THE marine disaster that has ravaged the natural ecology of Mirs Bay has spread to Hong Kong's rare coral reefs, causing damage that may take decades to reverse. Peter Collinson from the Swire Marine Institute, who is just completing a PhD on Coral Ecology based on research at the Hoi Ha Wan Marine Park, said a diving expedition last week showed much of the seabed had been turned into a coral graveyard. ''At least 30 per cent of all the coral was dead, with some areas completely destroyed,'' Mr Collinson said. ''There was a cut-off point of about two metres. Below that, everything had been killed,'' he said. Mr Collinson said when he visited the same sites only two weeks ago, the water was full of dead crabs, fish and urchins ''but the coral was OK''. ''It is going to take years to grow back . . . if the pollution problem in Mirs Bay gets worse, it might never grow back.'' Tests by the Agriculture and Fisheries Department into the massive loss of marine life in Mirs Bay have led experts to believe the damage was caused by a massive body of cold water moving in from the open sea following the torrential rains of the past month. The water was low in oxygen, killing marine life such as coral and starfish. At first, the huge belt of cold water hung at a depth of 12 metres or more. But then it began to rise, creating a sudden temperature change that killed fish. Mr Collinson said the coral had probably been killed because it had been deprived of oxygen for several weeks. ''The cold, deep oceanic water is quite nutrient-rich, which sends the algae mad and robs the coral of oxygen.'' Dick Choi Kwong-chuen of the Agriculture and Fisheries Department said similar conditions had been reported in the past off the coast of China, but had rarely occurred on such a massive scale in Hong Kong waters. He said the recent heavy rains had caused surface water to flow strongly out to sea. This, in turn, was replaced by a deep column of cold water being sucked closer to Hong Kong's coastline. ''The level of this cold water is now rising, which is why fish are being killed.'' However, Mr Choi disputed claims that the damage was potentially irreversible: ''We don't believe this will have a long-term effect on marine life in the area - which would have happened if there had been toxins or oil in the water. ''Once the sea conditions change, the organisms will return to their habitat.'' Mr Choi said the department was monitoring the situation, but added it was impossible to say for how long the sea conditions would continue to kill marine life. Scientists say virtually all marine life has been killed in an area covering about 60 square kilometres. The secretary of the Marine Conservation Society in Hong Kong, Ian Thomas, said he was extremely concerned by the massive loss of aquatic life in what was ''one of the best areas left in Hong Kong for marine life''.