There are certain maritime areas that are subject to a compulsory obligation to use the pilotage services as provided by the national port authorities, although there are also certain type of ships that are exempted from this obligation. The areas where it is compulsory to use local pilotage are specified in article 6 of the Law-Decree and included, fro instance, the six miles after the end of the river Tajo in Lisbon or the are two miles inside after ria Formosa in Faro. Moreover it is provided by this law that the pilotage service is a public one which can be exercised by the competent authorities or through concessionaries.