Water quality parameters
The water quality management in the nursery and hatchery systems are bit critical than the grow out ponds since, the larval forms are more susceptible to toxic metabolites. Though there was no significant difference (p>0.05) in the temperature and DO levels of treatments and control they were within the optimum for growing
P. vannamei (
Emerenciano et al., 2013). DO and salinity is inversely proportional hence, DO levels were slightly higher at 5 ppt treatments
(Boyd et al., 2018). The pH, TA and TSS values showed significant difference (p<0.05) between treatments, controls and within treatments. BFV showed significant difference (p<0.01) between and within treatments. The pH, TA and TSS levels were within the optimum range for growing
P. vannamei (
Van Wyk and Scarpa, 1999;
De Morais et al., 2020). Regular addition of carbon sources in the BFS with higher DO levels resulted in lower pH due to degradation of organic matter and formation of heterotrophic biomass
(Emerenciano et al., 2013). TA was lower than the controls since the assimilation of nitrogenous matter consumed alkalinity in the treatments
(Furtado et al., 2014). In the present experiment a reasonable level of TSS content was maintained and PL of
P. vannamei might have consumed the bioflocs
(Schveitzer et al., 2013; Khanjani et al., 2020).
Nutrient dynamics in bioflocs nursery rearing systems
The TAN, NO
2-N, NO
3-N and PO
4-P values showed significant difference (p<0.01) between treatments and controls, whereas significant difference was not observed within treatments (Fig 3, 4 and 5a to c). It was observed that TAN values started increasing slowly till 5
th week, after that it showed downtrend, while NO
2-N, NO
3-N started increasing. But in the controls TAN, NO
2-N and NO
3-N was gradually increasing from the first week and attained the peak at the 7
th week. So, after the bioflocs were established in the treatments, the assimilation was dominated whereas, nitrification was solely responsible for TAN assimilation in the control tanks. This indicated the fact that the heterotrophic bacterial biomass (HBB) in BNS outwitted the nitrifiers in the nitrogenous nutrient assimilation which was an impediment for the development of the nitrifiers in BFS
(Cortes-Lorenzo et al., 2015). In BFS insignificant levels of NO
2-N and NO
3-N were observed in treatments, which used to happen due to assimilation of nitrogenous compounds. Though there was an increase in the TAN, NO
2-N and NO
3-N concentrations in the treatment groups during the first 2 to 3 weeks, it was within optimal level for rearing and growing PL of
P. vannamei (
Lin and Chen, 2001;
Lin and Chen, 2003). The distinction in the levels of TAN, NO
2-N and NO
3-N values between treatment and controls served as an evidence that HBB assimilated the TAN in BNS (
Avnimelech, 1999). A significant difference in the THC between treatments and controls also expounded the fact very well
(Khanjani et al., 2020). The results exhibited that nitrification was the major source of nitrogenous matter oxidation in controls whereas in BNS, assimilation as well as nitrification also served as the major nitrogenous matter oxidation process
(Emerenciano et al., 2013).
The PO
4-P values were higher than the controls and similar observations were made in the earlier studies also (Fig 6a to c). Unlike the plankton based systems the microbial flocs in the BNS often unable to assimilate the accumulated phosphorus arising out of non-ingested feed, fecal matter and decomposed organic matter hence, periodical removal of excess phosphorus is necessary in such systems
(Panigrahi et al., 2019).
Growth characteristics
Salinity, carbon sources and their interaction showed significant influence (p<0.05) between different treatments in the ABW and survival rate of
P. vannamei BNS (Table 2). A significant difference (p<0.01) was found in the ABW, between treatments and controls and within the treatments (p<0.05). Sugar treatments recorded significantly higher growth rate than molasses treatments irrespective of salinity. ABW was 2.23 to 2.87 g in sugar treated tanks and 1.93 to 2.19 g in molasses treated tanks and showed an increasing trend with decrease in salinity. Growth rate was higher at 5 ppt than 35 or 20 ppt. Earlier studies reported that use of BFS as nurseries for Penaeid shrimps yielded better growth of cultured organisms with higher ABW and survival rate if optimal water quality is maintained. In the present experiment irrespective of carbon sources 5 ppt showed higher growth rate than 20 and 35 ppt
(Bray et al., 1994). Among various carbon sources applied in the present experiment sugar showed higher ABW irrespective of salinities, this is in accordance with the earlier studies conducted by
Xu et al., (2012).
A significant difference (p<0.01) in the survival rate was observed between the control, treatments and within treatments (p<0.05) except between 35 and 20 ppt. The survival rate was increasing with increase in salinity, similar results were observed with previous studies conducted with low salinities
(Li et al., 2007). But
P. vannamei can be cultured from 1 to 45 ppt if it is acclimatized for the salinity during the post larval development stage which would modify the ensuing salinity tolerance limits of juveniles
(Criales et al., 2011). Among the carbon sources sugar showed higher survival rate ranging from 88.3 to 96.3% followed by molasses with 79.8% to 90% (Table 2).