THOUSANDS of fish were yesterday discovered dead, mysteriously lining the banks of the Shing Mun River in Sha Tin - previously considered one of the best examples of the Government's efforts to clean up rivers in the New Territories. Mostly about 15 to 30 centimetres long, there were 'many, many thousands' of the fish concentrated in the area around New Town Plaza, according to a witness. 'This is very disappointing,' said Lisa Hopkinson, conservation officer with Friends of the Earth. 'This was one of the most improved rivers.' Although a foul-smelling gully a decade ago, fish have been returning to the Shing Mun River as the Government cut down both industrial and farm waste running into the river and Tolo Harbour, which it empties into. The river is partly tidal and a variety of both salt and freshwater species have been returning to it. Although no formal complaint has yet to trigger an investigation, Dr Malcolm Broom, of the Environmental Protection Department, said the river was at risk from chronic oxygen shortages associated with algae. 'If there are a lot of dead fish, I'd suggest it's probably to do with a bloom of algae which has then died and caused the water to be deoxygenated,' he said. Sudden increases in the amount of algae in water were also blamed for the red tides such as the one which wiped out marine life over two square kilometres of Mirs Bay in 1994. The algae growths are triggered by a sharp rise in the amount of organic nutrients in the water, but Dr Broom said this did not necessarily indicate a major pollution incident. The sediments in Tolo Harbour had absorbed nutrients from decades of dumping, and a sudden temperature change or other factors could mean they were re-absorbed into the river. Since unveiling an action plan in 1987, organic pollution emptying into Tolo Harbour had been cut by 45 per cent as of last April. Nutrient pollution had been cut by 36 per cent. 'It is disgusting,' said one local resident. The dead fish 'just float up and down with the tide'.