The sole survivor has told rescuers that the boat left northern Africa in September with 34 people on board.
At least four migrants have died and 29 others are missing after a boat sank off Spain's Canary Islands.
Spain's marine rescue service said on Sunday that it had picked up four bodies from a rubber dinghy that was adrift in waters off northwest Africa.
A merchant ship rescued one person after it spotted the boat on Saturday, around 280 kilometres south of the Canary Islands archipelago.
The marine rescue service sent a helicopter to take the man to Gran Canaria, where he was treated in a hospital.
The survivor -- a 27-year-old man from the Ivory Coast -- told rescue services that there had been 34 people on the boat when it set sail.
The NGO Caminando Fronteras said on Twitter that it had received a distress call from the same boat on 24 September only a few hours after it had left the Western Sahara coast "with 34 people on board".
From January to July, at least 9,308 people reached the Canary Islands from Western Africa, according to Spanish government figures. The figure represents a 27.5% rise compared with the same period in 2021.
The voyage across the Atlantic Ocean is one of the most dangerous migration routes with trips in often overloaded boats taking more than a week to reach European soil.
According to Caminando Fronteras, 4,404 migrants died or disappeared last year at sea while trying to reach Spain, twice as many as in 2020.