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A growth trial was conducted to estimate the optimum requirement of dietary available phosphorus (P) for Chinese sucker juveniles. Triplicate groups of juveniles Chinese sucker (initial mean weight: 1.77 ± 0.02 g, mean ± SD) were fed diets containing graded levels (3.1, 5.3, 7.5, 9.6 and 11.8 g kg−1) of available phosphorus. The basal diet (diet 1), containing 3.1 g kg−1 available P, was supplemented with graded levels of monocalcium phosphate to formulate four experimental diets. The fish were fed twice daily (08:00 and 17:00 h) to satiation for 8 weeks. During the experimental period, the water temperature fluctuated from 27.5 to 30.5 °C and dissolved oxygen was more than 6 mg L−1. The specific growth rate, protein efficiency ratio were all significantly increased by dietary available phosphorus up to 7.5 g kg−1 (P < 0.05) and then levelled off beyond this level. Feed conversion ratio significantly decreased with dietary available phosphorus level up to 7.5 g kg−1 (P < 0.05). Dietary treatments did not significantly affect feed intake (P > 0.05). Efficiency of phosphorus (P) utilization significantly decreased with dietary available phosphorus level (P < 0.05). Body composition analysis showed that the whole‐body lipid, ash, calcium, phosphorus, magnesium (Mg), zinc (Zn) and copper (Cu) contents were all significantly affected by dietary available P concentration (P < 0.05); however, no significance were found for manganese (Mn) concentration and calcium/phosphorus (Ca/P) ratios in whole‐body among all the treatments (P > 0.05). Dietary phosphorus levels also significantly affected the mineralization of vertebrae and scale (P < 0.05), and Ca/P ratios in scale were not influenced by dietary P supplementation, while vertebrae Ca/P ratio decreased with dietary available P levels (P < 0.05) (quadratic effect, P < 0.001). Signs of phosphorus deficiency were characterized by poor growth, slightly reduced mineralization and an increase in body lipid content. The blood chemistry analysis showed that dietary available P had distinct effects on enzyme activities of alkaline phosphatase, as well as contents of triacyglycerol and total cholesterol (P < 0.05). Broken‐line analysis based on weight gain indicated the minimum available phosphorus requirement for the optimal growth of juvenile Chinese sucker was 7.4 g kg−1. Based on the phosphorus content in whole body, vertebrae or scale indicated that the requirements were 8.3, 8.8 and 8.6 g kg−1 respectively.

Six isonitrogenous diets (420 g kg−1 crude protein on dry matter basis) with six levels of soybean meal (SBM) inclusion (0, 112, 225, 336, 449 and 560 g kg−1) in substitution of fish meal (0%, 20%, 40%, 60%, 80% and 100%) were evaluated in Chinese sucker of 1.81 ± 0.01 g initial weight for 8 weeks. There existed a significant difference (P < 0.001) in fish weight gain, feed conversion ratio, feed intake, protein efficiency ratio and specific growth rate when the replacement level for fish meal protein was increased from 40% to 60%, indicating that up to 40% of fish meal protein could be replaced with SBM protein without causing reduction in growth and protein utilization. Lipid contents in the Chinese sucker body decreased significantly (P < 0.001) as dietary SBM increased. Digestive enzyme activities in the intestine and hepatopancreas of the fish were significantly different (P < 0.001) as the SBM protein replacement level was increased. Results of the present study appear to indicate that 40% FM can be replaced by SBM in practical feeds of Chinese sucker.

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