But slavery is often harder to pin down. With almost half the population living on less than $2 a day, many slaveowners work alongside their slaves.
Boubacar Messaoud grew up in a grey area between slavery and freedom, paid a token salary in return for farming. "One day when we were about seven, the slaveowner's son, whose name was also Boubacar, said I should be called Boubacar abd [the black slave], so people didn't confuse us. That was when I understood."
And many do not identify themselves as slaves. "When people talk of slavery, they talk of chains, prisons, and threats. That was the slavery of those who had known liberty – the Africans who jumped into the sea rather than be enslaved in America," said Messaoud, who founded the abolitionist organisation SOS Slaves. "Today we have the slavery American plantation owners dreamed of. Slaves believe their condition is necessary to get to paradise."
Some former slaves like Malaka, 28, tended his owner's goats unsupervised for weeks at a time in the desert. "I didn't want to leave because I was scared to leave my family behind. And I was scared because I had heard about money, but I had never seen it in my life," he said.
Escape is no guarantee of freedom. When Ahmeid went to her local magistrate, her mother testified against her. Her uncle beat her savagely. After weeks shuttling between sympathizers, she found herself crouching in a two-story building in Nouakchott late one evening as truckloads of policemen stormed an anti-slavery organisation where she had been sheltering. The group's leader, Birame Ould Abeid, and three others were jailed after publicly burning religious texts that have been used to justify slavery, and calling for black Moors and black Africans to unite.