Microbe-driven farming system enhances production, water consumption efficiency, and economic returns in a sustainable high-density nursery rearing of Labeo rohita (Hamilton, 1822)

Abstract

In the present study, spawn of rohu, Labeo rohita, of size 5.6 ± 0.15 mm (2.40 ± 0.06 mg) were stocked at four densities, i.e., 4000 spawns m⁻³ (T1), 6,000 m⁻³ (T2), 8 000 m⁻³ (T3), and 10,000 m⁻³ (T4) to evaluate the suitable density for the microbe-mediated biofloc system. Rohu spawn was reared in 20,000L poly-lined circular biofloc tanks (5 m diameter) in triplicates for 28 days. The spawn was fed with dust feed (36% CP, 4% fat) at 4 kg per one million of seeds in two split meals with a 10% daily increment. The floc was prepared with the use of molasses, and a C/N ratio of 12:1 was maintained, which enabled maintenance of floc volume at 8–10 ml⁻¹/L. High fry survivals of 79.7–86.8% were achieved and plotted an inverse relation with rearing densities from T4 to T1, and T1 was significantly different from other treatments (P < 0.05). The fry length after 28 days of rearing were 20.13 ± 1.7, 17.8 ± 1.6, 15.6 ± 1.5, and 13.0 ± 1.3 mm, and weights were 90.97 ± 5.52 mg, 74.10 ± 4.85 mg, 67.17 ± 2.62 mg, and 41.07 ± 2.99 mg in T1, T2, T3, and T4, respectively. While the fry growth, particularly in T1, T2, and T3, is comparable to those observed in rohu in concrete tanks (17–21 mm, 55–62% survival after 21 days). Higher survival was achieved, even at 2–4 folds higher densities in the biofloc system. However, the total length was relatively low and demanding an extension of the rearing period in T4 at fivefolds higher density despite the higher survival rate. The water consumption showed an inverse relation with rearing density, and accordingly, the water productivity increased with increased densities from T1 to T4. The level of stress biomarkers, like Superoxide Dismutase (SOD), Catalase, Glucose, and Alkaline Phosphatase (ALP) was significantly higher in T4 in comparison to other treatments. The calculated net profit and cost–benefit ratio were significantly highest in T3 (135.37 ± 8.63 USD) and significantly lowest in T1 (82.5 ± 5.92 USD) whereas T2 (109.7 ± 17.03 USD) and T4 (115.33 ± 8.11 USD) were statistically homogenous. Therefore, the study recommended a rearing density of rohu spawn in the biofloc of up to 8000 m⁻³.