Monitoring the effect of water temperature on the heart rate of olive flounder (Paralichthys olivaceus) using a bio-logger

Abstract

Olive flounder (Paralichthys olivaceus) is a major aquaculture fish species in South Korea and is primarily cultivated in running water culture systems. However, these systems undergo continuous water temperature changes due to environmental variations, and abrupt changes in water temperature can lead to physiological stress. This study was conducted to monitor the health conditions of farmed olive flounder according to water temperature. A bio-logger (DST milli-HRT, Star-Oddi) was inserted near the heart of olive flounder to observe heart rate changes in response to water temperature under two scenarios. In Experiment 1, the water temperature was increased (1 °C/day) from 15 °C to 25 °C. In Experiment 2, the water temperature was constant (28 °C). Control fish were cultivated at 24 °C. All logged heart rate measurements were graded with a data verification quality index (QI) to calibrate the electrocardiogram data. All experiments were conducted in duplicate. In Experiment 1, the average heart rates at 15 °C and 25 °C were 35 ± 8.70 beats min−1 and 59 ± 20.99 beats min−1, respectively, demonstrating a significant (p < 0.05) increase of 69%. No significant changes in heart rates relative to those at 15 °C (baseline) were observed from 15 °C to 22 °C (p > 0.05); however, a significant difference was observed at 23 °C (p < 0.05). In Experiment 2, heart rates fluctuated between 66 and 69 beats min−1 at 28 °C. In the control experiment, heart rates remained relatively constant with an average of 46 ± 1.66 beats min−1. Notably, heart rates were higher in Experiment 2 than in Experiment 1, suggesting that cardiac output may increase with the increased oxygen requirement of olive flounder under certain conditions, especially during growth at high water temperatures. Moreover, the variations in the heart rate of olive flounders according to the diurnal cycle were found to increase from day to night when the fish were kept within an optimal water temperature range (control experiment). Additionally, the heart rate of olive flounders was found to increase during feeding.