Pre-slaughter transport density and seasonal effects on Nile tilapia (Oreochromis niloticus): welfare and filet quality outcomes

This study evaluated the effects of pre-slaughter transport density on physiological welfare indicators and fillet quality of Nile tilapia (<i>Oreochromis niloticus</i>) during summer and winter. The experiment followed a completely randomized 3 × 2 factorial design, with three transport densities (375, 425, and 475 kg/m3) and two seasons. Stress biomarkers (glucose and lactate), oxidative stress indicators (reduced glutathione, catalase, and lipid oxidation), water quality parameters, and fillet quality traits (pH, water-holding capacity, color, and texture) were assessed. Significant density × season interactions were observed for plasma lactate (<i>p</i> = 0.0472), muscle pH (<i>p</i> = 0.0091), water-holding capacity (<i>p</i> < 0.0001), and fillet resilience (<i>p</i> = 0.0043). In summer, fish transported at 375 kg/m3 showed lower lactate concentrations and higher water-holding capacity, whereas in winter, higher muscle pH and resilience were observed at 425 and 475 kg/m3. Water quality variables also exhibited significant density × season interactions (<i>p</i> < 0.05). Overall, seasonal conditions were the primary drivers of physiological stress responses and postmortem fillet traits. Environmental temperature and associated water quality changes modulated metabolic demand and muscle characteristics, even under short transport duration with adequate oxygenation. Although transport density influenced specific attributes, particularly texture parameters, no consistent progressive pattern was observed across the tested range. These findings indicate that, under short-distance transport (~1.5 h), seasonal thermal management and water quality control are more critical determinants of welfare and fillet quality than moderate density adjustments within 375-475 kg/m3.