Effects of elevated salinity levels on the biological alterations of rohu (Labeo rohita): initiative for developing salinity tolerant line

Salinity is a critical abiotic factor in aquatic environments imposing severe stress on farmed freshwater species, and negatively impacting growth, metabolism, immunity, and overall wellbeing. The Indian major carp, rohu (Labeo rohita) has been facing various production-related issues associated with salinity intrusion in the upper coastal regions of Bangladesh. The current study examined the effects of four different salinity levels (0‰ as control, 2‰, 4‰, and 6‰) on selected cellular (ultra-structure of gill), physiological, biochemical, and gene expression traits of rohu. Experimental salinity levels significantly affected different biological parameters of rohu (P < 0.05) including moderate to severe levels of gill tissue damage, 16–25% slower growth, 12–30% lower survival, and a 15–56% higher O₂ consumption for the treatment groups (2‰, 4‰, and 6‰) when compared to the control (0‰). Blood glucose (≈55 µg/ml for control while 76–128 µg/ml for the treatments) and stress hormone (cortisol) levels (≈75 ng/ml for control and 87–145 ng/ml for the salinity treatments) increased with increasing salinity levels whereas blood cell counts were in a declining trend (23 × 10⁶ to 17 × 10⁶ cells/µl). Expression levels of the six selected genes showed salinity-specific differential changes; 1.5 to 2 - fold higher expression of growth, immunity, and metabolic genes were observed for control conditions while the ion regulatory genes showed 2 to 3 fold higher expression levels for the salinity treatments (2‰, 4‰, and 6‰) compared to the control. Results of this study indicate that different salinity levels impose stress at varying orders of magnitude on experimental rohu. It was also evident that rohu deploys different biological responses to cope with the rising salinity. Therefore, this study provides important clues to develop salinity tolerant lines/strains of rohu that may help to reduce potential adverse effects related to climate change (i.e., inland salinity intrusion) on the farming of this species.