Arab slave traders travelled through northern Mozambique both to reach the interior and to mount slave hunting parties. There is evidence of their passage in the oral tradition. The ruins of slave houses along the coast are also a powerful reminder of the trade. African slaves ended up as sailors in Persia, pearl divers in the Gulf, soldiers in the Omani and Ottoman armies and workers on the salt pans in what is now Iraq. Many people were domestic slaves, working in rich households. Women were often taken as sex slaves.
In the second half of the 18th century, the slave trade expanded and became more organised. Omani farmers needed slaves to work in the clove plantations on Zanzibar and Pemba set up by Sultan Seyyid Said. Brazilian traders were finding it difficult to operate in West Africa because the British navy was intercepting slave ships. Brazilian slave traderes made the journey round the Cape of Good Hope and bought slaves from the markets along the coast of nowadays Mozambique. The French also needed workers for their sugar and coffee plantations in Mauritius and Reunion islands.
In this period, Arab merchants were joined by Portuguese and other European adventurers who went to the interior searching for slaves and ivory to later trade along the coast. The prazeros, descendants of Portuguese and Africans, operated in the Zambesi basin. They counted on the local support of the Yao and Makua, two autochthonous ethnic groups who controlled the northern Zambezi and the coastal region, respectively. Arab traders established a trading and raiding state in the 1850’s south of Lake Tanganyika. Aided by the local population, they organized raid as far as modern Angola.
When Mozambique became a Portuguese colony, the trade intensified. Slaves were captured in the interior and divided into three categories. The first class of slaves was given to Portuguese officers; they were about 5% of the total slave population. The second class of slaves was offered to Portuguese settlers who used them to till their farms. This second group amounted to 35% of captured slaves. The third class, the great majority, was earmarked for export. They were sold to plantation owners in Mauritius or Cuba, but also other destinations in the Caribbean Islands and the Indian sub continent.
While it is impossible to quantify the number of Mozambicans reduced to slavery through the centuries, there are records that give us an idea of the size of the trade. In 1843, there were 22,122 slaves owned by 2,160 colons. However, this number is an underestimation, since colons always reported fewer slaves than they owned. A recent study by José Capela, researcher of the University of Porto (Portugal) and with extensive experience in Mozambique (O trafico de escravos nos portos de Mozambique, Edicoes Afrontamento, Porto), gives a broad treatment to the trade in Mozambique and the list of all boats departing from Mozambique ports from 1721 to 1860. At its peak, as many as forty slave carrying boats left Mozambique every year. Unfortunately, not all records were kept in regards to the number of slaves that left the country. Slavery in Mozambique was finally abolished in 1904.