Approximately 1.3 billion fish are raised in off-shore and land-based aquaculture systems in the United States each year and slaughtered for food. Numerous methods are used to stun and/or kill farmed fish, but scientific research has found most to be inhumane. Methods such as asphyxiation on ice or air, carbon-dioxide stunning, gill-cutting (exsanguination) without prior stunning, and live chilling do not cause immediate insensibility, and studies on the physiological and behavioral responses of fish show that these slaughter practices are likely to cause suffering as animals respond with highly aversive behavior. Both percussive and electrical stunning and killing systems, if applied correctly, can induce immediate and irreversible insensibility, thereby subjecting the animals to less pain, stress, and undue suffering as compared with other methods. At present, however, no slaughter technique is without welfare problems.
Yue, Stephanie, "An HSUS Report: The Welfare of Farmed Fish at Slaughter" (2008). IIA. 3.