Biomarker Responses in the Freshwater Fish European Chub (Squalius cephalus): Pollution Effects and Implications for Biomonitoring

Biomarkers are used to assess pollutant-associated responses in European chub, Squalius cephalus (Linnaeus, 1758), yet their interpretive reliability under field conditions remains uncertain. This critical narrative review examines research on European chub, reporting biotransformation, oxidative stress, genotoxicity, histopathology, and other endpoints alongside pollution data. 7-Ethoxyresorufin-O-deethylase (EROD) and glutathione S-transferase (GST) responses differed among locations and tissues; an EROD–hexachlorocyclohexane (HCH) association was reported in Czech rivers; however, mixed exposures limited attribution. In metal-affected sites, antioxidant induction sometimes accompanied oxidative damage, whereas increased hepatic glutathione peroxidase (GPx) activity in Slovak chub occurred without a change in thiobarbituric acid-reactive substances (TBARS). Per- and polyfluoroalkyl substances (PFAS) studies reported renal antioxidant and protein-oxidation responses; transcript and enzyme endpoints did not always show concordant patterns. Comet and micronucleus assays differed in their ability to distinguish sites, while histopathology documented gill, liver and kidney alterations. Czech male European chub exhibited variations in plasma vitellogenin (VTG) and 11-ketotestosterone (11-KT) concentrations, while gonadal abnormalities, including intersex characteristics, were observed at polluted Elbe River site. These findings support the integration of complementary, pollutant- and tissue-relevant biomarkers with individually matched chemical measurements, while accounting for biological and seasonal variation, to improve the interpretation of biomarker responses in European chub.