Entangled and drowned in a fishing net off the coast of Brazil, these green sea turtles in an undated picture are just a few of the millions of sea turtles that have been unintentionally killed by fishing operations over the past 20 years, according to a study released today by the journal Conservation Letters.

“Of all the threats sea turtles face right now, bycatch is the most serious,” said Bryan Wallace, a marine biologist with Conservation International and lead author of the study.

The study pulls together data from around the world on sea turtle deaths from nets, hooks, and trawls – and questions the estimates of previous reports.