Evidence of spawning among Pacific bluefin tuna, Thunnus orientalis, in the Kuroshio and Kuroshio–Oyashio transition area

Reproductive characteristics are one of the key factors for understanding population dynamics in fishes. This study describes the spawning habitat of Pacific bluefin tuna (Thunnus orientalis) in the Kuroshio and the Kuroshio–Oyashio transition area. A total of 551 adult fish (fork length > 120 cm) were collected during 1994–2007 (except in 2001 and 2003) and 2016. Based on a histological analysis of the gonads, we concluded that actively spawning females were mainly present in late May to early July. Relative batch fecundity was significantly lower than that reported in the Sea of Japan but was similar to that observed in the Nansei area. Whereas most spawning adults in the Sea of Japan are 3–6 years old and those in the Nansei area are over 10 years old, the majority of the specimens in this study ranged from 160 to 180 cm in length, which corresponds to 6–8 years of age. Our results indicate that Pacific bluefin tuna spawn in the study area and provide important data for understanding the spatiotemporal differences in spawning strategy with growth in this species

Distributional and reproductive aspects of the bigeye thresher shark (Alopias superciliosus) in the Atlantic ocean

Captura asociada a la pesquería de palangre de superficie dirigida a pez espadaThe bigeye thresher shark, Alopias supercilious is sometimes caught as bycatch in pelagic longline fisheries targeting tunas and swordfish in the Atlantic Ocean. As part of an ongoing cooperative program for fisheries and biological data collection, fishery observer data from various fishing nations and projects were compiled and analyzed. Those data sets include information on geographic location of the observations, as well as size, sex and in some cases maturity stage. A total of 4371 bigeye threshers were recorded throughout the Atlantic Ocean between 1992 and 2013, with the sizes ranging from 70 to 305 cm FL (fork length). Considerable variability was observed in the catchat- size, with particular emphasis on the tropical region where the mean sizes tended to be smaller than in the other regions. The expected distribution of juvenile and adult specimens also showed considerable variability, and the sex-ratios varied between regions and size classes. Maturity ogives were fitted to data from 642 specimens, with the median sizes at maturity estimated at 208.6 cm FL (corresponding to 349.1 cm TL) for females and 159.2 cm FL (corresponding to 269.8 cm TL) for males. In addition, a segmented regression model (SRM) was used for males, and two breakpoints (Bk1: 122.5cm FL, Bk2: 173.3cm FL) estimated, identifying transitions between the three different maturity stages for male sharks (immature, maturing and mature). Only a few pregnant females were recorded, always with the presence of two embryos (one per uterus), and were distributed predominantly in the tropical northeast Atlantic closer the African continent, and in the southwest region, with those regions possibly serving as nursery areas for this species. These reproductive parameters, and especially the estimated median sizes at maturity and low fecundity, highlight the vulnerability of this species, reinforcing that the bigeye thresher tends to mature at a larger size than the other species of the Alopiidae family. The biological and distributional patterns presented can help managers adopt more informed and efficient conservation measures for this species.En prensa0,000

A Simplified Gas Chromatographic Fatty-Acid Analysis by the Direct Saponification/Methylation Procedure and Its Application on Wild Tuna Larvae

A method for the direct preparation of fatty‐acid methyl esters (FAME) was simplified for fatty‐acid analysis of a single fish larva using gas chromatography (GC). The method included the isolation of a larval trunk and drying in a glass vial, followed by saponification of all the contents without prior lipid extraction. Thereafter, the fatty acids released were methylated by trimethylsilyldiazomethane. This method has advantages over another method, direct acid‐catalyzed transesterification, because both the saponification and methylation at room temperature can reduce loss of unsaturated fatty acids and formation of artifacts unavoidable in acidic reaction at high temperature. GC of the products showed that the simplified method can yield methyl esters without artifacts interfering analysis. More than 50 fatty acids were determined, which are twice as many as those previously analyzed using high‐performance liquid chromatography. Observation of consistent small impurities in GC of blank tests allowed the accurate determination of fatty acids by correcting the peak areas. Dry matter weights (<3 mg) and the total fatty‐acid contents displayed a linear relationship. Fatty‐acid analysis of wild larvae of bluefin tuna, yellowfin tuna, and skipjack tuna collected from the waters around Japan (n = 100) revealed that the eicosapentaenoic acid (EPA) level in bluefin tuna collected from the Japan Sea was significantly higher than that in the three species collected from Nansei Islands. The simplified direct saponification/methylation method will be a powerful tool for investigating growth and survival of individual larval tuna and other fish species

Geographic, seasonal and ontogenetic variations of δ15N and δ13C of Japanese sardine explained by baseline variations and diverse fish movements

&lt;p&gt;&lt;span&gt;Understanding and predicting variability in the stable isotope ratios of nitrogen and carbon (δ15N and δ13C, respectively) of small pelagic fish is crucial to enable isotopic studies of a variety of marine predators that feed on them. However, because the isotope ratios reflect plastic feeding habits and fish migration in addition to baseline variation, their predictions require a mechanistic understanding of how each factor contributes. Here, we investigated the habitat-wide variability of δ&lt;sup&gt;15&lt;/sup&gt;N and δ&lt;sup&gt;13&lt;/sup&gt;C of the Japanese sardine &lt;em&gt;Sardinops melanostictus&lt;/em&gt; in the western North Pacific and its marginal seas (the East China Sea and the Sea of Japan). By combining this with the archived particulate organic matter (POM) dataset as a baseline, we aimed to understand how ecological processes and baseline fluctuations affect isotope ratios of the sardine. Both δ&lt;sup&gt;15&lt;/sup&gt;N and δ&lt;sup&gt;13&lt;/sup&gt;C of sardine showed significant geographical and seasonal trends, with higher values in southern nearshore areas, including the Seto Inland Sea, intermediate values in marginal seas and lower values in Pacific offshore areas. As the variations were largely consistent with the geographic and temporally integrated seasonal trends of isotope ratios of POM, respectively, the baseline variations are the main determinant of sardine isotope composition. The trophic levels of sardine are therefore not significantly different between regions, with possible minor increases in the southern nearshore area. Adults showed less geographic variation than larvae and juveniles, likely due to slower turnover periods and wider migration ranges. Although larval and juvenile isotope ratios in marginal seas mostly reflected the local baseline, those in the Pacific offshore often reflected the baseline in the neighbouring southern region, suggesting contrasting juvenile movements between regions. Our results suggest that the δ&lt;sup&gt;15&lt;/sup&gt;N and δ&lt;sup&gt;13&lt;/sup&gt;C of Japanese sardine strongly reflect baseline variations, but can also be influenced by life-stage- and region-dependent fish movements, thereby demonstrating both the possibility and difficulty of mechanistically modelling the isoscapes of lower trophic level species.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;Funding provided by: Ministry of Agriculture, Forestry and Fisheries&lt;br&gt;Crossref Funder Registry ID: https://ror.org/02zdz1m23&lt;br&gt;Award Number: &lt;/p&gt;&lt;p&gt;Funding provided by: Fisheries Research Agency&lt;br&gt;Crossref Funder Registry ID: https://ror.org/02gmwvg31&lt;br&gt;Award Number: &lt;/p&gt;&lt;p&gt;&lt;span&gt;Tissues were freeze-dried and ground into powder. Lipids were extracted from all samples using a 2:1 chloroform:methanol solution, freeze-dried again, and 800 μg of a subsample was extracted for isotope analysis. The δ&lt;sup&gt;15&lt;/sup&gt;N and δ&lt;sup&gt;13&lt;/sup&gt;C values of the samples were determined at Fisheries Resources Institute (Yokohama, Japan) or GeoScience Laboratory (Nagoya, Japan) using a continuous-flow stable isotope ratio mass spectrometer (IsoPrime100, Elementar, Stockton, UK; Delta Plus Advantage, Thermo Fisher Scientific, Waltham, Massachusetts, USA) coupled to an elemental analyser (vario MICRO cube, Elementar; FLASH2000, Thermo Fisher Scientific, Yokohama Japan). The δ&lt;sup&gt;15&lt;/sup&gt;N and δ&lt;sup&gt;13&lt;/sup&gt;C values were reported in δ-notation against the atmospheric N2 standard and the VPDB reference standard (Vienna Pee Dee Belemnite), respectively, and given as a ‰ value. Analytical accuracies were ± 0.2‰ for δ&lt;sup&gt;15&lt;/sup&gt;N and δ&lt;sup&gt;13&lt;/sup&gt;C in both laboratories. The agreement of the reported values between the two laboratories was tested using a blind standard (powder of fish eye lens) where the differences in the reported values for both δ&lt;sup&gt;15&lt;/sup&gt;N and δ&lt;sup&gt;13&lt;/sup&gt;C were less than the analytical precisions.&lt;/span&gt;&lt;/p&gt