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

Abstract

Abstract 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\u202f±\u202f0.84\u202fg) was fed five isonitrogenous (303.20\u202fg\u202fkg−1 crude protein) and isocaloric (19.44\u202fMJ\u202fkg−1 gross energy) diets under thermal winter stress for 60\u202fdays. The diets contained five levels of propolis-extract (also known as bee glue): 0 (control), 1, 2, 3 and 4\u202fg\u202fkg−1 diet. The range of water temperature throughout the experimental period was (16–19\u202f°C). Results indicated that the survival rate was higher in fish fed with the diet supplemented with 4\u202fg\u202fkg−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\u202f=\u202f.017; P\u202f=\u202f.051). Feed conversion ratio was significantly improved with the addition of dietary propolis (quadratic, P\u202f=\u202f.031). A linear response in hematocrit (linear, P\u202f=\u202f.001), hemoglobin (linear, P\u202f=\u202f.031), red blood cells count (linear, P\u202f=\u202f.041) and white blood cells count (linear, P\u202f=\u202f.003) of fish was also observed with the increasing level of propolis-extract. On its turn, significant quadratic decreases in alanine aminotransferase (P\u202f=\u202f.001), aspartate aminotransferase (P\u202f=\u202f.001), alkaline phosphatase (P\u202f=\u202f.002) and lactate dehydrogenase (P\u202f=\u202f.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\u202f=\u202f.071 and P\u202f=\u202f.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\u202f=\u202f.001 and P\u202f=\u202f.012). The concentration levels of potassium was linearly (P\u202f=\u202f.001), and sodium quadraticly (P\u202f=\u202f.001) increased with dieatary proplis-extract. The expression of Δ 9D gene of fish under cold stress was upregulated (linearly, P\u202f=\u202f.001) with increasing dietary propolis-extract levels. To conclude, the optimum dietary inclusion level of propolis was estimated at 4\u202fg\u202fkg−1 based on the maximization of the growth parameters and survival rate.