Effect of tank shape on survival and growth of Pacific bluefin tuna Thunnus orientalis larvae

Abstract

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.