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.