Lethal dose of plastics for sea creatures smaller than you might think

Less than three sugar cubes of plastic can kill a seabird, according to comprehensive new analysis

Peer-Reviewed Publication

Ocean Conservancy

“Ocean Plastics are an Existential Threat to the Diversity of Life on Our Planet”: Data Show that Nearly Half of Animals that Ingested Plastics were Red-Listed as Threatened Species

WASHINGTON — The Proceedings of the National Academy of Sciences today released a new study, “A quantitative risk assessment framework for mortality due to macroplastic ingestion in seabirds, marine mammals, and sea turtles.” Led by Ocean Conservancy researchers, the peer-reviewed paper is the most comprehensive study yet to quantify the extent to which a range of plastic types — from soft, flexible plastics like bags and food wrappers; to balloon pieces; to hard plastics ranging from fragments to whole items like beverage bottles — result in the death of seabirds, sea turtles, and marine mammals that consume them.

The study reveals that, on average, consuming less than three sugar cubes’ worth of plastics for seabirds like Atlantic puffins (which measure approximately 28 centimeters, or 11 inches, in length); just over two baseballs’ worth of plastics for sea turtles like Loggerheads (90 centimeters or 35 inches); and about a soccer ball’s worth of plastics for marine mammals like harbor porpoises (1.5 meters, or 60 inches), has a 90% likelihood of death. At the 50% mortality threshold, the volumes are even more startling: consuming less than one sugar cube’s worth of plastics kills one in two Atlantic puffins; less than half a baseball’s worth of plastics kills one in two Loggerhead turtles; and less than a sixth of a soccer ball kills one in two harbor porpoises. 

“We’ve long known that ocean creatures of all shapes and sizes are eating plastics; what we set out to understand was how much is too much,” said lead author of the study Dr. Erin Murphy, Ocean Conservancy’s manager of ocean plastics research. “The lethal dose varies based on the species, the animal’s size, the type of plastic it’s consuming, and other factors, but overall it’s much smaller than you might think, which is troubling when you consider that more than a garbage truck’s worth of plastics enters the ocean every minute.”

To arrive at their findings, Ocean Conservancy scientists analyzed the results of 10,412 necropsies, or animal autopsies, conducted worldwide in which cause of death and data on plastic ingestion were known. Of the animals studied, 1,537 were seabirds representing 57 species; 1,306 were sea turtles representing all seven species of sea turtles; and 7,569 were marine mammals across 31 species.

They then modeled the relationship between the plastics in the gut and likelihood of death for each group, looking both at total pieces of plastics as well as volume of plastics. Based on data availability, they also looked at different plastic types to determine which are particularly lethal to each group. They found that rubber and hard plastics are especially deadly for seabirds, soft and hard plastics for sea turtles, and soft plastics and fishing gear for marine mammals.

“This study reminds us that plastic bags, lost fishing gear, and other larger items can be dangerous to animals big and small,” said Ocean Conservancy’s Director of Ocean Plastics Research and study co-author Dr. Britta Baechler, who co-authored a study in 2024 that showed microplastics are present in both animal and plant-based proteins eaten by humans. “One in 20 sea turtles that we studied died from ingesting plastics. I wouldn’t take those odds.”

Nearly half (47%) of all sea turtles; a third (35%) of seabirds; and 12% of marine mammals in the dataset had plastics in their digestive tracts at their time of death; overall, one in five (21.5%) of the animals recorded had ingested plastics, often of varying types. Additional findings included:

Seabirds

• Of seabirds that ate plastic, 92% ate hard plastics, 9% ate soft plastics, 8% ate fishing debris, 6% ate rubber, and 5% ate foams, with many individuals eating multiple plastic types.

• Seabirds are especially vulnerable to synthetic rubber: just six pieces, each smaller than a pea, are 90% likely to cause death.

Sea Turtles

• Of sea turtles that ate plastic, 69% ate soft plastics, 58% ate fishing debris, 42% ate hard plastics, 7% ate foam, 4% ate synthetic rubbers, and 1% ate synthetic cloth.

• Sea turtles, which on average weigh several hundred pounds, are especially vulnerable to soft plastics, like plastic bags: just 342 pieces, each about the size of a pea, would be lethal with 90% certainty.

Mammals

• Of marine mammals that ate plastic, 72% ate fishing debris, 10% ate soft plastics, 5% ate rubber, 3% ate hard plastics, 2% ate foam, and 0.7% ate synthetic cloth.

• Marine mammals are especially vulnerable to fishing debris: 28 pieces, each smaller than a tennis ball, are enough to kill a sperm whale in 90% of cases.

The study also found that nearly half of the individual animals who had ingested plastics are red-listed as threatened — that is, near-threatened, vulnerable, endangered or critically endangered — by the IUCN. Notably, the study only analyzed the impacts of ingesting large plastics (greater than 5 millimeters) on these species, and did not account for all plastic impacts and interactions. For example, they excluded entanglement, sublethal impacts of ingestion that can impact overall animal health, and microplastics consumed.

“This research really drives home how ocean plastics are an existential threat to the diversity of life on our planet,” said Nicholas Mallos, vice president of Ocean Conservancy's Ending Ocean Plastics program and a study co-author. “Eating plastics is just one way that marine life is threatened by the plastic pollution crisis. Imagine the dangers when you also consider entanglement and the everpresent threat of toxic chemicals leaching from plastics.”

Scientists estimate that more than 11 million metric tons of plastics enter the ocean every year. Much of those plastics are single-use items like those commonly found by volunteers during Ocean Conservancy’s annual International Coastal Cleanup®. Since 1986, more than 19 million volunteers have removed more than 400 million pounds of trash from beaches and waterways worldwide. 

“Every year, volunteers collect massive numbers of balloons, plastic bags, straws, food wrappers, and other items that are lethal to wildlife even in small amounts, according to this research,” said Ocean Conservancy’s Senior Director of Conservation Cleanups Allison Schutes. “When you pick up just a few pieces of plastic, you are helping to protect the life of a marine animal. And when we all clean up together, we are helping to protect countless lives.”

Scientists have determined that to successfully address the plastic pollution crisis, we need to reduce plastics production, improve waste collection and recycling, and clean up what does get into the environment.

“We are thrilled to have this new research quantifying the wildlife impacts of plastic pollution,” said Ocean Conservancy’s Director of Plastics Policy Dr. Anja Brandon. “While there is no single solution to this issue, these hard numbers reaffirm that our work addressing particularly problematic items like balloons and plastic bags are truly meaningful. In the fight to protect our marine wildlife, every policy and every individual action matters.” 

“Governments around the world are grappling with how to address plastic pollution, and they are looking for science-based targets to inform policy decisions,” said Dr. Chelsea Rochman, associate professor in the Department of Ecology and Evolutionary Biology at the University of Toronto, scientific advisor to Ocean Conservancy, and senior author of the study. “This research provides an important foundation for decision-makers to understand thresholds for risk to better protect biodiversity.”

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For more than 50 years, Ocean Conservancy has delivered effective, evidence-based solutions for the ocean and all who depend on it. Today, we continue to unite science, people and policy to protect our ocean from the greatest challenges it faces: climate change, plastic pollution and biodiversity loss. We are a 501(C)3 headquartered in Washington, D.C. that inspires a worldwide network of partners, advocates and supporters through our comprehensive and clear-eyed approach to ocean conservation. Together, we are securing a healthy ocean and a thriving planet, forever and for everyone.