Red Crescent officials in Mauritania say some of the immigrants were in canoes, one of which sank when it hit another boat coming to rescue them.
At least 26 people were saved by fishing boats during the incidents but dozens more died. At least 17 people have been reported missing.
The victims had sailed from Mauritania in an attempt to reach the Canary Islands which belong to Spain.
It was unclear what caused the accidents but heavy winds have buffeted the northern coast of West Africa for days making the seas treacherous.
The immigrants were riding in wooden canoes called "pirogues" and survivors were plucked from the sea by larger fishing ships.
Many of the rescued passengers are now under care in Mauritania's second city of Nouadhibou.
The passengers from the doomed boat were from several countries in West Africa including Gambia, Guinea Bissau, Ivory Coast, Mali, Mauritania and Nigeria.
The Canary Islands are about 1,000 kilometres north of Nouadhibou.
African immigrants looking to enter Europe without proper documentation often try to sneak into the Spanish islands or an enclave north across the Sahara Desert, in Morocco. Many die each year trying.
At least four died and more than two dozen people were missing and presumed dead in a similar incident in December, when a boat capsized off Mauritania's coast in strong winds.