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Using historical fish collections to track microplastic contamination in Costa Rican freshwater ecosystems
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Scientists studied 180 fish specimens collected over 65 years to see how microplastic pollution in Costa Rica's rivers has changed over time. Every single fish had plastic fibers in its gut, and the amount has grown significantly, especially in rivers near heavily developed areas, showing that freshwater fish (a food source for many communities) are absorbing more plastic pollution as human activity increases. This matters because when we eat fish, we may also be consuming these accumulated microplastics, and the study highlights that reducing plastic waste and improving wastewater treatment could help curb this growing contamination.
Introduction: Microplastics are now pervasive in aquatic ecosystems and represent a growing environmental concern. Their persistence and biological uptake threaten aquatic biodiversity and may affect food webs. In Costa Rica, research on microplastics has mainly focused on coastal and marine systems, leaving freshwater environments less explored. Museum specimens offer a unique opportunity to reconstruct historical patterns of microplastic contamination in these inland waters. Objectives: To examine temporal and spatial variation in microplastic ingestion in Astyanax aeneus, an abundant and widely distributed freshwater fish in Costa Rica. To (1) evaluate long-term changes, (2) compare basins with contrasting levels of human disturbance, and (3) test whether body size influences microplastic ingestion rates. Methods: We analyzed 180 specimens from a national collection spanning seven decades (1960–2025) across three river basins (San Carlos, Tárcoles and Térraba). Gastrointestinal tracts were digested with KOH and examined under stereomicroscopy to identify microplastic particles. Temporal and spatial variations were tested using ANOVAs, and relationships between fish size and microplastic load were assessed with correlation analyses. All procedures included strict contamination control. Results: All specimens contained microplastics, mainly fibers. Microplastic loads increased markedly over time in the San Carlos basin and were substantially higher in the heavily impacted Tárcoles basin. No relationship was detected between fish body size and the number of ingested particles. Conclusions: Freshwater fishes in tropical ecosystems are experiencing increasing microplastic exposure, primarily driven by watershed-level disturbance. The absence of a size-dependent pattern suggests that environmental availability and ecological context play a central role in shaping exposure. Moreover, this study supports the use of museum collections for retrospective monitoring of microplastic pollution and underscores the need for management strategies targeting plastic inputs and wastewater contamination.
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