Meanwhile, six more beaches were swamped by the algal blooms yesterday, bringing the total number affected to 11, including all the beaches in Tuen Mun.

A red tide expert warned that heavy rain forecast for tomorrow would spread the problem 'all over Hong Kong'.

The extensive accumulation of algae was discovered at four western beaches in Tuen Mun - Cafeteria Old Beach, Cafeteria New Beach, Gold Beach on Castle Peak Road and Butterfly Beach on Lung Mun Road - at about 10am. Kadoorie Beach was affected an hour later, followed by Castle Peak Beach at about 2pm.

The species of algae, Cochlodinium sp, is the same one that affected five beaches on southern Hong Kong Island on Monday and Ma Wan last Friday. It is non-toxic, but in the concentrations that form a red tide it can deprive fish of oxygen.

An AFCD spokeswoman said fish farmers in three fish culture zones - Lo Tik Wan, Sok Kwu Wan and Ma Wan, all in the East Lamma Channel - had reported red tides in their waters.

Legislator Wong Yung-kan, who represents the agriculture and fisheries sector, said the channel had five fish culture zones that supported up to 400 fish farmers.

The red tide is believed to be the biggest since 1998, when the worst red tides in history killed 90 per cent of the city's farmed fish.

The department, which came under fire on Monday for declining to alert all fish farmers about red tides, released a city-wide warning.

'We have phoned the fish farmer representatives and associations and through them all fish farmers. We reminded them to report any irregular fish deaths to us,' the spokeswoman said. The department was not worried that the red tide would spread to all Hong Kong waters, she said.

But red tide expert Ho Kin-chung of the Open University of Hong Kong said that if the forecast heavy rains came tomorrow and the following days, polluting nutrients would be washed from the Pearl River to Hong Kong, stimulating the growth of algae and causing all Hong Kong waters to be swamped by the red tide.

Dr Ho, who has studied red tides for more than two decades, said he believed tidal surges had helped spread the red tide from the south to Tuen Mun in the west.

Lai Tak-chuen, one of 60 fish farmers in the Ma Wan area, said farmers were ready to run boat engines to oxygenate the water if the problem worsened.

So far there have not been many fish deaths, but farmers are worried the algae will spread further and kill their fish. 'Some of us have raised the fish for a few years and are waiting for harvest at the end of this year. These are all our assets,' Mr Lai said.

Red Zones

A total of 11 beaches are now affected

- Cafeteria Old Beach, Cafeteria New Beach, Gold Beach, Kadoorie Beach, Castle Peak Beach, Butterfly Beach - (all affected yesterday)

- Deep Water Bay, Repulse Bay, Middle Bay, South Bay, Chung Hom Kok Beach (first hit Monday)

Source: HK Government