Heart rate variability as a neurophysiological biomarker of stress and welfare in olive flounder (Paralichthys olivaceus) under varying stocking densities

This study sought to evaluate the physiological stress responses of olive flounder (Paralichthys olivaceus) to varying stocking densities by analyzing heart rate and heart rate variability (HRV). All experiments were conducted in flow-through tanks under both normal- and high-density conditions. Bio-loggers were implanted in the fish to continuously monitor their heart rates and HRV over a 25-day period. The collected data were processed using the Mercury software, using only high-quality heart rate data. To evaluate HRV, the coefficient of variation (CV) was calculated, offering a standardized measure of heart rate variability. The fish in the high-density environments exhibited significantly higher heart rates and lower HRVs than those in normal-density environments. Elevated plasma cortisol levels in the high-density group (p < 0.05) further validated the stress responses indicated by heart rate and HRV. These findings suggest that high-density conditions induce significant stress in olive flounder, potentially reducing physiological adaptability and autonomic nervous system balance due to sympathetic dominance. This underscores the importance of density management in aquaculture to enhance fish welfare and highlights HRV analysis as a reliable, non-invasive tool for stress assessment.