Ultrastructural, Osmoregulation, Physiological Metabolism, and Antioxidative Response of Red Tilapia (Oreochromis spp.) Under Acute Alkaline Stress

With increasing global demand for fishery products, saline-alkaline aquaculture has emerged as a potential solution. However, limited information exists regarding the tolerance and adaptability of red tilapia (Oreochromis spp.) to alkaline environments. This study evaluated the acute semi-lethal toxicity of alkaline water and its physiological effects on juvenile red tilapia (44.80 ± 1.09 g; 12.84 ± 1.02 cm). Fish were exposed to NaHCO3 for 96 h at five alkalinity levels (50, 55, 60, 65, and 70 mmol/L). Survival declined significantly with increasing alkalinity, and the 96 h LC50 was 60.66 mmol/L, indicating relatively high alkaline tolerance. Gill tissue and serum samples were collected at 0, 12, 24, 48, and 96 h under 50 mmol/L NaHCO3 and freshwater (control). Under prolonged alkaline exposure, gill structure exhibited marked alterations, with SLL and ILCMT progressively decreasing over time. Serum antioxidant responses were impaired, showing a sustained decline in SOD activity and a rise in MDA content, indicating the onset of oxidative stress. TAOC, CAT, and GSH-Px displayed a biphasic response, rising initially before declining with continued stress. TP and GLU levels remained largely stable throughout. Energy metabolism indices also shifted dynamically: TC showed a transient increase followed by a decrease, whereas TG and LDH initially declined before rising later in the exposure period. Stress-related markers, including cortisol, T3, ALP, and ACP, were notably elevated during the exposure. Additionally, serum ion concentrations (K+, Ca2+, Na+, Cl−) increased gradually over the 96 h period, reflecting progressive osmoregulatory disturbance under alkaline stress. Overall, this study shows that juvenile red tilapia (Oreochromis spp.) have high alkaline tolerance (96 h LC50 = 60.66 mmol/L NaHCO3), but exposure to alkaline water causes significant physiological stress, affecting gill structure, antioxidant defense, metabolism, and ion balance. These results offer valuable insights into their adaptability to saline-alkaline aquaculture.