Abstract
In China, the yield of the hybrid of blunt snout bream (Megalobrama amblycephala) and topmouth culter (Culter alburnus) is very high, but the requirement for protein in the feed of these hybrids is still unknown. Thus, a 12-week feeding trial was conducted to determine the optimal protein requirement in diets for juvenile backcross variety, M. amblycephala ♀ × (M. amblycephala ♀ × C. alburnus ♂) ♂. Five experimental diets containing 25%, 30%, 35%, 40% and 45% crude protein (being abbreviated as P25, P30, P35, P40, P45) were formulated by adjusting the percentage of fish meal and starch was used as the fish meal replacer to give feeds that were iso-lipidic and iso-caloric. Fifty juveniles with the same sizes (initial weight, 6.50 ± 0.05 g) were reared in 15 concrete ponds (5 treatments in triplicate groups) and fed the test diets to apparent satiation two times a day. Weight gain (WG) in fish fed with P30 and P35 was significantly higher (p < .05) than those fed with P25 and P45. Fish fed with P45 diet had the lowest WG. Based on the quadratic curve model, the maximum WG and the minimum feed conversion ratio (FCR) of backcross juveniles were observed at 33.89% and 33.50% protein levels, respectively. The optimal dietary protein levels for backcross juveniles were estimated to be 33.69%. Protein content of dressed carcasses was highest at P35 and lipid content decreased significantly (p < .05) from P30 to P45. Furthermore, intestinal protease activity increased significantly (p < .05) from P25 to P35 and then stabilized from P35 to P45. In line with the trend of protease activity, expression of protease genes (TRY, CTRL1, ELA1, CPA2) was initially up-regulated significantly (p < .05) and then stabilized from P35 to P45. The intestine showed the best morphology with increasing villus height, width and muscular layer thickness at P35, which was closest to the optimum feed protein level of 33.69% based on the maximum WG and minimum FCR.
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