Physiological Stress Responses and Recovery Dynamics of Labeo rohita (Hamilton, 1822) Fingerlings under Varying Packing Densities and Transport Durations

The study was conducted during September–October, 2024 at the Fish Seed Production Center and Fish Farm, College of Fisheries Science, Nagpur, Maharashtra, to evaluate the effects of varied densities during transportation of Labeo rohita fingerlings on stress and mortality. Two transport durations (12 h and 24 h) were tested under five and seven packing densities, respectively. Post-transport, fingerlings were reared in hapas at a uniform density for 24 h, 48 h, 1 week, and 2 weeks to monitor stress recovery through physiological and hematological assessments. Transportation significantly elevated glucose and lactate levels, with the highest glucose recorded at higher densities: 147.50 mg dl^-1 in 8 fingerlings l^-1 (12 h) and 144.00 mg dl^-1 in 4 fingerlings l^-1 (24 h). Corresponding lactate concentrations were lower in 6 fingerlings l^-1 (6.38±0.28 mmol l^-1) and 3 fingerlings l^-1 (6.07±0.06 mmol l^-1), suggesting reduced anaerobic stress. Dissolved oxygen levels approached hypoxic limits: 3.36±0.25 mg l^-1 (6 fingerlings l^-1) and 3.01±0.06 mg l^-1 (3 fingerlings l^-1), while total ammonia nitrogen remained lower in these groups. Mortality exhibited a density-dependent pattern, with 6 fingerlings l^-1 (12 h) showing only 5% mortality at 48 h post-transport, compared to ~15% in 8 fingerlings l^-1, primarily due to physical stress and post-transport weakness. Overall, 6 fingerlings l^-1 for 12 h and 3 fingerlings l^-1 for 24 h were identified as optimal densities. Extended post-transport monitoring revealed delayed stress and mortality responses that were often overlooked in short-term studies.