooms, though rare, can be debilitating or even fatal.

Between May 7 and 10, 2016, a red flag was hoisted for red tides at Stanley Main Beach.

In early February this year, two red tides were sighted at Shing Mun River Channel near Lion Bridge and East Lamma Channel and West Lamma Channel. No associated death of fish has been reported.