Bluefin tuna (Thunnus thynnus) are captured from the wild and ranched in sea cages to market weight. Their welfare is exceptionally difficult to maintain due to their extreme physiological requirements.
Bluefin tuna welfare in ranching is severely compromised. Initial capture by purse seine causes extreme physiological stress with 30-50% mortality in some operations. Survivors are towed in tow cages to ranch sites over days, during which chronic stress causes ongoing mortality. In ranch cages, minimum swimming speed requirements mean that any net collision or crowding causes physical damage. Slaughter methods (ike jime — brain spiking) when properly applied are among the most humane available for large tuna but require skilled implementation.