Effects of salinity on cold tolerance of Malaysian red tilapia

This study was conducted to investigate the effect of salinity on growth, survival, osmoregulation, metabolism, hematological and biochemical parameters and T-complex protein (TCP-1) mRNA expression levels in Malaysian red tilapia at declining tank water temperatures. Five salinity concentrations (0, 5, 10, 15, 20 aEuro degrees) were evaluated during the study period. Samples were collected on a range of biochemical parameters at 16.5 (week 2), 12 (week 4) and 9.5 A degrees C (week 6). No significant growth (body weight) was showed in the study (P > 0.05). Survival was monitored in the groups of fish until the holding tank temperature reached 5.6 A degrees C. The lowest temperature in the study population of Malaysian red tilapia tolerated was 9 A degrees C, although the fish were able to survive in temperature of 5.6 A degrees C if they had been reared at 10 aEuro degrees water salinity. The salinity and change in water temperature had significant effects on hematological and biochemical parameters and the TCP-1 gene expressions in Malaysian red tilapia (P < 0.05). The effect of temperature and salinity on superoxide dismutase (SOD), malondialdehyde (MDA) and hemoglobin (HGB) levels was significant (P < 0.05 or P < 0.01); with decreasing temperature, SOD and MDA activities increased, whereas HGB concentrations decreased. With increasing level of salinity, the SOD, MDA and blood glucose levels significantly increased (P < 0.05). The highest cholesterol (TC), triglycerides (TG), red blood cells (RBC) and HGB values were apparent in the moderate salinity groups, whereas the highest high-density lipoprotein cholesterol (HDL-C) was in the moderately high-salinity group. Rearing at low temperature, Na+-K+-ATPase (NKA) increased with increasing level of salinity at 16.5 A degrees C, although it initially declined before rising again at water temperatures 12 and 9.5 A degrees C. Rearing at low temperature, fish in the low-salinity groups significantly up-regulated the expression of the TCP-1 genes (P < 0.05), whereas fish in mild to moderate salinity groups significantly down-regulated the expression (P < 0.05). Taken together, these results suggest that exposure to environmental conditions of low temperature, low salinity and high salinity influence osmoregulation, metabolic parameters and the fish physiology. However, a change in the isotonic point at cold temperatures was observed by rearing in a salinity of under 10 aEuro degrees. This could potentially provide a means to increase cold tolerance of overwintering Malaysian red tilapia.