First, although the original House resolution is vague about the number of Africans who made the Middle Passage (“millions”), and historians do not all agree, 25–50 million is two to four times larger than what is generally accepted. David Eltis, in his Atlas of the Transatlantic Slave Trade (2010), suggests that between 1500 and 1866 about 12.5 million Africans were loaded onto ships bound for the New World, with about 1.8 million of them dying along the way (for a mortality rate of about 15 percent). Shark's migratory patters were changed because these predators followed the ships in the Middle Passage because when a slave died they were thrown overboard, or if they were killed because they were protesting or if they committed suicide, the sharks knew that they could follow the ships, and it changed the migratory patterns of sharks during this period of time.
In the meantime, additional information on sharks and the slave trade can be found in The Slave Ship: A Human History (2007) by Marcus Rediker. He writes how the predators fed on crew and slave alike. And while they were especially dangerous in the warm waters of Africa, sharks also sometimes “followed the slavers all the way across the Atlantic into American ports, as suggested by a notice from Kingston, Jamaica, that appeared in various newspapers in 1785:
“The many Guineamen [slave ships] lately arrived here have introduced such a number of overgrown sharks. (The constant attendants on the vessels from the coasts) that bathing in the river is become extremely dangerous, even above town. A very large one was taken on Sunday, along side the Hibberts, Capt. Boyd.” Abolitionists would do much to publicize the terror of sharks in the slave trade, but this evidence comes from a slave society, before the rise of the abolitionist movement. More came from Captain Hugh Crow, who made ten slaving voyages and wrote from personal observation that sharks “have been known to follow vessels across the ocean, that they might devour the bodies of the dead when thrown overboard.”