By Agence France-Presse
Vincent Nichols, Archbishop of Westminster and president of the Santa Marta anti-trafficking group, said: "We estimate there are 42 million victims worldwide."
The Santa Marta Group is formed of both religious and non-religious figures, bringing together international police chiefs and church leaders in the fight against human trafficking and modern slavery.
"What struck me particularly was the extent of modern day slavery on the seas," Nichols said.
"There are around 4.4 million fishing vessels in the world, and on most of those or many of those, there will be people in enslaved labour."
The conference focused on finding tailored solutions for each region with greater cooperation between countries facing similar challenges.
Pope Francis, who has made the fight against human trafficking one the priorities of his papacy, held an audience with conference participants on Friday morning.
"We ask you Holy Father to urge financial and business institutions to do all they can to eliminate slavery and its profits from their transactions," said the group.
The International Labour Organisation estimated that in 2016 over 40 million people were victims of modern slavery globally.
During the audience the pope said that "these modern forms of slavery are much more widespread than one can imagine, including, to our great shame, within the most prosperous of our societies".