Red tide left two tonnes of fish dead off Tai O, western Lantau, as the algal bloom spread into southern Hong Kong waters in the past week.
The algae in this case is not toxic, fisheries officials say, but hot weather has sped up its decomposition, reducing oxygen in the waters off the fishing village. That causes the fish to suffocate.
The Agriculture, Fisheries and Conservation Department said the red tide, a natural phenomenon, had this time been caused by an algae called Protopolykrikos distortus, which was also recorded in Hong Kong waters from 2005 to 2007 and in 2009 but did not kill any fish.
A departmental spokeswoman said the summer heat and Tai O's geography had caused the red tide to spread. 'The recent hot weather has raised water temperatures and sped up decomposition of algae,' she said. 'The decomposition then consumes all the oxygen in the water.
'Tai O waters are shallow and water flow is static.' This caused the oxygen depletion, she said.
On receiving complaints from residents about the smell of the dead fish, the Marine and Food and Environmental Hygiene departments had begun clearing them from Tai O on Monday. In two days, they collected 1,870kg of dead fish in the sea and on the shore.
Ho Pui-han, a Tai O resident, said the odour had gone yesterday after the fish were removed.