El Hierro, a small island in the Atlantic Ocean, is Europe's latest frontline in the struggle to cut irregular migration. Nearly twice as many migrants as residents have landed this year on the southernmost part of Spain's Canary Islands.

On a Sunday in late October, a group of 30 teenagers from Mali and Senegal, some in soccer shirts with headphones around their necks, ambled across a deserted town square in the capital Valverde. A few locals watched silently.

Across town, chairs were piled up in the assembly hall of the Nuestra Señora de los Reyes hospital to make space for beds for smuggled migrants, who often suffer from hypothermia, dehydration or injuries after the roughly eight-day crossing from Africa.