How many Black Africans were sold as slaves outside the Americas?

Aug 2016
I remember my old history professor going on about how the amount of black African slaves shipped to areas such as North Africa and the Middle-East was much smaller than the amount that went to the Americas. Admittedly, the people in the Americas had a very strongly agricultural economy that largely continues today.

However, surely the Ottoman empire and other regional powers had plenty of need for agricultural workers and other menial jobs that slaves could fill. Native Americans could and did make up much of the early slave work-force, but they seem to have largely died out. Why was the importation of slaves to North Africa and the Middle-East so much lower than countries in the Americas?