LAUSR

Rivers play a crucial role in collecting and transporting microplastics. Nonetheless, the degree to which microplastic pollution of freshwaters affects its biota remains understudied. Sampling of wild fishes has so far demonstrated that microplastic ingestion occurs commonly across species with alternate feeding modes, as well as in different environmental compartments. Due to the exploratory nature of many preceding studies, drawing insight about factors driving microplastic ingestion has remained difficult. It continues unknown for instance, what the importance of varying environmental microplastic concentrations is to predict ingestion rates in fish from those areas. Here we show that ingestion rates of microplastic particles (>300 μm) in the benthic round goby from the Rhine river were negligible (1 particle in 417 fish). Among the 535 visually selected putative microplastic fragments, stringent data processing steps to reduce the number of false positives during reference library searches, revealed the importance of taking such steps into account in comparison with other data processing routines. Our observations remained consistent, despite having collected fish from a strongly polluted site of the lower Rhine, which served as contrast to a significantly cleaner site upstream. These results demonstrate that higher environmental microplastic concentrations are not necessarily mirrored by higher ingestion rates in a given fish species.