Commonwealth Scientific and Industrial Research Organisation (CSIRO) scientists carried out post-mortem examinations (necropsies) on 246 sea turtles from Queensland, Australia, and analysed 706 more necropsy records from StrandNet, the Queensland Government's stranding database, comparing the amount of plastic in each animal's gut with whether plastic had killed it.
They found that a turtle that had eaten a single piece of plastic had about a 22% chance of death from it. The risk rose steeply with each additional piece: at 14 pieces the chance of death reached 50%. Young turtles were more likely to have eaten plastic than adults.
The study matters because it was the first to put a number on how much plastic it takes to kill a turtle, turning a well-known hazard into a clear measure of how the risk rises with each extra piece.
The data are from Queensland waters and from turtles that stranded or died in care, which may not represent healthy populations elsewhere.