More than 30 would-be migrants were missing and believed dead on Sunday after their boat collided with a fishing vessel as it set out from this African desert country toward Europe, according to officials and doctors who treated survivors. The boat was carrying 57 people and had been at sea for about 17 days before the accident late on Saturday, said Commandant Sidi Ould Ahmed of Mauritania's military police. He said 32 people were still missing.