Effects of the 2022 Oder River environmental disaster on fish gill structure

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 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 2 common cyprinid fish species, namely vimba bream Vimba vimba (L.) and roach Rutilus rutilus (L.), collected from the lower Oder River at 3, 4, and 6 mo 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 in toxin concentrations resulting from reduced water levels together with elevated temperatures and low precipitation, triggered the formation of ILCM, increasing the susceptibility of fish to hypoxia. Fish species with a capacity for adaptive interlamellar hyperplasia, such as common bream Abramis brama, roach, and common perch Perca 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, whereby the gills attempt to protect themselves by developing ILCM, appear to be particularly vulnerable in conditions of aquatic hypoxia.