Abstract
In order to determine the optimal feed zinc requirement for Monopterus albus, we formulated basic experimental diets using fish meal and soybean meal as protein sources, and fish oil and soybean oil as fat sources. Seven experimental diet groups were created by sequentially adding glycine zinc as the zinc source to the basic group, resulting in zinc concentrations of 0, 10, 20, 40, 80, 160, 320 mg/kg. Monopterus albus with an initial body weight of (23.17±0.04) g were fed these diets for 60d, resulting in actual zinc contents of 48.24, 60.79, 70.25, 88.04, 123.88, 204.35, and 381.65 mg/kg dry matter, respectively. The zinc demand of Monopterus albus was comprehensively evaluated by weight gain rate, specific growth rate, vertebral bone zinc content and serum alkaline phosphatase activity. The results showed as follows: (1) The weight gain rate and specific growth rate initially increased and then decreased with dietary zinc level. Survival rate exhibited a declining trend. The 60.79 and 70.25 mg/kg groups showed higher weight gain and specific growth rates compared to other groups. The 381.65 mg/kg group had the lowest rates, significantly lower than the 48.24—70.25 mg/kg group. Based on gain rate and specific growth rate, the zinc demand for Monopterus albus was 56.85 and 56.79 mg/kg, respectively. (2) With increasing dietary zinc level, whole fish crude protein initially increased and then decreased, peaking in the 70.25 mg/kg group. Whole fish crude fat exhibited an initial decrease followed by a slowdown, with the 48.24 mg/kg group significantly higher than others. Zinc content in the whole body increased, and zinc deposition rate followed an initial increase and subsequent decrease. The content of zinc in various tissues (vertebrae, liver, skin and kidney) increased significantly, with vertebrae zinc content peaking before leveling off. Bone phosphorus and calcium content increased and then decreased. Considering zinc content in vertebral bone, the zinc demand for Monopterus albus was 81.01 mg/kg. (3) With the increase of dietary zinc level, Monopterus albus exhibited a pattern of initially increasing and subsequently decreasing serum alkaline phosphatase activity and albumin content. The apex of these parameters was observed in the 88.04 mg/kg group, surpassing significantly the range of 204.35—381.65 mg/kg. Utilizing serum alkaline phosphatase activity as the evaluation index, the zinc demand of Monopterus albus juveniles was determined to be 76.73 mg/kg. Lysozyme activity demonstrated an initial rise followed by a decline, with the 60.79 mg/kg group exhibiting a marked superiority over the other groups. The serum alanine aminotransferase activity in the 381.65 mg/kg group reached the highest levels, significantly higher than that in the 48.24—70.25 mg/kg group. In conclusion, considering the effects of dietary zinc levels on growth performance, nutrient composition, tissue znic deposition, and serum biochemical indexes of Monopterus albus, the appropriate zinc demand in the diet ranges from 56.79 to 81.01 mg/kg. Additionally, the skin, spine, and liver of Monopterus albus exhibit higher susceptibility to zinc deposition than muscle.
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