Microplastic concentration and composition in surface waters and in stream-rearing Steelhead Trout (Oncorhynchus mykiss) in a rural coastal California stream

Microplastic (particles <5 mm) is an emerging environmental concern that threatens aquatic ecosystems worldwide. While extensive research has documented the ubiquity of microplastic in marine and coastal environments and its assimilation by organisms at all trophic levels, comparatively little is known about microplastic occurrence in rural riverine ecosystems. Even less information exists for salmon-bearing streams, where microplastic pollution may hinder salmonid conservation and recovery efforts. We examined the presence and concentration of microplastic in Scott Creek, a small coastal California (USA) watershed with minimal urban development. Our study quantified suspended microplastic in surface waters and in the gastrointestinal (GI) tracts of juvenile Steelhead Trout ( Oncorhynchus mykiss ). Microplastic was detected in 93% of water samples, indicating that Scott Creek routinely transports and cycles microplastic. Concentrations were higher in late summer than in spring, although monthly spatial patterns were largely consistent among sites. In juvenile steelhead, 60.6% of GI tracts contained one or more types of microplastic. For both surface waters and juvenile steelhead, microplastic was predominantly small (<5 mm) dark blue, black, and transparent fiber-type particles, with fragments, shards and foams encountered less frequently. These findings suggest that fish ingest microplastic at low frequencies, or that ingested particles may pass rapidly through their digestive systems. This study provides one of the first assessments of microplastic in a minimally urbanized, salmon-bearing watershed. Our results serve as a critical baseline for evaluating the ecological implications of microplastic pollution in small coastal creeks and its potential to influence native salmonid populations. The surface water and steelhead microplastic datasets generated and analyzed during the current study are publicly available via Zenodo (DOI: 10.5281/zenodo.18038857). • Microplastic was detected in 93% of surface water samples in a rural watershed. • Fibers (>86%), primarily blue and black, dominated microplastic morphotypes observed. • Microplastic levels peaked in low-flow July samples, indicating seasonal accumulation. • 60% of juvenile steelhead ingested microplastic, almost exclusively as fibers.