Effect of Illuminance Transition Rate on Survival of Juvenile Pacific Bluefin Tuna,Thunnus orientalis

To investigate the reason for the high mortality of cultured juvenile Pacific bluefin tuna (PBT), especially during land‐based culture, we examined the effect of the rate of light‐intensity change during the dawn period. Juvenile PBT kept in a natural light environment (control) were compared with those exposed to an artificially slowed change in light intensity during the dawn period (test group). For the test group, lamps, which were connected to automatic timers and placed above the rearing tank, were switched on from 0345 to 0515 h at 15‐min intervals. After 9 d, the survival rate of the test and control group was identical. Between Day 1 and Day 6, whole‐body cortisol levels were not different between groups. However, whole‐body glucose levels in the test group were significantly higher than those in the control group. Both groups displayed a diel rhythm in plasma cortisol concentration that peaked at 0300–0600 h. These results suggest that slowing the increase in light intensity during the dawn period did not improve survival and that low light intensity itself induces high mortality in cultured PBT juveniles.     

Effects of Brief Summer or Winter Fasting on the Muscle Quality of Cultured Pacific Bluefin Tuna (Thunnus orientalis)

The effects of fasting on the quality of the dorsal and ventral ordinary muscles from cultured Pacific bluefin tuna (Thunnus orientalis) during chilled storage were investigated. Tuna were subjected to fasting for 2 days in the summer or 6 days in the winter prior to harvesting. The breaking strength of the dorsal ordinary muscle sampled in the summer increased until 24 h and then decreased. There were no significant differences in the lipid and glycogen content of the ordinary muscle after 9 h of storage between the controls and either fasting group. The pH of the ordinary muscle subjected to summer and winter fasting was higher than in the controls after 24–48 h of storage. However, the relationship between the pH and glycogen content was unclear. The metmyoglobin content during chilled storage was lower in the ordinary muscles from either fasting group than in the controls. In conclusion, fasting for 6 days in the winter improved the color stability of the ordinary muscle without a decline in its lipid content.     

Completion of the Pacific bluefin tuna Thunnus orientalis (Temminck et Schlegel) life cycle

Tuna aquaculture is currently dependent on the wild capture of juveniles for production. The development of hatchery technology for bluefin 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 Pacific bluefin 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 artificially hatched and reared spontaneously spawned in captivity. The resulting eggs hatched and were subsequently reared to the juvenile stage. The spawning fish 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 fish in 1995 and 1996. Resulting juveniles (the artificially hatched first generation) were reared to maturity and spawned in 2002. Over the summer of 2002, 1.63 million eggs from these fish were used for a mass rearing experiment, and 17 307 juveniles were produced and transferred to an open sea net cage. Of these artificially hatched second‐generation PBT, 1100 grew to approximately 95 cm total length and 14 kg body weight in 22 months. This procedure means the completion of PBT life cycle under aquaculture conditions, which was first attained among large tuna species. The problems awaiting solution in PBT hatchery production are their unpredictable spawning in captivity, to improve survival during the first 10 days post hatch, to reduce cannibalism in larval and juvenile stages, and to solve collision problem causing high mortality during the juvenile stage.     

Optimal periods of night‐time lighting in the sea cage culture of Pacific bluefin tuna,Thunnus orientalis,juvenile (Temminck and Schlegel)

We have previously reported that night‐time lighting prevents the mass death of Pacific bluefin tuna (PBT) juvenile due to collision and/or contact with the walls of sea cages, immediately after transfer to the cages, and that night‐time lighting does not negatively impact fish stress levels. However, the period of night‐time lighting should be limited to minimize negative impacts on the surrounding environment and aid management. Therefore, we investigated the optimal period of night‐time lighting by evaluating the whole‐body cortisol and glucose levels as stress parameters, growth performance and survival of PBT juvenile in four cages with different periods of night‐time lighting (i.e. unlit, 4‐day, 8‐day and 12‐day lighting). The results showed that almost all fish were died 1 day after transfer to the unlit cage. In comparison, the other groups (4‐day, 8‐day and 12‐day lighting) had high survival rates (92.5–96.0%) without significant difference. However, in the 4‐day‐lighting group, an obvious stress response was recorded on day 5, and growth performance was significantly lower. In the 8‐day‐lighting group, whole‐body cortisol levels were slightly elevated on day 9; however, significantly elevation was not recorded on day 12. These results indicate that the recommended lighting period of night‐time lighting in sea cages is 8–12 days.     

Improving rearing performance in sea‐cage culture of Pacific bluefin tuna Thunnus orientalis juveniles (Temminck and Schlegel) using small sea cages

High levels of mortality occur in large net‐cage culture of Pacific bluefin tuna (PBF) Thunnus orientalis due to poor growth on commencement of sea‐cage culture obstructing the mass production of fingerlings. Therefore, we carried out this study to develop a countermeasure by using small sea cages (square with 13‐m sides). PBF juveniles were directly transferred to a 30‐m‐diameter cage (control) and compared them to fish transferred to small sea cages for 12 days before being merged into another 30‐m‐diameter cage. The survival rate, growth performance and potential factors increasing mortality were examined. The results of our study showed that survival rate in small sea cages was approximately 20% higher than that of the control. Poor growth also occurred in the small sea cages; however, its frequency and daily mortality rate were half those in the control. In addition, growth performance such as specific growth rate and weight gain of PBF juveniles in small sea cages significantly increased compared to the control. These results suggest that using small sea cages could be an effective countermeasure for poor growth, which may mitigate the high‐mortality conditions of current sea‐cage culture systems for PBF.     

Effect of fasting on physical/chemical properties of ordinary muscles in full-cycle cultured Pacific bluefin tuna Thunnus orientalis during chilled storage

Using the full-cycle cultured (FC) Pacific bluefin tuna [body weight 16.3±1.9 kg (pre-fasting group, pre-FG), 14.2±0.9 kg (post-fasting group, post-FG)], changes in the physical/chemical properties of the cephalal parts of dorsal (D) and ventral (V) ordinary muscles (OM) by fasting (6 days) during chilled storage (4°C) were investigated. Condition factors were 26.7 (pre-FG) and 20.3 (post-FG, P < 0.05). Fasting changed the liver color to green. Fasting also decreased the amount of protein and lipid contents of the DOM and VOM of FC tuna. The breaking strength and pH of the DOM and VOM of post-FG tuna were higher (P <0.05) than for pre-FG tuna during storage. In contrast, the glycogen contents of DOM and VOM of post-FG tuna were lower than for pre-FG tuna. The color values (L*, a* and b*) of DOM of post-FG tuna were lower than for pre-FG tuna throughout the storage period. In addition, the metmyoglobin (metMb) content of DOM of post-FG tuna was lower (P <0.05) than that of pre-FG tuna, and the metMb content of VOM of post-FG tuna remained low after fasting. These results indicate that fasting suppresses deterioration (especially meat color) of FC tuna muscles during chilled storage.     

Ontogenetic changes in RNA,DNA and protein contents of laboratory-reared Pacific bluefin tuna <Emphasis Type="Italic">Thunnus orientalis</Emphasis>

ABSTRACT:     The ontogenetic changes in the growth potential of larval and juvenile laboratory-reared Pacific bluefin tuna were examined based on RNA–DNA and protein–DNA ratios. Experimental fish were reared at the Ohshima Experiment Station of Kinki University Fisheries Laboratory in August 2002. Samples were taken from 13 to 35 days after hatching (DAH). Metamorphosis from larva to the juvenile stage was observed around 23 DAH. Somatic growth of Pacific bluefin tuna was accelerated after metamorphosis. The value of the RNA–DNA ratio from 13 to 19 DAH increased slightly from 3.77 ± 0.58 (mean ± SD) to 7.28 ± 2.23. After that, the ratio markedly increased from 13.89 ± 3.71 on 21 DAH to 19.11 ± 4.27 on 23 DAH, which was the end of the metamorphic period. After 25 DAH, the ratio remained at a high level of 15–20. The protein–DNA ratio showed a similar tendency to the RNA–DNA ratio. These results suggest that the rapid increase in the RNA–DNA ratio in the metamorphic period supports the consequent rapid somatic growth in the juvenile stage. The high ratio after the metamorphic period could be because of the species-specific traits large prey exhibit for their survival and because of the tuna's fast -growth after the juvenile stage.     

Testes maturation of reared Pacific bluefin tuna Thunnus orientalis at two-plus years old

ABSTRACT:   Stable reproduction is essential for supplying artificially hatched fish to tuna aquaculture. We observed testes maturation in reared Pacific bluefin tuna (PBT) Thunnus orientalis at 2+ years of age. The incidence of males with mature testes was 25.0%, and 40% of the males had developing testes that contain spermatozoa, while oocytes of the same aged females were not mature. These fish were wild-caught at 0+ years old in August 1997 and the gonads were examined in October 1998 and January–February 2000. Therefore, the age at examination in 2000 was estimated to be 2 years and 7–10 months old considering the spawning season of the wild PBT and the size when captured. Histological examination of the matured and developing testes showed that they contained spermatozoa, spermatids, spermatocytes, and spermatogonia. All the spermatozoa were observed to be motile in sea water under light microscopy. From the results of this and previous studies, matured males are probably fertile for at least 5 months a year in Kushimoto. The testes maturation observed at young age in captivity is considered promising to reduce the cost of broodstock maintenance for the juvenile production of PBT, especially if the sperm are cryopreserved.     

Effect of tank wall colour and pattern on the survival rate of juvenile Pacific bluefin tuna Thunnus orientalis (Temminck and Schlegel) during ship transportation

Juvenile Pacific bluefin tuna (Thunnus orientalis; PBT) often experience high mortality during ship transportation. This study investigated whether the addition of colours or patterns to the walls of tanks affected survival rate. In the first experiment, three colours and lattice patterns were tested: dark blue single‐colour, red single‐colour, and red–blue lattice pattern. Fish in all tanks exhibited abnormal behaviours when sunlight entered the tanks between 0800 and 1000 hours, but mortality only increased in the single‐coloured tanks as a result of collision with the tank walls. In the second experiment, four colours and patterns were tested: dark blue single‐colour, red–blue lattice pattern, red–blue lattice pattern with shade sheet and red–green lattice pattern with shade sheet. Again, we visually observed that fish in all treatment groups exhibited abnormal behaviour when sunlight entered the tanks, but there were no collision deaths in the lattice‐patterned tanks and survival in this group was significantly higher than in the single‐coloured tanks. Thus, the use of a high‐contrast colour pattern can prevent mass death of juvenile PBT during ship transportation.     

Visual spectral sensitivity of photopic juvenile Pacific bluefin tuna (Thunnus orientalis)

Although Pacific bluefin tuna is a species that relies on vision, its photopic visual function is not well known; we therefore recorded electroretinograms to investigate photopic spectral sensitivity in juveniles of this species (49-81?days post-hatch; standard length 74-223?mm). The peak spectral sensitivity wavelength was 505?nm. We estimated that two (λ(max)?=?512-515?nm and 423-436?nm) or three (λ(max)?=?512-515?nm, 423-436?nm, and 473?nm) types of cone visual pigments contribute to photopic vision; these spectral sensitivities are adapted to surface water habitats in clear ocean and coastal water.