After a migrant boat sank and about 50 of its passengers went missing in the Atlantic Ocean off northwestern Africa, nine survivors endured two days on the semi-submerged wreck before they were found, according to Spanish rescuers. The rescue happened on Monday near the coast of the Canary Islands, a Spanish archipelago and a destination along a migration route on which, experts say, many other shipwrecks are likely to have gone unreported. The rescue occurred after a merchant ship reported a sinking vessel 60 nautical miles south of El Hierro, an island in the far west Canaries, said Carmen Lorente Sánchez, a spokeswoman for the Spanish maritime safety and rescue organization. She said rescuers found nine people on board and took them to the island’s airport. The survivors later told the authorities that the shipwreck had occurred two days earlier and that around 60 people were on board when they departed from Senegal, Ms. Sanchez added. The Canary Islands received about 40,000 migrants last year, a sharp increase from the previous year, according to the …