Physico-chemical parameters of water
The physic-chemical characteristics of all the experimental pond water are shown in Table 2. All water quality criteria for the culture of brackishwater fish and shrimp were within the ideal range throughout the experiment;
(Biswas et al., 2012; Biswas et al., 2017). There was no difference (p>0.05) in the mean values of pH, temperature, alkalinity, salinity, or hardness between the treatments. Other measures including DO, TAN, NO
2 and NO
3 showed a statistically significant difference (p<0.05) between treatments. In contrast to the polyculture system, a significantly (p<0.05) lower DO level was observed in the IMTA-based treatments (T1 and T2). Compared to the polyculture system, the IMTA-based system showed significantly (p<0.05) lower values for the nutritional parameters TAN, NO
2, NO
3 and phosphate-phosphorus. DO is essential for aquaculture production
(Rahman et al., 2020; Mahmudi et al., 2022). All treatment ponds recorded a DO level greater than 5 mg L
-1 (
MoEF, 2005;
Boyd, 1992). Shrimp, fish and microbes all consume the DO produced by photosynthesis during the day (
Boyd, 1992). In all treatments, the DO concentration was higher than 8 mg L
-1 during the day, with a decrease in oxygen levels at night. The mangrove treatments had a low oxygen concentration; This could be because treatments with mangrove-based vegetation resulted in higher respiratory rates. Oxygen depletion at night was not a serious problem due to the presence of aerators
(Moroyoqui-Rojo et al., 2012). Total ammonia levels (TAN) increased concomitantly with the higher stocking density of Nile tilapia, which negatively impacted growth, immunity, dissolved oxygen levels and digestive enzyme function
(Dawood et al., 2019). The bacterial breakdown of organic materials such as feed waste, feces and other organic waste is the main source of ammonia in any aquatic system. Nitrite is a by-product of nitrification and its concentration is inversely correlated with that of ammonia (
Bhatnagar and Devi, 2013). Therefore, the IMTA system is successful in reducing the inorganic nitrogenous compounds (NO2-N, NO3-N and TAN) and phosphate-phosphorus (PO
4-P) concentration when
M. casta,
A. officinalis and B. gymnorhiza were used as extractive species
(Moroyoqui-Rojo et al., 2012; Oliveira et al., 2021). BOD and COD were found to differ significantly between treatments (p<0.05); the T1 treatment had the lowest values, followed by the T2 treatment and control ponds had the highest values, which may be related to an increase in the concentration of decaying materials (
Nandan and Azis, 1990). The presence of filter feeders such as clams, which reduce TSS (p<0.05), may account for the lower values in the halophyte-based system (
Cloern, 1982). In an integrated cage aquaculture system,
Kaspar et al., (1985) found that bivalves can reduce phytoplankton bloom intensity and turbidity and oysters can decrease eutrophication
(Viji et al., 2014). Studies by
Cloern (1982) and
Officer et al., (1982) in the San Francisco Bay area further demonstrated that the abundance of bivalve suspension feeders directly decreased the amount of suspended solids available for remineralization by pelagic consumers and bacterioplankton.
Monitoring of microbial population in pond water
In this work, it was discovered that the presence of bivalves is crucial in reducing both THB and TVC in culture water. The total vibrio count and total heterotrophic bacteria are shown in Table 3, Fig 2 and 3. Significantly (p<0.05) lower TVC and THB were found in T1 and T2 than in control. The presence of bivalves such as
M. casta in T1 and T2 treatments helps to reduce THB and TVC. Bivalves have effective filtration abilities that can remove excess nitrogenous waste from feed and feces, reducing the nutrients available for bacterial growth. Unlike nitrate, a higher concentration of ammonia or organic nitrogen provides a better environment for microbial growth
(Zhuang et al., 2020). Many species of the genus Vibrio are known to transmit various diseases to farmed shrimp and finfish
(Alavandi et al., 2004; Austin and Zhang, 2006). The co-culture of
P. vannamei and
C. gigas appeared to improve the physiological circumstances that reduced the prevalence of Vibrio in the intestine, which was vulnerable to environmental factors
(Xing et al., 2013). Bivalves raised in a co-culture system had gut bacterial populations that were more diverse than the ones raised in a monoculture system
(Omont et al., 2020). The eastern oyster (
C. virginica) may have the ability to control microbial populations in the tidal creek of South Carolina’s North Inlet
(Wetz et al., 2002). One of the main causes of the disappearance of micro-sized protists is believed to have been the grazing activity of
C. gigas, which efficiently retains particle size >5 µm
(Dupuy et al., 2000). As a result, the decline in Vibrio populations in the presence of bivalves can be used as a sign that aquatic species are in good health
(Xing et al., 2013).
Growth performance
The growth parameters of
P. vannamei and
M. cephalus are presented in Table 4. There were significant (p<0.05) changes in final body weight, SGR (%), FCR, PER and total production in
P. vannamei and
M. cephalus. The higher final weight, SGR, PER and total production in the T1 and T2 group of fed species due to better water quality in IMTA resulted in better growth performances of shrimp and fish (
Rejeki et al., 2016). On the other hand, when seabass was exposed to high concentrations of ammonia,
Lemarie et al., (2004) observed weight loss and growth retardation. In the present study, the FCR was lower in the halophyte-based IMTA system than in the control due to better feed utilization of fed species in the IMTA systems,
Biswas et al., (2020) reported that in IMTA there was 22% reduction in FCR compared to the polyculture system. In the present study, FCR was reduced by 14% in T1 and 7% in T2 treatment compared to control. The survival rate of
P. vannamei is significantly higher in the halophyte-based IMTA than in the control, this could be attributed to the better water quality created by the extractive species in IMTA, the same result was reported by
Biswas et al., 2019 and
Naskar et al., 2022. While in the case of
M. cephalus, no significant difference was observed among the treatments.