You've never heard of the Middle Passage? If you want firsthand primary sources, you can find trader accounts on various academic databases or through nifty google searching. If you just want research sources, I recommend checking out books on the Middle Passage at your local library or, again, via google.
Those casualties were factoring in centuries of slave trade. I believe the number of total slaves traded is estimated at around 16,000,000 with 2-3 million in casualties. As someone pointed out earlier, it wasn't all European traders. Many African tribes such as the Dahomey bought firearms from the Portuguese and used them to capture hundreds of thousands of neighboring natives who would later be sold to Europeans.