Most of the deaths took place en route to Spain's Canary Islands after increased patrols along Europe's southern coast dramatically reduced crossings to the continent via the Mediterranean Sea.
Nearly 2,100 migrants died trying to reach Spain by sea during the first six months of 2021, five times more deaths than was registered during the same period of last year.
The huge number is also the highest number for a first half of the year since non-governmental organisation Caminando Fronteras, which monitors migratory flows, started keeping records in 2007.
"It's been a horrendous year," Helena Maleno, an activist with the Spanish NGO, told a news conference called to present the group's latest report.
The migrants are from 18 countries, mainly in West Africa. Over 90 percent of the deaths this year took place during 57 shipwrecks on the route to Spain's Canary Islands.
Such tragedies are recurring with migrant arrivals on the Atlantic archipelago surging since late 2019 after increased patrols along Europe's southern coast dramatically reduced crossings to the continent via the Mediterranean.
The shortest route to the islands is more than 100 kilometres from the Moroccan coast, but it is notoriously dangerous because of the strong currents in the Atlantic.
The Spanish NGO worker blamed the rise in deaths this year on the greater use of inflatable boats on this route, which is less safe because migrants were struggling to get their hands on wooden vessels and on inadequate cooperation between the rescue services of Spain and Morocco.
The cooperation between the two nations has worsened since a crisis in May that resulted in the entry of over 10,000 people from Morocco into Ceuta, a tiny Spanish enclave, she added.
"There is no coordination. Information is not flowing between the two states," Maleno said.
In many cases, even after the NGO sounds the alarm about a struggling boat, no one comes to the rescue, she added.
"Migrants may spend an entire day drowning," she said.
The higher death toll is also due to the fact that more family members of migrants are reporting to the NGO when their loved ones go missing at sea, Maleno added.
Spanish interior ministry figures show that between January 1 and June 30, a total of 12,622 migrants arrived in Spain by sea, compared with 7,256 a year earlier. But a spokesman said the ministry had no tally for how many migrants went missing or died in the attempted crossing.