• A new study looking at the impacts of plastic ingestion by seabirds, sea turtles and marine mammals found that relatively small amounts of consumed plastic can be deadly.
• The research analyzed the necropsy results for more than 10,000 animals and quantified the amount of plastic that could prove deadly as well as the types of plastic with the biggest impact, which included synthetic rubber, soft plastics (such as plastic bags and wrappers) and discarded plastic fishing gear.
• Overall, one in five of the deceased animals had consumed plastic (affecting 50% of all studied sea turtles, 35% of seabirds and 12% of marine mammals); nearly half of the species studied were considered threatened or near threatened on the IUCN Red List.
• The researchers didn’t consider other health impacts of plastic, such as chemical exposure and entanglement, which led the lead author to conclude the study likely underestimates the “existential threat that plastic pollution poses to ocean wildlife.”

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New research has found that even small amounts of plastic can be deadly to seabirds, sea turtles and marine mammals. While previous research has established that plastic can lead to mortality in many species, this new study identifies the types and amounts of plastic that pose the greatest danger, and estimates how likely an animal is to die after ingesting it. The study authors found the lethal dose to be much smaller than expected.

The team of international researchers, including several from U.S.-based environmental advocacy group Ocean Conservancy, conducted a literature review of more than 50 studies, drawing together the necropsy results for more than 10,000 animals that included data on the cause of death and on plastic ingestion. The mortality data included 1,537 seabirds from 57 species; 1,306 sea turtles from all seven marine turtle species; and 7,569 marine mammals representing 31 species, including whales, dolphins and seals.

The team used the data to generate modeling that analyzed the relationship between the plastic in the animal’s gut and the likelihood of death for each animal group, looking at both the total number of pieces of plastic and their volume. Where possible, they also considered the type of plastic the wildlife ingested to understand which types were most lethal to the animals.

Plastic found inside a dead black-footed albatross. Image by Dan Clark /USFWS.

“That lethal dose is much smaller than we expected,” lead author Erin Murphy, manager of ocean plastic research at the Ocean Conservancy, told Mongabay.

She and her co-authors were only looking at “acute mortality events,” which occurred when the animal’s gastrointestinal tract was perforated, cut or obstructed by plastic, preventing movement of food through the stomach and intestines.

“It’s harder to quantify the potential deadliness of the longer-term chronic exposure of plastic ingestion with these types of studies,” Murphy explained. “But I think a key point is that our numbers are only focusing on acute mortality events from plastic ingestion. So as a result, it’s likely an underestimate of the actual existential threat that plastic pollution poses to ocean wildlife, because we’re not considering chemical exposure, we’re not considering chronic exposure, and we’re not considering entanglement.”

Even excluding these other kinds of plastic exposure, the study, published Nov. 17 in PNAS, found that relatively small amounts of ingested plastic could be deadly. For instance, seabirds such as the Atlantic puffin (Fratercula arctica) only had to consume about three sugar cubes’ worth of plastics to have a 90% chance of death. Sea turtles such as the loggerhead (Caretta caretta), which can weigh about 124 kilograms (275 pounds), needed to consume more than two baseballs’ worth of plastics, while various marine mammal species, including harbor porpoises (Phocoena phocoena), which can weigh about 58 kg (130 lbs), had to consume about a soccer ball’s worth of plastics to have a 90% chance of death.

The research also identified the plastic categories deadliest to these animals. For seabirds, it was synthetic rubber; for sea turtles, it was soft plastics (such as plastic bags and wraps); and for marine mammals, it was discarded plastic fishing gear.

The study determined that one in five of the studied deceased animals had consumed plastic, accounting for nearly 50% of the studied sea turtles, 35% of the seabirds, and 12% of the marine mammals. Notably, nearly half of the studied species were on the IUCN Red List, with classifications ranging from near threatened to critically endangered.

“These are species that are already vulnerable,” Murphy noted. “Their populations are already at risk, and plastic ingestion is another thing that is impacting these already stressed species.”

Harbor seals and gulls off the eastern coast of the U.S. close to Canada. Image © Julie Larsen.

Study co-author Lauren Roman, a marine scientist at the University of Tasmania in Australia, told Mongabay the research “identifies some important patterns, such as the proportion of different animal groups that die from plastic and the volume of different types of materials likely to be lethal.”

She added the findings can help “shape evidence-based policies” to reduce high-risk plastic items — ranging from single-use plastic bags and food wrappers to fishing debris — from entering the marine environment and impacting animals there.

While the study’s findings focus on mortality events, Brendan Godley, a conservation scientist at the University of Exeter, U.K., who wasn’t involved in the research, noted the resilience seen in some species, despite consuming plastic. For instance, he “marveled at the robustness of sea turtles with over 100 plastic pieces being tolerated by 50% of individuals.”

“I suspect this threshold will increase as further data from more polluted parts of sea turtle habitats are included in subsequent studies,” Godley told Mongabay. “In the Mediterranean, we often encounter juvenile green turtles with extremely high burdens that have been otherwise healthy until bycaught in fishing nets.”

Godley said the study provided a “fantastic step towards quantitatively assessing the likelihood of mortality due to plastic ingestion in seabirds.”

“Given the ubiquity of the plastic threat, it is important to start to understand the likely impacts to individuals,” he said, “so that we can understand the likely implications for different populations, and subsequently whole species.”

Banner image: A seaturtle consumes a plastic bag. Image by Troy Mayne / Ocean Conservancy.

Elizabeth Claire Alberts is a senior staff writer for Mongabay and was recently a fellow with the Pulitzer Center’s Ocean Reporting Network. Find her on Bluesky and LinkedIn.

Citation:

Murphy, E. L., Baechler, B. R., Roman, L., Leonard, G. H., Mallos, N. J., Santos, R. G., & Rochman, C. M. (2025). A quantitative risk assessment framework for mortality due to macroplastic ingestion in seabirds, marine mammals, and sea turtles. Proceedings of the National Academy of Sciences, 122(48). doi:10.1073/pnas.2415492122