This area was the source of an estimated 150,000 African slaves transported by the Portuguese, mainly from Upper Guinea before 1500, some used to grow cotton and indigo in the previously uninhabited Cape Verde islands. Portuguese traders and exiled criminals penetrated the rivers and creeks of Upper Guinea forming a mulatto population using Portuguese-based Creole language as their lingua franca. However, after 1500 the main area of Portuguese interest, both for gold and slaves, was further south in the Gold Coast.