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Selective ingestion drives divergence between environmental and gut microplastics in riverine fishes
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Scientists studying fish in a Bangladesh river found something surprising: how much microplastic pollution is in the water doesn't match how much ends up in fish guts. Instead, fish seem to preferentially swallow certain types of plastic—especially larger pieces, foam bits, fibers, and blue or black particles—meaning the plastic risk isn't just about how polluted the water is, but what *kind* of plastic is floating around. Since many of us eat fish, this matters because it suggests we should track the specific types of microplastics in waterways, not just total amounts, to better understand what's actually ending up on our plates.
Microplastics (MPs) are pervasive in freshwater rivers, yet biologically relevant exposure may not be captured by environmental concentrations alone. Here, we quantified MPs in river water and in the gastrointestinal tracts of three riverine fishes (Puntius ticto, Mystus cavasius, Sperata seenghala) across five sites along the Barnoi River (Bangladesh) to test whether gut MP characteristics decouple from ambient availability. Water and fish were sampled monthly (February-April 2025). MPs were extracted by digestion, density separation, and filtration and then characterized by shape, size, and color using microscopy. Environmental MP concentrations differed significantly among sites (2.67 ± 0.33 to 51.67 ± 9.21 MP L⁻; Kruskal-Wallis, p < 0.05), while gut MP abundance showed no interspecific difference (H = 1.63, p = 0.443; ηH = 0.003). Negative binomial GLMs indicated that gut MP abundance varied with site but not with species identity or fish body size. In contrast, gut MP composition deviated strongly from random uptake expectations, with significant non-random patterns for shape (χ = 254.09, p < 0.001), size (χ = 234.87, p < 0.001), and color (χ = 335.33, p < 0.001). Large particles (4-5 mm), foam, and fibers were enriched in fish guts relative to river water, and blue/black particles were disproportionately retained. Collectively, these findings provide field evidence that selective ingestion structures gut MP composition independently of exposure gradients, demonstrating trait-mediated modification of exposure between environmental and internalized MPs. Trait-based metrics (size, shape, color) should therefore be incorporated into freshwater MP monitoring and ecological risk assessment to better reflect biologically relevant exposure.
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