Hematological and histopathological toxicities of silver nanoparticles (Ag-NPs) to rainbow trout were assessed in three water salinities: 0.4 ppt (low salinity), 6 ± 0.3 ppt (moderate salinity), and 12 ± 0.2 ppt (high salinity). The concentrations of Ag-NPs in the low salinity were 0.032, 0.1, 0.32, and 1 ppm, and in the moderate and high salinities were 3.2, 10, 32, and 100 ppm. The results indicated a concentration-dependently increased (thrombocyte, monocyte, and large lymphocyte) and decreased (neutrophil and small lymphocyte) white blood cell count in the Ag-NP treatments in the low salinity than the other ones in the moderate and high salinities. Red blood cell volume significantly increased in all of the experimental groups exposed to higher Ag-NP concentrations, especially those in the low salinity. In the moderate and high salinities, blood plasma total protein decreased in 10 and 32 ppm Ag-NP treatments, but albumin increased in the groups in the low salinity. Blood plasma ions (Cl−, Na+, K+, Ca2+, and Mg2+) showed high changes in the higher Ag-NP treatments. In all treatments, gill histological analysis demonstrated a time- and Ag-NP concentration-dependent extent of abnormalities, with the highest epithelial lifting in 1 ppm Ag-NPs in the low salinity and also the highest necrosis and aneurism in the 32 ppm treatments in other salinities. Lower Ag-NP concentrations in the low salinity led to fibrosis, villus fusion, inflammation, vacuolization, and microvillus hyperplasia in the gut, yet villi lifting and necrosis in 0.32 and 1 ppm of Ag-NPs were the main anomalies. In addition to the mentioned alterations, villi abolitions predominantly occurred in 32 ppm Ag-NP concentrations in the moderate and high salinities. Overall, despite exposing to lower Ag-NP concentrations, the fish kept in the low salinity demonstrated more vulnerability to Ag-NPs than those in the other salinities.
