Electrofishing-induced cardiac disturbance and injury in rainbow trout

Cardiac output (CO), heart rate (HR) and stroke volume (SV) were monitored in rainbow trout Oncohynchus mykiss electroshocked (pulsed DC) with various voltage, frequency, pulse width and shock duration settings. Additionally, behavioural recovery times and internal haemorrhaging were examined. During electroshock, heart function became erratic and typically ceased for much of the event (cardiac arrest). Following electroshock, CO increased almost entirely due to an increase in SV. Cardiac function generally returned to resting levels within 2-3 h. Behaviourally, recovery was much more rapid, typically taking only a few minutes for fish to regain equilibrium and to begin swimming normally, and only rarely was >1 h. Internal injury ranged from 0 to 7cm 2 of haemorrhaging along the spine and surrounding musculature, although only 4% of the fish had corresponding damage to the vertebrae. Comparisons across the various electrofisher settings indicate that response and injury are highly variable, but generally increased with more intense settings. Higher frequencies and voltages appear to most negatively affect behavioural recovery and injury while longer shock durations increased the length of cardiac arrest and the duration of cardiac recovery. Because of the variability in results and small sample sizes, however, the strongest conclusion that can be drawn from this work is that electrofishing, regardless of the settings, has a considerable negative impact on rainbow trout that is often not apparent externally. Additionally, this study has shown how electrofishing directly effects fish physiologically through impairment of cardiac function. Studies examining the physiological and behavioural response of fishes and subsequent recovery will be necessary for the development of electrofishing guidelines which minimize the disturbance to fishes.