Researchers examined the diet and plastic ingestion of green sea turtles inhabiting waters around the Ogasawara Islands, Japan, and detected plastics in 7 of the 10 individuals studied. By integrating genetic, isotopic, and plastic analyses, they estimated that the ingested plastics originated from areas beyond the turtles' migratory range, indicating the influence of transboundary marine pollution.
Plastics have been found in a wide range of marine organisms, from pelagic fishes and whales to even zooplankton. Among these organisms, sea turtles are frequently highlighted as particularly affected by marine plastic pollution. Plastics ingested by sea turtles are thought to originate from two main pathways: non-selective ingestion of plastics mixed with their natural food items, such as algae, and selective ingestion of plastics that resemble one of their typical prey, jellyfish. However, detailed data on the diet of sea turtles remain limited.
The green turtle is globally distributed species found mainly in tropical and subtropical waters. In Japan, it nests in the Ogasawara Islands, located approximately 1,000 km south of the main island. Green turtles migrate from the Pacific coasts of the Japanese mainland to the Ogasawara Islands for mating and nesting.
"During their long journey to Ogasawara Islands, the green turtles are likely to ingest and accumulate plastic distributed across a wide range of marine areas in various forms, including floating debris, sedimentary particles, and items mixed with seaweed," Prof. Lee explains. "To estimate the factors influencing plastic ingestion and the origin of the ingested plastics, we integrated morphological and genetic analysis of gut contents with isotopic and plastic analyses of ten green turtles captured in the Ogasawara Islands."
At the time of capture, the turtles' primary food source was macroalgae, and the DNA analysis indicated that the feeding grounds could be traced to three locations where their preferred seaweed species (Ectocarpus crouaniorum, Sargassum muticum, and Lobophora sp.)predominate. Stable isotope ratios of carbon and nitrogen in muscle tissue, together with DNA analysis, further suggested that they may have fed on drifting seaweed and gelatinous plankton during southward migration toward the Ogasawara Islands.
Plastics were detected in the digestive tracts of 7 out of 10 individuals examined. The average number of plastic items found was 9.2±8.5 (range: 0–31). Notably, macroplastics measuring 10 cm² to 1 m² accounted for 56.5% of all items.
Most plastics reported in marine organisms are microplastics (<5 mm). In sea turtles, however, ingestion is not limited to microplastics; previous studies have shown that they also consume larger mesoplastics (5 mm to <2.5 cm) and macroplastics (2.5 cm to <1 m), which are considered to have more severe negative impact on individual organisms and ecosystems. The findings of this study provide further evidence supporting this pattern.
Based on these results, the captured individuals likely to have ingested meso- and macroplastics associated with large and drifting seaweeds, mistaking them for gelatinous plankton such as jellyfish and salpas. Analysis of the printed characters on the ingested plastics suggest that they originated from region extending beyond the turtles' migratory range, indicating that this issue arises from cross-border pollution.
"This study demonstrates that plastic pollution is a transboundary issue," Prof. Lee emphasized. "Efforts to reduce plastic pollution—including reduction in the production, use, and disposal of plastic products—must be pursued through international cooperation, alongside continued research."
