An estimated 4,400 migrants, including about 205 children, were lost at sea while trying to cross to Spain in 2021.
A monitoring group, Walking Borders, also known as  Caminando Fronteras, in a statement  said the figures doubled that of 2020, and is the highest since the group began counting in 2018.
The group blamed such factors as increasingly dangerous routes, inferior boats and the fear of some vessels to help migrants at sea for the surge in deaths.
According to Spanish official statistics, 39,000 undocumented migrants successfully reached Spain by sea or land last year, a similar figure to the previous year.
Walking Borders said more than 90% of those missing or dead were lost on 124 shipwrecks en route Spain’s Canary Islands in the Atlantic Ocean as of December 2020.
Since 2020, the islands off the coast of Africa have become the main destination for migrants trying to reach Spain, with a much smaller share trying to cross the Mediterranean Sea to the Spanish mainland.
Helena Maleno, founder of Caminando Fronteras, said the group gathered its data from hotlines set up for migrants on vessels in trouble to call for help, and from family members seeking information.
The group investigates the fate of each boat. It presumes those missing at sea for at least a month to be dead, noting that close to 95% of the figures represent people who are missing.
The United Nations International Organization for Migration (IOM) counted 955 people who died or went missing in 2021 trying to reach the Canary Islands by December 22, the highest figure it documented since 2014.
The statement said the IOM obtains its data from official records, media reports and other groups, including Walking Borders, but says its numbers are conservative and the real toll is probably higher.
The Interior Ministry’s spokesperson  said, Spain does not keep figures for people who die trying to reach its shores, as he  declined to comments on the latest figures.