"We can never know the exact number,'' Roman said.
The 25 survivors included five women and five children. One woman said her husband and three small children died at sea, Roman said.
"She was in shock, in a state of great anguish,'' he said.
Mainland Spain and its Canary Islands off the coast of West Africa are a common destination for destitute Africans who risk their lives for a chance to reach Europe and find work.
Two similar events claimed nearly 30 lives in July, also in waters off southern Spain.
Traditionally, migrants travelled through Morocco across the Strait of Gibraltar or crossed the Mediterranean further east to get to the Spanish mainland.
But a crackdown in recent years has prompted traffickers to move operations west to countries such as Mauritania, now the launching point for much-longer voyages to the Canary Islands.
The number of those who actually make it to the Canary Islands is down drastically because of better surveillance of African coastal waters, Spanish officials say.