March, marking the formal Portuguese entrance in World War I. In the beginning of the World War I, the Portuguese fleet included five cruisers, one aviso (sloop-of-war), one destroyer, one submarine, 12 gunships, seven river gunboats, four torpedo boats, two training ships and other seven armed vessels. During the war, two additional destroyers, three submarines and three gunships would be received. A number of liners, cargo ships, trawlers and tugs would be mobilized and fitted for naval service, these including three auxiliary cruisers, one hospital ship, four transport ships and more than 20 naval trawlers (patrol boats and minehunters). The Portuguese Naval Aviation as well as its Lisbon Naval Air Station would be created in 1917, receiving more than 25 seaplanes during the wartime. From February 1916, the Portuguese naval forces operating in Mozambique included the cruiser and the gunship Chaimite, besides several river gunboats and other small vessels of the Zambezi Flotilla. During May 1916, Adamastor and Chaimite led an operation against the German forces in the mouth of the Rovuma River, bombarding the enemy positions and launching landing parties that took the German fortifications of the Namaca island. An amphibious assault against the northern bank of the river was however repulsed by well entrenched German forces, with intense machine-gun fire that caused a high number of casualties in the Portuguese landing parties. The northern bank of the Rovuma would finally be occupied by the Portuguese forces three months later. The cruiser São Gabriel was also sent to Mozambique in April 1918, incidentally detecting and engaging a U-boat, while en route. When it was anchored in Cape Town, South Africa, São Gabriel was requested to collaborate in the security of the city, that was under the threat of an uprising and only guarded by 50 police officers, disembarking 116 of the members of its crew, that assumed the defense of the port for four days, until the return of the British naval forces that had left for the sea. One of the main roles of the Portuguese Navy was to defend the ports of Mainland Portugal and of the Portuguese North Atlantic isles (the Azores, Madeira and Cape Verde), specially against naval mines and U-boat attacks. The main effort was made on the defense of the Lisbon harbor, the principal Portuguese seaport and naval base, with intense naval patrolling done by surface vessels, submarines and naval aircraft, naval surveillance