DAO prepress abstract   -  DOI: https://doi.org/10.3354/dao03836

ABSTRACT: The 2022 Oder River disaster was one of the most significant harmful events in recent European river history, with an estimated 60% reduction in fish population biomass in the lower section of the river. While the prevailing hypothesis attributes associated fish kills to toxins from golden algae, Prymnesium parvum, our histopathological study on the gills of two common cyprinid fish species vimba bream Vimba vimba (L) and roach Rutilus rutilus (L), collected from the lower Oder River at 3, 4, and 6 months after the disaster, suggests another mechanism. Vimba bream showed damage to the epithelial layer of lamellae and increased mucus production. Roach exhibited interlamellar cell mass (ILCM), lamellar damage, including hypertrophy of epithelial cells, lamellar fusion, as well as significant thickening of the water-blood barrier compared to controls. These findings suggest that adverse factors, most likely the increase of toxin concentrations, as they result from the reduction of water levels, together with elevated temperatures and low precipitation triggered the formation of ILCM, increasing fish susceptibility to hypoxia. Fish species with a capacity for adaptive interlamellar hyperplasia, such as common bream (Abramis brama), roach, and perch (Perch fluviatilis), accounted for the largest number of deaths during the disaster. Vimba bream, which showed no ILCM, were observed only sporadically, with mortality confined to a single area of the Oder. In conclusion, fish capable of adaptive hyperplasia, when the gill is attempting to protect itself by developing ILCM appear to be particularly vulnerable in conditions of aquatic hypoxia.