Estimated time for recovery from transportation stress and starvation in juvenile Pacific bluefin tuna Thunnus orientalis

It is important to understand the physiological stress responses in juvenile Pacific bluefin tuna (PBF), Thunnus orientalis, when transporting them from land-based tanks to sea cages, to develop countermeasures against mass mortality and enhance survival. We investigated the estimated time required to recover from transportation stress in PBF juveniles (32 and 35 days post hatch (DPH)) transported over two different distances (Exp 1; long distance (15.2 km), Exp 2; short distance (2.3 km)). Additionally, we examined the effects of simulated transport in fish that were transported from tank-to-tank, as well as starvation tolerance to determine the causes of stress responses during transportation. Whole-body cortisol levels and HIF-1α expression levels in gills increased from pre-transport levels shortly after juveniles were released into the sea cage in Exp 1, but recovered within 24 h. Simultaneously, whole-body glucose levels fell over the 48 h after transport. Gene expression of the Hsp70 family in the liver was upregulated 48 h after fish were released into the sea cages in Exp 1, and was significantly upregulated after 48 h in Exp 2. These results indicated that 72 h were required for PBF juveniles to recover from transportation to sea cages. The starvation experiment suggested that PBF juveniles were significantly affected after 2 days of fasting and that the environmental conditions in the sea cage were the main factors resulting in a decrease in whole-body glucose levels and upregulation of Hsp70-family gene expression. Furthermore, changes of proximate compositions indicated that PBF juveniles consumed crude lipids and proteins, which were significantly affected by fasting for 1-2 days. We suggested that the first 3 days after stocking are a critical period for recovery from transportation stress and it is necessary to develop appropriate management techniques to alleviate the high mortality caused by these stress responses.