DIETARY SUPPLEMENTATION OF NON-STARCH POLYSACCHARIDES AFFECTS DIGESTIBILITY AND GROWTH OF PACIFIC WHITE SHRIMP Litopenaeus vannamei IN BIOFLOC AND RAS SYSTEMS

Introduction

Energy transfer in aquaculture systems depends on the  interactions between  the biota contributing to the food web , e.g.  the  culture species, algae, bacteria, and fungi. The interacting biota are part of an ecological trophic cascade exchanging energy and nutrients . The efficiency of energy transfer through the food web can be  influenced by the diet and   the  resulting fecal composition . In  Pacific  white shrimp culture, when starch dominates the dietary carbo n  input, the majority will leave the system as CO2, limiting access for further carbon utilization in the system . S upplementing  non-starch polysaccharide (NSP) in the diet  results in more carbon retained in the shrimp feces due to its low digestibility in shrimp . However, the h eterotrophic  bacteria  in the rearing tank  can utilize this carbon and transfer part of its energy to the food web . Together with algae, heterotrophic bacteria are at the base of the food  web. In a b iofloc system both biotas are present . This research compared  the effect of  a high and a low NSP diets on biofloc performance and shrimp production. Furthermore, the study also estimated the contribution of biofloc  to  shrimp growth reared in biofloc systems by estimating  the  growth in a zero biofloc system  - recirculating aquaculture system (RAS).

Materials and methods

This experiment compared two diets  differing  in the NSP level: high  (NSP) and low (CON). The first mentioned  diet contains wheat bran as the NSP source . Two experiments,  a  growth and  a digestibility experiment, were carried out.  In  the growth experiment , two  culture systems were compared (Table 1) :  biofloc  and RAS  (with clear water rearing tanks) system, for 6 weeks. The digestibility experiment lasted  for  5 weeks and  was done only in  RAS only . The biofloc treatments were performed in circular 1000-L fiber mesocosm tank in triplicates . The clear water rearing tanks  were 120-L aquaria all part of the same RAS , with 4 replicates per treatment. The stocking  densities were 100 ind/tank in biofloc tanks , 30-25 ind/aquaria in RAS, both for the growth and digestibility experiment. The shrimp were fed iso proteinic, once daily.

Results

NSP: high NSP diet, CON: low NSP (control) diet, BF: biofloc system, RAS: recirculating aquaculture system with clear water aquaria, SGR: specific growth rate, FCR: feed conversion ratio, PER: protein efficiency ratio, P-value: probability value . Different letters  following mean values  within  the same row indicate significant differences (P<0.05).

The results showed that NSP diet increased the carbon to nitrogen (CN) ratio of the shrimp feces from 11.8 in CON treatment to 20.5, which later stimulated  more biofloc formation (P<0.05).  The algae  concentration  in the biofloc at week-6 was higher when shrimp was fed with NSP diet (P<0.05).  The NSP diet  had a  lower apparent digestibility coefficient (ADC) of dry matter,  protein, energy, carbohydrate, and minerals (P<0.05). The experimental d iet  did not show any effect on the shrimp production (P>0.05), but the system did (P<0.05), due to the higher stocking density in tanks (Table 2 ). Feed conversion ratio  was  higher  with  NSP diet (P<0.05).  Biofloc presence as  an  additional natural food  that  contributed to an increase of the shrimp biomass by 32% in NSP and 46 % in CON diet in BF tanks .  The  fraction  of egested energy  stored  in shrimp biomass  based on  biofloc consumption  was  higher  BF tanks fed the  CON  diet  than in BF tanks fed the NSP diet  (P<0.05). The NSP diet resulted in a 24% higher carbohydrate content in shrimp biomass at harvest (P<0.05).Future research should focus on the effect of NSP type in the diet on shrimp performance in biofloc tanks.

Conclusions

-                       Increasing the NSP  content  in the diet  increases the C:N ratio in the feces.

-                        Although  the NSP diet is less digestible than the CON diet, when f ed isoproteinic, the final weight reached was similar , in either  RAS or BF system.

-                       In BF tanks fed the NSP diet, more biofloc was present than in BF tanks fed the CON diet ,  but  shrimp  did  not realize more growth based on biofloc in NSP fed BF  tanks  than in CON fed  BF tanks.