Evaluation of parasitic and tissue-specific accumulation of emerging contaminants in wild European chubs (Squalius cephalus) across the Seine River system

• Sixty emerging contaminants quantified in water, fish tissues, and parasites • Gills showed the highest pollutant diversity and concentrations among tissues • Acanthocephalan parasites bioaccumulated pharmaceuticals, including paracetamol • Parasite biomass altered host pollutant loads in gills and muscle differently • Fish size, condition and habitat correlated with tissue contaminant burden Aquatic ecosystems are increasingly affected by chemical contamination from anthropogenic activities, yet the fate of pesticides and pharmaceutical residues within organisms remains poorly understood. Parasites can also significantly influence pollutant dynamics, through a high capacity for bioaccumulation. In this study, we quantified 60 emerging contaminants, including pesticides, antibiotics, and other pharmaceuticals, in surface waters, fish tissues (gills, liver, muscle), and intestinal acanthocephalan parasites of wild European chubs ( Squalius cephalus ) across multiple rivers of the Seine river system, France. Forty compounds were detected in water samples, whereas 28, 18, 17, and 8 were detected in parasites, gills, liver, and muscle tissues, respectively. Six compounds, mainly pharmaceuticals and one pesticide (diflufenican), were detected in more than 50% of samples in at least one biological compartment. Pollutant detection in water was associated with increased occurrence and levels in fish tissue, although tissue levels did not directly correlate with waterborne levels. Gills exhibited the highest pollutant diversity and levels among tissues. Habitat type influenced the number of pollutants detected in water, with higher values in agricultural areas, but did not directly affect tissue concentrations. Acanthocephalans accumulated several pharmaceuticals, especially paracetamol and fluoxetine, at high levels, supporting their bioaccumulation potential. Moreover, parasite biomass influenced host contamination in a tissue-specific manner, decreasing pollutant levels in gills but increasing them in muscle. Fish size positively, and body condition negatively, correlated with pollutant load. These findings highlight the importance of considering both host physiology and parasitism when assessing contaminant fate in freshwater ecosystems.