THE Government has been asked to report on the cumulative impact of all the dredging activities in Hongkong to provide a better assessment of how local waters, fisheries and beaches are affected. Currently, each dredging project is assessed in isolation and the Environmental Pollution Advisory Committee (EPCOM) yesterday said it was concerned it was not getting an overview of the situation. Dredging can create a high level of silt in the water, which affects water quality and fisheries and it can also affect beaches if the dredging goes too deep and disrupts wave depths. Dredging for marine sand to be used in reclamation for the airport and West Kowloon is underway at Tat Tong Channel. The Government is considering doing the same at East Lamma Channel after rejecting proposals for Mirs Bay and Tsing Chau. Reports were given yesterday on the impact of dredging all four sites. EPCOM chairman Dr Wang Gung-wu said all of the technical reports ''seemed to be very good and professionally done''. ''But members felt uneasy about what the total effect will be,'' he said. ''We're looking for an overall picture that the cumulative effect won't be doing damage to the waters as a whole.'' The committee was told that more information on the cumulative impact would be compiled and presented, but no date was given. Tat Tong Channel was not subject to as detailed a study as the other sites before dredging began and EPCOM was told work was continuing on a report. Water quality at fish farms around Tung Lung Chau is said not to have been affected, but the beaches along eastern Hongkong Island and the Cape D'Aguilar marine laboratory are being watched for any effects. Meanwhile, the Industry Department yesterday published a guide to pollution control legislation affecting manufacturing industries. The guide provides a summary of major legislation and includes references to major international environmental measures which may affect the manufacturing sector in the territory. It also lists where technical advice on legal compliance can be obtained.