Nighttime Lighting Influences on the Plankton Feeding and Growth of Juvenile Pacific Bluefin Tuna, Thunnus orientalis

SIMPLE SUMMARY: This study validated the effect of nighttime lighting on the growth and survival of Pacific bluefin tuna (PBF) juveniles using land-based tanks. As heavy mortality of PBF juveniles occurs in sea cages when they are transferred from nursery tanks, nighttime light can effectively prevent collision deaths. Here, we clearly demonstrate the secondary benefit of nighttime lighting; i.e., nighttime lighting also promotes the growth of PBF juveniles. Various live prey including zooplankton and positive phototoxic larvae aggregate under nighttime lighting at practical sea cage cultures of PBF, allowing the juveniles to take in these feed items even during nighttime. Thus, providing nighttime lighting is an essential technique for increasing the viability as well as growth of PBF juveniles. ABSTRACT: During fingerling production of Pacific bluefin tuna (PBF) Thunnus orientalis, heavy mortality can occur immediately after juveniles are transferred from nursery tanks to sea cages; however, nighttime lighting can moderate this mortality. Additionally, various live prey aggregate due to nighttime lighting in practical sea cage culture of PBF. Here, we investigated whether the growth and viability of PBF juveniles could be improved through promoting feeding on live prey that aggregate under nighttime lighting. Two treatment groups were established using land-based tanks under constant environmental conditions, one in which the juveniles were fed live prey at night (night-feed in four replicate tanks) and the other in which juveniles were not fed during the night (control in four replicate tanks). Although the survival rate did not differ significantly between the two groups, growth was significantly improved in the night-feed group, in which 69–78% of the juveniles showed evidence of feeding during the night. Thus, nighttime lighting plays a vital role in the aggregation of various live prey. PBF juveniles consume these prey in sea cages, which promotes their growth. This may partially serve as a countermeasure against the heavy mortality observed in sea cages.