Effects of stocking density on growth and survival of tilapia cultured in biofloc technology system in brackish water

nt. The 
environmental factors (T
o
C, DO, pH, S‰) in 
experimental treatments with biofloc systems 
(I, II and III) show no significant difference 
compared to the control treatment (IV). This 
environmental condition was suitable for tilapia 
culture and biofloc growth [8–10]. 
Monitoring results of nutrient factors 
Monitoring results of total ammonia 
nitrogen (TAN) in table 2 showed that the 
mean value of TAN in the treatment I was 0.53 
mg/l, with a range from 0.16–1.55 mg/l; in the 
treatment II was 0.70 mg/l with a range from 
0.22–1.82 mg/l; in the treatment III was 0.83 
mg/l with a range from 0.14–2.28 mg/l; in the 
control treatment IV was 1.42 mg/l with a 
range from 0.12–3.22 mg/l. TAN tended to rise 
in the treatments, then gradually decreased, 
when adding carbon and biofloc it grew rapidly 
as heterotrophic bacteria had a large biomass to 
absorb nitrogen to produce biofloc particles. 
TAN value in the control treatment tended to 
be higher than that in the treatments with BFT 
application due to no carbon adding. The 
treatments with higher density had higher TAN 
value than the treatments with lower density, 
but there was no statistically significant 
difference (P < 0.05). 
Figure 1 showed that, from the 7
th
 week of 
culture onwards, the fish food intake was 
needed more along with biofloc decomposition, 
because fish did not used up, it caused the 
process of high N accumulation, resulting in 
increasing TAN value. TAN value was the 
highest in the 9
th
 week in culture systems and 
biofloc sediment needed to be removed. In the 
control treatment, TAN value decreased due to 
the water replacement by 20% in the 4
th
 and 5
th
weeks and by 50% in the 9
th 
week. 
These experimental results were consistent 
with the results of Emerenciano et al., (2017). 
Emerenciano et al., (2017) and Azim and Little 
(2008) [4, 10] also recommended that the 
amount of TAN is less than 1 mg/l when 
applying BFT. There is no TAN limit in the 
environmental regulation on tilapia culture. 
Effects of stocking density on growth and survival 
225 
Table 2. Monitoring results of the nutrient factors in experiments 
Nutrient factors 
Stocking density treatments 
I II III IV (Control) 
TAN (mg/l) 
0.53 ± 0.4a 
(0.16–1.55) 
0.7 ± 0.49a 
(0.22–1.82) 
0.83 ± 0.67ab 
(0.14–2.28) 
1.42 ± 0.94cb 
(0.12–3.22) 
TSS (mg/l) 
247.1 ± 97.3a 
(57.3 – 409.0) 
307.5 ± 84.6a 
(132.7–437.3) 
330.9 ± 85.2a 
(142.9–445.7) 
188.8 ± 82.4b 
(38.7–331.3) 
NO2-N (mg/l) 
0.13±0.09a 
(0.01–0.36) 
0.16 ± 0.11a 
(0.02–0.41) 
0.20 ± 0.16a 
(0.02–0.56) 
0.28 ± 0.21b 
(0.02–0.84) 
NO3-N (mg/l) 
1.98 ± 1.32a 
(0.21–4.35) 
2.39 ± 1.69a 
(0.24–05.66) 
2.7 ± 1.91ab 
(0.22–6.27) 
3.36 ± 2.35cb 
(0.25–7.79) 
Notes: Values with different lowercase letters in the same row show statistically significant 
differences (P < 0.05). Values with same lowercase letters in the same row show no significant 
difference (P > 0.05); I, II, III with BFT included I: 6 fish/m
3
; II: 8 fish/m
3
; III: 10 fish/m
3
; IV 
(control without BFT): 3 fish/m
3
. 
(control without BFT): 3 fish/m
3
. 
Figure 1. The variation of TAN value during the experiment 
The monitoring results of total suspended 
solids (TSS) in table 2 showed that the mean 
value of TSS in the treatment I was 274.1 
mg/l with a range from 57.3–409 mg/l; in the 
treatment II was 307.0 mg/l with a range 
from 132–437 mg/l; in treatment III was 
330.0 mg/l with a range from 142–445 mg/l; 
in the control IV was 188.8 mg/l with a range 
from 38.7–331 mg/l. 
TSS was produced right after fish stocking 
because the biofloc formation of TSS tended to 
increase during adding more feed and biofloc 
growth. TSS in the control was lower than in 
other treatments because the control did not add 
carbon, causing less biofloc. 
In the 4
th
 and 5
th
 monitoring of the control 
treatment, the water replacement by 20% in the 
4
th 
week and the 5
th 
week also caused the 
decrease of TSS. In the next monitoring, TSS 
increased rapidly due to the more feed intake 
and the biofloc decomposition, and TSS was 
the highest in the 9
th 
week. In the experimental 
treatments, the biofloc sediment was then 
removed and clean water was added. In the 
control treatment, water was replaced by 50% 
to reduce TSS, then TSS continued to rise 
during feeding and adding carbon (figure 2). 
The experiment result in table 2 and fig. 2 
showed that the amount of TSS in the biofloc 
system ranged from 16.6–560 mg/l, which was 
consistent with the result of Azim and Little 
(2008) [10]. TSS value in the treatments was 
maintained less than 500 mg/l, which was within 
the proposed limit of Emerenciano et al., [4]. 
Nguyen Xuan Thanh et al. 
226 
rise during feeding and adding carbon. 
Figure 2: Variation of TSS in the experiment 
The experiment result showed that the amount of TSS in the biofloc system ranged from 16.6- 
560 mg/L, which was consistent with the result of Azim and Little (2008) [10]. TSS value in the 
treatments was maintained less than 500 mg/l, which was within the proposed limit of 
Emerenciano et al., (2017) [4]. 
Figure 2. Variation of TSS in the experiment 
Figure 3: Variation of nitrite (mg/l) in the treatments 
Figure 3. Variation of nitrite (mg/l) in the treatments 
The monitoring result of nitrite (NO2-N) 
(mg/l) in figure 3 showed that the nitrite ranged 
from 0.01–0.84 mg/l. Nitrite tended to increase 
in the very first weeks, then decreased in the 4
th
week and increases in the 8
th 
week, then 
dropped and stabilized in the next weeks. The 
amount of nitrite was maintained less than 1 
mg/l, within the proposed limit of Emerenciano 
et al., (2017) [4]. 
The monitoring result of nitrate (NO3-N) 
(mg/l) in figure 4 indicated that the amount of 
nitrate in the high density treatments was 
higher than in the low density treatments. The 
control treatment had higher nitrate than the 
other treatments. Nitrate tended to rise in the 
very first weeks, then decreased and increased 
again in the 8
th 
week, then dropped and 
stabilized in the next weeks. The nitrate in the 
Effects of stocking density on growth and survival 
227 
treatments ranged from 0.01–0.84 mg/l, which 
was less than 20 mg/l within the proposed limit 
of Emerenciano et al., (2017) [4]. 
Figure 4: Variation of nitrate (mg/l) in the treatments Figur 4. Variation of nitrate (mg/l) in the treatments 
The growth rate and the survival rate of 
tilapia 
The growth rate 
The result in table 3 showed that, after 86 
days of tilapia culture with BFT at different 
densities, the average weight of tilapia in the 
treatments I, II, III was 263.2 g/fish, 248.7 
g/fish and 212.3 g/fish, respectively. The 
growth rate of tilapia in the control treatment 
with low density was higher than that in the 
other treatments, the average weight of tilapia 
was 269.4 g/fish. 
The result in figure 5 and table 4 showed 
that in the same BFT system with the 
allowable environmental conditions, the 
growth rate of fish in the low density 
treatment was higher than that in the high 
density treatment. 
Table 3. The monitoring result of the growth rate of tilapia (gram) 
Date of monitoring I II III IV 
Initial fish (2/5/2019) 2.22 ± 0.38a 2.23 ± 0.29a 2.25 ± 0.39a 2.22 ± 0.29a 
1st (17/5/2019) 6.3 ± 0.23a 6.1 ± 0.47a 5.9 ± 0.60a 5.5 ± 0.35b 
2nd (3/6/2019) 23.1 ± 2.68a 22.9 ± 4.06a 18.3 ± 2.68a 30.6 ± 7.34b 
3rd (17/6/2019) 74.1 ± 4.39ac 72.4 ± 3.56a 65.3 ± 5.85b 77.7 ± 9.05c 
4th (3/7/2019) 147.2 ± 5.54ac 144.5 ± 6.85a 121.3 ± 13.97b 149.1 ± 8.07c 
5th ( 18/7/2019) 194.3 ± 5.47ac 191.7 ± 4.80a 160.4 ± 10.29b 198.1 ± 9.03c 
6th ( 26/7/2019) 263.2 ± 4.2ac 248.7 ± 9.1a 212.3 ± 12.5b 269.4 ± 5.1c 
Notes: Values with different lowercase letters in the same row show statistically significant 
differences (P < 0.05). Values with the same lowercase letters in the same row show no 
significant difference (P > 0.05); I, II, III with BFT included I: 6 fish/m
3
; II: 8 fish/m
3
; III: 10 
fish/m
3
; IV (control without BFT): 3 fish/m
3
. 
Nguyen Xuan Thanh et al. 
228 
Figure 5. The growth of tilapia in the experiments 
The result in table 4 showed that, after 86 
days of tilapia culture with BFT at different 
densities, the average SGR of tilapia in the 
treatments I, II, III was 5.72 %.day
-1
, 5.62 
%.day
-1
 and 5.43 %.day
-1
, respectively. The 
average SGR of tilapia in the control treatment 
was 5.71 %.day
-1
; The average DGR of tilapia 
in the treatments I, II, III and IV (control 
treatment ) was 3.13 g.day
-1
, 2.98 g.day
-1
, 2.55 
g.day
-1
 and 3.27 g.day
-1
, respectively. 
Table 4. Specific growth rate - SGR (%.day
-1
) and daily growth rate - DGR (g.day
-1
) 
Days 
I II III IV 
SGR 
(%.day1) 
DGR 
(g.day-1) 
SGR 
(%.day-1) 
DGR 
(g.day-1) 
SGR 
(%.day-1) 
DGR 
(g.day-1) 
SGR 
(%.day-1) 
DGR 
(g.day-1) 
14 8.69 0.34 8.39 0.32 8.03 0.30 7.56 0.27 
16 8.66 1.12 8.82 1.12 7.55 0.83 11.44 1.67 
15 7.77 3.40 7.67 3.30 8.48 3.13 6.21 3.14 
15 4.58 4.87 4.61 4.81 4.13 3.73 4.35 4.76 
15 1.85 3.14 1.88 3.15 1.86 2.61 1.89 3.27 
11 2.76 6.26 2.37 5.18 2.55 4.72 2.79 6.48 
TB 5.72 3.19 5.62 2.98 5.43 2.55 5.71 3.27 
Notes: I, II, III with BFT included I: 6 fish/m
3
; II: 8 fish/m
3
; III: 10 fish/m
3
; IV (control without 
BFT): 3 fish/m
3
. 
The survival rate 
The results showed that the survival rate of 
tilapia was 100% in the treatments I, II (6 
fish/m
3 
and 8 fish/m
3
) and it was 95.75% and 
88.9% in the treatment III and in the control, 
respectively. Tilapia cultured with BFT at 6 
fish/m
3
 and 8 fish/m
3 
indicated the similar 
survival rate of fish, which was higher than that 
when cultured at 10 fish/m
3
 and without BFT 
(figure 6). 
The results in table 5 showed that after 86 
days, the feed conversion ratio (FCR), daily feed 
intake (DFI) and protein efficiency ratio (PER) 
in treatments I and II were nearly equivalent. 
FCR in the treatments I and II was less than that 
in the treatment III and in the control treatment. 
In the treatment I, the size of fish was more 
uniform than that in the three remaining 
treatments. The dry feed intake in the treatments 
I, II, III, and control was 333.3 g/fish/86 days; 
Effects of stocking density on growth and survival 
229 
312 g/fish/86 days; 275 g/fish/86 days and 416.7 
g/fish/86 days, respectively. The PER in the 
treatments I, II, III and IV control was 2.24 
gram fish/gram protein; 2.25 gram fish/gram 
protein, 2.07 gram fish/gram protein; 1.83 
gram fish/gram protein, respectively. 
higher than that when cultured at 10 fish/m
3
 and without BFT (Fig. 6). 
Figure 6: The survival rate of tilapia (%) in the experiments 
Figure 6. The survival rate tilapia (%) in th experiments 
Table 5. The criteria for evaluation of the stocking density after 86 days 
Criteria 
Stocking density treatments 
I II III IV 
Initial weight (g/fish) 2.22 ± 0.38 2.23 ± 0.29 2.25 ± 0.39 2.22 ± 0.29 
Final weight (g/fish) 263.2 ± 4.2 248.7 ± 9.1 212.3 ±12.5 269.4 ± 5.1 
FCR after 86 days 1.28 1.27 1.38 1.56 
DFI (g/fish/86 days) 333.3 312.5 275.0 416.7 
PER (g/g) 2.24 2.25 2.07 1.83 
Productivity - 86 days (g/m3) 1579.2 1989.6 2016.9 808.2 
Notes: I, II, III with BFT included I: 6 fish/m
3
; II: 8 fish/m
3
; III: 10 fish/m
3
; IV (control without 
BFT): 3 fish/m
3
. 
CONCLUSIONS 
The values of TAN, TSS, NO2, NO3 in the 
treatments with high density tended to be 
higher than in the treatments with low density. 
The control with low density and without BFT 
had TAN, NO2, NO3 higher and TSS lower than 
with BFT. 
The tilapia cultured with BFT in the 
brackish water at treatment I (6 fish/m
3
) had 
values of growth rate, survival rate, and PER 
higher than those in the treatments II, III (8 
fish/m
3
; 10 fish/m
3
). FCR of the tilapia cultured 
with BFT was lower than that without BFT. 
The study proposed that the density of 
tilapia culture with BFT in brackish water is 6–
8 fish/m
3
. However, when applying BFT in the 
production scale, it is necessary to find out the 
appropriate farming model and improve 
practical skills, monitoring and quick response 
to the problem in the culture system. 
Acknowledgements: The authors would like to 
thank the project “Research on building an 
intensive tilapia culture model in brackish 
water with biofloc technology in Hai Phong 
city”, Institute of Marine Resources and 
Environment (IMER), Vietnam Academy of 
Science and Technology (VAST) and Hai 
Phong Department of Science and Technology 
for the support to accomplish the research. 
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