The number of deaths recorded on migratory routes fell this year, although COVID-19 difficulties and so-called "invisible shipwrecks" mean the real number is probably much higher, officials at the U.N. migration agency said on Friday.
The IOM's Missing Migrants report showed 3,174 deaths compared to 5,327 in 2019.
"The decrease in recorded migrant deaths is not necessarily an indication that the number of lives lost truly dropped in 2020 as COVID-19 also challenges our ability both to collate data on deaths during migration and monitor specific routes," the IOM said.
Crucially, the data does not include losses from at least 15 so-called "invisible shipwrecks" in the Mediterranean - events that cannot officially be corroborated because the vessels cannot be located and information is insufficient.
If officials learn about them at all, it is often through bereaved family members.
Sometimes, the only indication is floating bodies.
In a poignant indication of the problem, the bodies of four children washed up on the shores of Libya this week from a boat believed to be carrying North and West African migrants.
"Incidents like this happen way too often. These are the ones we know about and the number of lives lost on the crossing are much higher (than reported)," said the IOM's Safa Msehli.
The report said 729 deaths had been confirmed in the central Mediterranean in 2020.
Msehli estimated that at least 600 more people have drowned in the Mediterranean this year in unrecorded incidents.
"There are gaps, serious life-threatening gaps in the monitoring of these routes," said the IOM's Paul Dillon, calling for rigorous, state-led search and rescue capacities.
The IOM data also showed an increase in fatalities on some routes, such as the journey to Spain's Canary Islands, where 593 deaths were recorded compared with 210 last year.
More migrant losses were also recorded in South America, with many of the 104 recorded from Venezuela.