Land-based Circular Tank RAS Experiment for Largemouth Bass: High Yield, Efficiency & Economic Analysis

Experiment on Land-based Circular Tank Recirculating Aquaculture System for Largemouth Bass

Abstract

Largemouth Bass (Micropterus salmoides), commonly known as California bass or black bass, belongs to the order Perciformes, suborder Percoidei, family Centrarchidae, and genus Micropterus. Native to North America, it is a popular game

fish worldwide. It was introduced to Taiwan, China, in the late 1970s, successfully artificially bred in 1983, and introduced to Guangdong Province in the same year. After years of development, it has become one of China's important freshwater aquaculture species. Current farming modes include pond culture and cage culture. However, these modes, constrained by production capacity and environmental protection concerns in large water bodies, have limited room for development. Land-based circular tank culture is a novel aquaculture model. Its construction is not limited by terrain, does not change land use nature, allows for centralized tail water treatment, and can be intelligently upgraded. It has gained widespread popularity among farmers in southwestern China. This system typically consists of circular culture tanks, an aeration system, water inlet/drainage systems, and a tail water treatment system. Compared to pond engineering and land-based container RAS models, the land-based circular tank RAS model offers advantages in tail water treatment, water quality control, and cost reduction. This experiment aimed to culture Largemouth Bass using a land-based circular tank RAS.

1. Materials and Methods
 

1.1 Time and Location

March 7 to September 7, 2023. The experiment was conducted at the Nama Freshwater Pilot Base of the Guangxi Academy of Fishery Sciences.

1.2 Materials

1.2.1 Water Source
The culture water source was from the nearby BaChi River. The water was clear, and according to the "Environmental Quality Standards for Surface Water" (GB 3838-2002), its quality classified as Class III. During the trial, salinity was <0.05‰, dissolved oxygen (DO) ranged from 4.6 to 6.8 mg/L, and temperature was maintained between 24–29 °C.

1.2.2 Facilities
The aquaculture system comprised one culture tank, oxygen supply equipment, a microscreen drum filter, a nitrification biofilter, and an ecological filter tank. The culture tank had a diameter of 6 m, an effective water depth of 1.4 m, and a total water volume of 40 m³. During the culture period, pure oxygen was supplied by an oxygen generator via air supply pipes and nano-diffuser aerators.

1.3 Experimental Fish

Largemouth Bass fingerlings were purchased from a hatchery in Nanning, Guangxi. The average body weight was (80.21 ± 0.16) g, totaling 2,000 individuals. The fingerlings were uniform in size, with intact scales and fins, healthy, active, and showed no obvious signs of disease or injury.

1.4 Experimental Methods
 

1.4.1 Stocking
Before stocking, the circular tank was disinfected using a 10 g/m³ potassium permanganate solution. The water treatment system was debugged and run for 24 hours, monitoring DO and pH. Before introducing the fish into the tank, they were bathed in a 5% salt solution for 10 minutes to reduce pathogens. The stocking density was 50 fish/m³.
After stocking, the fish were fasted for 24 hours and acclimated for one week before the formal experiment began.

1.4.2 Feeding
"Rongchuan" brand extruded compound feed for Largemouth Bass was used. Feeding followed the principle of "fixed timing, fixed quantity, fixed quality," using different pellet sizes according to the growth stage. Feeding occurred twice daily at 09:00 and 18:00. During the first two months, the daily feeding rate was 5% of the fish's body weight. For the remaining four months, it was gradually reduced to 2%. After feeding, tanks were inspected, and any residual feed was promptly removed.

1.4.3 Water Quality Management
An Oakland multi-parameter water quality analyzer was used to monitor and record dissolved oxygen (DO), pH, and water temperature daily. Daily tank inspections were conducted. If fish were seen gasping at the surface, aggregating abnormally, or water quality deteriorated, blowers were activated immediately to aerate the water, and backup water sources were used for water exchange. During the culture period, 80% of the bottom water in the culture tank was replaced monthly, the tank bottom was cleaned, and solid waste discharged from the microscreen filter was collected and treated.

2. Results and Analysis
 

2.1 Water Quality

Water quality monitoring results are shown in Table 1.
As seen in Table 1, the water quality parameters remained within the acceptable range for land-based high-density recirculating aquaculture. Water quality did not adversely affect the growth of the Largemouth Bass.

2.2 Harvest

Fish were harvested on September 7th. The harvest results are shown in Table 2. From Table 2, the weight gain rate of Largemouth Bass over the 6-month culture period was 567.8%, achieving a production density of 26.3 kg/m³.

2.3 Economic Benefit

Aquaculture costs are shown in Table 3. The total water usage in this trial was 232 tons. Compared to the water usage for culturing the same number of Largemouth Bass (2,000 fish, approx. 356.82 t) in a land-based high-level pond (non-recirculating system), water utilization efficiency was significantly improved. The economic benefit is shown in Table 4, with an input-output ratio of 0.877.

3. Discussion

Literature exists on culturing Largemouth Bass using the land-based circular tank RAS model, focusing on optimizing aspects like pond ratio matching and adjusting aquatic plant density in tail water purification ponds, achieving certain results. Chen Nairui et al. utilized this model in hilly areas to culture Largemouth Bass, obtaining high aquaculture profits and ecological benefits, indicating this model is an ecologically efficient industrial project. Yang Rui et al. found that when Largemouth Bass reached around 500 g, the growth rate in the land-based circular tank model was superior to that in pond culture. Jie Baifei et al., studying Largemouth Bass at different densities, found that a density of 65 fish/m² (equivalent to 50 fish/m³ by volume) resulted in the lowest feed conversion ratio (FCR) and highest unit yield. Therefore, this experiment adopted a density of 50 fish/m³.

The land-based circular tank RAS model is easy to manage. In this experiment, the Largemouth Bass exhibited good growth, and corresponding aquaculture profits were achieved after six months. Compared to the study by Zeng Jiajia et al., the FCR in this experiment was slightly higher, but water use efficiency was improved. This might be because the fingerlings used were relatively large and not acclimated to recirculating conditions beforehand. Furthermore, the system did not maintain ideal water quality; some residual feed and feces accumulated at the bottom, requiring regular manual cleaning, which affected water quality and likely contributed to the increased FCR.

Under land-based circular tank RAS conditions, the operating parameters of water treatment equipment should be adjusted according to the growth characteristics and water quality requirements of Largemouth Bass. This ensures key water quality indicators (e.g., DO, ammonia nitrogen, nitrite nitrogen) remain within the optimal range, supporting healthy growth. During culture, stocking density should be adjusted promptly. Fish should be graded and separated into different tanks based on size to provide a better growing environment and ensure welfare. Land-based circular tank RAS achieves significantly higher water resource utilization efficiency. However, management practices for Largemouth Bass under RAS conditions and the corresponding aquaculture equipment still require further refinement. This is necessary to reduce operating costs and steer the development of land-based circular tank RAS towards greater intelligence and energy efficiency.