Nutritional mitigation of winter thermal stress in Nile tilapia by propolis-extract: Associated indicators of nutritional status, physiological responses and transcriptional response of delta-9-desaturase gene

A feeding trial was conducted to investigate the ability of diet supplemented with propolis-extract (winter feed, WF) to assist Nile tilapia, Oreochromis niloticus in coping with winter thermal stress. Nile tilapia (average initial weight, 25.40 ± 0.84 g) was fed five isonitrogenous (303.20 g kg⁻¹ crude protein) and isocaloric (19.44 MJ kg⁻¹ gross energy) diets under thermal winter stress for 60 days. The diets contained five levels of propolis-extract (also known as bee glue): 0 (control), 1, 2, 3 and 4 g kg⁻¹ diet. The range of water temperature throughout the experimental period was (16–19 °C). Results indicated that the survival rate was higher in fish fed with the diet supplemented with 4 g kg⁻¹ of propolis-extract compared to the remaing diets. Weight gain and specific growth rate were linearly improved with increasing dietary propolis-extract inclusion (linear, P =.017; P =.051). Feed conversion ratio was significantly improved with the addition of dietary propolis (quadratic, P =.031). A linear response in hematocrit (linear, P =.001), hemoglobin (linear, P =.031), red blood cells count (linear, P =.041) and white blood cells count (linear, P =.003) of fish was also observed with the increasing level of propolis-extract. On its turn, significant quadratic decreases in alanine aminotransferase (P =.001), aspartate aminotransferase (P =.001), alkaline phosphatase (P =.002) and lactate dehydrogenase (P =.002) were detected with increasing dietary propolis-extract levels in the diet. There was a linear response in triglyceride and cholesterol of fish as the level of propolis-extract increased in the diet (linear, P =.071 and P =.003, respectively). The responses of serum cortisol and glucose in fish under cold stress tended to decrease in response to increasing dietary propolis-extract (respectively, quadratic, P =.001 and P =.012). The concentration levels of potassium was linearly (P =.001), and sodium quadraticly (P =.001) increased with dieatary proplis-extract. The expression of Δ 9D gene of fish under cold stress was upregulated (linearly, P =.001) with increasing dietary propolis-extract levels. To conclude, the optimum dietary inclusion level of propolis was estimated at 4 g kg⁻¹ based on the maximization of the growth parameters and survival rate.