Tuna aquaculture is currently dependent on the wild capture of juveniles for production. The development of hatchery technology for blue¢n and other tunas would be a major step forward in improving sustainability of their aquaculture. The present study overviews the technology in the life cycle completion of the Paci¢c blue¢n tuna (PBT) Thunnus orientalis (Temminck et Schlegel) under aquaculture conditions in Kinki University, and the problems to be solved for the establishment of tuna hatchery technology. On 23 June 2002, broodstock of PBT that were arti¢cially hatched and reared spontaneously spawned in captivity. The resulting eggs hatched and were subsequently reared to the juvenile stage. The spawning ¢sh were the result of a research project started in 1987 to rear wild-caught juvenile PBT that were several months old. Fertilized eggs were obtained from these ¢sh in 1995 and 1996. Resulting juveniles (the arti¢cially hatched ¢rst generation) were reared to maturity and spawned in 2002. Over the summer of 2002, 1.63 million eggs from these ¢sh were used for a mass rearing experiment, and 17307 juveniles were produced and transferred to an open sea net cage. Of these arti¢cially hatched second-generation PBT, 1100 grew to approximately 95 cm total length and14 kg body weight in 22 months. This procedure means the completion of PBT life cycle under aquaculture conditions, which was ¢rst attained among large tuna species. The problems awaiting solution in PBT hatchery production are their unpredictable spawning in captivity, to improve survival during the ¢rst 10 days post hatch, to reduce cannibalism in larval and juvenile stages, and to solve collision problem causing high mortality during the juvenile stage.
Diel and ontogenetic changes in larval body density related to swim bladder volume were investigated in Paci¢c blue¢n tuna, Thunnus orientalis, to determine the causality of larval mortality^adhesion to the water surface and contact with the tank bottom during seedling production. The density of larvae with de£ated swim bladders increased with total length and days post hatch. Diel density change was observed after day 2 post hatch; owing to daytime de-£ation and night-time in£ation of the swim bladder, the density was relatively higher during the daytime. Increased swim bladder volumes clearly reduced larval density during the night-time after day 9 post hatch. However, the density of larvae with in£ated swim bladders was greater than rearing water density (Dr40.0099). The small density di¡erence between larvae and rearing water (Dr 5 0.0022 À 0.0100) until day 4 post hatch may have caused larval mortality by adhesion to the water surface because larvae can be easily transported to the water surface by aeration-driven upwelling in rearing tanks. Density increased noticeably from day 5 to day 9 post hatch. The increased density di¡erence (Dr 5 0.0065 À0.0209) in larvae and rearing water possibly induced mortality by contact with the tank bottom because larvae sink particularly during the night-time on ceasing swimming.
Abstract.— Indoor and outdoor experiments were conducted to improve the rearing techniques for artificially hatched northern bluefin tuna Thunnus thynnus during growout culture. Collisions with the walls of tanks or nets caused mass mortality that occurs during growout. The period when collisions frequently occur and the types of injury caused by collision were examined in this study. Juveniles were reared in indoor tanks from 30 to 120 d after hatching, and in an open sea net cage from 42 to 150 d after hatching. Dead fish were collected and counted daily in both of the experiments. In the indoor experiment, the sampled fish were preserved in 10% formalin solution, and each of 10 specimens of about 30, 50, 70, 85, 100, 130, 160 and 225 mm in body length (BL) were examined using x‐rays to detect injury of the bones. Juvenile and young adult bluefin tuna showed a reduction in numbers caused by collision with the tank or the net wall during the experiments. In the indoor tank, there were 1,200 fish on day 30 but only eight on day 120. The daily mortality increased from day 30 after hatching, when juveniles reached 50‐mm BL and remained over 4%/d until day 60 when juveniles grew to 300‐mm BL. The proportion of dead fish with injuries of bone, especially of the vertebral column and the parasphenoid, increased after fish reached 50‐mm BL, and exceeded 60% in fish with BL 85 mm or greater. In the open sea net cage, there were 3,841 fish at the start of the experiment on day 42 and only 65 on day 150. In this experiment, the reduction was greatest from the start of the experiment until day 80, when fish grew to approximately 25 cm in total length. Significant bacterial, viral or parasitic diseases were not observed in these fish; the only findings were dislocations of the vertebral column and injuries to the upper and lower jaws. These results show that the loss of juvenile and young adult bluefin tuna was caused by collision with the tank or net wall that fatally damaged the bones of the vertebral columns and the parasphenoid.
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