Slavery and the slave trades had a significant impact on the size of the population and the gender distribution throughout much of Africa. The precise impact of these demographic shifts has been an issue of significant debate. The Atlantic slave trade took 70,000 people, primarily from the west coast of Africa, per year at its peak in the mid-1700s. The Arab slave trade involved the capture of peoples from the continental interior, who were then shipped overseas through ports on the Red Sea and elsewhere. It peaked at 10,000 people bartered per year in the 1600s. According to Patrick Manning, there was a consistent population decrease in large parts of Sub-Saharan Africa as a result of these slave trades. This population decline throughout West Africa from 1650 until 1850 was exacerbated by the preference of slave traders for male slaves. It is important to note that this preference only existed in the transatlantic slave trade. More female slaves than male were traded across the continent of Africa. In eastern Africa, the slave trade was multi-directional and changed over time. To meet the demand for menial labor, Zanj slaves captured from the southern interior were sold through ports on the northern seaboard in cumulatively large numbers over the centuries to customers in the Nile Valley, Horn of Africa, Arabian Peninsula, Persian Gulf, India, Far East and the Indian Ocean islands.