Pacific Bluefin Tuna Thunnus Orientalis Research Articles

Identification of cymothoid isopods probably responsible for mass mortality of hatchery-reared Pacific bluefin tuna Thunnus orientalis juveniles after transfer to sea cages

Identification of cymothoid isopods probably responsible for mass mortality of hatchery-reared Pacific bluefin tuna Thunnus orientalis juveniles after transfer to sea cages

Effects of triploidization on light sensitivity in larval stage of Pacific bluefin tuna Thunnus orientalis.

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.

Spawning performance of 3-year-old Pacific bluefin tuna Thunnus orientalis exposed to a phase-shifted photoperiod regime

The effects of a phase-shifted photoperiod regime to advance maturation and spawning were examined in 3-year-old Pacific bluefin tuna Thunnus orientalis. Fish in three broodstock groups reared in sea cages were separately transferred at age 2 years to land-based indoor tanks, in June or July, in each of the three trial years; they were then exposed to a 2- or 2.5-month advanced photoperiod cycle. In all three trials, when the fish were 3 years old, the spawning was initiated 2–3 months before the normal spawning season. During the continuous spawning, the fish were maintained under a constant long photoperiod of 15 h light and water temperature of 24–25 °C. In the 2019/2020 trial, 114 spawning days were recorded between March and July 2020 in a group of 28–30 females and 23–28 males; in the 2020/2021 trial, 100 days were recorded between April and July 2021 in a group of 30–32 females and 20–22 males; and in the 2021/2022 trial, 138 days were recorded between April and September 2022 in a group of 6–8 females and 7–10 males. The number of eggs produced daily peaked 1–2 months after the onset of spawning and thereafter decreased. Egg diameter showed a negative relationship with days elapsed from the onset of spawning. The analysis of parentage assignment of the larvae in the 2019/2020 trial showed that at least 50% of females and 75% of males contributed to the spawning, whereas histological observation of gonads in the 2020/2021 trial showed that 33% of females completed vitellogenesis and 85% of males reached spermiation. The analysis of parentage assignment also showed that the spawning interval of females declined between March and June, and the mean number of eggs produced daily per female peaked in April and decreased until June. Our observations demonstrate that manipulation of a phase-shifted photoperiod regime is a powerful technique for advancing maturation and spawning in the Pacific bluefin tuna at age 3 years.

Lateral bone ridge expansion and internal tissue replacement for vertebral body growth in Pacific bluefin tuna Thunnus orientalis.

Vertebral growth is an essential developmental process to support the expansion of the vertebrate body. In teleosts, the lateral side of the vertebral bodies develops to form different structures among species in the late stages of vertebral growth, although lateral structures are not apparent in the early stages. Lateral structures are one of the structural features that determine the diversity of teleost vertebrae. However, explanations for the formation of lateral structures are conflicting because few reports have investigated the growth of teleost vertebral bodies. To clarify the growth process, we analyzed the morphological changes in the vertebral body of Pacific bluefin tuna Thunnus orientalis at different developmental stages using micro-computed tomography(CT) scans. The micro-CT scans showed that the vertebral centrum formed a plate-like ridge on the lateral side along the cranial-caudal direction and extended laterally with increasing thickness. Simultaneously, the proximal region of the lateral ridges became porous as the vertebrae grew to form bone marrow cavities. Furthermore, we used histological observations to describe the relationship between these morphological changes and osteoblast and osteoclast activities. Osteoblasts accumulated on the distal edges of the lateral ridges, whereas osteoclasts were distributed in the bone marrow cavities. These observations suggest that bone resorption occurs proximally to form bone marrow cavities in addition to bone synthesis at the edges of the lateral ridges. The bone marrow cavities were occupied by blood vessels, extracellular matrix, and adipocytes, and the internal tissue composition changed to increase the area of adipose tissue. Because the ratio of bone volume decreases in large vertebrae, bone formation and resorption are regulated to separate the external cortical and internal trabecular bones to support the vertebrae. This study is the first to report the formation of lateral structures and can be applied to similar lateral structures in the vertebrae of other teleost species.

Nighttime Lighting Influences on the Plankton Feeding and Growth of Juvenile Pacific Bluefin Tuna, Thunnus orientalis.

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.

Revisiting the spawning fraction of Pacific bluefin tuna (Thunnus orientalis) caught in the Sea of Japan based on diurnal changes in spawning markers

The spawning fraction (SF) is an important reproductive trait used to assess the reproductive potential of indeterminate spawning fishes. To estimate the SF more accurately, the durations of spawning markers in ovaries need to be determined. The SF of the Pacific bluefin tuna Thunnus orientalis (PBF) in the Sea of Japan (SOJ) is higher than the SFs in its other main spawning grounds. The present study aimed to revisit the SF of the PBF in the SOJ by focusing on diurnal changes in spawning markers and estimating the duration of spawning markers in PBF ovaries. According to diurnal changes in the postovulatory follicle (POF) degeneration process and final maturation process of oocytes, POFs were observed in ovaries up to 38.5 h after ovulation. The revisited total SF throughout the spawning season was 0.34–0.43 in Area 1 and 0.50–0.64 in Area 2; these values are smaller than the SF previously estimated in the SOJ. The estimated SF of the PBF in the present study was equal to or slightly higher than that in the water between the Nansei Islands, Japan, and the northeast of the Philippines and higher than that in the Kuroshio-Oyashio transition area.

Otolith geochemistry reflects life histories of Pacific bluefin tuna.

Understanding biological and environmental factors that influence movement behaviors and population connectivity of highly migratory fishes is essential for cooperative international management and conservation of exploited populations, like bluefin tuna. Pacific bluefin tuna Thunnus orientalis (PBT) spawn in the western Pacific Ocean and then juveniles disperse to foraging grounds across the North Pacific. Several techniques have been used to characterize the distribution and movement of PBT, but few methods can provide complete records across ontogeny from larvae to adult in individual fish. Here, otolith biominerals of large PBT collected from the western, eastern, and south Pacific Ocean, were analyzed for a suite of trace elements across calcified/proteinaceous growth zones to investigate patterns across ontogeny. Three element:Ca ratios, Li:Ca, Mg:Ca, and Mn:Ca displayed enrichment in the otolith core, then decreased to low stable levels after age 1-2 years. Thermal and metabolic physiologies, common diets, or ambient water chemistry likely influenced otolith crystallization, protein content, and elemental incorporation in early life. Although similar patterns were also exhibited for otolith Sr:Ca, Ba:Ca and Zn:Ca in the first year, variability in these elements differed significantly after age-2 and in the otolith edges by capture region, suggesting ocean-specific environmental factors or growth-related physiologies affected otolith mineralization across ontogeny.

Natal origin of Pacific bluefin tuna Thunnus orientalis determined by SIMS oxygen isotope analysis of otoliths.

Accurate understanding of changing population dynamics associated with climate change is critical for managing highly migratory fish species. However, long-term data on population dynamics and the resulting recruitment variability is still lacking for many species, making it difficult to predict and integrate the effects of ocean warming into management schemes. In this study, high-resolution stable oxygen isotope (δ18O) analysis was performed on the otoliths of adult Pacific bluefin tuna Thunnus orientalis using secondary ion mass spectrometry (SIMS) to determine the natal origin of an individual fish. The core δ18Ootolith corresponding to the larval stage greatly varied among the individuals, indicating that the larvae experienced a wide range of thermal environments. The non-hierarchical cluster analysis performed on the core δ18Ootolith grouped fish into those with higher δ18Ootolith (lower temperature) and those with lower δ18Ootolith (higher temperature), most likely representing relative temperature difference experienced between fish born in the Sea of Japan and in the Nansei Islands area. The Nansei Islands area cluster showed more variability in the early otolith growth indicating a longer spawning season, which is consistent with the observed longer spawning duration in this area. The absolute temperature estimates based on the SIMS-measured core δ18Ootolith were significantly higher than those expected from sea surface temperature data, suggesting the effects of matrix-related bias on the temperature offsets. The relative temperature difference, however, matched well with the known spawning temperature range of the two spawning grounds. The recruitment contribution from each spawning ground (all year-classes pooled, n = 51) was 45% in the Sea of Japan and 55% in the Nansei Islands area. Overall, this study demonstrated the effectiveness of SIMS δ18Ootolith analysis for investigating the natal origin of fish and its potential application in fish population dynamics studies.

Recent and historical data show no evidence of Pacific bluefin tuna reproduction in the southern California Current system.

Despite their broad distribution across the North Pacific Ocean, the only known spawning grounds for Pacific Bluefin Tuna (Thunnus orientalis) are around coastal Japan and the East China Sea. However, an increase in the prevalence of large bluefin tuna up to 10 years old in the California Current System during exceptionally warm ocean conditions has led to speculation that they may be spawning in this region. To investigate this possibility, we collected samples from 36 females (estimated 3–8 years old) between 2015 and 2019. Histological analyses revealed no signs of imminent, active, or recent spawning. Further examination of historical ichthyoplankton collections showed no records of larval bluefin tuna, but confirmed the presence of the larvae of other tuna species in waters > 24°C. Fishery-dependent records showed that bluefin tuna are rarely recorded in purse seine catches where surface temperatures exceed 23°C. Our study, therefore, provided no evidence of bluefin tuna reproduction in the CCS. However, more comprehensive sampling, in particular off southern Baja California, may be required to confirm the absence of spawning.

Effects of Non-Heated and Heat Processed Krill and Squid Meal-Based Diet on Growth Performance and Biochemical Composition in Juvenile Pacific Bluefin Tuna Thunnus orientalis

This study investigated the effects of krill and squid meal and their heat processing on the growth performance and biochemical composition of juvenile Pacific bluefin tuna (PBT) Thunnus orientalis. An experiment using a 2 × 2 factorial design examined the effects of two dietary protein sources (squid and krill meal) and heat treatment (heated and non-heated). Prey fish were provided to a reference group. Fish with an initial mean weight of 74.1 mg were fed one of the five diets. After six days of the feeding trial, the fish fed with krill meal and non-heated diets showed improved growth compared to those fed with the squid meal and heated diets. Fish fed the non-heated diets showed significantly higher whole-body crude protein and crude lipid contents than fish fed the heated diets. These results suggest that nutrient availability could be improved by using krill meal and the non-heated treatment to improve the growth performance of juvenile PBT.

Peruvian fish meal has comparative potential to enzyme-treated Chilean fish meal as protein source of diet for larvae and juvenile Pacific bluefin tuna Thunnus orientalis

Peruvian fish meal has comparative potential to enzyme-treated Chilean fish meal as protein source of diet for larvae and juvenile Pacific bluefin tuna Thunnus orientalis

Association between fatty acid signature and growth rate of larval Pacific bluefin tuna in two major spawning grounds

Fatty acid composition and total fatty acids of field-caught Pacific bluefin tuna (PBF) Thunnus orientalis larvae were investigated to identify relationships between spawning ground, physiological condition, and growth rate. Multivariate statistical analyses revealed that both growth rate and environmental conditions at spawning grounds were substantially associated with variations in fatty acid compositions. Fast-growing larvae typically contained more α-linolenic acid and 22:5n-3, which are important as metabolic precursors of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Larval PBF fatty acid compositions differed by spawning ground; larvae caught around the Nansei Islands contained more 15:0, 17:0, 19:0, and 22:5n-6, and less EPA and arachidonic acid (ARA) than those from the Sea of Japan. Differences in larval odd-numbered fatty acid compositions might indicate different degrees of dependence on microbial loop energy supply between spawning grounds. Environments subject to sudden changes, such as those in water temperature and prey density in the Sea of Japan, might cause variability in fatty acid profiles, including extremely low %DHA. We suggest that continuous food intake and subsequent fatty acid catabolism for energy generation would be needed to facilitate fast growth.

Molecular cloning of myosin chaperone uncoordinate-45B (UNC-45B) and its heat stress response in the muscle of Pacific bluefin tuna Thunnus orientalis

Molecular cloning of myosin chaperone uncoordinate-45B (UNC-45B) and its heat stress response in the muscle of Pacific bluefin tuna Thunnus orientalis

Onset of nutrient consumption during early life stage digestive system development of two tuna species (Thunnus orientalis and Thunnus albacares)

To specify the timing of exogenous nutrient consumption in the larvae of two commercially important tuna species, the Pacific bluefin tuna (PBF) Thunnus orientalis and the yellowfin tuna (YFT) Thunnus albacares, the gene expressions of peptide transporter 1 (PEPT1) were examined. The mRNA expressions of PEPT1 first occurred at 2 days post hatching (dph) in PBF larvae and 3 dph for the YFT, and PEPT1 was found to only be expressed in the intestinal tract. The histological changes of the digestive tract of the YFT larvae were observed and compared to PBF larvae from a previous study. The intestines were developed at the hatching day for both species. It was found that the developmental timing of internal organs differed between the species, with the YFT showing an approximately one-day delay. The major organs such as liver, pancreas and gall bladder that excrete digestive enzymes appeared at 1 dph for PBF and 2 dph for YFT. The development of external morphological features was similar to organ development timings, with mouth-opening and first feeding starting at 2 dph for PBF, and 3 dph for YFT. Growth during the first month is rapid and variable for both species, ranging from 1.06 to 1.56 mm/d. Our findings provide new information about the early onset of feeding and larval development for the two species which would contribute to future aquaculture.

Seasonal changes and endocrine regulation of gonadal development in hatchery-produced Pacific bluefin tuna Thunnus orientalis broodstock in sea cages

We examined the annual reproductive cycle of both female and male hatchery-produced Pacific bluefin tuna (PBT, Thunnus orientalis) broodstock at 3 years of age, including the associated changes in plasma levels of sex steroid hormones and the gene expression of key endocrine factors. The females progressed to vitellogenesis in April, and approximately 50% of the females were observed in late vitellogenesis between June and July at water temperatures between 24.1 °C and 29.0 °C. Approximately 90% of the males were in spermiation for at least 7 months between April and October, which included the period when vitellogenic females were present. Furthermore, the gonadosomatic index of both females and males peaked between June and July. These results indicate that the reproductive season of hatchery-produced PBT peaks between June and July, which is in agreement with the reproductive season reported for PBT broodstock originating from wild-caught juveniles. Furthermore, the percentage of hatchery-reared females that reached late vitellogenesis in the present study was higher than that in wild-caught captive-reared females reported in previous literature. Analysis of plasma sex steroid levels and gene expression profiles suggested that oocyte and ovarian development were associated with 17β-estradiol (E2) control, whereas spermatogonial differentiation towards meiosis seemed to be mediated by 11-ketotestosterone (11-KT). Furthermore, the production of E2 and 11-KT is likely regulated by luteinizing hormone and follicle-stimulating hormone, respectively, via their related receptors. The findings of this study are expected to contribute to improvements of technologies for obtaining fertilized eggs from hatchery-produced PBT broodstock.

Estimation of tuna population by the improved analytical pipeline of unique molecular identifier-assisted HaCeD-Seq (haplotype count from eDNA)

Many studies have investigated the ability to identify species from environmental DNA (eDNA). However, even when individual species are identified, the accurate estimation of their abundances by traditional eDNA analyses has been still difficult. We previously developed a novel analytical method called HaCeD-Seq (Haplotype Count from eDNA), which focuses on the mitochondrial D-loop sequence. The D-loop is a rapidly evolving sequence and has been used to estimate the abundance of eel species in breeding water. In the current study, we have further improved this method by applying unique molecular identifier (UMI) tags, which eliminate the PCR and sequencing errors and extend the detection range by an order of magnitude. Based on this improved HaCeD-Seq pipeline, we computed the abundance of Pacific bluefin tuna (Thunnus orientalis) in aquarium tanks at the Tokyo Sea Life Park (Kasai, Tokyo, Japan). This tuna species is commercially important but is at high risk of resource depletion. With the developed UMI tag method, 90 out of 96 haplotypes (94%) were successfully detected from Pacific bluefin tuna eDNA. By contrast, only 29 out of 96 haplotypes (30%) were detected when UMI tags were not used. Our findings indicate the potential for conducting non-invasive fish stock surveys by sampling eDNA.

Characterization of digestive physiology in Pacific bluefin tuna Thunnus orientalis juveniles fed a raw fish feed and a commercial diet

This study examined the biochemical characteristics of key digestive enzymes in artificially produced Pacific bluefin tuna juveniles and revealed details of the digestive physiology of juveniles fed a commercial pellet diet and raw fish over long-term (225 days) and short-term (response to feeding) periods. pH and thermal profiles of key digestive enzymes of pepsin, trypsin, chymotrypsin, lipase and amylase were examined in juvenile bluefin tuna and the characteristics differed from those previously reported for larval and adult tuna. Long-term feeding (225 days) of the commercial diet markedly increased stomach and pyloric caeca size as well as storage/synthesis levels of the digestive enzymes. Highly increased gene expression levels of cholecystokinin (cck) and its receptor (cck1r) were also found in juveniles fed the commercial diet, suggesting cck might be involved in the gastrointestinal hypertrophy and digestion enhancement seen in the present study. In the short-term experiment (response to feeding), juveniles fed the commercial diet displayed faster gastric emptying, higher pH in the intestinal content and increased digestive enzymes storage/synthesis whereas the enzymes secretion was similar with those of juveniles fed raw fish. These indicate that juvenile bluefin tuna morphologically and physiologically adapted their digestive system to the artificial diet, however the adaptation was not sufficient to obtain a comparable performance as tuna fed raw fish feed. Biochemical characteristics of the digestive enzymes and the observed effects of artificial diet on digestive physiology could be good indicators to improve feeding strategies such as diet composition, feeding ration and frequency in juvenile tuna.

Differences in the reproductive traits of Pacific bluefin tuna Thunnus orientalis among three fishing grounds in the Sea of Japan

Differences in the reproductive traits of Pacific bluefin tunaThunnus orientalis(PBF) in the Sea of Japan (SOJ) were investigated to evaluate the reproductive potential in 3 fishing grounds (i.e. northern, central and southern areas). The size distribution of PBF differed among the fishing grounds, with smaller fish (100-130 cm in fork length, FL) being caught in the central and northern areas of the SOJ. Spawning-capable females were found at sea surface temperatures (SSTs) between 17 and 25°C, with most SSTs >21°C. Seasonal changes in SST differed among the fishing grounds, and the period when SST exceeded 21°C differed in each fishing ground. Spawning-capable females were found at different times among the fishing grounds, and were observed throughout the fishing season in the southern area of the SOJ. The mean ± SD relative batch fecundity was estimated to be 69.8 ± 45.3 and 92.7 ± 43.1 oocytes g-1in the central and southern areas of the SOJ, respectively. The total spawning fraction was 0.2, 0.38 and 0.73 in the northern, central and southern areas of the SOJ, respectively. SST and FL were positively correlated with the batch fecundity. In addition, FL, SST and location affected the probability of occurrence of spawning-capable fish among mature fish. Therefore, the difference in the observed reproductive potential of PBF among fishing grounds in the SOJ can be linked to the differences in seasonal increases in SST and the size distribution of adult fish.

Habitat use of adult Pacific bluefin tuna Thunnus orientalis during the spawning season in the Sea of Japan: evidence for a trade-off between thermal preference and reproductive activity

To examine the habitat usage of adult Pacific bluefin tuna (PBF), electronic tagging was conducted in the Sea of Japan during May and June of 2012-2017. Archival tags were internally implanted and pop-up satellite archival transmitting tags were deployed; data on the horizontal movements and diving behaviours of 36 individual PBF were successfully retrieved. In the summer spawning season, the tagged PBF were concentrated near Sado Island and Oki Island in the Sea of Japan, and they were distributed widely to the southwest (near Tsushima Island) or northeast (near the Tsugaru Strait) in the autumn and winter. We obtained the first long-term tracking record (246 d) for adult PBF, and this individual exhibited residency in a known spawning region during the spawning season in the proximity of warm-core eddy features. This fish spent most of the daytime below the thermocline between 30 and 150 m depths where the surface ambient temperature was 26.0 ± 1.5°C, but at night it ventured into the warm surface layer. Its whole-body heat transfer coefficient increased when it experienced warm waters (≥24°C), which we suggest is a physiological response to avoid overheating. The mean peritoneal cavity temperature was only 1.8°C higher than the ambient temperature, compared with 6.9°C higher during the cooler autumn-winter period. Our hypothesis is that the warm surface temperatures found in the spawning grounds induce a physiology-reproduction trade-off in adult PBF, which must behaviourally and physiologically thermoregulate their body temperature to gain spatial and temporal access to oceanographic conditions that may promote larval survivorship and growth.

Establishment of a tracing technique for transplanted bluefin tuna germ cells in recipient’s gonads using monoclonal antibodies specifically recognizing bluefin tuna spermatogenic cells

Our ultimate aim is to establish a small-bodied surrogate broodstock (e.g., mackerel) that produces functional gametes of the Pacific bluefin tuna Thunnus orientalis using xenogeneic germ cell transplantation. To accomplish this goal, we need to be able to sequentially trace transplanted donor cells in recipients. Here we succeeded in identifying two monoclonal antibodies that could specifically recognize the germ cells of bluefin tuna but not of the chub mackerel Scomber japonicus or the eastern little tuna Euthynnus affinis, which are promising recipient species. Using these antibodies, we developed a method to detect transplanted bluefin tuna germ cells in the gonads of xenogenic recipients. Since all stages of testicular germ cells can be recognized by these antibodies, donor germ cells can be observed throughout gametogenesis in the recipient’s gonads. Thus, the antibodies established in this study could be a powerful tool to observe survival, proliferation, and differentiation of the transplanted bluefin tuna germ cells in the gonads of small Scombridae recipients, and evaluate transplantation efficiency.