Preliminary investigation into the contribution of bioflocs on protein nutrition of Litopenaeus vannamei fed with different dietary protein levels in zero-water exchange culture tanks

A 7-week experiment was conducted to investigate the contribution of bioflocs on protein nutrition of Litopenaeus vannamei juveniles (average 6.68 +/- 0.09 g) through feeding with different crude protein (CP) levels in zero-water exchange culture tanks. Four bioflocs treatments (BFT) and one control group were managed: BFT fed diets of 20% CP (BFT-20%), 25% CP (BFT-25%), 30% CP (BFT-30%) and 35% CP (BFT-35%), and clear water control without bioflocs fed with 35% CP (CW-35%). Each group consisted of quadruplicate tanks (125 L) and each tank contained 28 shrimp (equivalent to a shrimp density of 224 m(-3) water volume). During the culture period, brown sugar was added into the BFT tanks to promote the development of bioflocs based on initial inoculation, which not only maintained suitable water quality but also provided supplemental protein source (protein content of the bioflocs ranging from 25.61 to 31.14%) for cultured shrimp. At the end of the experiment, the shrimp in all the bioflocs treatments showed higher activities of total proteinases and trypsin in the digestive gland than those in the control (P<0.05). The feed conversion rate of the shrimp in BFT-35% was significantly lower than that in the control (P<0.05); and for protein efficiency ratio and protein productive value, better values were observed in all the bioflocs treatments compared to those in the control (P<0.05). Moreover, the shrimp in both BFT-30% and BFT-35% gained better growth performance (in terms of final weight, weight gain and specific growth rate) than that in the control (P<0.05). Survival rates of the shrimp were above 85%, with no significant differences among all experimental groups (P>0.05). Besides, for all above indicators, there were no significant differences among BFT-25%, BFT-30% and BFT-35% (P>0.05). Present results demonstrated that bioflocs could provide a supplemental source of food protein and stimulate activities of digestive proteinases, both of which made contributions on protein nutrition of cultured shrimp as indicated by improved feed utilization, protein retention and growth performance. Results also showed that, when L. vannamei juveniles were reared in zero-water exchange bioflocs-based tanks, dietary protein level could be reduced to 25% without affecting shrimp growth. (C) 2012 Elsevier B.V. All rights reserved.