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Flow field in a larviculture tank is often based on the empirical assumptions by fish culturists and science-based information had been very limited. Therefore, we studied and reviewed research on the flow field dynamics in larviculture tanks of groupers and Pacific bluefin tuna, and compared them with results from our own and other studies. For radial symmetry tanks (cylindrical or octagonal), aeration by one aerator at the bottom of the tank forms vertical circulation. We quantified and visualized this flow at different aeration rates in a 1 kL tank and compared survival rates of seven-band grouper Epinephelus septemfasciatus and devil stinger Inimicus japonicus. The highest survival with the lowest surface tension-related death (STRD) was achieved when vertical flow velocity above an aerator was 8?cm/s (200?mL/min) for larvae of E. septemfasciatus, while better survival was observed at >8?cm/s in I. japonicus. Larviculture tank proportions also influence both flow field and performance of fish larvae. When tanks were set with the same water volume (100L) and aeration rate (50?mL/min) but different aspect ratios (AR: water depth/tank radius), survival of larvae for the above two species in a tank with AR >2.0 was found to be significantly higher with lower STRD. Flow field in a vertical cross-section of a tank changed from a single-pair vortex system to two-pair vortex systems as AR changed from 1.0 to 2.0. However, sinking syndrome, which causes high mortality by sinking of larvae to the tank bottom during darkness in some marine fishes (i.e., Pacific bluefin tuna Thunnus orientalis), cannot be prevented with the above conventional flow field management. Two flow field control methods have been proposed in order to prevent sinking syndrome. One is increasing the aeration rate at darkness for vertical mixing of the rearing water. The other is a ‘water pump system’, where a water pump was put in a small net cage with fine mesh connected to a drain at the center of the tank, and water from the water pump was discharged via a cross-shape pipe on the tank bottom. Both were found to be effective to prevent sinking syndrome. Flow fields in the conical-cylindrical tanks and the rectangular tanks, which are used widely and have many different proportions, have not been well quantified yet. We have expanded our approach investigating the flow patterns in rectangular tanks by three dimensional two-phase aerated flow simulations, and found that velocity fields were different from those of cylindrical tanks., Aquaculture, 498, pp.513-521; 2019

This study aimed to seek for the optimal condition for small-scale larviculture of red seabream Pagrus major larvae. We examined the effects of tank shapes and aerations, which were assumed to influence the larval survival, growth and swim bladder inflation of P. major larvae. Seawater (50-l) was filled into three cylindrical (CT: 1.7 × 103 cm2 water surface area, 30 cm deep)and six rectangular (RT: 1.8 × 103 cm2 water surface area, 28 cm deep) tanks. One air stone with 100 ml/min aeration rate was set at the bottom center of three CT and RT (RT1AS), and two air stones with 50 ml/min aeration rate were set at the half bottom center of three RT (RT2AS). Five hundred eggs were distributed into each experimental tank. Rotifers were fed to larvae at 10 individuals/mL on 3 days post hatching (dph) and their distribution in tanks were measured. Survival rate at 14 dph in CT (54.7 ± 11.0 %) and RT1AS (55.3 ± 6.0 %) were significantly higher than that in RT2AS (29.6 ± 9.3 %, p < 0.05). However, the growth of larvae was not significantly different between tank shapes and aerators. Swimbladder inflation rates were not different between tank shapes and aerations, however, CT (58.9±28.3 %) showed lower trend (RT1AS 94.4±6.9 %, RT2AS 92.2±10.7 %). Rotifer distribution in tanks was higher at tank bottom (p < 0.05). Low flow regions were observed along the side walls of the tanks and bottom areas in CT and RT1AS due to a single-pair vortex system and formed at the center (between air stones) and from the air stone to the tank walls in RT2AS due to two single-pair vortex systems. These low-flow areas were coincided with higher rotifer distribution areas at the tank bottom indicating that measuring rotifer density can estimate the flow in a tank. We recommend the rectangular tank with one air stone system for red seabream larvae., Aquaculture Reports, 18, art.no.100451; 2020

We examined the effect of rearing tank shape on survival and growth of Pacific bluefin tuna Thunnus orientalis larvae. Cylindrical (1.7 × 103 cm2 water surface area; 30 cm deep) and rectangular (1.8 × 103 cm2 water surface area; 28 cm deep) tanks (n = 3 each) were filled with 50 l of seawater. One air stone with a 100 ml/min aeration rate was set at the bottom center of each tank. Light intensity at the water surface was 2000 lx with a photoperiod of 24 L:0D. Larvae were introduced into each tank at a rate of 10 individuals/l at 2 days post-hatching (dph). Rotifers were fed at 10 individuals/ml and their distribution in tanks was measured. Survival of larvae in cylindrical tanks (CT; 52.7 ± 5.1%) at 8 dph was higher than that in rectangular tanks (RT; 0.8 ± 0.7%, p < .01). Meanwhile, larvae growth was not significantly different between tank shapes either in body length (CT: 4.23?±?0.26?mm; RT: 4.09 ± 0.20 mm) or dry weights (CT: 95.1 ± 17.6 μg; RT: 67.7 ± 10.9 μg). The swimbladder inflation rate of larvae also did not differ significantly between tank shapes (CT: 16.5 ± 14.5%; RT: 56.9 ± 3.47%). Rotifer distribution was higher at tank bottom in both shapes (p < .05). Two-phase bubbly flow simulations in the tanks revealed that the low-flow area was larger in the RT. The low-flow area at tank bottom varied by tank shape, occurring at the edge of the tank wall on the bottom in the CT, and from the center of the tank (air stone) to the tank wall in the RT. These low-flow areas at tank bottom coincided with areas of higher rotifer distribution, which may be a cause of sinking syndrome in fish larvae. Our results indicate that small-scale (50-l) PBT larviculture experiments can be conducted using a CT with the present aeration system, and that an RT requires an improved aerator in place of the single air stone., Aquaculture, 524, art.no.735283; 2020