Andy Schroeder, Scott Farling, Kenzie Hahne, Kayla West, Tyler Brasington, Melanie Clapham

Plastic accounts for approximately 80% of all human-produced waste materials found in marine environments, with approximately 14 million tonnes of plastic entering the ocean each year. Macroplastic pollution (plastics >10 mm) poses significant threats to marine ecosystems through entanglement, habitat degradation and ingestion. As these materials fragment into microplastics (<5 mm), they become increasingly widespread and bioavailable, facilitating ingestion and trophic transfer across marine food webs. Accumulation of microplastics within organisms has been linked to oxidative stress, intestinal damage, energetic depletion, and reduced reproductive output, with potentially irreversible consequences for animal physiology. Brown bears (Ursus arctos) are a highly generalist species that exploit a wide range of complementary food resources, including ungulates, fruits, herbaceous vegetation, and numerous marine-derived foods such as salmon, shellfish and marine mammals. As a result, coastal brown bears represent an important yet understudied model for assessing microplastic exposure at the interface of marine and terrestrial ecosystems and may act as vectors for the transfer of microplastics from marine to terrestrial environments. We tested and compared brown bear scat samples from coastal populations in Alaska and Canada with samples collected from an interior North American population in Wyoming to assess: (a) whether microplastics are present in brown bear diets, and (b) whether microplastics are more prevalent in coastal populations that consume marine-derived foods. Of the 30 scat samples analysed from coastal populations, 93% contained microplastic particles, and an additional 93% contained anthropogenic fibres. While the physiological consequences of these contaminants for brown bears remain unknown, our findings highlight a potentially widespread and previously unrecognized exposure pathway that warrants urgent further investigation.