Over the past four decades (1970-2012), only a few scientific studies have ocused on stowaways found on board merchant ships (Walters, 2008 ; Burtin and Maquet, 2012). Because migrants are not allowed to be on these ships, they are usually held in cells. Often it is private individuals or societies (captain, P & I (protection & indemnity) club, etc.) who take charge of them. Public authorities are gradually delegating this control function. The surveys conducted in several French ports shed light on the role merchant navy boats and port areas play in a broader system of detention measures for migrants. In the context of increasingly restrictive legislation, this article describes “hybrid” measures for the detention of migrants which shift between the informal and the official depending on the period and the places where the stowaways are found (on the high seas or in harbours). The externalization of controls of “unwanted migrants” is placed under the responsibility of private actors. They wish to make them disembark as quickly as possible for the purpose of repatriating them to their home country or to their port of embarkation.