The first known transatlantic slave ship sailed from São Tomé to New Spain in 1525, outsourced by Genoese bankers who paid the Spanish court for the "Asiento de Negros". Portuguese slavers had a near monopoly on the export of slaves from Africa until about 1640, while in the second half of the 17th century competing Dutch, British and French traders concentrated the trade in slaves on the Caribbean islands of Curaçao, Martinique and Barbados from where they would be exported to the mainland. During the 18th century, when the slave trade transported about 6 million Africans, British slavers carried almost 2.5 million.