Preventing injuries and recovery for electrically stunned Atlantic salmon (Salmo salar) using high frequency spectrum combined with a thermal shock

•Stunning with frequency spectrum prevents spinal injuries and haemorrhaging.•Recovery and mortality were significantly influenced by the frequency spectrum.•A thermal shock can ensure that the fish remains unconscious until death occurs.

To understand the importance of electrical frequencies on stunning, recovery and inflicting injuries, Atlantic salmon (Salmo salar) were exposed for 5 s to either 217 V, 50 Hz, AC or 107 V coupled AC + DC at 200 Hz, containing a high frequency spectrum or not. Post stun the fish were placed back into water, either at ambient seawater temperature (10.4 °C) or cold water (− 1.3 °C), to look upon recovery or mortality. Results show that a high frequency spectrum at low decibels prevents the muscles from contracting in such a degree that spinal injuries and haemorrhaging were prevented in all individuals. Injury rates of 14 and 18% were observed when using electrical signals containing only low frequencies of 200 Hz AC + DC and 50 Hz, AC. The high frequency spectrum also reduced the stimulation of the brain as fish recovered faster with no mortality. Adding a cold shock post stunning delayed or prevented recovery of all groups within the time span required to kill the fish by exsanguination.