Combined physiological effects of high temperature and salinity stress on genetically improved farmed tilapia (Oreochromis niloticus) reared in inland saline water

Freshwater scarcity and increasing salinisation of inland water bodies pose significant challenges to agriculture and traditional freshwater aquaculture. Using inland saline water (ISW) for aquaculture offers a promising option to expand farming into salt-affected and non-arable regions. An experimental trial (60 days) was conducted to evaluate the combined effect of temperature and ISW of varying salinities on genetically improved farmed tilapia (GIFT) fingerlings (initial weight: 2.73 ± 0.02 g). Fish were distributed randomly across 6 treatments in triplicate tanks (n=15/tank). The experiment included combinations of two temperatures (Ambient: 28.5 ± 0.39 °C, AT; High: 33.5 ± 0.17 °C, HT) and three salinities (Freshwater, 0-2.25 ppt; ISW, 10 ppt; ISW, 15 ppt). Growth parameters, survival, osmoregulatory parameters and stress markers were analysed. The temperature and salinity interaction was significant, with the poorest growth and survival in the HT X 15 group. Serum osmolality and ions (Na⁺, Ca²⁺ and Cl^-) increased progressively with salinity, and were further elevated at HT, while K⁺ showed the opposite trend. mRNA expression of branchial ion transporters such as sodium potassium ATPase, nka1α, and chloride channel, clc2, showed an increase with salinity, while aquaporin, aqp1, was downregulated. Serum stress markers (cortisol and glucose) increased dose-dependently with both stressors. Serum thyroid hormones declined, reaching minimum levels at HT X 15. Antioxidant enzymes showed tissue-specific responses: liver antioxidant enzymes increased 3- 4 fold at HT X 15, while gill enzymes showed moderate elevation. Hepatic hsp70 gene expression was upregulated (3-fold) in the HT X 15 group. These findings help define environmental limits to support fish health, growth, and sustainability in inland saline aquaculture under thermal and salinity stress.