Comparative study on direct stocking versus nursery rearing in Litopenaeus vannamei culture systems

Introduction

The global aquaculture industry has emerged as the fastest-growing animal food-producing industry in the last five decades, with farmed aquatic products exceeding capture fisheries as the latter has plateaued (FAO, 2022) [1] . This growth is attributed to population pressure, rising seafood demand, technological advancements, and overexploitation of fish resources. In this industry, penaeid shrimp farming has assumed exceptional economic importance as one of the most valuable globally traded aquaculture products. Its rapid expansion in tropical and subtropical coastal areas has profoundly affected rural economies, creating millions of jobs, especially in developing countries where it forms the backbone of blue economy projects (Raju et al., 2021; CIBA, 2020) [3,2] . Of the farmed penaeid shrimp, the Pacific white shrimp (Litopenaeus vannamei) is the most cultivated due to its desirable biological traits. It grows very fast, reaching marketable size much sooner than other species (Wyban et al., 1991) [4] . Its extraordinary euryhaline tolerance allows it to thrive in a wide range of salinity levels, from almost freshwater to hypersaline conditions (Lavens et al., 1996) [5] . The species also has high fecundity and tolerance of different production systems, from low-intensity extensive ponds to hyper-intensive biofloc systems (Wyban et al., 1991; Lavens et al., 1996) [4,5] . In India, shrimp farming has emerged as a significant sector along both coasts, with export earnings and production increasing considerably after the development of pathogen-free strains (CIBA, 2020) [2] . According to Raju et al. (2021) [3] , the sector has created millions of jobs in coastal areas, with substantial effects on women's employment in hatcheries and processing units. However, increased focus on shrimp farming has also accentuated biological, environmental, and economic risks. These issues are particularly pronounced at early life stages, where post-larvae and juveniles are highly sensitive to environmental changes, nutritional imbalances, and disease. Lightner (2011) [6] reported catastrophic effects of viral diseases in the Western Hemisphere, where mortality outbreaks can decimate entire crops. Flegel (2012) [7] reviewed pathogen emergence in Asia, where production intensification has in creased disease transmission efficiency, especially due to bacterial pathogens like Vibrio parahaemolyticus (causing Early Mortality Syndrome) and viruses like White Spot Syndrome Virus. The transition from controlled hatchery conditions to variable growout conditions is a key stress point in shrimp culture. In hatcheries, factors like temperature, salinity, and water quality are held within tight optimal limits. When shrimp are moved to grow-out ponds, they face environmental variability, competition, predation, and new microbial communities. Boyd et al. (2015) [8] quantified pond variability, including daily changes in dissolved oxygen, pH, and temperature. Similar patterns of early-life management improving later performance have been documented in other aquaculture systems (Dar et al., 2022; Ratnakar et al., 2024) [9,10] . The optimization of production has focused on two strategic methods: direct stocking and nursery rearing. Direct stocking involves transferring post-larvae directly from hatcheries to final grow-out ponds without an intermediate step. This method was common in early shrimp culture when production intensity was low (Hopkins et al., 1993) [11] . Its advantages include simplicity and lower capital costs. However, direct stocking subjects post-larvae to drastic environmental changes, causing physiological stress and mortality risk. Lightner (2011) [6] explained how stress triggers neuroendocrine pathways that mobilize energy stores and modulate immune responses. Flegel (2012) [7] reported Early Mortality Syndrome causing > 70% mortality within weeks of direct stocking. In contrast, nursery rearing is an advanced management technique that mitigates hazards associated with early pond access. This sequential approach involves stocking post-larvae in nursery conditions for 15-30 days, then transferring larger, stronger juveniles to grow-out ponds. The effectiveness of nursery rearing was proven by Wasielesky et al. (2006) [12] . Nursery systems provide environmental control, optimized feeding, and health management (Jory et al., 2020; Samocha et al., 2022) [13,14] . Benefits include gradual acclimation to salinity and temperature, optimized feeding schedules, intensive water quality management, and immune system conditioning through controlled microbial exposure (Crab et al., 2012; Wasielesky et al., 2013) [15, 16] . Given this background, we designed this study to compare direct stocking versus nursery rearing in Litopenaeus vannamei culture systems.

Materials and Methods

Study location and duration

The experiment was conducted at an aquaculture research facility in a coastal area of India with filtered seawater supply, bore well freshwater source, hatchery facility, nursery tanks, and earthen grow-out ponds. The study lasted 100-120 days, comprising a 25-day nursery stage (only T₂) and a 75-90 day grow-out stage (both treatments).

Experimental design

We used a completely randomized design (CRD) with two treatments and three replicates each.

Nursery phase protocol (T₂ only)

Stocking density: 1500 PL/m³ (6,000 PL per tank in 4 m³ working volume). Water quality was monitored daily for temperature (28-30 °C), DO (> 5 mg/L), pH (7.8-8.2), salinity (25 ± 2 ppt), TAN (< 1.0 mg/L), nitrite (< 0.5 mg/L), nitrate (< 10 mg/L), and alkalinity (> 100 mg/L). Feeding used commercial nursery crumble feed (45% protein) four times daily, starting at 25% of biomass daily and reducing to 15% by the end. Weekly sampling monitored growth.

Grow-out phase protocol (both treatments)

Ponds were prepared by drying (7-10 days), liming, fertilization, water filling to 1.2 m, and aeration installation. Both treatments were stocked at 35 PL/m². Water quality was monitored twice daily for temperature and DO, and weekly for TAN, nitrite, and transparency. Feeding used starter crumble (40% protein, 3 times daily) for weeks 1-4, grower pellet (35% protein) for weeks 5-8, and finisher pellet (32% protein) for weeks 9-harvest. Weekly sampling monitored growth.

Growth parameters

Statistical analysis

Data were analyzed using independent t-test for growth parameters and survival, repeated measures ANOVA for growth trajectories, and paired t-test for within-treatment survival. Normality was tested using Shapiro-Wilk test (p > 0.05). Homogeneity of variance was tested us-ing Levene's test (p > 0.05). Significance was set at α = 0.05. Effect size was calculated using Cohen's d.

Results

Water quality parameters

All water quality parameters were maintained within acceptable ranges throughout the study. No significant differences were found for any water quality parameter between treatments during grow-out (p > 0.05). Therefore, differences in growth and survival can be attributed to stocking strategy, not environmental variation.

Growth performance

At stocking (Week 0), T₂ shrimp were already 0.85 g while T₁ shrimp were only 0.012 g-a massive 70-fold difference. At Week

Biomass production and feed efficiency

Biomass yield in T₂ was 6,168 kg/ha compared to only 3,728 kg/ha in T₁-a 65.4% increase. This is the bottom line for farmers: nursery rearing produced 2,440 kg more shrimp per hectare. This increase comes from two sources: higher survival (36.8% improvement) and larger individual shrimp (21.1% improvement). The multiplicative effect of these two factors (1.368 × 1.211 = 1.654) explains the 65.4% increase. No other management intervention in shrimp farming typically achieves such a large production increase.

Statistical summary

The t-values are all well above the critical value (approximately 2.78 for df = 4 at α = 0.05). The highest t-value (8.45 for grow-out survival) indicates an extremely strong treatment effect. The negative tvalues for FCR and CV simply indicate that T₂ had lower (better) values than T₁. These results confirm that the observed differences are real, not due to random variation or chance.

Discussion

Our study demonstrated that nursery rearing significantly improves growth, survival, feed efficiency, size uniformity, and biomass production in Litopenaeus vannamei compared to direct stocking. These results are consistent with Wasielesky et al. (2006) [12] , who first proved the effectiveness of nursery rearing over direct stocking. Growth performance: Nursery-reared shrimp (T₂) showed 21.1% higher final harvest weight (21.85 g vs. 18.05 g) and 21.3% higher daily growth rate. This improvement can be attributed to several factors. First, nursery systems enable gradual acclimation to desired salinity and temperature conditions, reducing osmoregulatory stress (Lavens et al., 1996) [5] . Second, optimized feeding schedules in nurseries promote fast, simultaneous growth, ensuring all individuals receive sufficient nutrients (Jory et al., 2020) [13] . Third, reduced metabolic stress allows shrimp to allocate more resources to growth rather than stress response (Lightner, 2011) [6] . Similar patterns of early-life management improving later performance have been documented in other aquaculture systems (Dar et al., 2022; Ratnakar et al., 2024) [9,10] . [17] showed that although nursery infrastructure requires investment, enhanced survival, growth, and FCR compensate for the expenditure. Survival: Grow-out survival was 80.7% in T₂ vs. 59.0% in T₁ (p < 0.001). This 21.7 percentage point increase is substantial. Higher survival in nursery-reared shrimp can be attributed to larger initial size, better physiological condition, and enhanced immune competence at transfer. Lightner (2011) [6] explained how stress-induced immunosuppression predisposes shrimp to opportunistic pathogens. Flegel (2012) [7] reported that Early Mortality Syndrome causes > 70% mortality within weeks of direct stocking. Nursery systems facilitate immune system conditioning through controlled microbial exposure (Crab et al., 2012; Wasielesky et al., 2013) [15,16] . Size uniformity: CV was significantly lower in T₂ (15.2% vs. 22.8%, 33.3% improvement). Montoya et al. (2006) [18] found that nurseryreared shrimp produced highly uniform groups because all individuals had unlimited access to feed in a low-competition environment. This uniformity persists through grow-out, as equal competitors grow in synchrony. Brito, Vinatea, and Wasielesky (2016, 2020) also found that nursery-reared L. vannamei in biofloc systems consistently had better size uniformity. Biomass production: Biomass yield increased by 65.4% in T₂ (6,168 kg/ha vs. 3,728 kg/ha). This increase results from combined effects of higher survival (36.8% improvement) and higher harvest weight (21.1% improvement). Kumar et al. (2018) [19] showed similar benefits of 20-25 day nursery phases in Indian semi-intensive systems. The 25-day nursery phase increased total production cycle by 25 days but reduced grow-out phase by 5 days (75 days instead of 80 days). This shorter grow-out period, combined with higher survival and growth, may allow an additional crop per year in tropical regions (Raju et al., 2021) [20] . Water quality: No significant differences were found between treatments for any water quality parameter during grow-out (p > 0.05). This confirms that performance differences were due to stocking strategy, not environmental variation. Boyd et al. (2015) [8] quantified pond variability and emphasized the importance of standardized conditions for valid comparisons. Economic implications: The 65.4% increase in biomass yield, 17.4% improvement in FCR, and 21.7% increase in survival all contribute to improved profitability. Although nursery infrastructure requires capital investment, enhanced production efficiency compensates for the expenditure (Brown, Boyd, & Chappell, 2009; Wasielesky et al., 2006) [17,12] . Shorter production cycles may allow additional crops per year, further improving economic returns (Raju et al., 2021) [20] .

Conclusion

This 100-120 day comparative study proved that nursery rearing significantly improves growth performance, survival, feed utilization, size uniformity, and biomass production of Litopenaeus vannamei compared to direct stocking. Nursery-reared shrimp (T₂) showed 21.1% higher final harvest weight (21.85 g vs. 18.05 g), 21.3% higher daily growth rate, 17.4% better feed conversion ratio (1.42 vs. 1.72), 33.3% better size uniformity (CV 15.2% vs. 22.8%), 36.8% higher grow-out survival (80.7% vs. 59.0%), and 65.4% higher biomass yield (6,168 kg/ha vs. 3,728 kg/ha) compared to directly stocked shrimp (T₁). Overall survival from post-larvae to harvest was 67.8% in T₂ vs. 59.0% in T₁. Water quality parameters showed no significant differences between treatments during grow-out (p > 0.05), confirming that performance differences were due to stocking strategy, not environmental variation. The 25-day nursery phase added 25 days to the total production cycle but reduced grow-out phase by 5 days. All effect sizes were classified as "large" (Cohen's d > 0.8). We conclude that nursery rearing is a superior stocking strategy for L. vannamei culture, offering significant biological and production advantages over direct stocking.

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