The perilous sea passage from West Africa to Europe was once a major route for migrants seeking jobs and prosperity.
The sinking is one of the deadliest incidents since the mid-2000s when Spain stepped up patrols and fewer boats attempted the journey.
The boat carrying at least 150 people ran out of fuel and was stranded for days when approaching Mauritania before it capsised.
Some 83 people swam to shore. The survivors were being helped by Mauritanian authorities in the northern city of Nouadhibou, the International Organization for Migration (IOM) said.
Survivors said the vessel left The Gambia on November 27.IOM's Leonard Doyle said the boat was unseaworthy and overcrowded when it overturned.
"It speaks really to the callousness of the smugglers who of course have made their money and disappeared into the wilderness. That's the problem here, people are being exploited, people are looking for a better life," Doyle told Al Jazeera.
An unknown number of injured were taken to hospital in Nouadhibou.
There was no immediate statement from authorities in The Gambia, a small West African nation from which many migrants set off in hopes of reaching Europe.
Al Jazeera's Mohamed Vall, reporting from Mauritania's capital Nouakchott, said the military police discovered the survivors - most of whom came from The Gambia - and that is when the extent of the tragedy became clear.
"It's a very horrible story and one of the deadliest incidents in regard to migrants trying to cross the Atlantic Ocean or the Mediterranean Sea towards Europe this year," said Vall. "It's been confirmed that women and children were on that boat and some of them lost their lives."
Although home to some of the continent's fastest-growing economies, West Africa is struggling to generate enough jobs for its growing population of young people.
Doyle said the survivors would likely be returned to their home countries.
"We can imagine that they're deeply traumatised. People will need some medical care and our staff will need to establish their origin and try to help them return in the most dignified way as possible. The tragedy in all this is there is no happy solution for people who take these routes. Once things like this happen, they eventually end up back where they started. The only winner is the smuggler," he said.
Despite the Gambia's small size, more than 35,000 Gambian migrants arrived in Europe between 2014 and 2018, according to the IOM.
Onyekachi Wambu, executive director of the African Foundation for Development, said the issue of economic migration is in part caused by demographics with many young people looking for work.
"What you're seeing is Africa exporting its labour in a very dysfunctional manner," Wambu told Al Jazeera.
"West Africa is enjoying a period of growth, but it's largely jobless growth. A lot of people are heading north because they can see by getting into Europe they can transform their lives."
The 22-year long oppressive rule by then-president Yahya Jammeh severely affected the country's economy, especially for The Gambia's young people, prompting some to look to migrating.
Since Jammeh was voted out of office in 2016 and fled into exile in January 2017, European countries have been pushing to return asylum seekers, but the country's economy has still to recover.
The coastal nation, a popular tourist destination, was shaken earlier this year by the collapse of British travel company Thomas Cook.
At the time, The Gambia's tourism minister said the government convened an emergency meeting on the collapse, while some Gambians said the shutdown could have a devastating impact on tourism, which contributes more than 30 percent of the country's GDP.