Atlantic and Pacific bluefin tuna ranching involves capturing wild juvenile tuna and growing them to market size in sea cages. The welfare implications of confining one of the ocean's most active fish are profound.
Confining bluefin tuna in sea cages represents one of the most severe mismatches between species biology and farming conditions in global aquaculture. These fish are obligate ram ventilators that must swim continuously to oxygenate their gills. Cage confinement prevents the sustained high-speed swimming that is fundamental to their physiology. The trauma of purse seine capture, crowding in transport nets and introduction to sea cages causes extreme acute stress. Fish that survive this period face chronic confinement stress in a space that is orders of magnitude smaller than their natural range. The welfare cost per fish in bluefin ranching is among the highest in any aquaculture system.