Reproductive biology of female Pacific bluefin tuna Thunnus orientalis from south-western North Pacific Ocean

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 incre ased with fork length; and (vii) preliminary estimates of spawning frequency between batches ranged 2–4.5 days based on analysis of postovulatory follicles.     

Partial characterization of the digestive enzymes of Pacific bluefin tuna <Emphasis Type="Italic">Thunnus orientalis</Emphasis> under culture conditions

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.     

Effect of docosahexaenoic acid enrichment in Artemia on growth of Pacific bluefin tuna Thunnus orientalis larvae

Feeding enriched Artemia induces growth failure in Pacific bluefin tuna (PBT) Thunnus orientalis larvae; however, feeding of yolk-sac larvae of marine fish promotes larval growth. It is considered that this growth failure partly results from dietary docosahexaenoic acid (22:6n-3, DHA) deficiency. Therefore, we examined the effect of DHA contents in enriched Artemia on the growth of PBT larvae. Artemia nauplii were enriched with graded levels of DHA ethyl ester, and fed to PBT larvae for 9 days. Yolk-sac larvae of Japanese parrotfish Oplegnathus fasciatus were used as a reference diet. The DHA contents in Artemia increased with the enrichment from 0 mg g^− 1 dry weight basis (DW) to 25 mg g^− 1 DW, while the content in the reference diet was 21 mg g^− 1 DW. Feeding of enriched Artemia significantly improved the growth of PBT larvae. However, this improvement was negligible when compared with the excellent growth of the larvae that were fed the reference diet. PBT larvae that were fed the reference diet accumulated approximately twice or much higher levels of DHA in the neutral and polar lipids in the body when compared with the larvae that were fed enriched Artemia. These results show that PBT larval growth cannot be promoted by feeding enriched Artemia even if the DHA contents in Artemia are elevated to the same levels as those of yolk-sac larvae. The incorporation of dietary DHA into phospholipids in the fish body may be desirable for the normal growth of PBT larvae.     

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

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

利用鳍条研究北太平洋长鳍金枪鱼的年龄与生长

根据2013—2014年在大连远洋渔业金枪鱼钓有限公司的超低温金枪钓渔船“天祥”16号船上采集的258个鳍条样品为基础(从845尾长鳍金枪鱼样本中随机选取了258个鳍条样本),研究了北太平洋长鳍金枪鱼的年龄和生长情况。研究首次采用了3个不同的切割位置(鳍条全长的10%、20%、50%标记段)对鳍条截面进行处理,利用线性函数、指数函数和幂函数分别拟合长鳍金枪鱼叉长和鳍条截面半径之间的关系,采用赤池信息准则(AIC)来获得最优化的函数,并计算其逆算叉长,最后得到北太平洋长鳍金枪鱼的生长方程、生长拐点以及生长率和生长指标。研究表明,北太平洋长鳍金枪鱼的主要年龄段集中在4~6龄,不分雌雄的生长方程为L_t=112.231×[1-e^{-0.277 132×(t+1.435)}],拐点年龄为2.53,生长率和生长指标随着年龄的增长呈递减的趋势,且年龄越大变化越小。本研究可为北太平洋长鳍金枪鱼的资源状况评估及渔业可持续发展提供生物学信息。     

Effects of local scale perturbations in the Atlantic bluefin tuna (Thunnus thynnus L.) trap fishery of Sardinia (W. Mediterranean)

Time series (1825–1973) of the Atlantic bluefin tuna, Thunnus thynnus, landings from the trap fishery of Sardinia (W. Mediterranean) were analyzed to identify (i) patterns of catch variability in the historical traps of Isola Piana, Portoscuso and Porto Paglia, and (ii) effects of three local environmental variables: Mining, Run-off and Sea Surface Temperature.We applied different time series analysis techniques: auto-correlation functions, spectral analysis and Dynamic Factor Analysis (DFA). Moreover, an asymmetrical design using a Before/After and Control/Impact location approach was used to test the perturbations due to mining processes that have characterized the fishery area.A sharp decreasing trend of Porto Paglia trap landings beginning in 1880, coincided with mechanized mining. This result was confirmed by the ANOVAs contrasting Before vs. After (P < 0.01) and Controls vs. Impact location (P < 0.01).We found a slight cyclical behaviour in the same time series. Spectral analysis highlighted that pattern synchronized with the Run-off cycles with periods corresponding to 7 years. We hypothesized that the effect of Run-off from watershed mine tailing dams creates a pulsing boundary ‘reflective’ effect for tuna migration schools on the in-shore trap of Porto Paglia, resulting in a periodic oscillation of captures.We have identified (i) a reversal of trends in captures between Portoscuso and Isola Piana traps and (ii) significant differences of the monthly distribution of capture events and timing of landings (P < 0.01) with a higher proportion now in May, rather than in June as in the past. We assume there is a relationship with climatic changes because warmer waters can induce precocity in gonad maturation. DFA did not show any significant differences among environmental factors and landings probably because the cyclical component in Run-off is masked. We emphasize the importance of Mediterranean traditional traps as ‘sentinels’ of the homing reproductive behaviour of bluefin tuna and we argue the case for a plan to link the remaining traps in a common monitoring network.     

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

Tuna aquaculture is currently dependent on the wild capture of juveniles for production. The development of hatchery technology for bluefin and other tunas would be a major step forward in improving sustainability of their aquaculture. The present study overviews the technology in the life cycle completion of the Pacific bluefin tuna (PBT) Thunnus orientalis (Temminck et Schlegel) under aquaculture conditions in Kinki University, and the problems to be solved for the establishment of tuna hatchery technology. On 23 June 2002, broodstock of PBT that were artificially hatched and reared spontaneously spawned in captivity. The resulting eggs hatched and were subsequently reared to the juvenile stage. The spawning fish were the result of a research project started in 1987 to rear wild‐caught juvenile PBT that were several months old. Fertilized eggs were obtained from these fish in 1995 and 1996. Resulting juveniles (the artificially hatched first generation) were reared to maturity and spawned in 2002. Over the summer of 2002, 1.63 million eggs from these fish were used for a mass rearing experiment, and 17 307 juveniles were produced and transferred to an open sea net cage. Of these artificially hatched second‐generation PBT, 1100 grew to approximately 95 cm total length and 14 kg body weight in 22 months. This procedure means the completion of PBT life cycle under aquaculture conditions, which was first attained among large tuna species. The problems awaiting solution in PBT hatchery production are their unpredictable spawning in captivity, to improve survival during the first 10 days post hatch, to reduce cannibalism in larval and juvenile stages, and to solve collision problem causing high mortality during the juvenile stage.     

Carrying capacity and survival strategy for the Pacific bluefin tuna, Thunnus orientalis, in the Western Pacific

The carrying capacity for the Pacific bluefin tuna at each life stage is estimated and its survival strategy is examined numerically, using a new method to define the hypothetical capacity, the standard population, and the search volumes that are necessary and are feasible for the tuna. The carrying capacity for the adult is estimated at 1–2 × 10⁶ individuals, which corresponds with 5–10% of the hypothetical capacity and is comparable with the maximum levels of the southern and the Atlantic bluefin tuna populations. It is hypothesized semiquantitatively that the migration at each life stage and the remarkable decrement of growth at 120 days and about 40 cm occur as an evolutionary response to population excess over the carrying capacity. It is also hypothesized semiquantitatively that the early larvae have minimal food available in the Subtropical Water and develop the predatory morphology, high growth rate, and high mobility, however, at the expense of a high mortality as an evolutionary response to the tuna spawning in the Subtropical Water. This method may be an available tool to not only investigate the carrying capacity and survival strategy of a specific fish species, but also predict when and in how much abundance the fish species occurs in a specific area of its habitat.     

Diel and ontogenetic body density change in Pacific bluefin tuna, Thunnus orientalis (Temminck and Schlegel), larvae

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.     

Relationship between prey utilization and growth variation in hatchery‐reared Pacific bluefin tuna,Thunnus orientalis (Temminck et Schlegel), larvae estimated using nitrogen stable isotope analysis

In mass culture of Pacific bluefin tuna Thunnus orientalis, a marked growth variation is observed after they start feeding at 6–7 mm in body length (BL) on yolk‐sac larvae of other species, and the growth variation in tuna larvae is a factor leading to the prevalence of cannibalism. To examine the relationship between prey utilization and growth variation, nitrogen stable isotope ratios (δ¹⁵N) of individual larvae were analysed. A prey switch experiment was conducted under two different feeding regimes: a group fed rotifers (rotifer fed group), and a group fed yolk‐sac larvae of spangled emperor, Lethrinus nebulosus (fish fed group) from 15 days after hatching (6.87 mm BL). The fish fed group showed significantly higher growth than the rotifer fed group. Changes in the δ¹⁵N of the fish fed group were expressed as an exponential model and showed different patterns from those of the rotifer fed group. The δ¹⁵N of fast‐growing tuna larvae collected in an actual mass culture tank after the feeding of yolk‐sac larvae was significantly higher than those of the slow‐growing larvae, indicating that slow glowing larvae depended largely on rotifers rather than the yolk‐sac larvae.     

Sizes of adult bluefin tuna Thunnus orientalis in different areas of the western Pacific Ocean

ABSTRACT:   The length frequencies of adult Pacific bluefin tuna Thunnus orientalis caught near Japan, near Taiwan, in the tropical western Pacific Ocean, and in the South Pacific Ocean, were compared. The largest catches consisted of medium-sized adults (160–209 cm fork length) taken near Japan and large adults (≥210 cm) taken near Taiwan. Small adults (120–159 cm) were occasionally caught in the Sea of Japan. Small numbers of medium-sized and large adults were caught in the tropical western Pacific and the South Pacific. The length frequencies of the fish greater than 160 cm in length caught in different areas around Japan were similar to each other. As adult bluefin tuna grow older they spawn earlier, and their distribution, movements, and spawning areas change.     

Promotion of initial swimbladder inflation in Pacific bluefin tuna,Thunnus orientalis (Temminck and Schlegel), larvae

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 (17.8–7.5%). 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.     

Influence of initial swimbladder inflation failure on survival of Pacific bluefin tuna,Thunnus orientalis (Temminck and Schlegel), larvae

This study investigated the influence of initial swimbladder inflation (ISI) failure on survival of Pacific bluefin tuna (PBT), Thunnus orientalis, larvae to prevent mass mortality due to sinking death. In Experiment 1, swimbladder inflation frequency and survival within an ISI promoted (PS) group, for which surface film of rearing water was removed, and a group without ISI promotion (NPS) were compared in 20 and 30 kL tanks. The PS group demonstrated significantly higher swimbladder inflation frequency and increased survival than the NPS group within 20 kL tanks at 9 days post hatching (dph). Similar tendencies were observed within 30 kL tanks. In Experiment 2, larval distribution and swimbladder inflation frequency in the night‐time were examined through larval sampling within the upper and middle layers of the water column and tank bottom within 1.6 and 30 kL tanks at 5 dph. Larvae at tank bottom had higher distributional density and significantly lower swimbladder inflation frequency than those distributed in the upper and middle layers within 1.6 kL tanks. Similar tendencies were observed within 30 kL tanks. Results of this study suggest the improvement of larval survival due to prevention of sinking death through ISI promotion in mass‐scale PBT larviculture.     

北太平洋金枪鱼延绳钓钓钩垂直分布及浸泡时间对渔获物的影响

为积累黄鳍金枪鱼 (Thunnus albacares) 幼鱼陆基循环水养殖基础数据,测定了不同体质量的黄鳍金枪鱼幼鱼血清、肌肉、肝脏、胃、肠道等组织的酸性磷酸酶 (ACP)、碱性磷酸酶 (AKP)、淀粉酶、脂肪酶、蛋白酶等活性。结果显示,ACP活性在各组织中由高至低为肠道>肌肉>肝脏>血清,AKP活性则为肠道>肝脏>肌肉>血清。随着体质量的增加,ACP活性在肌肉、肠道和肝脏组织中差异较小,而AKP活性在肝脏中呈逐渐上升的趋势,且相邻两组间差异显著 (P<0.05),其在肠道、肌肉中呈波动性变化;黄鳍金枪鱼幼鱼消化酶活性整体稳定,组间差异不显著 (P>0.05),仅在体质量达1 000 g时肠道胰蛋白酶活性显著下降 (P<0.05),随后小范围浮动,组间差异不显著 (P<0.05)。综上,ACP和AKP活性在黄鳍金枪鱼幼鱼组织中较高且稳定,这种稳定的高活性模式可保证机体对磷 (P) 高效分解、吸收、再利用,提供对外源物的持续免疫力;黄鳍金枪鱼在养殖期间营养需求稳定,且蛋白质需求旺盛。     

Tank culture of yellowfin tuna, Thunnus albacares: developing a spawning population for research purposes

A land-based culture facility for research on yellowfin tuna, Thunnus albacares, was developed at the Achotines Laboratory in the Republic of Panama. Six concrete tanks, and seawater and life support systems were built to maintain a yellowfin broodstock. On average, 50% of the yellowfin caught survived capture and handling, and approximately 30% became broodstock in Tank 1 (17 m diameter, 6 m depth) or Tank 2 (8.5 m diameter, 3 m depth). Each fish was tagged with a microchip implant tag, then weighed, measured, and injected with oxytetracycline (OTC) prior to stocking. Daily rations of primarily market squid, Loligo opalescens, and Pacific thread herring, Opisthonema spp., were regulated based on the feeding activity and energy requirements of the fish. Feeding activity of the broodstock decreased when the water temperatures decreased, and the fish ate decreasing daily rations and increasing calories with increasing size. Spawning occurred in both tanks within 6–8 months of capture. Spawning first occurred in Tank 1 when 24 females ranged in size from 6 to 16 kg and 65 to 93 cm fork length (FL). Spawning was intermittent during the first 2 months and occurred near daily thereafter. Tank size appeared to affect survival rates, the types of mortalities that occurred, and the growth of the fish. Survival rates after 1 year in captivity were higher, and the fish were larger, on average, in Tank 1 than in Tank 2. Most of the mortalities in Tank 1 were the result of wall strikes, which occurred more frequently after the fish reached their highest density of 0.64 kg m⁻³ and sizes greater than 96 cm FL and 19 kg. Non-linear growth models were fitted to the initial stocking sizes and final sizes of fish that died or were removed from Tank 1 during 1996–1999. Estimated growth rates in length (11–48 cm year⁻¹) for fish between 51 and 150 cm FL decreased with increasing length. Estimated growth rates in weight ranged from 9 to 19 kg year⁻¹ for fish less than 19 kg and 20–23 kg year⁻¹ for fish greater than 19 kg. The results of this work demonstrate that the stable environment of a land-based culture facility may be the preferred system for long-term maintenance of a yellowfin broodstock.     

北太平洋长鳍金枪鱼卵巢的发育特征

根据2013年10月–2014年2月在北太平洋海域(29°08′~41°08′N,163°50′~144°19′W)采集的364尾长鳍金枪鱼的卵巢样本,利用组织学分析,详细描述了长鳍金枪鱼卵巢、卵细胞的发育阶段。结果显示,北太平洋长鳍金枪鱼卵巢内同时存在不同时相的卵细胞,为分批产卵类型;组织学上,长鳍金枪鱼的卵细胞发育过程分为6个时相,卵巢发育过程分为6个时期;卵巢成熟指数在成熟期为Ⅰ~Ⅴ期时逐渐增大,在Ⅵ期时减小;北太平洋长鳍金枪鱼产卵高峰为12月中旬和1月初,其卵巢成熟指数随纬度的升高呈递减趋势,随经度变化规律不明显。研究表明,通过对北太平洋长鳍金枪鱼卵巢的发育特征的分析与探讨,可为北太平洋长鳍金枪鱼的资源状况评估及渔业可持续发展提供生物学信息。     

Long‐term changes in recruitment of age‐0 Pacific bluefin tuna (Thunnus orientalis) and environmental conditions around Japan

Recruitment of age‐0 Pacific bluefin tuna (Thunnus orientalis) from 1952 to 2014 was examined by a sequential regime shift detection method. The regime shifts in recruitment were detected in 1957, 1972, 1980, 1994 and 2009. The durations of regime shift ranged from 8–15 years and averaged 13.0 years. In both the total (1952–2014) and data rich (1980–2014) periods, negative relationships were found between recruitment and the Pacific Decadal Oscillation in autumn, and positive relationships were found between recruitment and sea surface temperature (SST) anomalies in the northern part of the East China Sea, in the southwestern part of the Sea of Japan, and in the waters off Shikoku and Tokai in summer and autumn. The 1994 and 2009 regime shifts in recruitment occurred in the same years as shifts in SST anomalies in the northern part of the East China Sea in summer. These results suggest that the ocean conditions in the northern part of the East China Sea are closely related to recruitment of Pacific bluefin tuna, and that the warmer conditions result in higher recruitment of the species.