Artificial lighting prevents high night-time mortality of juvenile Pacific bluefin tuna, Thunnus orientalis, caused by poor scotopic vision

High mortality following transfer of early juvenile Pacific bluefin tuna (PBT) aged around 30 days post hatching (about 5 cm total length) from hatchery tanks to sea net cage, particularly during the first few days, is a major problem inhibiting development of large-scale culture operations. Survival rate of the fish after stocking from land tank to a small sea cage was dropped to about 50% or less by the next morning, and became about 10% thereafter. In addition, the whole body cortisol levels increased significantly the next day. In a lab-based study using an infra-red camera, death was associated with fish touching and/or colliding with the net wall, then exhibiting abnormal swimming behavior, then becoming moribund during the night and dawn. In trials in sea net cages, overhead lights providing 200–3000 lx intensity at the water surface, significantly improved survival to 73% up to 23 days post-transfer compared to 12% survival in un-lit control sea cage. Skin injuries were observed in most of the dead fish. The scotopic visual threshold of juvenile PBT was 11.1 log photons/cm2/s, at least 40 fold inferior to the threshold of 8.6–9.6 log photons/cm2/s exhibited by four marine teleosts, juvenile Pagrus major, Takifugu rubripes, Seriola dumerili, and Epinephelus septemfasciatus. It was thought that the PBT had scotopic vision that was much less developed than that of other marine fish in the early juvenile stage, and they touched and/or came into collision with the net wall during the night and dawn.