The biggest oil slick in two years polluted four bays on southern Hong Kong Island yesterday. But authorities were unsure whether to blame heavy rains for washing out drains or a ship for dumping oil. A spill of 200 by 500 metres covering the whole of Chung Hom Kok Bay, one off Repulse Bay measuring five by 100, one at South Bay 15 by 100, and another off South Horizons, Ap Lei Chau, measuring 10 by 200 metres were reported. Combined, the slicks are the biggest in Hong Kong waters since a Philippine-registered vessel dumped oil off Tsuen Wan in July 1996. That action cost the Government more than $1 million and took several days to clean up. The rainbow-hued substance which covered the bays yesterday could have been fuel dumped from a ship overnight, said a Marine Department spokesman. However, he said Tuesday's torrential rains might have washed grease from nullahs and drains to cause the pollution, something which marine ecologist Professor John Hodgkiss, from the University of Hong Kong, said was unlikely. 'Added together, it's a lot of oil. I think it would be more likely that a ship released some gunge in its ballast water,' he said. However, attempts to trace any rogue vessel in the area proved fruitless. Four launches were sent out to help the clean-up, with officers pouring chemical dispersants into the water to break up the spills. Professor Hodgkiss said dispersants could cause more problems by creating a toxic, chemical stew in the water. Some absorbent cloth was used to soak up oil from the surface, while the Environmental Protection Department tested water samples. Fifty Urban Services Department staff cleaned up the oil as it came ashore on Repulse Bay, Chung Hom Wan and South Bay beaches, which were closed. Open University ecologist Professor Gordon Maxwell said the authorities should be trying to suck up the oil rather than dumping more chemicals in the water to disperse it. 'It's such a huge amount that surely it would be better to try and remove some of it. I'm a bit disappointed with the usual rapid-dispersal approach.' Better Environment Hong Kong spokesman Steve Choi Sau-yim has repeatedly called on the Marine Department to regulate how ships dispose of oil. 'Every time this happens we ask how they can make sure it doesn't occur again. Sooner or later nature is just going to say 'no',' he said.