The level of fish measured by biomass - a calculation based on fish weight and numbers - in the corals around Hoi Ha Wan is now about 10 times lower than in Australia's Great Barrier Reef, parts of the Philippines and substantially lower than coral areas in Thailand.

Dr Cornish said the fish population in the marine park in Sai Kung was 'among the lowest ever recorded globally from a coral habitat' and warned it would continue to fall unless action was taken.

In 1997, the biomass reading in Hoi Ha Wan was 16 grams per square metre. Last year, the reading had fallen to 8 grams per square metre, Dr Cornish found.

'The only place worldwide I could find where there were actually lower levels than Hong Kong was a coral reef in Kenya which had very little live coral remaining because it had been heavily blasted by dynamite,' said Dr Cornish.

'If things carry on as they are, we will see further decline. It is impossible to say how much further that decline will go. If we compare Hong Kong to other places, you would think we had already reached rock bottom, but I suspect the fish population could continue to decline to really quite disturbingly low levels.'

Dr Cornish conducted more than 600 separate surveys of the fish life in Hoi Ha Wan in 1997-98 and 2002-03. He also studied coral communities at Sharp island (Port Shelter) where there are no restrictions on fishing and found biomass had decreased by 80 per cent from 1997 to 3 grams per square metre last year. In Tung Ping Chau Marine Park (Mirs Bay), which differs from Hoi Ha in that it has two core areas where all fishing is banned, fish populations remained the same, Dr Cornish found. But the ban should have brought about a rise in the population, he said.

'The primary reason for the low biomass is a lack of large fish,' he said. 'We know from interviews with people who have been diving in Hong Kong for decades that large reef fish used to be found among these shallow coral communities. A decrease in the size of fish is one of the main indicators of overfishing.'

There are still 280 fishermen with permits to fish in the Hoi Ha Marine Park area and Dr Cornish said there should be less permits and wider exclusion zones around coral areas.

'Most people in Hong Kong don't realise there is still commercial fishing in marine parks,' he said. 'There needs to be some debate about what we are trying to achieve with the marine parks ... The Agriculture, Fisheries and Conservation Department may need to rethink its management strategy.'

Dr Cornish did not think it would be 'politically possible' to ban all fishing, but stricter controls were needed to prevent fish stocks being further devastated. 'I would like to see coral communities within the marine parks, and possibly some outside, made off-limits to fishing to protect their biodiversity and to allow juvenile fish to mature and reproduce before they are caught,' he said.

'These proposals would benefit Hong Kong fishermen in the long term. Besides they have little to gain by continuing to fish among corals as there are so few large fish remaining.'

Clarus Chu Ping-shing, assistant conservation officer with the World Wide Fund for Nature Hong Kong, said his group backed Dr Cornish's recommendations. 'We have been working very hard to try to persuade the government to limit the number of fishing licences inside the marine parks and would like to see the setting up 'no-take' zones.

'When people go diving they like to see big fish swimming around the corals, but there is no way they can do that in Hong Kong now.'

The Agriculture, Fisheries and Conservation department yesterday said it installed measures to protect biodiversity in Hoi Ha Wan Marine Park.

It also said it had designated two more vessels to monitor illegal fishing. It said the number of fishing permits had dropped from more 800 in 1997 to 280 now.

A spokesman said the department was concerned about the findings of Dr Cornish's survey and would hold discussions with him.