An estimated two thirds of the world's fisheries depend ultimately on shallow seas - especially estuaries and mangrove swamps. These provide the essential nurseries for fish, crustaceans and molluscs as well as being the homes of many birds and other animals. Coral reefs too are of great local importance. In fact, if it were not for the coral building organisms more than 400 islands would not exist! In Sri Lanka repeated removal of corals for the production of lime has led to the collapse of a whole fishing industry. Coastal areas are very susceptible to pollution by industrial works and sewage.