MARINE Police will step up stop and search operations on vessels leaving Hongkong waters following a growing number of thefts from ships moored in Victoria Harbour. Since April 1, there have been four cases of marine theft and burglaries - including two early yesterday - but police believe the problem may be under-reported. In the first quarter of the year, the figure was 12. Harbour Division commander Superintendent Laurence Knox said it was important to take action to stop the number of thefts from increasing. Mr Knox, who had asked for assistance from other police divisions to stop and search outgoing vessels, said police faced an operational difficulty when stopping a suspected vessel. ''Unless the goods on board are found to have been reported stolen, we can't do anything,'' he said. ''All we can do is detain the vessel for a while as we try to ascertain whether the goods had been reported stolen.'' Mr Knox believed many minor thefts were not reported by ship masters because of tight shipping schedules. ''If they did then it would mean holding up the shipping schedule which means a lot of money,'' he said. Another problem facing Marine Police is that the victims often do not immediately report a theft. ''The crew usually report the case to us very late as they want to find out what has been stolen first. ''By the time we arrive, the marine thieves have left the scene for a while, making it difficult for us to chase after the suspected getaway boat,'' Mr Knox said. He said many cases were ''opportunistic'' thefts as the suspects had taken advantage of a ''not well attended'' vessel. Marine Police operations against the thefts include mounting observation posts on land and sea. Meanwhile, in the first robbery yesterday, a crew member of a steamship, the Balpimar Neptune anchored between Green and Stonecutters islands, alerted police at 1.45 am after seeing five men climbing on board. When he returned with other crew members, the suspects had disappeared. Four or five containers had been forced open and an unknown quantity of silk stolen. Police on launch No 75 stopped a shrimp trawler at about 2.25 am off Kennedy Town and arrested nine mainland men. The men were taken back to the steamship but the witness could not identify them. They were later discharged. In another incident, officers were informed by the crew of a cargo ship, the Grace Liberty anchored off Stonecutters Island, at about 7.30 am that five men had climbed up from a 15-metre boat at about 2 am. Nothing was reported stolen although five containers had been prised open. The captain of another vessel, the Shanghai-registered Shau Shan, alerted police at 10.17 am that seven containers on board had been forced open. No loss was reported.