## 3 Results and Discussion
### Histopathology  
All tissues presented specific histological changes that affected the morphology of the gills, liver, and kidneys of each species (Figure 3). The most relevant bronchial changes were represented by vasodilation, epithelial necrosis, and desquamation, occasionally resulting in lamellar fusion and epithelial lifting. Many epithelial cells appeared marked by hypertrophy and hyperplasia. Contamination of the freshwater with metals caused histopathological changes in fish gills in a metal concentration and exposure time-dependent manner, mainly because these are the primary target organs of aquatic pollutants [23; 24]. The increased bioaccumulation of Cr and Zn in the gills of all analyzed species could be correlated with the observed histopathological changes. According to [PERSON] et al. [25], the impacts of metals could be of major importance and are significantly correlated with filament epithelium proliferation, lamellar fusion, and epithelial necrosis. Hepatocyte vacuolization and sinusoid dilatation are more common than other pathological changes. Structural changes less severe than those found in the gills and kidneys suggested that the liver has a higher capacity and adaptability to counteract the metal  
Figure 3: Microphotographs of histopathological changes observed in the captured fish: **Gills—**cartilaginous core (C), secondary lamellae (SL), epithelial edema, epithelial necrosis and desquamation (arrow), lamellar fusion (LF), epithelial lifting (EL), lamellar aneurism (LA); **Liver—**centrilobular vein (CV), dilation of blood sinusoid (arrow), vacuolated hepatocytes (arrowhead); **Kidneys—**necrosis of renal tubules epithelium (arrowhead), shrinkage of glomerulus and increase in Bowman’s space (arrow); BB—_Barbus babus_; CN—_Chondrostoma nasus_; SC—_Squalius cephalus_. H&E stain. Scale bar: 20 μm. induced damage based on its stronger antioxidant system, thus preventing injuries induced by these toxins [14]. On the other hand, the kidney morphology was more affected compared to that of the gills and liver. Vacuolization of the renal epithelial cells and atrophy of the renal tubular lumens were noticed in all tested species. Glomeruli contraction and disruption of the hematopoietic tissue was also highlighted. Atrophy of renal tubules and degeneration of the Bowman's capsule were more evident for CN. The histopathological changes ranged between 1 and 3 for each organ and species and correlated with the specific bioaccumulation of metals into the fish tissues (Figure 4). The metals were simultaneously accumulated in the gills, kidneys, and liver, and this reflects their uptake by the organisms, their subsequent distribution, and significant interspecies histopathological damage differences. The extended histopathological lesions recorded in the kidneys of the predatory fish (2.8 \(\pm\) 0.2) are significantly correlated with the high concentrations of Cd, Pb, and Zn at the renal level (\(p<0.001\)), suggesting that these metals are mainly involved in the damage  
Figure 4: Correlations of the histopathological lesion grade (range 1–3) in the gills, kidneys, and liver of _Barbus barbus_ (BB), _Squalius cphalus_ (SC), and _Chondrostoma nus_ (CN) with metal concentration [cadmium (**A**) chromium (**B**), zinc (**C**), copper (**D**), lead (**E**)] determined in the same tissues of all three analyzed species. Statistical results mark the calculated significance between the metal concentration and the lesion grade in the respective organ per each species: *** significant difference at \(p<0.001\); *** significant difference at \(p<0.01\); * significant difference at \(p<0.05\). of this tissue. A previous study noticed Cd and Pb accumulations mainly in the kidneys, while the aquatic environment and fish habitats affect the metals' bioavailability and favor its significant accumulation in predatory fish [26]. Moreover, the highest amount of renal Zn determined in our experiment confirmed [PERSON]'s data [27], which showed that predators accumulated this heavy metal more than benthivores.