Effects of water temperature and feed iron level on nitrite resistance of spotted seabass <i>（Lateolabrax japonicus）</i>

After 56 days of culture experiment, spotted seabass ( Lateolabrax japonicus ) (10 fish per tank) fed diets with similar dietary iron levels (63, 188, and 554 mg/kg) were subjected to a 24-hour nitrite stress challenge at 27 °C and 33 °C, respectively. Nitrite stress significantly reduced the levels of hematocrit (HCT), red blood cell (RBC) and hemoglobin (Hb). The reduction in the 33 ℃ group was greater than that in the 27 ℃ group, and the decrease in the 63 mg/kg iron level group was the largest. The main impact analysis revealed that when the dietary iron level rose from 63 mg/kg to 554 mg/kg, the contents of red blood cell (RBC), white blood cell (WBC), and hemoglobin (Hb) in the blood and the activities of total antioxidant capacity (T-AOC) and catalase (CAT) in the serum significantly increased (P&lt; 0.05). The group with an iron level of 188 mg/kg showed a considerably lower serum malondialdehyde (MDA) content than the group with an iron level of 63 mg/kg (P&lt; 0.05). The 63 mg/kg iron level group observed substantial decreases (P&lt; 0.05) in hemoglobin (Hb) and total antioxidant capacity (T-AOC) under 27 °C, but dietary 188 and 554 mg/kg iron mitigated this phenomenon. Iron deficiency and high temperatures facilitated the formation of methemoglobin (MetHb). This study demonstrated that spotted seabass is more susceptible to the physiological harm caused by nitrite at 33 ℃ than at 27 ℃. Iron deficiency exacerbated the risk of hemocytopenia, oxidative stress and hypoxia. Dietary iron fortification could enhance tolerance by enhancing antioxidant capacity and blood homeostasis.