During fingerling production of Pacific bluefin tuna (PBF) Thunnus orientalis, heavy mortality can occur immediately after juveniles are transferred from nursery tanks to sea cages; however, nighttime lighting can moderate this mortality. Additionally, various live prey aggregate due to nighttime lighting in practical sea cage culture of PBF. Here, we investigated whether the growth and viability of PBF juveniles could be improved through promoting feeding on live prey that aggregate under nighttime lighting. Two treatment groups were established using land-based tanks under constant environmental conditions, one in which the juveniles were fed live prey at night (night-feed in four replicate tanks) and the other in which juveniles were not fed during the night (control in four replicate tanks). Although the survival rate did not differ significantly between the two groups, growth was significantly improved in the night-feed group, in which 69–78% of the juveniles showed evidence of feeding during the night. Thus, nighttime lighting plays a vital role in the aggregation of various live prey. PBF juveniles consume these prey in sea cages, which promotes their growth. This may partially serve as a countermeasure against the heavy mortality observed in sea cages.

Changes in nutritional constituents and enzyme activities were clarified in yellowfin tuna (YFT, Thunnus albacares) eggs during embryonic development, from eggs immediately after fertilization to hatching. The protein levels in the eggs gradually increased with development until the completion of hatching. In contrast, the triglyceride (TG) and free amino acid (FAA) levels in the eggs gradually declined with embryonic development until hatching was complete, although the energy composition of the FAAs was lower than that of the TGs throughout embryonic development. These results indicate that endogenous TGs are preferentially expended as an energy source during embryonic development. Overall, changes in the activities of aspartate aminotransferase, alanine aminotransferase, creatine kinase, and alkaline phosphatase showed similar patterns throughout development. First, the enzyme levels diminished; then, they remained at constant, low levels just before hatching, when they rapidly increased. This rapid increase was consistent with the protein content, suggesting that organ differentiation and functionalization were promoted during this period. These results will contribute to the establishment of mass-seeding production of YFT.

This study aimed to examine the effect of different water temperatures and light intensities on swim bladder inflation (SBI) and growth of red sea bream Pagrus major larvae to improve rearing techniques for this species. Two sets of experiments were conducted: different rearing temperatures were used in experiment 1 (19, 21, 23, and 25 °C), and different light intensities in experiment 2 (250, 1000, 4000, and 16,000 lx). Water temperature did not affect SBI frequency, but SBI initiation was accelerated at higher temperature, i.e., it was initiated on 3 days post-hatching (dph) at 25 °C and on 6 dph at 19 °C, suggesting that the promotion period for SBI, which needs a surface skimmer to be run, also accelerated with increasing temperature in red sea bream larviculture. A higher temperature also significantly promoted larval growth, although the notochord of larvae at SBI initiation was shorter at higher temperatures. Light intensity had no effect on either the initiation or the frequency of SBI. However, light intensity of 250 lx significantly reduced early larval growth compared to light intensities higher than 1000 lx. These results indicate that light at an intensity greater than 1000 lx at the water surface is suitable for the early larviculture of red sea bream.

Pacific bluefin tuna (Thunnus orientalis) has high market value, but its wild populations have decreased in recent years. The broodstock of Pacific bluefin tuna that were hatched artificially and reared under aquaculture conditions is beginning to be used for production. The creation of broodstock with commercially valuable traits, such as rapid growth, is therefore of great interest. Genetic linkage map-based identification of markers associated with quantitative trait loci (QTLs) facilitates marker-assisted selection (MAS) breeding and allows efficient genetic improvement of broodstock. Single nucleotide polymorphism (SNP)-based genetic linkage map construction using the genotyping-by-sequencing method can expand the number of mapped markers and help identify growth-related QTLs. In this study, we constructed sex-specific maps for 24 linkage groups consisting of 677 SNP and 651 microsatellite markers. The total lengths of 93 progenies in the mapping population followed normal distribution, with an average length of 9.4 mm. We performed composite interval mapping in the mapping population. QTL analysis revealed one significant QTL in LG10 on the female linkage map. The genetic linkage map—the second such map generated for Pacific bluefin tuna—and the growth-related QTLs detected in this study will be useful for tuna aquaculture MAS programs.

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-1 alpha 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.

Three experiments were conducted to investigate the growth, survival, and standardized cohort biomass of Pacific bluefin tuna, Thunnus orientalis, larvae fed nutritionally enhanced prey during the first week of feeding using two commonly used, commercially available enrichment media, AlgaMac Enrich and Marine Glos. T. orientalis larvae exhibited exponential growth in standard length and dry weight. The daily specific growth rates in length and weight are the first reported for T. orientalis larvae and the averages ranged from 3.8 to 4.1% and 27.5%, respectively, for larvae in the AlgaMac treatment and from 4.1 to 6.1% and 31.5%, respectively, in the Marine Glos treatment. Average daily growth rates in length ranged from 0.16 to 0.23 mm/d for larvae in the AlgaMac treatment and from 0.17 to 0.27 mm/d for those in the Marine Glos treatment. Daily growth rates in length were similar to those reported for other tuna larvae reared in the laboratory but slower than most published estimates for larval tunas in situ at similar water temperatures. Mean prey number per gut was positively associated with mean prey level in the tank. Both enrichment media appear to be good sources of nutritional improvement of planktonic prey for T. orientalis larvae.

Success of swim bladder inflation (SBI) is crucial for early survival of Pacific bluefin tuna (PBF) larvae, because it reduces larval sinking death by enhancing buoyancy. In Experiment 1, we examined the effect of photoperiod on SBI and survival in PBF larvae by comparing photoperiods of 9L: 15D (9L), 14L: 10D (14L: natural photoperiod), 19L: 5D (19L) and 24L: 0D (24L) during 2-10days post hatch (dph). In Experiment 2, the combined effects of photoperiod (24L and 14L) and nighttime aeration rate (enhanced night-time aeration: ENA of 1300mLmin(-1) as a countermeasure for sinking death and 130mLmin(-1)) on the survival and SBI were also examined during 2-10 dph. Moreover, in Experiment 3 the effect of photoperiod on vertical distribution of larvae in night-time was examined on 3-5 dph. Photoperiod of 24L in Experiment 1 significantly inhibited SBI compared with 14L and 19L; nevertheless, it significantly improved survival compared with other photoperiods with a dark period. On the other hand, the shortened light period (9L) showed significantly reduced SBI and also survival. In Experiment 2, the countermeasure for sinking death of ENA under 24L did not further improve the survival; rather it tended to reduce the survival. In Experiment 3, larvae distributed less in the bottom layer in 24L than in 14L, suggesting the reducing effect of 24L on sinking death. The results indicate that 24L without ENA is suitable for survival which is the most serious problem in PBF larviculture.

Pacific bluefin tuna (PBF), Thunnus orientalis, is commercially one of the most important species of tuna. In this study, amplified fragment length polymorphism (AFLP) screening was conducted to find the growth-related polymorphic DNA in cultured PBF. Fish hatched in 2007 were harvested at an age of 818-1994days. They were categorized into superior, average and inferior growth groups, depending on their growth score at the time of harvest. On AFLP screening of 24 fish, with eight fish from each group, 215 polymorphic DNA fragments were observed. A second amplification, with EcoRI+ACC and MseI+CCC primers, generated a polymorphic fragment of 630bp at a rate of 80.0% (n=15) in the superior, 56.3% (n=16) in the average and 20.0% (n=15) in the inferior growth groups. Polymerase chain reaction (PCR) primers, which could amplify both AFLP-positive and AFLP-negative loci, were developed using the consensus sequence outside the AFLP target fragment. Eleven haplotypes were obtained by sequence analysis of the PCR product at the AFLP target loci. Among those, haplotype 1 was statistically significant in the superior and average growth groups and could be used as a molecular marker for distinguishing the individuals with superior and average growth from those with inferior growth.

To investigate the dietary effect of taurine on the larval stage of tuna species, Pacific bluefin tuna (PBF) and yellowfin tuna (YFT), larvae were reared until 16 days after hatching (dAH) and 14 dAH, respectively, and replicate samples were fed either non-taurine-enriched rotifers (T-0) or rotifers enriched with 800 mg taurine L-1 (T-800). Most PBF and YFT larvae were at the preflexion stage until 7 and 8 dAH, and there were no differences in the growth performance and total protein content of larvae between the T-0 and T-800 groups (t-test; P > 0.05). Thereafter, however, for larvae of both species, these parameters in the T-800 group significantly increased with enhanced notochord development compared to those in T-0 group (t-test; P < 0.05). Except for the RNA content in PBF larvae, there were no significant differences in changes of DNA and RNA content with larval growth between the T-0 and T-800 groups, but both PBF and YFT larvae showed increased protein DNA(-1) and protein RNA(-1) ratios in the T-800 group compared to the T-0 group after notochord flexion. This indicates that taurine is an important nutrient for the rapid growth of early stage PBF and YFT larvae, and we conclude that the growth improvement of PBF and YFT larvae by dietary taurine supplementation is due to the increase in protein synthesis efficiency after notochord flexion.

Early-stage mortality due to surface water tension-related death and due to sinking to the tank bottom was investigated for yellowfin tuna, Thunnus albacares (YFT), larvae. Different aeration rates and rearing water surface conditions were examined to evaluate the effect on larval survival, swim bladder inflation and growth. The percentage survival of yolk sac larvae was significantly higher when the rearing water surface was covered with fish oil at aeration rates of 0 and 50 mL min(-1). The highest mortality occurred at the highest aeration rate of 250 mL min(-1) regardless of surface water condition. A second experiment was conducted twice under different water surface conditions: the water surface was covered by fish oil (FO), skimmed of fish oil (SS), and was not treated (NC). The percentage survival was not significantly different between treatments after 7 days of feeding. In contrast to the survival, the proportion of larvae with inflated swim bladders was significantly higher for the NC and SS groups than that of the FO group. Results of these experiments indicate that the addition of oil to the rearing water surface without its removal interferes with the initial swim bladder inflation in YFT larvae. These results also indicate that YFT larvae need to obtain (gulp) air at the water surface for initial swim bladder inflation, and success of initial swim bladder inflation may be crucial for their survival.

This study investigated the effect of feedings taurine-enriched rotifers on the growth and development of larval red sea bream (RSB). Rotifers incubated in taurine-enriched water at a taurine concentration of 800mgL(-1) (T-800) and 0mgL(-1) (T-0) were fed to larvae from 3 to 20days after hatching (DAH). Notochord length, body weight and specific growth rate of T-800 group were significantly greater than those of T-0 at 14, 17, 9-11 and 18-20 DAH. Taurine content of larvae in the T-800 group increased rapidly from 11 DAH and thereafter remained significantly higher than T-0. Flexion larvae firstly appeared in both groups at 8 DAH, however, at 20 DAH post-flexion larvae were significantly more abundant in T-800 than T-0. While nucleic acid and protein contents (gmg(-1) wet fish) showed remarkable changes, ontogenetic growth in RSB larvae stage was observed to switch from hyperplastic growth to hypertrophic growth with the start of the flexion stage. Although a similar change in nucleic acid contents was observed between the two groups, the protein content (gfish(-1)) and protein/DNA ratio of T-800 remained higher than that of T-0 during the hypertrophic growth period. These results suggest that dietary taurine accelerates the growth and development in RSB larvae especially during hypertrophic growth (flexion stage) after the early hyperplastic growth.

Effects of temperature and salinity on hatching rate and normal larval rate at hatching, and survival of fasting larvae after hatching (survival activity index; SAI) were investigated using spontaneously spawned eggs of captive yellowfin tuna (Thunnus albacares, YFT). Within the range of experimental temperatures, 23-35 A degrees C, at 32 psu salinity, hatching and normal larval rates and SAI were highest at 23 and 26 A degrees C. In the experiment exploring the most suitable salinity within the range 23-38 psu, 35 and 38 psu gave the highest hatching rate and normal larval rate; however, SAI was highest at 26 psu. The results of multi-factor experiments in each temperature (23, 26, and 29 A degrees C) with each salinity (32, 35, and 38 psu) indicated interactive effects of temperature and salinity on the three indices and within the experimental ranges gave an optimal combination of 23 A degrees C and 38 psu for YFT hatching and survival.

This study examined the influence of swimbladder inflation (SBI) failure on mortality (Experiment 1), lordotic deformity (Experiment 2) and growth (Experiment 1, 2) in Pacific bluefin tuna, Thunnus orientalis, postflexion larvae and juveniles by producing larvae lacking inflated swimbladders. Experiment 1 was conducted for postflexion larvae to juveniles (from 18 to 30days-post-hatch; dph). Mortality was not significantly different between the fish with (WIS) and without (WOIS) inflated swimbladders. Standard length (SL) and body weight (BW) were significantly smaller in WOIS than WIS. Moreover, the SBI was found in WOIS after postflexion stage. In Experiment 2, two examination trials were conducted on SBI, vertebral deformity and growth for juvenile stage. Lordotic deformities were found neither in the WOIS nor in the WIS. Although SL and BW were significantly smaller in WOIS than WIS at 22dph, after 37dph no significant differences were found between them. The results of Experiment 1 and 2 indicate that SBI failure causes growth retardation until juveniles of 30dph; however, it does not cause growth retardation in juveniles after 37dph and mortality of postflexion larvae and juveniles. Moreover, SBI failure did not cause lordotic deformity in PBT differently from many other fish.

This study investigated the optimal timing of day to promote initial swimbladder inflation (ISI) for improved Pacific bluefin tuna (PBT), Thunnus orientalis, larval survival. Larval swimbladder inflation frequency was compared based on three experiments using different time schemes of surface film removal (SFR) from 3 to 9days post hatch (dph). SFR was conducted from 05:00 to 19:00hours (light period: S.5-19), 19:00 to 05:00hours (dark period: S.19-5), 08:00 to 19:00hours (S.8-19) and the entire day (S.24) in Experiment 1; from 08:00 to 19:00hours (S.8-19-E2), 08:00 to 13:00hours (S.8-13), 13:00 to 19:00hours (S.13-19) in Experiment 2; and from 13:00 to 16:00hours (S.13-16), 16:00 to 19:00hours (S.16-19), 18:00-19:00hours (S.18-19) in Experiment 3. The swimbladder inflation frequency at the experiment termination (9dph) was significantly higher (P<0.001) in S.24 (91.1 +/- 5.7%), S.5-19 (92.2 +/- 5.1%) and S.8-19 (93.3 +/- 3.4%) than in S.19-5 (11.1 +/- 5.1%) in Experiment 1, and remarkably higher in S.8-19-E2 (81.7%) and S.13-19 (88.3%) than in S.8-13 (0.0%) in Experiment 2, and significantly higher (P<0.001) in S.16-19 (84.4 +/- 5.1%) and S.18-19 (70.0 +/- 12.0%) than in S.13-16 (7.8 +/- 3.9%) in Experiment 3. These results suggest that the optimal timing to promote larval ISI by SFR is a few hours before the end of light period (16:00-19:00hours) from 3 to 9dph.

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 1000hours, 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.

Efforts to develop techniques for breed improvement in aquaculture have been conducted on numerous species, including salmonids, tilapias, and carps. Although attempts at breed improvement for tuna only recently began, rearing technologies used to close the Pacific bluefin tuna (PBFT) life cycle could potentially expedite the development of PBF breed improvement. The PBFT breeding program at Kinki University currently targets the following desirable traits: improved reproduction, increased survival rate during early larval development, faster growth rates, enhanced disease resistance, and improved meat quality. This technology emphasizes the use of marker-assisted breeding programs based on genomic and mitochondrial DNA analysis conducted on carefully selected fish held in bio-secure oceanic net pens. This approach prevents the potential "genetic pollution" of natural PBFT populations from encounters with escaped fish. Together with PBFT breed improvement, the simultaneous development of stress-free (reduced) handling technologies for activities such as juvenile tagging is also essential as tuna species have a low tolerance for handling stress. Finally, a comprehensive strategy for intellectual property protection including copyrights, patents, and trademarks should be implemented in any novel breeding program such that the hard-earned products and technologies developed can be protected from unfair competition.

A male-specific marker of a DNA sequence for the Pacific bluefin tuna Thunnus orientalis, Male delta 6 (Md6), was identified by comparative study of the sequences obtained by F3-cultured male and female AFLP-selective DNA amplification products, followed by high-throughput DNA sequencing. Md6 was characteristic in continuous 6-bp nucleotide deletions compared to redundant sequences that could be seen in both sexes. The PCR genotyping primers were developed based on the Md6 sequence, which showed that 29 of 32 F3 males (90.6 %) and 9 of 13 (69.2 %) F2 parental males were genotyping positive, whereas in females, the genotyping test was negative in parental F2 (n = 12), and 1 of 32 (3.1 %) F3 female was Md6-positive. Parentage allocation tests suggested that Md6-positive F2 males were more attendant in spawning than Md6-negative males, suggesting that Md6 was passed from male parents to male progeny under aquaculture conditions.

The genetic population structure of the Pacific bluefin tuna (PBF) Thunnus orientalis and the yellowfin tuna (YFT) T. albacares in the North Pacific Ocean was investigated. The polymorphism of microsatellite (SSR) loci and sequences of mitochondrial DNA control region (mtCR) were analyzed for 71 samples of PBF from Japan and Mexico and 45 samples of YFT from Japan and Panama. In the SSR analyses, both single-locus (-0.010 to 0.008 in PBF and -0.023 to 0.020 in YFT) and global multilocus (0.003 in PBF and -0.002 in YFT) F-ST values among the geographic populations were low and not significant in these species. In the mtCR analyses, neither the neighbor-joining tree nor the minimum spanning network showed genetic differentiation among the geographic populations in each species. The pairwise F-ST values among the geographic populations of them (-0.005 in PBF and -0.020 to -0.014 in YFT) were low and not significant. Our SSR and mtCR data suggested that genetic differentiations were not evident among the eastern and western populations in the North Pacific Ocean either in PBF or in YFT. Mismatch distributions, demographic parameters, and neutrality tests suggested that sudden population expansion of PBF and YFT in the North Pacific Ocean occurred 628,000-731,000 and 450,000-525,000 years ago, respectively.

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.

With the development of successful aquaculture techniques, the Pacific bluefin tuna (PBT) has become the most important fish species in Japan. Early muscle growth and cellularity of muscle fibers have a profound effect on ultimate body size and meat quality of fish. Larvae and juveniles of full-cycle cultured PBT were sampled at 3, 15, 29, 41, 70, 128 and 218 days post hatch (dph). Transverse body sections until 41 dph and muscle sections from 41 dph thereafter from dorso-cranial or caudal areas were stained for muscle fiber morphological and morphometric analysis and muscle sections from the dorso-cranial region were used for ultra-structural observation. Red muscle fibers appeared by 15 dph at the horizontal septum both in epaxial and hypaxial regions. Other than a single layer of superficial red fibers and those at the horizontal septum, existence of red muscle fibers was apparent in the PBT myotome. The muscle fiber diameters varied in their size even in adjacent myotomes of PBT. Overall, growth throughout the various stages was a result of both hyperplasia and hypertrophy of muscle fibers. Muscle fiber diameters were greater in the dorso-cranial than in dorso-caudal region at 41, 70 and 128 dph. These results provide additional characterization of PBT growth, which may assist with managing fish production efficiency, harvest age and eating quality.

Type I collagen is widely distributed in most organs in teleosts. It plays a role not only in intercellular adhesion, but also in molecular signaling. In this study, Pacific bluefin tuna (PBT) procollagen alpha 1 (I) cDNA was cloned and characterized. The nine fragments of a procollagen alpha 1 (I) chain cDNA clone were prepared and spliced together to create the complete coding region. The resulting amino acid sequence was homologous with that of other teleosts. The mRNA expression profile of PBT procollagen alpha 1 (I) in various tissues and the phylogenetic analysis with other vertebrate procollagen alpha 1 (I) chains suggest that PBT procollagen alpha 1 (I) could be a precursor form of the PBT type I collagen alpha 1 chain. In addition, its level of expression in PBT larvae and early juveniles gradually increased with somatic growth. This increase was related to the standard length, wet body weight, and protein content of each individual fish. Therefore, the expression profile of procollagen alpha 1 (I) may be a useful indicator for somatic growth in fish larvae and juveniles.

Although the survival rate of juvenile Pacific bluefin tuna Thunnus orientalis (PBT) during 30 days after stocking in sea net cages has been as low as approximately 50%, the reason for this high mortality is not clear. The dead fish were collected and counted during 30 days after stocking in a sea net cage, and the growth performance, stomach contents, and bone injury were investigated. Nearly half (47.9%) had died within the first 4 days. The total length and body weight of the dead fish were significantly smaller than those of the live fish, and the weight of the dead fish did not increase during the first 7 days. Approximately one-third of the dead fish (21.9-42.9% per day) had ingested inorganic matter such as wood or Styrofoam waste during the first 7 days, and 25.0-45.0% of the dead fish sampled per day showed poor growth. The daily ingestion rate of inorganic matter decreased dramatically from the eighth day, when the daily mortality rate decreased. The percentage of dead fish with bone injuries due to collision or contact was low (less than 15%). These results suggest that one of the causes of high mortality during several days after stocking in a sea cage was accidental ingestion of inorganic matter by juvenile PBT. Improvement in feeding method can increase the survival rate of juveniles in sea net cages. (C) 2014 Elsevier B.V. All rights reserved.

The pacific bluefin tuna (PBT) Thunnus orientalis is one of the most important species for fisheries in the world. This species has been highly exploited in fisheries, resulting in dwindling of its natural resources. The artificial hatching of a third generation of fully cultured PBT was a success in 2007, suggesting the possibility of mass production of PBT for food and for seed release. To enhance stock structure studies and investigate genetic feature of seedlings, we isolated 25 polymorphic microsatellite loci (2-18 alleles/locus; expected heterozygosity, 0.042-0.941) from PBT.

The spectral sensitivity of the fish and the suitable light wavelength range for survival and growth performance of juvenile Pacific bluefin tuna (PBT) were investigated. The spectral sensitivity peak of PBT under photopic condition was observed between 449 and 503 nm, which corresponded to their natural habitat. The fish were reared in tanks irradiated continuously with 4 kinds of light emitting diodes (LEDs). The maximum wavelength of LEDs used for the rearing experiment were 460 nm (blue), 520 nm (green), 630 nm (red), and 450-680 nm (white). There was no notable difference in survival rate among fish in the four LED groups. However, the growth of juvenile PBT was lesser under red light compared to the green and white light wavelengths. These results suggest that PBT juveniles have low sensitivity to red light because the fish are rarely exposed to the red light wavelengths under natural ocean conditions. Thus, low sensitivity to red light negatively influenced the feeding behavior and growth of PBT juveniles.

Blood flukes of the genus Cardicola cause significant problems in the bluefin tuna farming around the world. In Japan, the mortality of the juvenile Pacific bluefin tuna Thunnus orientalis (PBT) caused by C. orientalis and C. opisthorchis has become a serious issue in the tuna farming industry in recent years. Oral treatment with praziquantel (PZQ) is highly effective against blood flukes and is indispensable for sustainable tuna farming. To develop a proper and safe drug treatment procedure, we investigated the minimum effective dose and pharmacokinetics of PZQ in juvenile PBT. Juvenile tuna were orally administered various doses of PZQ (0, 3.75, 7.5, 15 and 30 mg/kg body weight (BW)) once a day for 3 consecutive days, and the occurrence of C. opisthorchis before and after the treatment was compared. A significant decrease in the number of flukes was observed in the 3.75 mg/kg group, and complete eradication of flukes was achieved at dosages of 7.5 mg/kg and greater. This result indicates that the minimal effective dose of PZQ for treating C. opisthorchis is between 3.75 and 7.5 mg/kg, which is less than 1/20 of the treatment dose used for treating the skin fluke Benedenia seriolae in yellowtail culture. We used high-performance liquid chromatography analysis to assess the temporal changes in the concentrations of PZQ in serum, muscle, the liver and the kidney after a single administration of PZQ at 15 mg/kg BW. The PZQ concentration in the tissues peaked between 0.5 and 1.5 h following administration, and PZQ was undetectable after 24 h. The highest serum concentration was 2.0 mu g/mL, which is likely to be the lethal dose for C. opisthorchis. These results suggest that PZQ treatment is a highly effective and relatively safe method to control blood flukes in the tuna farming industry. (c) 2013 Elsevier B.V. All rights reserved.

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 (lambda(max) = 512-515 nm and 423-436 nm) or three (lambda(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.

Initial swimbladder inflation (ISI) of Pacific bluefin tuna (PBT), Thunnus orientalis, larvae was studied to increase the survival of cultured larvae. Experiment 1 was conducted to explore promotion and inhibition of ISI under different water surface conditions; including the use of surface skimmer to remove autogenous surface substances (SS), covering the water surface with liquid-paraffin-layer (LP) and oil film (OF), and a control (non-treatment, NT). Significantly higher inflation frequency was observed in SS (62.2%) than NT (11.9%), LP (2.7%) and OF (3.9%). This indicates that ISI in PBT larvae can be promoted by removal of surface substances on rearing water which inhibit larval air gulping. Experiment 2 aimed to elucidate proper day of larval age to start skimming for promoting ISI with four different periods of oil film removal: from 3 to 8 (SF3D), 4 to 8 (SF4D), 5 to 8 (SF5D), 6 to 8 (SF6D) days-post-hatch (dph). Significant improvement in ISI frequency was observed in SF3D (80.2%) but the frequency was very poor in SF4D, SF5D, and SF6D (17.87.5%). This implies the need of oil film removal without missing a narrow window, 1 day of 3 dph, to promote ISI in practical PBT larviculture.

Pacific bluefin tuna (PBT) peroxisome proliferator-activated receptor ? (PPAR?) gene was characterized to know the expression of its transcript contribution to the development of PBT, because PPAR? is the key molecule for adipose cell differentiation. Resulting cDNA and deduced amino acid sequence had high similarities to other teleosts that consisted of A to F domains. Phylogenetic tree analysis indicated a close relationship PBT PPAR? to cobia and flatfishes ones among the teleosts with the similarity of characteristic insertion sequences at amino terminal region of E/F domains. PPAR? transcripts quantification profile in the tissues of a 13-month-old PBT indicated the correlation between its quantity with the muscle types of different lipid content. The transcripts were also detected in the head kidney, digestive organs and red muscle with higher level inferred the PPAR? contribution to the multiple physiological processes. PPAR? transcripts were quantified for PBT juveniles fed artificial diets to investigate the effect of phospholipid supplementation on the growth, survival and PPAR? expression, to verify the efficacy of phospholipid source. No obvious difference in growth performance, survival and gene expression of PBT juveniles was identified, this may be due to DHA/EPA ratio analogy between diets and indicating that phospholipid sources in the juvenile diet might be adaptable.

We have shown that dietary bluefin tuna skin (TUS) protects against carbon tetrachloride (CCl4)-induced hepatic damage in mice. The CCl4-induced necrotic area was decreased in mice fed a TUS-containing diet. Consistent with the decreased necrotic area, dietary TUS markedly lowered the elevated serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities and the thiobarbituric acid-reactive substance (TBARS) formation induced by CCl4 injection. TUS diets also decreased phosphorylation of inhibitory kappa B-alpha and blocked the translocation of nuclear factor-kappa B to the nucleus. TUS is composed mainly (80.7 %) of type I collagen, and our results revealed that dietary tuna collagen peptides (TUCP) attenuated the increased hepatic necrotic area, serum AST and ALT activities, and liver TBARS levels induced by CCl4, similar to TUS, thus enabling us to attribute the hepatoprotective action of TUS in CCl4-intoxicated mice to tuna collagen. Therefore, TUS and TUCP may be potential food resources that are capable of alleviating hepatitis symptoms.

Morphogenesis is most active during fish embryonic development with increased susceptibility to environmental stressors. In order to elucidate the causalities of malformations in fishes that produce buoyant eggs, this study estimated the time change of dissolved oxygen (DO) and carbon dioxide (DCD) concentrations in the interstitial water of red sea bream, Pagrus major, egg aggregation. Since direct measurement of DO and DCD concentrations is not possible by current methodologies, we measured those in one liter water containing approximately 500,000 eggs for their estimation. Calculated oxygen consumption and carbon dioxide excretion rates from observed data were 1.32x10(-6) and 2.16x10(-7) mg/ind/min, respectively. Volumetric method with these numeric values estimated that DO decreases below 10% saturation within 21 s and DCD increases to more than 135 mg/L within 60 min in the interstitial water of egg aggregation. These results suggest that hypoxia and hypercapnia may become teratogenic within a short time in the aquaculture practices of egg collection, treatment, and transportation where eggs aggregate at the water surface in containers or rearing tanks, or in the wild when eggs are exposed to red tide in the open sea. (C) 2012 Elsevier B.V. All rights reserved.

It is important to produce safe cultured fish by reducing their dioxin content. We investigated the reduction of dioxin content in cultured red sea bream Pargus major by feeding artificial diets containing two sorts of fish oil with different dioxin content: CT (3 pg-TEQ/g) and MD (18 pg-TEQ/g). There was no significant difference in the growth, survival, and feeding indices between the CT and MD fed groups. The content of dioxins in the muscle significantly decreased in the CT fed group (0.07 pg-TEQ/g) compared with the initial content (0.14 pg-TEQ/g), whereas that in the MD fed group increased to 0.30 pg-TEQ/g. This effect was particulary clear in the liver. These results demonstrated the possibility of producing cultured fish with low content of dioxins by feeding low-dioxin fish oil.

Following the successful development of Pacific bluefin tuna (PBT) aquaculture, it is of considerable importance to determine the muscle fiber types and their growth patterns for future development. Muscle fiber profiles of dorsal ordinary, lateral ordinary (LO) and dark muscles and their growth patterns in PBT from 3.0 to 54.3 kg body weight were studied. Muscle fibers were histochemically stained for NADH-diaphorase and myosin adenosine triphosphatase activity (mATPase), and immunohistochemically stained with S-58 slow-muscle myosin antibody. All muscle fibers in dorsal and LO muscles showed low NADH-diaphorase activity, and acid-labile (pH 4.0 or 4.3) and alkali-stable mATPase activity. In LO muscle adjacent to dark muscle, three intensities of mATPase activity were observed after acid pre-incubation at pH 4.5 or 5.0, and the activity was related to the muscle fiber diameter. In dark muscle, all small and some large fibers stained intensely for NADH-diaphorase activity, related to their high aerobic metabolism. The high-active fibers with NADH-diaphorase in dark muscle were positive for S-58 antibody. Some large fibers in dark muscle showed intermediate NADH-diaphorase activity and high mATPase activity after alkali pre-incubations. These are fast-twitch oxido-glycolytic fibers in dark muscle and transformed to red muscle fibers with increasing body weight.

Aquaculture of tuna has shown exponential growth in the recent years. In association with the global development of tuna aquaculture, problems with parasitic infections have arisen. Among the parasites found in cultured tuna, blood flukes are considered to be a significant threat as their eggs accumulate in the gills and interfere with the gas exchange process. To date, two species of blood fluke from the genus Cardicola have been found in Pacific bluefin tuna (Thunnus orientalis, PBT) cultured in Japan. We conducted an experiment to develop a drug treatment against the blood flukes of cultured tuna. Artificially produced juvenile tuna that were naturally infected with blood flukes were orally administered with anthelmintic praziquantel (PZQ) of 150, 15, 1.5 and 0 mg kg(-1) BW for three consecutive days, and differences in the abundance of adults and eggs in the gills were assessed. In the fish given 150 and 15 mg kg(-1) BW PZQ, adult worms in the heart ventricle were eradicated by day 8 post treatment with the number of eggs in the gills gradually declining over time. However, small numbers of adults re-appeared by week 3 or 5 post treatment. The drug does not seem to affect eggs as viable miracidia were observed throughout the experiment. Mortalities in the two lower dose groups were significantly higher than the groups in which PZQ doses were shown to be effective. The study empirically demonstrated the efficacy of PZQ on Cardicola blood flukes and that the parasite significantly affects the survival of juvenile PBT. The oral PZQ treatment may be an essential practice for sustainable aquaculture of PBT. (C) 2011 Elsevier B.V. All rights reserved.

High levels of larval mortality are a significant barrier to the artificial mass production of Pacific bluefin tuna (Thunnus orientalis). Mortality may occur when larvae sink and come into contact with the bottom of the rearing tank during the first 10 days after hatching. We evaluated the effect of flow control by aeration on the survival of T. orientalis larvae. These larvae were held in 500-L tanks in which the aeration rate was varied during the night. Larval survival increased with air supply. We documented the cross-sectional flow pattern and gravitational sinking velocities of larvae to assess the correlation between survival and circulation patterns in the tank. The sinking velocity of T. orientalis larvae at night increased with larval body density, which varied with swimbladder volume. Larvae with uninflated swimbladders sank significantly faster than larvae with inflated swimbladders. Both water circulation speed and survival increased at higher aeration rates. Our results suggest that aeration rates >900 ml min(-1) may increase larval survival by counteracting sinking.

In cultured juvenile Pacific bluefin tuna (Thunnus orientalis), reducing the mass deaths caused by collision or contact with tank or net walls at night is a priority for seedling production. Pacific bluefin tuna is a visually dependant species, although its scotopic vision is poor. We recorded electroretinograms to investigate the visual function with growth in the dark-adapted eyes of juvenile Pacific bluefin tuna. Peak wavelengths of spectral sensitivity [38-62 days posthatch (dph), 77-167 mm standard length (SL)] were observed between 474 and 494 nm. Visual light sensitivity has a tendency to increase slightly with growth at 28-64 dph in individuals that measured 29-175 mm SL. However, visual temporal resolution did not significantly increase with growth at 38-62 days dph in individuals that measured 77-167 mm SL. These results suggest that the mass death continues between 28 and 64 dph because of low visual function and increasing swimming speed with growth.

AFLP, amplified fragment length polymorphism screening, was conducted to identify sex characteristic DNA markers of cultured Pacific bluefin tuna (PBT), Thunnus orientalis. The AFLP screening of 4 females and 4 males with 64 primer combinations produced a total of 1038 scorable bands, of which 183 were polymorphic between individuals. Male characteristic fragments were confirmed with EcoRI + AGG and MseI + CAT primers by gel image analysis, which corresponded to 437 bp in 29 of 32 males and 3 of 32 females. Fragment detail was analyzed by electropherogram analysis and DNA quantification. The 437 bp fragment was detected in 30 individuals of average DNA concentration 0.231 ng/mu L; while in females, 13 individuals were detected with average 0.080 ng/mu L (P<0.01).

To reveal the kinematical aspects of schooling development in the Pacific bluefin tuna Thunnus orientalis, changes in its schooling behaviour traits, local behaviour transmission among school members and morphological traits were investigated as fish developed from the larval to the juvenile stage. Schooling was first observed at around 24 days post-hatching (27 mm body length) in T. orientalis. Behaviour transmission among individuals took much longer among 24 d post-hatching individuals than among older fish. The compactness and polarity of the school progressively improved as the time required for behaviour transmission decreased. One cause of the reduction in the time required for behaviour transmission was the development of manoeuvrability, which resulted from further morphological development of the caudal fin and other organs related to swimming.

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.

Kudoa prunusi n. sp. (Myxozoa; Multivalvulida) is described from the brain of Pacific bluefin tuna Thunnus orientalis cultured in Japan. Numerous white cysts, up to 0.5 mm in size, were found on and in the brain. Spores having typically five spore valves and five polar capsules resembled a five-petal cherry blossom in apical view and were conical shape with a round bottom in side view. Average spore size was 9.63 (8.5-10.3)mu m in width and 7.50 (6.7-8.6)mu m in length. The spore dimensions of K. prunusi overlapped with those of Kudoa yasunagai ex Sillago ciliata having five to six spore valves, but they were clearly distinct in spore shape, 18S rDNA and 28S rDNA sequences (0.3% and 1.7% differences, respectively). Phylogenetic analysis of 18S rDNA revealed that K. prunusi grouped with the brain-infecting multivalvulid species, K. yasunagai, K. chaetodoni, K. lethrini and K. neurophila, rather than five-valved Kudoa spp. Combined with morphological, molecular and biological differences, K. prunusi was proven to be a new species. (C) 2010 Elsevier Ireland Ltd. All rights reserved.

Copyright (c) 2010 日本水産学会, rights: 本文データは学協会の許諾に基づきJ-STAGEから複製したものである, relation: isVersionOf: https://www.jstage.jst.go.jp/article/suisan/76/5/76_5_971/_article/-char/ja/

Schooling behavior traits during the process of retinomotor response from scotopic to photopic vision were examined in cultivated juvenile Pacific bluefin tuna (PBT) at 3 different ages. After a sudden change in illumination from darkness to 300 lx, retinal adaptations changed from scotopic to photopic vision. Retinomotor and schooling indices showed strong agreement, with juvenile PBTs forming polarized schools upon complete retinal adaptation to photopic vision. The behavioral and retinal adaptation to sudden illumination took 20, 15, and 10 min after illumination in PBT 25, 40, and 55 days after hatching (dah). At 40 dah, PBT took a longer time to adapt than fish aged 55 dab and showed the highest swimming speed, including momentary bursts of swimming immediately after illumination. This suggested that these fish were swimming at high speed under poor visibility conditions. In contrast, PBT at 55 dah showed a gradual increase in swimming speed that correlated with their retinal adaptation. Therefore, behavioral and retinal adaptation traits changed during growth, suggesting that the high mortality in PBT around 40 dah, due to collisions with the tank and net walls at dawn, may be because these adapt more slowly than fish at 55 dah and were swimming at a relatively high speed under conditions of poor visibility. (C) 2010 Elsevier B.V. All rights reserved.

We used electroretinogram recordings to investigate visual function in the dark-adapted eyes of the juvenile scombrid fishes Pacific bluefin tuna (Thunnus orientalis) and chub mackerel (Scomber japonicus) and the carangid fish striped jack (Pseudocaranx dentex). Despite the fast swimming speed of the Pacific bluefin tuna, analysis of flicker electroretinograms showed that visual temporal resolution in this species was inferior to that in chub mackerel. Peak wavelengths of spectral sensitivity in Pacific bluefin tuna and striped jack were 479 and 512 nm, respectively. The light sensitivity of Pacific bluefin tuna was comparable to that of chub mackerel but lower than that of striped jack. The Pacific bluefin tuna may not need high-level visual function under dim light conditions in natural habitat because it is a diurnal fish. However, this low temporal resolution and light sensitivity probably explain the mass deaths from contact or collisions with net walls in cultured Pacific bluefin tuna.

クロマグロ稚魚の大量死の原因が低い夜間視によること，電照飼育でそれを防止できることを明らかにした。

Twenty-three new polymorphic microsatellite markers were isolated in the Pacific bluefin tuna, Thunnus orientalis. Each locus comprised three to 34 alleles. The expected and observed heterozygosities ranged between 0.46 and 0.96 and between 0.44 and 0.97, respectively. The Kto9, Kto11, and Kto42 markers demonstrated significant deviation from Hardy-Weinberg equilibrium; high null allele frequencies (0.08-0.14) were observed in the deviating group. From the results of simulation of parentage assignment, a combination of four loci (i.e. Kto15, Kto23, Kto38, and Kto39) was considered the best for parentage assignment.

マグロ類特有の脂質蓄積機構は大回遊を可能にしている。一方でマグロ肉の脂質蓄積状態は食品としての肉質評価で重要である。本研究では、飼育クロマグロ仔稚魚で、脂質蓄積の発育変化を異なる飼育水温下で解明することを目的とした。飼育水温は、（I）一貫して27℃、（II）前屈曲期まで24℃、後屈曲期まで25℃、稚魚期以降27℃、の2系列とした。魚体の脂質分布は、凍結切片にオイルレッドO、ヘマトキシリンエオシン染色を施し、組織学的に調べた。2系列ともに、最初の脂質蓄積は脳周囲で、続いて内臓、稚魚期に入って筋肉で生じた。真性血合筋を除き、調べた全ての部位で、系列IでIIより早い発育段階で脂質蓄積が生じた。系列IIにおける発育初期の低温による脂質蓄積の遅延は、飼育水温が系列Iと同じ27℃にまで上昇した後も持続した。これらの結果は、低水温による脂質蓄積の遅延、発育初期の水温の脂質蓄積への影響の後の発育段階までの持続を示している。

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.

Schooling behaviour and histological retinal light adaptation in juvenile Pacific bluefin tuna Thunnus orientalis were examined under various light intensities to determine the effect of light intensity on behaviour. After monitoring the schooling behaviour of juveniles 35-36 and 45-46 days post hatching, schooling variables such as nearest neighbour distance and separation swimming index were measured under different light intensities. Furthermore, retinal indices of light adaptation were investigated histologically for each experimental light intensity. Under intensities >5 lx, schooling variables in the two juvenile growth stages were nearly constant, allowing schooling. In contrast, the schooling variables indicated that the fish gradually swam more widely and randomly with decreasing light intensities <5 lx. The retinal indices also showed a shift from light adaptation to dark adaptation at light levels <5 lx. From 5 to 0-01 lx, retinal adaptation and fish schooling behaviour changed with light intensity. These data suggest that the schooling behaviour of juvenile Pacific bluefin tuna is greatly affected by retinal adaptation. (c) 2007 The Authors Journal compilation (c) 2007 The Fisheries Society of the British Isles.

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.

Using the full-cycle cultured (FC) Pacific bluefin tuna [body weight 16.3 +/- 1.9 kg (prefasting 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 degrees 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.

Diel and ontogenetic changes in larval body density related to swim bladder volume were investigated in Pacific bluefin 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 deflated swim bladders increased with total length and days post hatch. Diel density change was observed after day 2 post hatch; owing to daytime deflation and night-time inflation 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 inflated swim bladders was greater than rearing water density (Delta rho > 0.0099). The small density difference between larvae and rearing water (Delta rho=0.0022-0.0100) 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 difference (Delta rho=0.0065-0.0209) 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.

A previous study elucidated that an extreme hypoxia during somitogenesis induced the most frequent skeletal malformation centrum defects in red sea bream (RSB), Pagrus major. In this study, details of the hypoxic conditions to induce them in RSB, dissolved oxygen (DO) concentration and exposure time to hypoxia, were investigated. Fertilized eggs were exposed to seawater of six DO concentrations (0%, 10%, 25%, 50%, 75% and 100% of saturation) for seven different periods (5, 10, 30, 60, 120, 240 and 360 min) during somitogenesis. Somitic disturbances in newly hatched larvae were induced by exposure to 0% and 10% DO concentration for 10 and 120 min and longer respectively. Rearing eggs exposed to hypoxic condition of 10% DO for 240 min for 40 days post-hatch showed that the location and the frequency of somitic disturbances in larvae and centrum defects in juveniles were significantly correlated (P < 0.01). Dissolved oxygen concentration of the interstitial water in the egg high density layer formed at the water surface in a stationary state abruptly decreased to 3.7% within 7 min. Centrum defect induction by exposure of eggs to extreme low DO concentrations for a short period, which is the probable situation in the practical juvenile production, suggests that careful maintenance of DO concentration is important in the incubating water of fertilized eggs during egg sorting and transportation, where eggs are made into a pile and undergo hypoxia, for the prevention of centrum defects.

クロマグロの種苗生産に関する基礎的知見を得ることを目的として, 人工孵化クロマグロの孵化後の成長に伴う消化系の発達および消化酵素活性について調べ, 次の結果を得た。孵化後36時間目までに, 肝臓, 胆嚢および膵臓が形成され, トリプシン様酵素およびアミラーゼ活性は摂餌開始後成長に伴い増加した。Preflexion期 (孵化後10日まで) に腸の回転が完了し, 咽頭歯および食道の粘液細胞が分化した。Flexion期 (孵化後11～17日) に機能的な顎歯がみられ, 胃盲嚢, 胃腺および幽門垂が形成され始めた。ペプシン様酵素活性は胃腺の形成に伴って増加し, Postflexion期から稚魚への移行期 (孵化後17～25日) にかけて胃の機能および幽門垂が著しく発達した。標準体長に対する肛門前長の比は孵化後11日目までは約40%で一定であったが, 26日目にかけて約65%まで増加し, 26日目から30日目まではほとんど変化しなかった。
以上の結果から, クロマグロの消化系は孵化後11日目まで (Preflexion期) は主として質的に発達し, それ以降26日目まで (Flexion期からPostflexion期) は量的な発達が伴うことが示唆され, このような量的および質的な消化系の発達が稚魚期の急速な成長に貢献するものと推察した。