The Ijaws were one of the first of Nigeria's peoples to have contact with Westerners, and were active as go-betweens in the slave trade between visiting Europeans and the peoples of the interior, particularly in the era before the discovery of quinine, when West Africa was still known as the "White Man's Graveyard" because of the endemic presence of malaria. Some of the kin-based trading lineages that arose among the Ijaws developed into substantial corporations which were known as "houses"; each house had an elected leader as well as a fleet of war canoes for use in protecting trade and fighting rivals. The other main occupation common among the Ijaws has traditionally been fishing and farming.