Consuming more than 14 pieces of waste plastic is 50 percent more likely to kill a sea turtle than other ways they die, new research has found.
The study was conducted by scientists from the University of the Sunshine Coast and and the Commonwealth Scientific and Industrial Research Organization's (CSIRO) Oceans and Atmosphere research unit, based in Hobart, Australia.
Kathy Townsend, USC lecturer in animal ecology, told Plastics News the study involved necropsies of almost 1,000 dead turtles from 2008 to 2015 found on beaches and in waters, mainly off the Queensland and northern New South Wales coasts.
She and her research team conducted 246 necropsies; another 706 were included in a database of turtle strandings, StrandNet, maintained by the Queensland government's environment and science department.
Townsend said the research was "one of the biggest of its kind" with in-depth statistical analysis of what kills turtles. It was published in the journal Scientific Reports.
"We didn't just examine 10 turtles. It's a very broad study with strong, convincing arguments," she said.
Turtle mortalities were separated into deaths that were not plastic debris related, for example: disease; boat strikes or entanglement in fishing gear; unknown causes; and deaths that could be directly linked to ingesting pieces of plastic.
Before the study was completed, Townsend said it was unknown whether plastic in the oceans was killing sea turtles or whether they ingested it without significant harm and it was possibly excreted.
Chris Wilcox, CSIRO Oceans and Atmosphere principal research scientist and lead author on the study, said in a statement: "We knew turtles were consuming a lot of plastic, but didn't know for certain whether that plastic actually caused turtles' deaths, or whether turtles just happened to have plastic in them when they died."
The research found a turtle had a 22 percent chance of dying from eating just one piece of plastic. One dead turtle had ingested 329 pieces.
Townsend said the research enables scientists to now focus clean-up efforts on areas that pose the highest risk to sea turtles. "Instead of trying to clean all the oceans, we can concentrate efforts on hot spots."
She is now conducting surveys that will test the effectiveness of Queensland's ban on free single-use plastic shopping bags, which started in July.
Townsend was surprised the research discovered such low numbers of pieces of plastic were potential killers. There are seven species of sea turtles globally, six of which are found in Australian waters. Townsend said all are threatened, some endangered and loggerhead turtle populations critically endangered.
"These animals have so many other human interactions, including boat strikes, habitat destructions, feral animal predation, and entanglement in fishing nets. Plastic debris ingestion is just another one. It's not sustainable for the turtle populations," she said.
But Townsend is optimistic because there is "a groundswell of people who want to do something about it."
She said plastic pollution has become part of "normal conversation" and scientists like her are "no longer yelling into a void."
The research report said globally it is estimated about 52 percent of all sea turtles have ingested plastic debris. The report suggests plastic physically resembling turtles' natural food is ingested at a higher rate than other types and "the rate at which debris is ingested may increase when natural food intake is compromised or reduced, suggesting a compounding effect."