Heart rate as an indicator of stress during the critical swimming speed test of farmed Atlantic salmon (Salmo salar L.)

Swim tunnel is to fish as treadmill to humans and is a device used for indirect measuring of the metabolic rate. This study aims to explore the fish stress (if any) during the critical swimming test routines (fish handling, confinement and swimming) using heart rate (f_H, heartbeat per minute) bio-loggers in farmed Atlantic salmon (Salmo salar L.). In addition, the recovery dynamics of exercised fish using f_H were explored for 48 h post-swim tests. Continuous f_H data were acquired following the surgical implantation and throughout the trials, such as during fish handling, swim tests (critical swimming speed, U_crit) and 48 h post-swim tests. Upon 3 weeks of surgical recovery, f_H stabilized at 46.20 ± 1.26 beats min⁻¹, equalizing a ca. 38% reduction in f_H recorded post-surgical tachycardia (74.13 ± 1.44 beats min⁻¹). Interestingly, f_H elevated by ca. 200% compared to baseline levels not only due to the U_crit (92.04 ± 0.23 beats min⁻¹) but also due to fish handling and confinement in the swim tunnel which was 66% above the baseline levels (77.48 ± 0.34 beats min⁻¹), suggesting fish stress. Moreover, significantly higher plasma cortisol levels (199.56 ± 77.17 ng mL⁻¹) corresponding to ca. 300% increase compared to baseline levels (47.92 ± 27.70 ng mL⁻¹) were identified after U_crit, predicting the post-swim test stress (physiological exhaustion). These findings reinforce the importance of fish acclimation in the swim tunnel prior to the swimming tests. However, f_H dropped over the course of 48 h post-swim test, although remained comparatively higher than the basal levels, suggesting fish should be given at least 48 h to recover from handling stress for better fish welfare. This study further explored the influence of fish tagging on U_crit which resulted in reduced swimming capabilities of tagged fish (1.95 ± 0.37 body lengths s⁻¹) compared to un-tagged fish (2.54 ± 0.42 body length s⁻¹), although non-significant (P = 0.06), and therefore, future tagging studies are warranted.