Effectiveness of cassava dregs ferment (CDF) to accelerate biofloc formation during the intensive farming of whiteleg shrimp, Penaeus vannamei (Boone, 1931)

Eficácia do fermento de borra de mandioca (CDF) para acelerar a formação de bioflocos durante o cultivo intensivo de camarão branco (Penaeus vannamei) (Boone, 1931)

Abstract

Cassava dregs are a byproduct of processing cassava into tapioca. These ingredients possess a simplified carbohydrate structure after fermentation, which also serves as an essential carbon source to support bacterial growth. This research aims to examine the utilization of cassava dregs ferment (CDF) in accelerating biofloc formation for intensive whiteleg shrimp (Penaeus vannamei) culture. The research was carried out over 60 days in two HDPE-lined ponds. Treatment A (with CDF) was stocked with 200 shrimp/m². Treatment B (without CDF) was stocked with 300 shrimp/m². Treatment A accelerated the formation of biofloc at DOC 26, whereas the biofloc formation in Treatment B started at DOC 38. Weight growth of shrimp was similar to Treatment A, averaging 6.21 ±1.27 g as compared to 6.21 ±1.73 g in Treatment B. Survival rates were significantly different, with 99.1% in Treatment A and 75.3% in Treatment B. Feed conversion ratio and total biomass were 1.4/1,153 kg (Treatment A) and 1.49/1,263 kg (Treatment B). Based on these findings, it appears that CDF can be used as an alternative source of exogenous carbon in biofloc technology and improve the productivity of intensive whiteleg shrimp culture.

Keywords:
exogenous carbon source; biofloc formation; intensive whiteleg shrimp

Resumo

A borra de mandioca é um subproduto do processamento da mandioca em tapioca. Esses ingredientes possuem uma estrutura simplificada de carboidratos após a fermentação, que também serve como uma fonte essencial de carbono para dar suporte ao crescimento bacteriano. Esta pesquisa tem como objetivo examinar a utilização do fermento de borra de mandioca (CDF) na aceleração da formação de bioflocos para o cultivo intensivo do camarão branco (Penaeus vannamei). A pesquisa foi realizada ao longo de 60 dias em dois tanques revestidos de PEAD-liner. O tratamento A (com CDF) foi estocado com 200 camarões/m². O tratamento B (sem CDF) foi estocado com 300 camarões/m². O tratamento A acelerou a formação de bioflocos no DOC 26, enquanto a formação de bioflocos no tratamento B começou no DOC 38. O peso final do camarão foi semelhante ao comparar o tratamento A, com média de 6,21 ± 1,27 g, com o tratamento B, com média de 6,21 ± 1,73 g. As taxas de sobrevivência foram significativamente diferentes com 99,1% no tratamento A e 75,3% no tratamento B. As taxas de conversão alimentar e biomassa total foram de 1,4/1,153 kg (tratamento A) e 1,49/1,263 kg (tratamento B). Com base nessas descobertas, parece que a aplicação de CDF pode ser usada como uma fonte alternativa de carbono exógeno na tecnologia de bioflocos e melhorar a produtividade do cultivo intensivo do camarão branco.

Palavras-chave:
fonte de carbono exógena; formação de bioflocos; camarão branco; intensivo

1. Introduction

Intensive farming of whiteleg shrimp (Penaeus vannamei) has been widely developed in the Asia-Pacific region to supply the global demand for frozen and fresh shrimp (Butt et al., 2021; Chaikaew et al., 2019). The farming system uses a high shrimp stocking density fry, which requires high feed inputs and the production of significant amounts of organic waste that has the potential to self-pollute the system and the environment if not appropriately controlled. The amount of feed to sustain a minimum of 110 shrimp/m² in an intensive system can release up to 34 tons of combined N and P wastes (Mustafa et al., 2023), representing a significant economic and environmental challenge for shrimp farmers and governing authorities.

The application of semi-floc, biofloc, and colloid technologies have been used in intensive shrimp farming systems to address these issues, focusing on improving feed conversion ratio (FCR), reducing waste release, and increasing shrimp overall health (Butt et al., 2021). Bioflocs, in particular, are very effective in improving water quality conditions in shrimp ponds by utilizing wasted organic and inorganic N in water (De Schryver et al., 2008). Constant aeration allows for aerobic decomposition and keeps floc bacteria in a suspended state, which is the defining characteristic of the activated suspension technique (AST) (Azim et al., 2008). Heterotrophic bacteria function as bioreactors that control water quality, and cultured aquatic organisms can specifically utilize concentrations of N and resulting masses of protein (Ekasari, 2009). In addition to these benefits, bioflocs can reduce the negative impacts of certain pathogenic bacteria, such as Vibrio parahaemolyticus, in shrimp aquaculture systems (Hostins et al., 2019).

The supplementation of exogenous carbon sources can, however, be detrimental to the development of biofloc by facilitating the blooming of heterotrophic bacteria and accelerating the assimilation of nitrogen concentrated in water (Huang et al., 2022). Despite its importance, different sources of exogenous carbons have distinctive effects on both culture media and farmed shrimps. For example, Huang et al. (2022) found that using soluble glucose and molasses could control ammonia and nitrite concentration in whiteleg shrimp culture media compared to starch-based carbon sources. Better shrimp growth was also observed in the former compared to the latter (Huang et al., 2022), signaling that the two simple carbons had better effects on the digestive enzyme activity and growth performance of organisms (Liu et al., 2019). This has led to the favorable application of both carbon sources in biofloc technology (BFT) systems, but the high cost of these carbon sources limits their use in practice. It is also worth noting that direct human use of exogenous carbons such as glucose and molasses competes with their application in shrimp farming.

An inexpensive and readily available alternative source of carbon that does not compete with direct human consumption could come from agricultural by-products such as cassava dregs. The dregs are a by-product in cassava (Manihot esculenta) starch production which are often discharged into rivers with little to no treatment. This rich source of nutrients often causes microbial contamination of receiving environments that leads to undesirable fermentation (Shang et al., 2018). Cassava dregs are available in large and stable quantities in many areas and are currently used as a cheap substitute for corn as a local feed ingredient. Fermentation is, however, required to increase nutrient content (crude fiber and protein) and simplify the carbohydrate structure of the dregs to make it more easily digestible by fish (Antika et al., 2014). For example, fermentation of cassava dregs flour with Rhizopus sp. resulted in a significant increase of nutrients in the form of protein, crude fiber, and fat (Antika et al., 2014). Bacillus subtilis and Bacillus cereus are also used as fermentation compounds (Sipayung et al., 2019; Yustinah et al., 2016). This microbial protein fermentation not only increases the digestibility of cassava but also reduces toxicity and improves shelf life (Caplice and Fitzgerald, 1999). Specifically, fermentation can reduce cyanide compounds and increase protein and nutrients compared to those obtained by conventional approaches, i.e., soaking, boiling, and drying (Hawashi et al., 2019). Fermentation also helps mitigate anti-nutrient levels (Aro, 2008), and xylooligosaccharides (XOS) derived from cassava dregs is a potent prebiotic (Sulistyaningsih et al., 2018).

The raw material for cassava dregs flour has a crude fiber content of 8.92% (Afebrata and Santoso, 2014) with a carbohydrate component that is indigestible and insoluble in water. Carbohydrates in fish feed are in the form of crude fiber and nitrogen-free extracts. The extract without nitrogen (BETN) in 25% cassava dregs flour serving as a substitute feed was 25.17%. Extracts without nitrogen (BETN) are organic compounds in soluble carbohydrates (Afebrata and Santoso, 2014). According to Sulistyaningsih et al. (2018), xylooligosaccharides (XOS) are polymers of the sugar xylose bound by β(1→4) glycoside bonds, which have the potential as a prebiotic and can be produced from agricultural waste such as cassava dregs. In addition to its high protein and fiber content, cassava also contains carbohydrates, which can serve as an essential carbon source to support the growth of biofloc. Despite its promising benefits, few studies have investigated the use of cassava dregs (including its derivatives such as enzyme-hydrolyzed cassava dregs) as an exogenous carbon source for biofloc technology in aquaculture (Abakari et al., 2021; Shang et al., 2018). It is even less common to find the application of cassava dregs ferment in actual intensive shrimp farming conditions. An exception is the study by Shang et al. (2018), who studied the application of enzyme-hydrolyzed cassava dregs conducted in controlled lab-like conditions. This study is one of the first to study the application of cassava dregs ferment to determine the role of cassava dregs ferment (CDF) in the formation of bioflocs, and its effect on the growth performance of whiteleg shrimp reared intensively under actual farming conditions.

2. Material and Methods

2.1. Research preparation

This research activity was carried out at the Takalar Brackishwater Aquaculture Development Centre (Takalar BADC) in South Sulawesi, Indonesia, during 2020-2021. The study used two shrimp ponds sized 900 m² and lined with high-density polyethylene (HDPE). Each pond had a 3.0 HP root blower and twelve 1.0 HP paddle wheels. The study also prepared molasses, cassava dregs, probiotics (Bacillus and Lactobacillus), yeast, PL9 seeds (domestically produced whiteleg shrimp), shrimp feed (D0-PV2), lime and vitamin C.

The cassava dregs were obtained from one of the leading tapioca flour processing factories in Gowa Regency, South Sulawesi, Indonesia. The CDF was made using cassava dregs mixed with yeast and Lactobacillus using the following simplified process: 1) 350 kg of wet cassava dregs were placed in several closed plastic containers; 2) fermentation solution was prepared by using 250 g of yeast (Saccharomyces cerevisiae mixed with emulsifier Sorbitan Monostearate E491), 100 g of lactobacillus product (containing Lactobacillus fermentum 2.4 x 10¹² cfu/gr, Lactobacillus plantarum 4.5 x 10¹² cfu/gr, Lactobacillus lactis 3.6 x 10¹⁵ cfu/gr, Bacillus coagulans 3.1 x 10¹² cfu/gr, and Bacillus subtilis 1.7 x 10¹⁰ cfu/gr), and 1.000 mL of molasses mixed with 5.0 liters of fresh water; 3) fermented solution was mixed with the cassava dregs; 4) mixed ingredient was then placed into clean, airtight plastic containers; and 5) fermentation took place for at least 72 hours with CDF applied to support the formation and stabilization of flocs in the shrimp pond media.

2.2. Research design

This study was designed as an on-farm experimental study in which no replication was administered to the treatments. The choice of a non-replicated experimental design was employed due to the limited availability of ponds and human resources to implement the research. Non-replicated trials have been verified to be valid and sometimes the only option to study the effectiveness of inorganic and organic nutrients and farming management practices (Girma et al., 2013; Machado and Petrie, 2006). Treatment A in this study used CDF, while Treatment B represented the control without CDF or replicates. Shrimp stocking densities in treatment A and B were 200 and 300 shrimp/m² within the 900 m² ponds. The main difference between the treatments was the application of CDF in the preparation and maintenance processes. CDF was introduced at the initial phase of shrimp stocking in Treatment A and continued weekly during the maintenance period, which lasted 60 days. CDF was introduced at the beginning of the rearing once per day for three consecutive days by adding 10 kg CDF/1,000 m³ (10.0 ppm) to the rearing pond. Subsequent CDF applications were performed by adding 5.0 kg CDF/1,000 m³ (5.0 ppm). Treatment B did not use CDF during the study period, but stocks of probiotic bacteria were regularly added to both treatment ponds. Stocks of probiotics that have been cultured for 2-3 days were stocked into the pond by keeping paddlewheels running to spread the biofloc evenly distributed. The amount of probiotics used was 50 liters twice a week with a total bacterial content of 1.5 x 106 CFU/ml. Feeding rates were determined based on the weight of the shrimp (weight).

2.3. Measured parameters and data analysis

The parameters measured in this study were: (1) Shrimp health and growth performance (average body weight/ABW, average daily growth/ADG, and survival rate/SR); (2) Biofloc level (using an Imhoff cone) and biofloc quality (using microscopic observation). Biofloc level was determined by the amount (mL) of the collected precipitated biofloc from the Imhoff cone that lasted for 15-30 minutes. This biofloc deposit was then observed using a microscope to determine the composition of the biofloc (heterotrophic bacterial floc, plankton, fungi, protozoa, ciliata, nematodes, particles, colloids and organic polymers); (3) Cassava fermentation performance (CDF). The CDF was assessed in the laboratory by determining the percentages of the CDF chemical compounds (water content, crude protein, fat and fiber content, and ash content) through proximate analysis. In addition, a visual examination of the CDF was also carried out, which showed the CDF appeared to be brownish-white with coarse fibers and a slightly sour smell; (4) feed usage (FCR); (5) water quality parameters (pH, temperature, salinity, alkalinity, dissolved oxygen/DO, and ammonia). Measurements of temperature, salinity, pH, and DO were carried out in situ using a YSI Pro multi-parameter meter. The alkalinity of the rearing media was determined in the laboratory via the titration method following the method described by APHA (2012). Ammonia concentration was measured using a spectrophotometer on water samples collected from the ponds. All data collected during this research period was then analyzed using descriptive statistics and presented in charts, except for productivity data and cassava dregs performance, which are presented in a table format.

3. Results

3.1. Biofloc formations

Biofloc formation data obtained from this research are shown in Figure 1. The application of CDF accelerated the formation of biofloc in the rearing pond, which started much earlier than in the non-CDF treatment pond. In Treatment A (using CDF), biofloc formation was already observed at DOC 26 at about 1.5 ml/l and relatively stable until the end of maintenance. In Treatment B (without CDF), biofloc was formed late at DOC 38 with a significantly higher concentration of 2 ml/L. The biofloc concentration in Treatment B continued to increase, reaching more than four times at DOC 56 compared to Treatment A.

Figure 1
Fluctuations of floc formations using CDF and non-CDF in the rearing media of intensive whiteleg shrimp.

3.2. Productivity performance

The production data (see Table 1) shows that shrimp in Treatment A surpassed the shrimp performance in Treatment B in most productivity variables. The final average of individual weight was the same in both treatments after 60 days of rearing. The total harvest of Treatment A was only 100 kg less than Treatment B despite having a lower stocking density. Higher survival rates in Treatment A (99.1% versus 75.3% in Treatment B) translated into similar shrimp biomass totals for both treatments.

Table 1
Description of the whiteleg shrimp farming system and productivity of intensive biofloc ponds treated with CDF and non-CDF applications.

No.	Description	Unit	Research Treatment
			A (Cassava dregs ferment/CDF)	B (Control or Non-CDF)
1	Ponds Area	m2	900	900
2	Number of Stock	PL	187,500	270,000
3	Stocking Density	PL/m2	200	300
4	Day Of Culture	Days	60	60
5	Survival Rate	%	99.1	75.3
6	Population	shrimp	185,727	203,402
7	Average Body Weight	g	6.21	6.21
8	Size*	shrimp/kg	161	161
9	Total Biomass (Productivity)	kg	1153	1263
10	Total Feed	kg	1,617	1,884
11	FCR		1.40	1.49

• Note: *Size is a market term for shrimp size based on the total number of shrimp per kg weight.

3.3. Growth performance

Shrimp growth performance during the 60-day rearing period (see Figure 2) shows that the absolute weight growth of shrimp in Treatment A and B were identical at 6.21 g. Despite these identical values, the weight of individual ranges in Treatment A and B ranged from 3.2- 8.5 gr/shrimp and 2.8 - 8.9 gr/shrimp, respectively.

Figure 2
Average body weight whiteleg shrimp cultured in intensive biofloc ponds treated with CDF and non-CDF applications.

The average daily growth (ADG) measured every week is provided in Figure 3. Treatment A had the highest daily growth at DOC 46 of 0.29 gr/day and DOC 60 of 0.36 gr/day, while in treatment B, the highest daily growth was obtained at DOC 46 of 0.35 gr/day and DOC 60 of 0.25 gr/day. Throughout the rearing period, the daily weight gain of shrimp in both treatments overlapped except during the DOC 39-58 period. Daily weight gain of shrimp in both Treatments A and B showed an extreme decrease during DOC 51-58. This reduction was particularly evident at DOC 51 for Treatment A and at DOC 53 for Treatment B. Subsequent measurements showed that weight gain in Treatment A rebounded and continued to increase until DOC 60, while weight gain in Treatment B lagged until DOC 60.

Figure 3
Average daily growth (ADG) of whiteleg shrimp reared cultured in intensive biofloc ponds treated with CDF and non-CDF applications.

The highest daily weight growth rates (see Figure 3) in Treatment A were obtained at DOC 46 (0.29 gr/day)and DOC 60 (0.36 gr/day), while the highest daily growth rates in Treatment B were obtained at DOC 46 (0.35 gr/day) and DOC 60 (0.25 gr/day).

3.4. Water quality of the Rearing Media

The pH fluctuations of the shrimp rearing media shown in Figure 4 ranged between 7.10-8.30 for Treatment A and 7.3-8.2 for Treatment B. Both are within the suggested threshold for whiteleg shrimp culture. Slightly more alkaline water was observed in Treatment B during DOC 38-60, suggesting a higher concentration of organic materials.

Figure 4
Fluctuations of pH, temperature and salinity in rearing media of intensive whiteleg shrimp ponds treated with CDF and non-CDF applications.

The measured water quality parameters are presented in Table 2 showing the maximum and minimum values of the measured parameters.

Table 2
Ranges of water quality parameters in intensive whiteleg shrimp treated with CDF or Non-CDF.

No	Water Parameters	CDF		Non-CDF
		Min	Max	Min	Max
1	Salinity (ppt)	35	40	34	36
2	Temperature (°C)	28	30	27	30
3	pH (0-14)	7.1	8.3	7.3	8.2
4	Dissolve Oxygen (ppm)	4.1	9.3	4.7	8.0
5	Alkalinity (mg/l)	112	177	99	135
6	Ammonia (mg.l)	0.006	0.006	0.006	0.006
7	Nitrate (mg/l)	< 0.0223	< 0.0223	< 0.0223	< 0.0223
8	Nitrite (mg/l)	< 0.0392	< 0.0392	< 0.0392	< 0.0392

Figure 4 also displays variations in temperature and salinity of the rearing media. Temperature was considered optimal for both treatments, ranging between 28-30 °C (Treatment A) and 27-30 °C (Treatment B). Salinity levels were within a suitable threshold for whiteleg shrimp, ranging between 35–40 ppt (treatment A) and 34–36 ppt (Treatment B). DO levels in the shrimp rearing media are illustrated in Figure 5. Treatment A varied between 4.1– 9.3 ppm, while Treatment B varied between 5.0 – 8.0 ppm. Fluctuations of water alkalinity in Treatment A and B ranged between 118–177 mg/l and 97–135 mg/l, respectively. Ammonia, nitrite and nitrate concentrations were measured during the middle and end of maintenance only and showed values of < 0.006 mg/l < 0.0223 and < 0.0392 mg/l, respectively.

Figure 5
Variation of dissolved oxygen and alkalinity of the rearing media of intensive whiteleg shrimp treated with CDF and non-CDF applications.

3.5. Performance of Cassava Dregs Ferment (CDF)

Table 3 shows that water, fat and ash contents in CDF were higher than those of non-fermented cassava dregs. In contrast, the protein and fiber contents in non-fermented cassava dregs were higher than that of CDF. The fiber content of fermented dregs was significantly lower (almost three times less) than non-fermented dregs.

Table 3
Proximate composition of cassava dregs and cassava dregs ferment.

No.	Parameters	Cassava Dregs	Cassava Dregs Ferment/CDF
1	Water content (%)	86.03	87.26
2	Protein content (%)	5.71	5.53
3	Fat content (%)	0.71	1.13
4	Ash content (%)	0.28	0.43
5	Fiber content (%)	2.94	1.08

4. Discussion

Biofloc formation was observed in both treatment ponds, but early and stable formation occurred in the CDF-treated pond. In contrast, the non-CDF pond had late biofloc formation and showed significant variation throughout the experiment (see Figure 1). In the CDF-treated pond at DOC 26, biofloc was formed at about 1.5 ml/l and was stable until the end of maintenance. In contrast, the non-CDF-treated pond had a new biofloc at DOC 38 with a volume of approximately 2 ml/l. At DOC 40, the formation of biofloc in Treatment B increased significantly. These different biofloc formations and characteristics could be caused by the different stocking densities in each of the two treatments, where the CDF-treated and non-CDF ponds had stocking densities of 200 shrimp/m² and 300 shrimp/m^2, respectively. However, the stable pattern of biofloc formation in the CDF-treated pond indicates the significant role of CDF in triggering an early and stable biofloc formation to control water quality conditions in intensive culture pond conditions (Shang et al., 2018). The absence of biofloc during the early rearing period and a sudden increase toward the end of the grow-out period in the non-CDF-treated pond could be an indication of late bacterial community responses to the carbon sources used, C/N ratio in the pond, and free bacterial cells in the rearing media (Luo et al., 2020). Mendez et al. (2021) revealed that biofloc generated by different carbon sources have different growth and settling velocity curves. Based on the microscopic observations, the formation of biofloc consisted of zooplankton, such as rotifers, plankton (Navicula and Skeletonema), fungi, protozoa, ciliates, and nematodes. This heterogeneous mixture of microbes, particles, colloids, organic polymers, and cations integrated well in pond water and could survive moderate water agitation (De Schryver et al., 2008).

The application of CDF also has a significant effect on the growth of whiteleg shrimp, particularly in the case of improving feed conversion ratio (FCR) and survival rate (SR). Shrimp reared in CDF treated pond had a slightly better FCR and superior SR than in Treatment B. These better FCR and SR values indicate the improved farming environment and more efficient feed usage due to CDF application. We argue that stocking densities did not play a significant role in FCR and SR differences between the treatments. The use of feed at the initial stage of the shrimp culture was low in quantity, and competition for space was negligible between the treatments to affect the FCR and SR values. Similar results showing improved FCR and SR were also obtained by Shang et al. (2018) when using cassava dregs as the carbon source for biofloc formation in a lab-controlled environment. The current study did, however, achieve a much better SR (almost double) and better FCR (35% higher) than produced by Shang et al. (2018) despite being carried out in an actual intensive shrimp farm setting. The high SR in the CDF-treated pond is suspected to be the product of improved water quality stemming from the early formation of biofloc. This biofloc formation could have accumulated poly-β hydroxybutyrate (PHB) compounds that strongly control pathogenic bacteria in aquaculture-rearing media (García et al., 2014; Luo et al., 2017). In addition, shrimp consuming the PHB-contained biofloc have enhanced immune and digestive system functions that improve resistance to pathogen disturbances (Duan et al., 2017).

Both treatments have similar absolute weight growth (see Figure 2). However, shrimp in the non-CDF pond had consistently higher weight gain than the CDF-treated pond from DOC 42 toward the end of the experiment. This consistent higher weight gain is suspected to result from high biofloc concentrations, surpassing those found in the CDF-treated pond starting at DOC 40. The high concentration of biofloc in the non-CDF pond might have improved the growth of the shrimp. Biofloc is rich in microbial protein and contains an organic polyhydroxybutyrate polymer that, when introduced with commercial feeds, forms a healthy and nutritious food chain that improves growth performance (Nisar et al., 2022).

Water quality parameters observed in this study consisted of pH, DO, water salinity, temperature, alkalinity, and ammonia. The temperature in both treatment settings was almost identical and relatively stable throughout the experiment. This is reasonable, considering both treatments used the same water source within the same farming cycle (see Figure 4). The water temperature variation recorded in this study did not exceed the optimal whiteleg shrimp growth temperature range between 29-32 ᴼC. In cases where the water temperature goes below 26 ᴼC, the feed consumption rate of whiteleg shrimp will decrease by about 50%. Salinity varied in both treatments, starting at DOC 35. The higher salinity of more than 2 ppt was recorded in the CDF-treated pond compared to that of the non-CDF pond during DOC 35-60. Despite these variations, water salinity in the CDF-treated (35-40 ppt) and non-CDF ponds (34-36 ppt) were both within an optimal range that supports the growth of whiteleg shrimp (see Figure 4). Robertson (2006) recommends that the salinity variation should not exceed 5 ppt to avoid increased stress levels in whiteleg shrimp.

Interestingly, higher salinity values were accompanied by lower pH values in the CDF-treated pond, while the non-CDF pond displayed an opposite profile. These pH differences could have been caused by the higher ionic solution in the rearing media in the CDF-treated pond due to the lower activity of floc compared to that of the non-CDF pond. The variation of water alkalinity ranges might explain the difference in pH fluctuation in both treatments. The water alkalinity in the CDF-treated pond showed an increased tendency toward the end of the experiment while relatively decreasing in the non-CDF pond. Higher concentrations of carbonate and bicarbonate in the non-CDF pond likely served as the pH buffer to neutralize acids.

In comparison, the CDF-treated pond possessed lower concentrations of these two compounds, resulting in more stable and lower pH values. Alkalinity measures water's capacity to neutralize acids and maintain pH stability. If the alkalinity of the water is low (which was the case in the non-CDF should be done twice a week. Adding carbonate through the application of 3-5 ppm dolomite lime can increase alkalinity values, and this should be done every 3-5 days until an alkalinity of at least 90 ppm is reached. Despite these differences, the pH levels in both treatments were within the threshold of optimum value for whiteleg shrimp farming.

DO levels in both treatments were relatively stable except for notable spikes that occurred during biofloc formation in both treatments. In the CDF-treated pond, DO concentration decreased sharply between DOC 28-35, coincidental with the spike of biofloc concentration at DOC 31. DO concentrations in the non-CDF pond displayed a significant reduction at DOC 56, which overlapped with the highest spike of biofloc concentration toward the end of the experiment at DOC 55. We suggest that the decrease in DO values was caused by increased biofloc biomass, which requires more oxygen for the respiration and aerobic decomposition of organic matter (Goldman and Horne, 1983).

Ammonia levels were in the range of 0.006 mg/l, nitrite content was 0.0223, and phosphate content of 0.0392 mg/l. Burford et al. (2003) and Schneider et al. (2005) reported that an increase in ammonia could be due to the transformation of nitrogen from feed waste and metabolites in the ammonification process by microbes that decompose organic matter. In a pond environment, ammonia content that exceeds a threshold of >0.1 mgL^-1 within certain time frames can kill cultured shrimp. One strategy to reduce ammonia concentrations in grow-out ponds is to increase heterotrophic bacteria by increasing the availability of organic C (Schneider et al., 2005).

Total ammonia nitrogen (TAN) is a combination of unionized ammonia (NH₃) and ammonium (NH₄). The high concentration of TAN in ponds can produce toxic conditions for cultured shrimp. The presence of heterotrophic bacteria in ponds also can reduce excess ammonia because it is used as food by bacteria. According to Hargreaves and Tucker (2004), bacteria in public waters can reduce ammonia to a form that is not toxic to fish. Increasing the number of heterotrophic bacteria has also been found to reduce total ammonia, nitrogen, nitrite, and nitrate in the media in laboratory and field conditions (Yun et al., 2019). Bacteria use ammonia during nitrification and ammonification to increase their density, and subsequently, this can reduce ammonia and nitrite, producing better water quality conditions for shrimp growth. According to Shang et al. (2018), biofloc containing >70% heterotrophic bacteria is considered healthy. Additionally, increasing heterotrophic bacteria abundance in the top 30 dominating microbial communities during cassava dregs ferment may reduce TAN and enhance water quality. Overall, our results suggest that the cassava dregs of enzyme hydrolysis could be used as a cheap and effective carbon source in BFT. Heterotrophic bacteria use the application of molasses in intensive shrimp ponds as a priming effect for decomposing organic waste. According to Shang et al. (2018), a good floc is composed of many bacteria with a high total of >10⁶ CFU per mL. A floc is less effective if the population is low (<10³ CFU per mL), and Vibrio dominance decreases with increased C:N ratios, thus confirming the dominance of heterotrophic bacteria in high C:N ratio groups.

Crude fiber is a carbohydrate compound that is indigestible and insoluble in water. Cassava deposited at the bottom of the pond can serve as a potential substrate to grow beneficial microbes, subsequently improving the conditions at the pond's bottom. The crude fiber in cassava also contains extracts without nitrogen. In the study by Afebrata and Santoso (2014), the extract without nitrogen (BETN) in 25% cassava dregs flour could substitute feed compounds by up to 25.17%. Extracts without nitrogen (BETN) are organic compounds categorized as soluble carbohydrates. In contrast to crude fiber, the nitrogen-free extract compound (BETN) contained in CDF is more soluble in water. Such characteristics indicate the competitive advantage of CDF as a cheaper and readily available exogenous carbon source for the BFT application in intensive whiteleg shrimp farming. Applying CDF in biofloc-based intensive whiteleg shrimp farming could improve protein and feed efficiency, reduce waste disposal rate into the aquatic environment, and conserve water usage.

5. Conclusions

The findings of this study demonstrate that the application of cassava dregs ferment during intensive whiteleg shrimp pond culture can accelerate the formation of plankton and biofloc formation. The biofloc was formed and relatively stable until the end of maintenance, contributing to the slightly higher FCR and superior SR compared to the non-CDF pond. Survival rate was the leading parameter in the production volume of shrimp as both treatments have similar weight gains. The high SR in the CDF-treated pond could be facilitated by the earlier biofloc formation. The stable biofloc formation in the CDF-treated shrimp pond also maintained water quality parameters such as pH, alkalinity, and DO, which support the growth of the cultured whiteleg shrimp.

Acknowledgements

The authors offer sincere thanks to Prof. Brian W. Szuster from the University of Hawaii at Manoa for reviewing the final draft of the manuscript. The authors also thank the Ministry of Marine Affairs and Fisheries for providing access to field site shrimp ponds, laboratory spaces, and equipment for the study. The authors are indebted to the technicians stationed at the Takalar Brackishwater Aquaculture Development Centre for their help during the field experimental study.

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Publication Dates

• Publication in this collection
  31 Jan 2025
• Date of issue
  2024

History

• Received
  14 July 2024
• Accepted
  28 Oct 2024