Tuna ranching — capturing wild juvenile tuna and growing them to market size in offshore sea cages — is practised for Atlantic bluefin and Pacific bluefin globally. The welfare of tuna in ranching operations is severely compromised by capture stress, crowding, and the physiological demands of intense musculature.
Tuna are among the most welfare-compromised farmed fish. Their physiology — requiring constant forward motion, high oxygen demand, and large thermal ranges — is fundamentally incompatible with sea cage confinement. Capture-related mortality of 10-30% means millions of fish die during transfer to cages. Those that survive face chronic confinement stress, circular swimming, and injury from cage walls. Slaughter by ice water immersion prolongs consciousness during cold stunning. Research into electrical pre-slaughter stunning adapted for tuna is advancing but not yet commercially deployed. Consumer demand for sashimi-quality bluefin drives market pressures that outweigh current welfare considerations.