At least 15 migrants have been found dead off Tunisia's coast and on its border with Algeria this week, authorities said on Friday as sub-Saharan Africans face increasing tensions in Tunisia and more and more people seek to leave or transit through the country en route to Europe.The Tunisian coast guard retrieved the bodies of 13 migrants on Wednesday night and rescued 25 others off the port city of Sfax, according to a statement from the Interior Ministry.