Millions of marine turtles have been killed over the past two decades through entrapment in fishing gear, according to a global survey.
Turtles must come to the surface to breathe.
When they are caught in a net or on a fishing hook, they cannot surface, and drown.
Three types of fishing gear are identified in the survey – long-lines, gillnets and trawls.
Debris in the oceans, such as plastic bags, can also cause drowning, while development in coastal regions can affect nesting and reproduction.
Some turtles are still targeted for meat, and their shells used for tourist souvenirs.
Numbers of adult leatherbacks – the largest species, growing to more than 2m long, and capable of journeys that span entire oceans – are thought to have declined by more than 75% between 1982 and 1996.
Lead researcher Bryan Wallace said the state of the world’s turtles was an indicator of the wider health of the oceans.
“Sea turtles are sentinel species of how oceans are functioning,” he said.