A RECORD $100,000 fine for illegally dumping mud in the sea was imposed yesterday after a dredger dropped 750 cubic metres of mud near Stonecutters Island. The mud smothers marine life on the sea bottom and is only supposed to be dumped at Cheung Chau, Ninepin Islands or Mirs Bay, but some dredgers have tried to take shortcuts by dumping in Victoria Harbour. Universal Dockyard Ltd was found guilty of doing so after a trial in the District Court in which it denied illegally dumping at Stonecutters Island in March 1992. Lai Chi-keong, employed by the company to work as the master of tugboat Universal 96, was fined $5,000 after he was also convicted of illegal dumping at sea. He had denied the charge. The company had 12 previous convictions, four of which were heard in the District Court in June 1992 and resulted in $50,000 fines for each offence, the previous record. The others were in magistrate's courts. The Environmental Protection Department (EPD) yesterday said it was satisfied with the $100,000 fine and it did not expect similar serious offences because it had introduced a more stringent control programme. Principal environmental protection officer Paul Holmes said serious offences had dropped from 13 in 1992 to three last year. ''This firm has been hit twice now [in the District Court] for offences. They obviously have realised in the intervening time that we do mean business,'' he said. In sentencing yesterday, Judge Hawkes said he was satisfied from the evidence given by expert prosecution witness Marc Smith-Evans that illegal dumping would have had an adverse environmental impact. Mr Smith-Evans, who has been working as an environmental protection officer for the EPD since 1991, said the dumping of mud would effectively kill marine life at the bottom of the sea. The scale of damage in this case would take at least a year to recover, he said. ''We can only kill something once,'' Mr Smith-Evans said after the hearing. Prosecutor Anthony Yuen said that on March 26, 1992, a barge loaded with marine mud dredged from near Shamshuipo was towed to the sea southwest of Stonecutters Island where it dumped its contents. The tugboat and the barge were subsequently intercepted by two environmental protection inspectors and it was revealed that Universal Dockyard Ltd was sub-contracted to carry out the work. Universal's counsel, Eric Kwok, submitted that it was Lai who breached the licence condition and did the actual dumping, and the company did not consent to the illegal dumping. But Judge Hawkes said Universal, which delegated the job of dumping mud to Lai, should also be held responsible for the offence. Lai's counsel, Vernon Eaton, urged the court to consider that his client, who had limited means, was low in the hierarchy of his company.