No one knows how many children die as they are shipped to the cocoa plantations of West Africa.
Those who have lived to tell of such things say they were left with a tiny amount of food and only filthy drinking water for a journey that lasts days.
The story of the MV Etireno, with its cargo of desperate children, is almost routine in the harsh context of West Africa. So prevalent is child slavery there - the UN estimates there are 200,000 children working as slaves in the region - that the boat's captain must be wondering what all the fuss is about.
The dilapidated Nigerian ship has been plying the west coast for years, transporting its cargoes of children to labour in the sprawling cocoa plantations, or to work as servants, and de facto sex slaves, in the homes of the rich. The Etireno sailed as freely as any of the slave galleys that once plundered the region of humans for the New World.
A fortnight ago, the vessel had no trouble loading its cargo of children in Benin. The ship had docked there many times before, one of a regular stream of such vessels. It is not clear exactly how many children were loaded this time, but they are likely to be very young and were destined to follow thousands of others to the plantations of Gabon. For once, though, the authorities in Gabon turned the ship away. Cameroon did the same thing, leaving the MV Etireno as a fugitive ship.
Children are loaded in the port of Cotonou, a thriving centre of the slave trade in times gone by. Today, selling children is illegal, but not uncommon.
Brokers scout the country persuading parents to part with their offspring for a few dollars in return for promises that they will get an education or send money home. Thousands of children, almost all younger than 12 years old, end up being shipped to cocoa plantations in Ivory Coast and Gabon.
Four years ago, the Gabonese authorities found 400 youngsters waiting to be unloaded from a ship anchored in the harbour. But mostly the trade is left uninterrupted.
There is a long history of child workers moving around West Africa. But according to Ms Esther Guluma, the Benin representative of the UN's children's agency, Unicef, it has taken on more alarming aspects in recent years.
"We have to distinguish between traditional patterns of child labour, that were positive and not negative, which involved children being taken from rural areas to work in their relatives' homes in urban areas so they got an education and a better life," she said.
"But in the past few years, because of the deteriorating economic situation there has been a much bigger trade in children where they are very badly abused. Particularly, these large plantations need a lot of cheap labour, obedient labour and children are perfect for that. They work hard all day, you don't have to pay them very much and they don't complain.
"They are subjected to hard physical labour, they are uprooted from their families, they don't have any access to education. It has a real impact on their physical and mental development. Those trafficked as domestic servants, a lot of them are sexually exploited," said Ms Guluma.
The trade is barely hidden, if at all, in many countries, which is why the captain of the Etireno is probably so mystified. There is an open market in children in Ivory Coast's capital, Abidjan, where women come to buy them to "help around the house".
Girls from Benin and Togo are particularly in demand in Lagos where they are taken in to the houses of the relatively well off and forced to work for a pittance.
But Nigeria also offers up its own, with a flourishing sex trade in young people to Europe and some wealthy Arab countries. Italy is a particularly favoured destination for the traffickers.
Most children, though, are destined for the plantations. Even if they survive the long journey there, within a few years of slave labour many children are considered too exhausted or too old to be of any value and are discarded.
According to Unicef, many never manage to find their way home.