The first European emigrants travelled in the holds of sailing cargo ships. These were usually served as motherships for deepwater fishing or as merchant or cargo ships. Harland & Wolff specialised in building tankers and cargo ships at Govan. Destroyer escorts were regular companions escorting the vulnerable cargo ships. These bases were easily resupplied by cargo ships. The line subsequently expanded through the purchase of second-hand cargo ships. The bulk of the escorts and two cargo ships eventually reached Malta. There is one commercial shipyard for cargo ships and passenger voyages. Passenger and cargo ships anchor in the Gibraltar Harbour. Ballast tanks are only used in cargo ships there. Debris is generated at sea from fishing vessels, stationary platforms, and cargo ships. Cargo ships transport dry and liquid cargo. Conditions improved after the prisoners had been transported in cargo ships to Italy. Severodvinsk now has the largest shipbuilding yard in Russia for large ships (tankers, cargo ships). Many cargo ships were converted into battle ships.