The present study confirmed that the DSW could promote floc development and positively affect shrimp production and water quality maintenance in a zero water exchange system. It is clearly explained that the key to BFT is the increasing conversion of microbial protein by carbon addition
(Zhao et al., 2012). Ammonia, nitrite and nitrate have significantly differed among the treatments (Fig 1). In all treatments, ammonia level was maintained at <0.4 ppm. In C/N 15 and 20, the ammonia level kept <0.1 ppm; at the end, the culture ammonia level increased. In C/N 10 treatment, NO
2 and NO
3 levels were higher than the other treatments. When ammonia-nitrogen and nitrite-nitrogen concentrations are higher than the 0.1 mg/l and 5 mg/l, respectively, it will harm the aquatic animals
(Qiao et al., 2006). Nitrate-nitrogen can also harm the shrimp when the animal is exposed for a longer period at a concentration of >60 mg/l
(Wang et al., 2015). When sugar cane, molasses and dextrose were used as the carbon source
(Samocha et al., 2007 and
Hari et al., 2004) in the culture system, ammonia was rapidly removed. Similar results were observed in this study. Alkalinity maintained above 110 mg/l and C/N 15 and C/N 20 having the alkalinity of >160 mg/l. This reduction of alkalinity occurs due to the consumption of inorganic carbon by the heterotrophic and nitrifying bacteria that form the bioflocs
(Ebeling et al., 2006). No significant difference was observed in dissolved oxygen, temperature, salinity, pH and alkalinity (Table 1). All water quality parameters remained within the limit for shrimp culture throughout the culture period (
Ponce-Palafox et al. 1997;
Zhang et al., 2006).
Floc volume, TSS and FVI were often used as indicators for the quantitative determination of the microbial floc
(De Schryver et al. 2008) as their changes over time can be reflected in the development of the floc in water. In the present study, floc density, in terms of floc volume, TSS and FVI was gradually increased over the culture period in all treatments. Total suspended solids, floc volume index and floc volume differed between treatments throughout the study (Fig 1). The average floc volume and TSS range from 8-16 ml L
-1 and 150 - 220 mg L
-1, respectively (Fig 1). Initially, floc volume was less in all treatments and after two weeks, floc volume increased based on the addition of C/N ratio and activity of heterotrophic bacteria and proliferation
(Liu et al., 2017). Floc volume steadily increased from C/N 10 to C/N 15. Total suspended solids significantly improved, based on the floc volume in the system and higher TSS was recorded in C/N 15 and C/N 20 at the range of 150-250 mg/ The lowest TSS was recorded in the C/N 10 treatment.
Furthermore, the microbial biomass yield per unit substrate of heterotrophic bacteria is 40 times greater than the nitrifying bacteria
(Ebeling et al., 2006; Hargreaves, 2006).
Avnimelech (1999) determined that using carbon sources to raise the C/N ratio within the culture system was a practical and inexpensive way to reduce inorganic nitrogen accumulation in the culture system. In the present study, use of distillery spent wash to raise the C/N ratio in treatments C/N10 to C/N25 has efficiently maintained the low level of nitrogen species without affecting the shrimp growth. This may result from high-density protozoans, bacteria, ciliates rotifer, zooplankton and organic matter present in the floc. The reduction in ammonia and development of floc volume shows a similar pattern when floc volume increases the ammonia concentration maintained below the average level and this strongly proves the utilization of ammonia-nitrogen by heterotrophic bacteria.
In the present study, biofloc was effectively utilized by the P.
vannamei nursery at high stocking density with a zero water exchange system. Shrimp survival was higher in the treatment C/N 15, suggesting that BFT resulting from the addition of a carbon source had a beneficial effect on the shrimp survival. A higher level of shrimp mortality was recorded in C/N 25 caused by the hypoxic condition of the culture system due to the over addition of carbon source in the initial phase of the culture system. These indicate that attention should be paid to a hypoxic condition in treatments with a high C/N ratio culture system and biochemical composition of the carbon source.
Growth performance was appraised through final weight, weight gain, specific growth rate and survival of the shrimp (Table 2) at the end of the culture period. The specific growth rate and PER, survival and net production of the shrimp in treatment C/N 25 significantly (P<0.05) lower than other treatments. The FCR of the shrimp was better in C/N 15 than in other treatments. This indicates that the appropriate quantity of carbon source addition was helpful in the good growth and survival of the shrimp. These might be due to the synergistic effect of the improved water quality, higher bacterial and zooplankton density. Studies have indicated that carbon addition can result in the production and accumulation of biofloc
(Avnimelech, 2007;
Emerenciano et al., 2011; Gao et al., 2012), which could serve as an essential food source for the zooplankton and thus could increase the growth of the shrimp. It has been demonstrated that zooplankton serves as an
in-situ food source for
P.
vannamei (Chen and Chen, 1992).
