Dietary mannitol improves salinity adaptation in Nile tilapia (Oreochromis niloticus) by enhancing ion regulation, energy metabolism and antioxidant defense.

Aquaculture in saline environments is expanding worldwide, but elevated salinity can alter growth performance and physiological homeostasis in cultured fish. Nile tilapia (Oreochromis niloticus) is a euryhaline freshwater species widely used in salinity tolerance research and aquaculture production. Here, this study evaluated whether dietary mannitol improves salinity adaptation in tilapia. Fish (1.86 ± 0.04 g) were fed isonitrogenous and isoenergetic diets containing 0, 1, 2, 5 or 10% mannitol for 8 weeks under freshwater or 20‰ salinity conditions. The results revealed that salinity treatment significantly reduced weight gain (WG) and whole-body moisture content and increased the hepatosomatic index (HSI), feed conversion ratio (FCR), and whole-body crude protein and lipid contents. Under salinity treatment, supplementation with 1% or 2% mannitol significantly increased WG, whole-body moisture, and crude protein and significantly reduced FCR and whole-body lipid content. These treatments also alleviated gill lamellar hypertrophy and hepatic cytoplasmic vacuolization. In addition, 1% or 2% mannitol significantly increased the activities of enzymes involved in glycolysis and the TCA cycle, as well as the lactate content in the liver. It also increased the expression levels of genes related to myo-inositol metabolism. Moderate mannitol supplementation also reduced oxidative damage. Regression analysis indicated that a mannitol content of approximately 1.5% can achieve optimal weight gain. These findings suggest that an appropriate level of mannitol improves growth and salinity adaptation in Nile tilapia, possibly through multiple physiological adjustments under salinity treatment, although the underlying mechanisms require further verification.