According to the department's statistics, the number of visitors to the marine park for dolphin watching increased sevenfold from about 1,200 in April to 8,160 in May, when there were more than 200 dolphin watching trips to the park, with 30 boats were spotted on one day. In June, there were 8,300 visitors to the park.

Conservationists fear the surge in boat visits - which has been linked to the economic downturn as more people stay at home rather than holiday abroad - will disturb the habitats of the dolphins and cause them stress.

Janet Walker, the operator of the privately run Dolphin Watch, is worried that the increasing number of boats will also lead to a rise in marine accidents.

She said some of the operators had no knowledge about the code of conduct governing dolphin watching and some had been very aggressive in their operations.

'There are six to eights boats in the sea at the same time. Some have cut in front of our boat, or shot past from the side,' she said. 'As they know we know where to find the dolphins, most of them follow us. Sometimes they go straight into the middle of a dolphin group and break up the group. They don't really care about the dolphins.'

Under the code of conduct drawn up by the Agriculture, Fisheries and Conservation Department, boats should travel at less than 10 knots per hour within the park. They should also keep a distance of 500 metres from each other. And dolphin watches should only take place 50 to 100 metres away from the dolphins.

Despite the apparent violations of the rules, there were only 500 reminders issued in May and only two verbal warnings.

Clarus Chu Ping-shing, assistant conservation officer of the World Wide Fund for Nature Hong Kong, which also organises dolphin-watch trips occasionally, said some boats had deliberately surrounded groups of dolphins during sightseeing trips.

'Under the law, these operators have already broken the law and conservation officers could have prosecuted them,' he said.

Mr Chu said tourists were also not fully educated about eco-tourism. 'Some tourists even throw their leftover bread into the sea to feed the dolphins,' he said.

In a written reply, the department said it had stepped up patrols in the marine park during holidays and organised seminars on the code of practice for dolphin watch operators. They have also sent undercover officers to join tours to collect evidence against any breaches by the operators. But the department said there was no need to regulate the number of boats inside the parks at this stage because they had steadily declined after the Sars outbreak.

It also said the Travel Industry Council would introduce training courses for tourist guides to improve their skills, ethics and knowledge.

A spokeswoman said: 'As compared to the immediate post-Sars period when there were up to 30 vessels per day engaged in such activity, the number of vessels has dropped and the tour operators are generally more conscious of following the code of practice.

'We will continue monitoring these activities in the marine park.'