More than 60 tonnes of dead fish were found yesterday at Grass Island (Tap Mun) fish farms, believed to be victim of the red tide build-up of micro-organisms. In the past month more than 327 tonnes of fish worth $20 million have been destroyed in five fish culture zones near Crooked Island (Kat O) by the red tides. The tides are caused by a massive build-up of microscopic sea organisms known as phytoplankton. They can increase in millions suddenly, turning the sea a brownish or reddish colour. Workers at the New Territories fish farms found the fish dead at about 6 am when they went to feed them. 'The water was rather murky and we could not see the fish from above the fish farms. And when we pulled up the nets, we saw all the fish dead at the bottom,' said Ho Yung, chairman of the Tap Mun Fish Culture Association. Mr Ho said the estimated loss could add up to more than $3 million, although some of the fish have been quickly shipped to the mainland. There they can be sold at about one-third the normal price. Staff from the Agriculture and Fisheries Department recorded the incident. The Democratic Alliance for the Betterment of Hong Kong's contestant for the Agriculture and Fisheries functional constituency Wong Yung-kan said the fish might have died because of a lack of oxygen. Some of the red tides are toxic, while others use up so much oxygen in the water that fish in the same area cannot survive.