Osmoregulation and Physiological Response of Largemouth Bass (Micropterus salmoides) Juvenile to Different Salinity Stresses

The distribution of saline-alkali water is extensive and is increasing globally each year. Fully utilizing saline-alkali water for aquaculture can help alleviate the scarcity of freshwater resources in global fisheries. As a major economic fish species, the largemouth bass (<i>Micropterus salmoides</i>) holds significant potential for aquaculture in saline-alkali water. In the present study, we evaluated its tolerance to different salinities (0 ppt, 6 ppt, 9 ppt, 12 ppt, 15 ppt, and 18 ppt) and investigated tissue pathology, serum biochemical indicators, enzyme activities of osmolality and antioxidant, and the relative expression of Na-K-2Cl 1a cotransporter (<i>NKCC1a</i>) under different saline stress (0 ppt, 6 ppt, 9 ppt, and 12 ppt). The largemouth bass 96 h mortality rate increased with increasing salinity, and the LC₅₀ for 96 h was 14.28 ppt based on the mortality results. High salinity group (12 ppt) caused gill and intestinal damage, including necrosis and cell shedding, while 6 ppt had no adverse effects, and the 9 ppt between the two salinities showed an adaptive change histologically. Serum osmolality, Na⁺, Cl^-, and cortisol levels of the high salinity group were significantly higher than of the low salinities (<i>p</i> < 0.05). Similarly, Na⁺/K⁺-ATPase (NKA), Ca²⁺-Mg²⁺-ATPase (CMA), and superoxide dismutase (SOD) activities of 12 ppt peaked at 24 h (15.7 U/mgprot, 11.5 U/mgprot, and 243 U/mgprot), which is significantly different compared to the other three groups (<i>p</i> < 0.05). The expression of <i>NKCC1a</i> was significantly upregulated at 9 ppt and 12 ppt, suggesting its role in osmoregulation. Furthermore, the expression of <i>NKCC1a</i> in the gill is 2-4 times higher than that in the intestine. These results suggested that largemouth bass can be cultured at 6 ppt and selectively bred for tolerance at 9 ppt. NKA activity, cortisol levels, and <i>NKCC1a</i> expression can be used as a marker of salinity suitability. These findings provide insight into the adaptive mechanisms underlying the physiological responses to acute salinity stress and will contribute to improving aquaculture in saline waters.