Most of those rescued are believed to be from both northern and central Africa.
At least 319 migrants have been rescued while trying to cross the Atlantic Ocean in several different boats, according to Spanish authorities.
Emergency services on the Canary Islands reported that 24 children were among those rescued on Wednesday.
But the NGO Walking Borders says that at least 18 people travelling on one of the migrants' boats had died.
“Rescue means were activated too late despite having the boat’s location,” Helena Maleno, the NGO’s founder, wrote in a tweet.
Spanish rescue vessels located the boats in different locations overnight on Tuesday, a Maritime Rescue Service spokesperson told The Associated Press.
The spokesperson added that rescuers did not find any bodies on or near the vessels but said that nine people were found alive, clinging to a capsized boat near the island of Lanzarote.
The Canary Islands' 112 emergency service said that nine adults and one baby among the 319 survivors were transferred to local health facilities on the archipelago. Most of those rescued were believed to be from both northern and central Africa.