The United Nations' migration agency says that 529 migrants have died attempting to reach Spain's Canary Islands this year.
But the agency says the true number could be much higher, as it does not take into account people who have disappeared at sea without being reported missing, or their bodies found.
At least 11 people are feared dead after another boat capsized crossing the Atlantic Ocean from North Africa, Spanish authorities and activists reported Tuesday.
Rescuers retrieved 32 survivors and one body overnight from a rubber boat in waters south of Fuerteventura, the Spanish delegate said. One of the survivors died on the rescue boat.
It was reported that as many as 60 migrants were on board when the boat set off from a beach near the southern Moroccan town of Tan-Tan.
But according to the Spanish NGO Caminando Fronteras (Walking Borders), the boat carried out 42 migrants when it set sail.
The Atlantic Ocean route is one of the most dangerous sea crossings to Europe.
In June, the United Nations' International Organisation for Migration (IOM) said 136 had died on the route to the Spanish islands since January. In just two months, the figure has increased almost threefold.
Survivors of other failed crossings also have reported that bodies of fellow passengers were thrown into the sea before rescuers arrived, which could increase the true death toll.
Walking Borders claims that as many as 2,000 people have died so far this year on their way to the Canary Islands.
The organisation has become one of the first contact points for African families trying to locate their relatives on the other end of the migration route.
"The human rights crisis at the border needs an urgent political response," the NGO's founder Helena Maleno said in a tweet addressed to Spanish Prime Minister Pedro Sánchez.
Walking Borders also said on Monday that at least 29 Africans, including seven children, had died on a boat following a rescue attempt on August 27.