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Female fishes allocate a certain quantity of lipids for egg production once mature. In aquaculture, such physiological changes often result in the deterioration of flesh quality. In the study reported here, we aimed to characterize the seasonal changes in the lipid contents in aquaculture-produced female Pacific bluefin tuna (PBT) Thunnus orientalis, which is one of the most commercially important species for aquaculture worldwide. We analyzed the seasonal changes in the lipid content and weight of tissues and organs responsible for lipid allocation towards egg production, including the muscles, liver, mesenteric fat (perigonadal fat), and ovaries. During the spawning season, the total lipid content of the muscles and liver of the mature females decreased, whereas that of the ovaries increased; similarly, the weight of perigonadal fat decreased. We hypothesized that muscle, liver, and perigonadal fats contribute significantly to the accumulation of lipids during the maturation of female PBT. This is the first study to report an association between a reduction in muscle lipid content and sexual maturation in aquaculture-produced female PBT. These findings provide important insights regarding efficient aquaculture operations for this economically important fish. In addition, this study is the first to provide data on the association between the lipid dynamics of female PBT and sexual maturation in aquaculture, further enhancing our understanding of the energy allocation strategy during maturation of female bluefin tuna. [ABSTRACT FROM AUTHOR]

Triploidization influences various biological characteristics of fish, which is associated with reductions in the number of multiple cell types in different tissues/organs. Our behavioral analyses revealed that triploid Pacific bluefin tuna (Thunnus orientalis) larvae exhibit lower sensitivity to light compared to diploids. Furthermore, histological analyses revealed a reduction in the number of ganglion cells and an increase in their size in the retinas of triploid T. orientalis larvae. Our findings provide the first evidence indicating that triploidization reduces sensory perception during the larval stage of fish. [ABSTRACT FROM AUTHOR]

Abstract Pacific bluefin tuna (PBT), Thunnus orientalis, is one of the most important species for aquaculture worldwide. For the conservation of this species and sustainable development of the tuna farming industry, production based on closed‐cycle aquaculture should be promoted. In this study, to develop an efficient method for transporting PBT eggs, we simulated egg transportation to optimize stocking density and shipping duration, which are the two variables critical to the transportation procedure of PBT. Based on the findings of this study, we concluded that PBT eggs should be transported at a density of 1 × 104 eggs/L within 12 h in the field. Furthermore, we showed that collisions between eggs during transportation negatively affected the normal hatching rates of eggs, and polyethylene glycol (PEG) treatment (at 1000 μg/mL for 12 h) could reduce such damage. Our results suggest that PEG treatment is a practical approach that contributes to the stable transportation of PBT eggs, particularly in cases where the effects of physical shock damage is considerably increased.

Pacific bluefin tuna (PBT), Thunnus orientalis, is one of the most important species for aquaculture worldwide. For the conservation of this species and sustainable development of the tuna farming industry, production based on closed‐cycle aquaculture should be promoted. In this study, to develop an efficient method for transporting PBT eggs, we simulated egg transportation to optimize stocking density and shipping duration, which are the two variables critical to the transportation procedure of PBT. Based on the findings of this study, we concluded that PBT eggs should be transported at a density of 1 × 104 eggs/L within 12 h in the field. Furthermore, we showed that collisions between eggs during transportation negatively affected the normal hatching rates of eggs, and polyethylene glycol (PEG) treatment (at 1000 μg/mL for 12 h) could reduce such damage. Our results suggest that PEG treatment is a practical approach that contributes to the stable transportation of PBT eggs, particularly in cases where the effects of physical shock damage is considerably increased. [ABSTRACT FROM AUTHOR]

Climate change impacts are projected to shift the distribution and abundance of global fisheries, affecting revenues and livelihoods worldwide (Barange et al., 2018; Lam et al., 2016). The creation of climate-resilient fisheries requires knowledge of how culturally and economically valuable species will be impacted by rising sea surface temperatures, ocean deoxygenation, and climatic variability. Two Western Pacific fisheries, the Pacific bluefin tuna (PBF; Thunnus orientalis) and Japanese anchovy (JA; Engraulis japonicus), may be particularly vulnerable to the impacts of climate change due to overfishing, sub-tropical spawning grounds, and declining prey availability (FAO, 2020; ISC, 2020). Chasing catch may become the new reality for commercial fleets around the world as PBF and JA inhabit new waters away from historic fishing grounds. In order to promote sustainable management of major global fisheries, policymakers must have the best tools and information available for how climate change will affect fisheries distribution and abundance through the 21st century.This study is aimed at improving the scientific basis for creating fisheries that are resilient to climate change by using climate change projections, species-specific life history traits, and ecological information to project potential fish stock distribution in response to changing coastal and oceanic conditions. Current projections focus on the use of suitable thermal habitat as a proxy for future species distribution and abundance (Morley et al., 2018). The aim of this study is to improve on these projections by evaluating the potential effects of changes in ocean mixing, nutrients, and other factors in addition to temperature as well as by incorporating information on how the target species (PBF and JA) are likely to respond to these changes using life history characteristics. Based on a decision tree provided by the Environmental Defense Fund, climate change is expected to impact every life stage of PBF and JA and centered around changes in sea surface temperature, ocean deoxygenation, and climatic variability. Future abundance of both species’ hinges on the survival of larval stages, recruitment success, and conservation measures to prevent the depletion of young age classes. Northward shifts in distribution are anticipated for both species into the next century, raising concerns about future international management. The entrance of new players into the PBF and JA fisheries may require international agreements based on the shift in distribution of each stock. International cooperation and adaptive management measures must be adopted by fisheries management entities to maintain productive fisheries that continue to generate social and economic benefits associated with PBF and JA.

Keywords: Dead end; Pacific bluefin tuna; Primordial germ cell; Sterile; Thunnus orientalis EN Dead end Pacific bluefin tuna Primordial germ cell Sterile Thunnus orientalis 3872 3876 5 06/14/22 20220701 NES 220701 The I dead-end i ( I dnd i ) gene was first identified in zebrafish, I Danio rerio i , as a germ plasm component encoding an RNA-binding protein (Weidinger et al., 2003). To examine germ cell deficiency in addition to observations by GFP labeling, we conducted whole-mount immunostaining using anti-germ cell marker Vasa antibodies, as described previously (Hayashida, Higuchi, Hashimoto, et al., 2021), and histological analysis. In fact, inhibiting I dnd i expression causes faulty migration and ablation of zebrafish PGCs, resulting in germ cell deficiency (Weidinger et al., 2003). The zebrafish Dnd protein binds to 3' untranslated regions (3'UTR) of germline RNAs, such as I nanos i mRNAs, thereby protecting them against microRNA-mediated degradation, so they can contribute to the migration and survival of primordial germ cells (PGCs) (Kedde et al., 2007; Weidinger et al., 2003). [Extracted from the article]

Pacific bluefin tuna (PBT), Thunnus orientalis , is one of the most important species for aquaculture in Japan. Recently, the reduction in muscle fat content associated with sexual maturation in farmed PBT has become a serious problem. To develop technologies for inducing sterility, detailed and reliable data on gonadal development in PBT are needed. Here, we demonstrated the process of gonadal sex differentiation, and of early ovarian and testicular development during the immature stages in PBT. Gonadal sex differentiation was first characterized by the formation of the ovarian cavity in female and of the efferent ducts in male 57 days post hatching (dph). The gonads then differentiated into ovaries or testes according to the genotypic sex until 83 dph. During this period, primordial germ cells, oogonia, and type-A spermatogonia were solitarily distributed in the gonads, and the number of germ cells did not differ between sexes. After gonadal sex differentiation, gonads of PBTs developed in a sexually dimorphic manner: proliferation and differentiation of germ cells occurred earlier in the ovaries than in the testes. The oogonia in ovaries formed cysts at 185 dph, but the type-A spermatogonia were solitarily distributed in testes at this stage, and cysts of type-A spermatogonia were first observed at 247 dph. Moreover, the oogonia entered meiosis and differentiated into chromatin-nucleolus stage oocytes until 247 dph, and subsequently into peri-nucleolus stage oocytes until 285 dph, whereas the type-A spermatogonia differentiated into type-B spermatogonia, spermatocytes, spermatids, and spermatozoa from 446 dph onwards. We believe the results of this study provide the necessary basis for future studies on sterile PBT production. • To develop technologies for producing sterile fish, detailed and reliable data on gonadal development are needed. • We demonstrated the morphological changes and timing of gonadal sex differentiation in Pacific bluefin tuna (PBT). • We also showed the developmental process of the ovary and testis during the immature stages of PBT. • This study will provide the necessary basis for future studies on sterile PBT production. [ABSTRACT FROM AUTHOR]

The consumption of fish and shellfish worldwide is steadily increasing, and tuna is a particularly valuable fish species. However, infection caused by Kudoa spp. is causing problems in many fish including the Pacific bluefin tuna (Thunnus orientalis), and there is much controversy about the association of these infections with foodborne disease. In this study, using haematological and histological analyses of the blood and internal organs (liver, spleen, kidney, heart, stomach, intestine, gill, and muscle) of Pacific bluefin tuna cultured in South Korea, infection with Myxosporea was first identified, and molecular biological analysis was conducted. In this study, Kudoa hexapunctata was finally identified. The Pacific bluefin tunas analysed in this study did not show any gross pathology lesions, such as visible cysts and/or myoliquefaction, of infection with this species. The histological analytical results can provide guidelines for the identification of K. hexapunctata. In the case of K. hexapunctata-induced infection, unlike other countries, such as Japan, there have been no reports in South Korea, and this study is the first to detect K. hexapunctata infection in Pacific bluefin tuna cultured in South Korea. The correlation between K. hexapunctata and food poisoning is not yet clear, however, it is thought that continuous observation of its infection is necessary.

Elucidating fishing effects on fish population dynamics is a critical step toward sustainable fisheries management. Despite previous studies that have suggested age or size truncation in exploited fish populations, other aspects of fishing effects on population demography, e.g., via altering life histories and density, have received less attention. Here, we investigated the fishing effects altering adult demography via shifting reproductive trade-offs in the iconic, overexploited, Pacific bluefin tuna Thunnus orientalis. We found that, contrary to our expectation, mean lengths of catch increased over time in longline fisheries. On the other hand, mean catch lengths for purse seine fisheries did not show such increasing trends. We hypothesized that the size-dependent energetic cost of the spawning migration and elevated fishing mortality on the spawning grounds potentially drive size-dependent skipped spawning for adult tuna, mediating the observed changes in the catch lengths. Using eco-genetic individual-based modeling, we demonstrated that fishing-induced evolution of skipped spawning and size truncation interacted to shape the observed temporal changes in mean catch lengths for tuna. Skipped spawning of the small adults led to increased mean catch lengths for the longline fisheries, while truncation of small adults by the purse seines could offset such a pattern. Our results highlight the eco-evolutionary dynamics of fishing effects on population demography and caution against using demographic traits as a basis for fisheries management of the Pacific bluefin tuna as well as other migratory species. [ABSTRACT FROM AUTHOR]

Background: Pacific bluefin tuna (Thunnus orientalis) spawning adults, juveniles and larvae are all reported to occur in areas between Philippine Sea and Japan Sea. However, no DNA evidence has been generated to support this. Context and purpose of the study: In this study, Pacific bluefin tuna caught North of Polilio Island in the Philippine Pacific Seaboard, was identified through DNA barcoding using mitochondrial DNA D-loop region and cytochrome oxidase 1 gene. Findings: The results show clustering of Philippine-caught bluefin tuna D-loop and CO1 sequences with published reference sequences for T. orientalis and supported by pairwise distances <0.034. Conclusion: This study provides the first DNA evidence on the occurrence of T. orientalis in Philippine waters. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: Since Kudoa septempunctata in olive flounder (Paralichthys olivaceus) was indicated to cause food poisoning in humans, other Kudoa species are suspected to have pathogenic potential. Recently, a myxosporean possibly associated with food poisoning in humans consuming raw Pacific bluefin tuna, Thunnus orientalis, was identified as Kudoa neothunni. This is a known causative myxosporean of post-harvest myoliquefaction in yellowfin tuna Thunnus albacares. Regardless of the significant differences in the 28S rDNA sequence and the pathological character (with/without myoliquefaction) between the two T. orientalis and T. albacares isolates, they were considered intraspecific variants of K. neothunni. However, the light and low-vacuum electron microscopic observations in the present study revealed that there were two morphotypes; pointed- and round-type spores, which were significantly differentiated by the ratio of suture width to spore width. Furthermore, the two morphotypes were genetically distinguishable by the 28S rDNA sequence analysis. This morphological and molecular evidence validates that the two Kudoa types are separate species, and thus the pointed- and round-types are referred to as K. neothunni and Kudoa hexapunctata n. sp., respectively. K. neothunni was detected solely from T. albacares, whereas K. hexapunctata n. sp. was found not only from T. orientalis but also from T. albacares. [Copyright &y& Elsevier]

Abstract: We found aporocotylid larval stages (sporocysts and cercariae) from five individuals of terebellid polychaete Terebella sp., which were collected from seabed substrate and ropes and floats attached to tuna cages in a tuna farm on the coast of Tsushima Island, Nagasaki, Japan. Nucleotide sequences of the regions of internal transcribed spacer 2 ribosomal DNA and 28S ribosomal DNA from these larval stages were 100% identical to those of Cardicola opisthorchis registered in GenBank. C. opisthorchis is a pathogen causing blood fluke infection of Pacific bluefin tuna Thunnus orientalis, which is considered to have a significant impact on the Japanese Pacific bluefin tuna aquaculture industry. This is the first description of the intermediate host of C. opisthorchis. This indicates that the life cycle of C. opisthorchis is completed within tuna farms in this area. [Copyright &y& Elsevier]

A feeding experiment was conducted to evaluate the efficacy of, and determine the requirements for dietary AsA ( l-ascorbic acid) using its popular derivative l-ascorbyl-2-monophosphate-Mg (APM) for juvenile Pacific bluefin tuna (PBT). The five test diets (with APM-0, -400, -800, -1200 and -1600) were prepared by adding 0, 400, 800, 1200 and 1600 mg of APM kg diet, respectively. These diets, together with chopped sand lance (the control diet), were administered, in duplicate to 0.27 g PBT at 25 days after hatching (300 juveniles per 15 m tank). The PBT were fed to apparent satiety 6 times a day for 2 weeks. While final body weight, feed conversion and survival showed no differences among diets APM 400-1600, PBT fed APM-0 indicated lower growth performance together with anorexia, dark pigmentation and ataxia from 5 days and reached 80 % mortality at 12 days after the start of the feeding trial. In addition, liver and brain ascorbic acid (AsA) concentrations increased with increasing dietary APM levels, the values were similar in PBT fed diets with APM-1200 and APM-1600, and liver and brain concentrations of AsA were significantly higher in these fish than in the PBT fed the other diets ( P < 0.05). The PBT responded to increasing dietary APM, and the fish fed APM-1200 had the highest weight gain, protein and lipid content. The data indicate that PBT can utilize APM as an AsA source and suggest that dietary supplementation with APM-1200 (454 mg AsA kg diet) would result in satisfactory growth. [ABSTRACT FROM AUTHOR]

Abstract: Although full-cycle aquaculture of Pacific bluefin tuna, Thunnus orientalis, has been established by Kinki University, Japan, most juvenile die after transfer to net cages offshore due to the stress associated with fluctuations in environmental conditions. We examined the effect of environmental conditions on mortality of juvenile using a generalized liner model (GLM). Juveniles were reared for more than 300days in a net cage and number of dead fish was examined over time and in relation to water temperature, salinity, moon phase, and water transparency. Mortalities soon after transferring the fish from the indoor rearing tank to the net cage occurred independently of environmental factors and were considered to be due to the handling stress and lack of acclimation. In subsequent periods, fluctuations in water temperature were the most critical factor affecting juvenile survival. Our results showed that optimal water temperatures for juvenile bluefin tuna aquaculture are considered to range between 15 and 25°C. Salinity and lunar irradiance (half moon to full moon) fluctuations had less marked effects on juvenile survival. Juveniles in net cages cannot move to avoid those unfavorable environmental fluctuations and this inability to locate areas of optimal temperature may complicate their body temperature maintenance because the capacity for endothermy in juveniles is undeveloped. We propose that sites selected for culturing juvenile Pacific bluefin tuna have water temperatures ranging from 15 to 25°C with constant temperatures that avoid large fluctuations. [Copyright &y& Elsevier]

Abstract: A new aporocotylid blood fluke is described, based on specimens from the ventricle of the Pacific bluefin tuna, Thunnus orientalis (Temminck et Schlegel), cultured in Wakayama and Nagasaki Prefectures, Japan. The new species is morphologically similar to the members of the genus Cardicola Short, 1953, but shows distinct differences in the body form, location of the testis and the orientation of the ootype. The body of the new species is long and slender, whereas other Cardicola species are small and generally lanceolate. The testis is mostly located posterior to the caeca and anterior to the ovary, occupying 31–45% of body length, in contrast to the known Cardicola species, whose testis is typically intercaecal. The ootype is oriented anteriorly, while in most congeners, it is directed posteriorly or horizontally. Phylogenetic analyses of this aporocotylid, together with Cardicola orientalis Ogawa, Tanaka, Sugihara et Takami, 2010 from the same host, were conducted based on DNA sequences of the ITS2 rDNA and the 28S region of ribosomal RNA. The analyses revealed that the new blood fluke belongs to the genus Cardicola despite the marked morphological differences. Thus, this aporocotylid is named Cardicola opisthorchis n. sp. and the generic diagnosis is emended in this paper. In addition, 100% identity among the ITS2 sequences from the present species, Cardicola sp. from T. orientalis in Mexico and Cardicola sp. from the northern bluefin tuna, Thunnus thynnus (Linnaeus) in Spain suggests that C. opisthorchis n. sp. has a broad geographical distribution and that it infects both the Pacific and northern bluefin tuna. [Copyright &y& Elsevier]

Abstract: 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 0.5mm 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)μm in width and 7.50 (6.7–8.6)μm 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. [ABSTRACT FROM AUTHOR]

The article discusses a study which investigated the natal origin of Pacific bluefin tuna Thunnus orientalis inferred from otolith oxygen isotope composition. The study demonstrated that otolith oxygen isotope values varied from -2.5 to -2.2% between the otolith core for age-0 tuna caught in the Japan Sea, while those caught in the Pacific Ocean varied between -2.8 and-2.2%. It was also observed that the temporal difference in experienced water temperatures produces an earlier increase of otolith oxygen isotope values for tuna caught in the Japan Sea than in the Pacific Ocean.

During the parasitological survey of cultured juvenile Pacific bluefin tuna (PBT) Thunnus orientalis in 2007 and 2008, two myxosporeans and one microsporidian were found. Morphological and molecular analysis showed that the heart-infecting and brain-infecting myxosporeans are identified as Kudoa shiomitsui and K. yasunagai, respectively. This is a new host record for both species. High prevalence of infection (77-100%) with K. shiomitsui was observed in October to December (1-2 months post-transfer to sea cages), whereas only a few fish were infected with K. yasunagai. A microsporidian observed as white 'cysts' in the trunk muscle of PBT had a resemblance to Microsporidium seriolae, the causative parasite of beko disease in yellowtail. Phylogenetic analysis showed that the microsporidian from PBT is closely related to but distinct from several other muscle-infecting species such as M. seriolae, Microsporidium sp. RSB, and Microsporidium sp. SH. Additionally, the spore dimension (2.7 × 1.5 μm on average) was remarkably smaller than the others. These results suggest that the present microsporidian is an undescribed species and designated Microsporidium sp. PBT. Prevalence and intensity of infection with Microsporidium sp. PBT reached a maximum of 100% and 20 cysts/fish, respectively. Although pathogenic effects of the two Kudoa species on fish health remain unknown, the microsporidian could be of concern to PBT aquaculture due to unsightly cysts in the musculature, reducing the market value of the fish. [ABSTRACT FROM AUTHOR]

Abstract: A new sanguinicolid blood fluke, Cardicola orientalis n. sp., is described from the afferent branchial artery and heart of Pacific bluefin tuna Thunnus orientalis (Temminck & Schlegel) cultured in Mie and Nagasaki Prefectures, Japan. The new species is most similar to C. ambrosioi Braicovich, Etchegoin, Timi et Sardella, 2006 from the Brazilian flathead, Percophis brasiliensis Quoy & Gaimard, but can be differentiated by the position of the female genital pore (in midline or slightly sinistral in C. orientalis vs. sinistral in C. ambrosioi) and much longer distance between male and female genital pore (101μm vs. 27μm). In wet mount preparations of infected fish, eggs were accumulated in great numbers in the gill lamellae and afferent filament arteries. Importance of this blood fluke infection of cultured Pacific bluefin tuna in Japan is discussed. [Copyright &y& Elsevier]

Abstract: Age determination of wild captured Pacific bluefin tuna, Thunnus orientalis, was conducted using sagittal otoliths of 806 specimens (47–260cm in fork length) caught in the waters off Japan and Taiwan. Otoliths were transversely sectioned and the opaque and translucent zones were analyzed. Opaque zones mainly appeared on the otolith edge from April to July, indicating that the opaque zone is formed annually. The opaque zones formed during later life (age 10+) were more distinct than the earlier zones. The estimated ages of specimens ranged from 1 to 26 years. Parameters of the von Bertalanffy growth function were estimated to be 249.6cm, 0.173, and −0.254 years for L ∞ , k, and t 0, respectively. Growth of younger fish was rapid up to 5 years old attaining about 150cm, and then growth rate decreased. After that, fish attained about 200cm at 9 years old and about 225cm (90% of L ∞ ) at 13 years old (50% of maximum age). This paper updates the biological information on length at age with a large size range to support stock assessment model analyses for this commercially valuable species. [Copyright &y& Elsevier]

Abstract: The amino acid sequences of three pepsinogens (PG1, PG2 and PG3) of Pacific bluefin tuna (Thunnus orientalis) were deduced by cloning and nucleotide sequencing of the corresponding cDNAs. The amino acid sequences of the pre-forms of PG1, PG2 and PG3 were composed of a signal peptide (16 residues each), a propeptide (41, 37 and 35 residues, respectively) and a pepsin moiety (321, 323 and 332 residues, respectively). Amino acid sequence comparison and phylogenetic analysis indicated that PG1 and PG2 belong to the pepsinogen A family and PG3 to the pepsinogen C family. Homology modeling of the three-dimensional structure suggested that the remarkably high specific activity of PG2 toward hemoglobin, which had been found previously, was partly due to a characteristic deletion of several residues in the S1′-loop region that widens the space of the active site cleft region so as to accommodate protein and larger polypeptide substrates more efficiently. Including the tuna and all other fish pepsinogen sequences available to date, the molecular phylogenetic comparison was made with reference to evolution of fish pepsinogens. It was suggested that functional divergences of pepsinogens (pepsins) occurring in fishes as well as in mammals, correlated with differences in various aspects of fish physiology. [Copyright &y& Elsevier]

In bluefin tuna aquaculture, collision of juveniles with the tank or net walls is a major cause of high mortality. This problem may be related to color sensibility of the visual mechanisms of this species. As a first step in understanding of color vision of Pacific bluefin tuna Thunnus orientalis, we applied a molecular technique and histology to study cone cell distribution in the retina of juvenile fish. We isolated three cone opsin genes encoding one blue-sensitive ( SWS2) and two green-sensitive ( RH2) visual pigments. In situ hybridization revealed that SWS2 mRNA is localized in the single-cone photoreceptors. The localization of the two RH2 signals in distinct cone cells was not determined, probably because of the high homology between the two RH2 genes. Single-cone photoreceptors appeared frequently in the ventral–temporal retina in approximately 80-mm fish and in the temporal retina in approximately 230-mm fish. These cone distributions may define a visual field with best color contrast vision in front and above the fish with a short wavelength (blue) reflecting target (sensed by single cones), and may be enhanced against the longer wavelength (green) background when fish see a target below them (sensed by double cones). [ABSTRACT FROM AUTHOR]

Abstract: We followed three patches of larval Pacific bluefin tuna (Thunnus orientalis) using Lagrangian drifters in 2004 and 2005 from May to June and determined the age-specific larval instantaneous mortality rates, which were estimated to be between 0.06 and 2.75day−1. The mortality rates differed significantly between patches and between ages within the same patch, which were higher than most of the estimations for other marine fish larvae. The results indicate that for the purpose of understanding the process of the larval survival, the use of larval patches is appropriate as a unit of observation. Evidence of the occurrence for net avoidance is presented and also it is concluded that net extrusion was observed for the larvae. Length-specific catch probability of the larvae was estimated to correct for length distribution biases derived from the net avoidance and net extrusion. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

The digestive enzyme activities of Pacific bluefin tuna Thunnus orientalis were evaluated for specific activity and characterized for pH and temperature optima in crude extracts of stomach, caecal mass, and proximal, middle and distal intestine. A higher level of alkaline proteolytic activity was detected in the caecal mass than in the proximal intestine. Total alkaline proteases, trypsin, chymotrypsin and leucine aminopeptidase (LAP) were tested. The temperature and pH analyses showed that proteolytic activity as well as lipase were maximal in the alkaline range, with a maximum at pH 9.0 and at temperatures between 35 and 60°C, except for the pepsin, which showed maximum activity at the same temperatures but in the acid range (pH 3.0). The α-amylase activity showed a broader range in activity, both for pH and temperature, with higher activity over the alkaline pH values and higher temperature. The lipase activity seems to be nondependent on bile salts under our assay conditions, resulting in a significant activity reduction in the presence of bile salts. This knowledge will allow the development of a gastrointestinal model (everted intestine) where food or feed will be hydrolysed with the fish’s own enzymes, a project that is being undertaken in our laboratory as a contribution to the development of novel diets for tuna fish. [ABSTRACT FROM AUTHOR]

Pacific bluefin is a highly valuable pelagic species that inhabits a broad range in the North Pacific Ocean. The reproductive biology, especially for the spawning aggregation in the south-western North Pacific Ocean, is not well understood. Thus, a total of 119 paired ovary specimens were collected from the Taiwanese longline fleet during the 1999 fishing season (late April through June) to gain a better understanding of important reproductivity-related stock parameters associated with this species. The following conclusions were made: (i) condition factor decreased from late May to early June; (ii) the sex ratio might be 1:1 for spawners; (iii) the gonadosomatic index stayed at a relatively high level and markedly increased from late May to early June; (iv) histological examination of oocytes indicated that all specimens were sexually mature; (v) spawning activity appeared to start in May and peak in late May to early June; (vi) batch fecundity increased with fork length; and (vii) preliminary estimates of spawning frequency between batches ranged 2-4.5 days based on analysis of postovulatory follicles. [ABSTRACT FROM AUTHOR]

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 (Δρ>0.0099). The small density difference between larvae and rearing water (Δρ=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 (Δρ=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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Blood fluke infection is one of the main causes of mortality in cultured juvenile Pacific bluefin tuna Thunnus orientalis (PBT) in Japan. We investigated 532 wild juvenile PBT of four different fish groups caught for tuna culture seed stock from off the Goto Islands, Nagasaki, Japan between 2011 and 2014, to determine whether they were infected with blood flukes. We detected the blood flukes, Cardicola orientalis and C. opisthorchis , from all four fish groups investigated. From this, it is suggested that bluefin tuna blood flukes are brought into tuna farms with the infected wild seed stock. [ABSTRACT FROM AUTHOR]

Highlights: [•] The juvenile PBT suffers high mortality after stocking in sea net cages. [•] Half the number of juveniles had died within the first 4 days. [•] One-third of the dead fish had ingested inorganic matter such as wood or Styrofoam waste during the first 7 days, and the dead fish showed poor growth. [•] Improvement in feeding method can increase the survival rate of juveniles in sea net cages. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

The market for Pacific bluefin tuna (Thunnus orientalis) is expanding as global consumption increases. Meeting this demand has led to overfishing and consequent depletion of wild populations, resulting in a focus on aquaculture as an alternative to fishing. However, under the crowded conditions typical of tuna aquaculture settings, infectious diseases are a common problem associated with considerable economic losses, especially with regard to Pacific bluefin tuna, which is a high-priced species. Studying diseases in Pacific bluefin tuna requires techniques that enable experimental infection; however, no promising methods are available due to the difficulty of handling these fish, including transportation, retention, and injection. In the present study, therefore, we developed a method for experimental infection of juvenile Pacific bluefin tuna with red seabream iridovirus (RSIV), which causes an acute and highly contagious disease in this species. The route of experimental infection was validated using Pacific bluefin tuna and greater amberjack (Seriola dumerili), which is used as a model species susceptible to the disease. Oral infection with the virus was found to be suitable for reproducing red seabream iridoviral disease (RSIVD). A high copy number of the RSIV genome was detected in Pacific bluefin tuna that died following RSIV experimental infection, indicating that the virus efficiently replicated in the infected fish. In addition to virus detection, a histopathologic abnormality specific to RSIV infection was detected in the infected fish. These results indicate that experimental infection via the oral route is effective for reproducing RSIVD in Pacific bluefin tuna. • The experimental infection route for red seabream iridovirus (RSIV) was determined. • Oral infection was optimal for reproducing RSIV disease in Pacific bluefin tuna. • RSIV efficiently replicated in RSIV-infected Pacific bluefin tuna. • Infected tuna exhibited RSIVD-specific histopathologic abnormalities. [ABSTRACT FROM AUTHOR]

Pacific bluefin tuna (Thunnus orientalis) is one of the most important species for aquaculture in Japan. In recent years, the reduction in muscle fat content during sexual maturation in farmed T. orientalis has become a serious problem. Herein, as a first step toward sterilization to avoid the deterioration of flesh quality associated with sexual maturation, we examined cold-shock conditions for inducing triploidy in T. orientalis. In this study, three treatment variables (initiation timing, duration, and water temperature of the cold-shock treatment) were considered, using the fertilized eggs collected immediately after spontaneous spawning in a land-based tank. We found that the optimal cold-shock conditions for T. orientalis were 5 °C for 15 min starting from 5 min after fertilization. Under these optimal conditions, we obtained high triploidization (96.6 ± 3.3%) and total hatching (64.5 ± 8.7%) rates. Therefore, using eggs obtained by spontaneous spawning in a land-based tank and the triploidization method established in this study, it is possible to repeatedly and efficiently produce triploid T. orientalis. In recent years, deterioration of flesh quality associated with sexual maturation in aquaculture-produced Thunnus orientalis has become a serious problem. To avoid this problem by sterilization technique, in this study, we have developed an efficient method for the induction of triploidy in T. orientalis , using the fertilized eggs collected immediately after the spontaneous spawning in a land-based tank. We believed that the results of present study are a valuable first step toward the sterilization of T. orientalis. • We established a technique for the mass production of triploid Pacific blue tuna. • The optimal condition was 5 °C for 15 min starting from 5 min after fertilization. • High triploidization (96.6%) and hatching rates (64.5%) were obtained. [ABSTRACT FROM AUTHOR]

Simple Summary: In this study, we detected Kudoa hexapunctata in Pacific bluefin tuna (Thunnus orientalis) individuals that did not show any gross pathology lesions. Giemsa staining was used to identify clearer lesions than haematoxylin and eosin (H&E) staining that is used in general histological analysis. K. hexapunctata was separated through molecular biological methods, other than haematolocial and histological analysis. Individuals infected with K. hexapunctata showed relatively low haemoglobin (Hb) and haematocrit (Ht) values, and histological analysis revealed clear pseudocysts in the abdominal and dorsal muscles. The consumption of fish and shellfish worldwide is steadily increasing, and tuna is a particularly valuable fish species. However, infection caused by Kudoa spp. is causing problems in many fish including the Pacific bluefin tuna (Thunnus orientalis), and there is much controversy about the association of these infections with foodborne disease. In this study, using haematological and histological analyses of the blood and internal organs (liver, spleen, kidney, heart, stomach, intestine, gill, and muscle) of Pacific bluefin tuna cultured in South Korea, infection with Myxosporea was first identified, and molecular biological analysis was conducted. In this study, Kudoa hexapunctata was finally identified. The Pacific bluefin tunas analysed in this study did not show any gross pathology lesions, such as visible cysts and/or myoliquefaction, of infection with this species. The histological analytical results can provide guidelines for the identification of K. hexapunctata. In the case of K. hexapunctata-induced infection, unlike other countries, such as Japan, there have been no reports in South Korea, and this study is the first to detect K. hexapunctata infection in Pacific bluefin tuna cultured in South Korea. The correlation between K. hexapunctata and food poisoning is not yet clear, however, it is thought that continuous observation of its infection is necessary. [ABSTRACT FROM AUTHOR]