Mauritania is no longer the world's slavery capital

The Walk Free Foundation's slavery index says the percentage of people living in modern slavery in Mauritania dropped from 4% in 2014 to about 1% this year. The country, which CNN featured in the 2012 documentary "Slavery's last stronghold," now has the world's seventh-highest incidence of slavery.

North Korea ranks worst on the index. Nearly one in 20 people there are thought to be enslaved.

"Slavery is not a thing of the past, and we must stop thinking that it is," the Walk Free Foundation said in a statement issued to CNN. "The very nature of modern slavery means it is clandestine and hidden from view, but that doesn't mean it isn't everywhere. Every country in the world is affected."

Globally 45.8 million people are held in slavery, according to the report, which is based on random surveys in 25 countries, including Mauritania, and statistical modeling. That's a 28% uptick since the group's last slavery index report was released in 2014. But Fiona David, the Walk Free Foundation's executive director of global research, cautioned against reading too much into that increase or the decrease in slavery's prevalence in Mauritania. The group's ability to estimate the prevalence of slavery is improving over time, she said, so the trends may result in part from better stats.

"It's too early for us to say whether or not there's been an absolute increase," she said.

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