Seven bodies were found on the boat, while an estimated 56 people are missing at sea and presumed dead, the International Organization for Migration (IOM) said Thursday.
"Generally, when people are reported missing following a shipwreck, they are presumed dead," IOM spokeswoman Safa Msehli said.
According to Cape Verde police, 38 survivors, including four children, were rescued when a Spanish fishing vessel found the boat adrift earlier this week.
People were at sea for over a month
The large fishing boat, called a pirogue, left the Senegalese fishing village of Fass Boye on July 10 with 101 passengers on board, Spanish migration advocacy group Walking Borders said.
Families of the people on board reached out to Walking Borders late last month after not hearing from loved ones on the boat.
The vessel was only located on Monday morning in the Atlantic Ocean, about 150 nautical miles (277 kilometers) from the Cape Verdean island of Sal.
Dangerous route to Europe
The boat was found on the Atlantic migration route from the coast of West Africa to Spain's Canary Islands. It is one of the world's deadliest.
Nearly 1,000 migrants died trying to reach Spain by sea in the first six months of 2023, Walking Borders says.
According to the Spanish Interior Ministry, around 10,000 people have made the perilous journey from the northwest coast of Africa to the Canary Islands by sea this year.
lo/nm (AP, AFP, dpa, Reuters)