This study provides updated information on the growth and maturation of longtail tuna (Thunnus tonggol) in the East China Sea and the western Sea of Japan. Around Japan, longtail tuna is caught in warmer southern and western waters, and the catch amount has been gradually increasing because of rising water temperatures caused by climate change. In this study, age was determined by using otoliths; otolith rings were counted independently by two readers. The precision of age determination was evaluated using the index of average percent error, which was 6.1%. We applied three growth models—von Bertalanffy, Richards, and Schnute–Richards—using a Bayesian estimation. The Schnute–Richards model produced the lowest DIC values among the candidate models, which is consistent with the rapid early growth observed in this species. As a result, the fork length at one year of age was estimated to be approximately 450 mm. Microscopic observations of the gonads indicated that the spawning period was from May to September; the gonadosomatic index increased during the same period. Hydrated eggs and postovulatory follicles were also observed during this period, demonstrating that spawning was occurring in this area. Individuals with a fork length of approximately 450 mm were first observed in a spawning-capable condition, suggesting that some individuals may contribute to reproduction at approximately one year of age. These findings should help with stock assessment and management of this species.

This study provides the first comprehensive long-term analysis of demersal fish community structure in the East China and Yellow seas using Japanese bottom trawl fishery data spanning four decades (1982–2023). Using nonparametric multidimensional scaling, we identified five distinct fish assemblages with spatial segregation patterns that changed significantly over time.Fish communities showed progressive spatial shifts, with clusters dominated by economically valuable species such as flatfishes and Japanese common squid in the Yellow Sea, and large Sciaenidae and conger pike along coastal China, undergoing substantial range contractions during the 1980s–1990s due to intensive fishing pressure. Sequential changes in target species were observed as fishing pressure transitioned from high-value species to alternatives including largehead hairtail, cuttlefish, and swordtip squid following initial species declines. Japanese fishing grounds reduced to waters near Japan after the mid-1990s due to international fishery agreements, coinciding with community structure changes. Current Japanese operations target yellow and red seabreams with stable catches under reduced fishing effort. This analysis demonstrates that fishing pressure and fleet redistribution due to international agreements considerably influenced demersal fish community dynamics. Future research requires multinational collaboration integrating data from China, the Republic of Korea, and Japan for comprehensive regional assessment of transboundary marine ecosystem changes.

The reproductive output of older repeat spawners is generally considered as a critical factor contributing to the productivity and stability of fish stocks. This study aimed to elucidate the seasonal and spatial changes in the reproductive activities of middle-aged and older adult Pacific bluefin tuna (Thunnus orientalis, PBF). Reproductive traits and the influence of environmental factors were examined in the waters between the Kuroshio-Oyashio transition area (KOTA) in northeastern Japan and the Nansei Islands, which are known spawning grounds for older PBF. The fork length (FL) of females in the Nansei Islands was significantly larger than that in KOTA, although the FL distributions in both spawning grounds overlapped, ranging from 160 to 230 cm. Spawning fish were first observed in the southern region of the Nansei Islands in April. As the season progressed, their distribution expanded northward, reaching approximately 40 degrees N in July. The spawning season differed between KOTA and the Nansei Islands due to seasonal changes in sea surface temperature (SST). Batch fecundity (BF) in the Nansei Islands was significantly higher than that in KOTA, and both FL and SST positively affected BF variation. The size at 50% maturity, estimated from samples collected in the waters between KOTA and the Nansei Islands, was 140 cm FL. These results suggest that the reproductive traits of PBF in the study region change seasonally, and that relatively larger adults tend to spawn with high BF in the Nansei Islands, where SST conditions favour egg hatching and larval growth.

ABSTRACT Understanding how pelagic fish assemblages reorganize in response to oceanographic variability is central to fisheries oceanography. We analyzed long‐term purse‐seine logbook data (1994–2022) from the East China Sea and Sea of Japan to link multivariate assemblage structure with species‐specific occurrence and habitat responses. Seasonal nonmetric multidimensional scaling (nMDS) ordinations followed by k ‐means clustering revealed pronounced temporal reorganization of assemblages, including contractions of sardine‐dominated groups and a marked northward expansion of Japanese amberjack. To explore the environmental factors underlying these shifts, we fitted generalized additive models (GAMs) for nine pelagic taxa. To evaluate robustness to fishing effort, additional CPUE‐based GAMs were also examined as a robustness check. Variable‐importance analyses showed that broad‐scale spatial variation (captured by the spatial smoother, representing geographic and oceanographic structure) was the dominant predictor of species occurrence, followed by subsurface temperature at 50 m. Thermal responses varied among taxa, including dome‐shaped responses indicating optimal temperature ranges, monotonic increases in warm‐affinity species, and negative responses in cold‐affinity species such as chub mackerel. The shelf indicator further distinguished species with contrasting affinities for coastal versus off‐shelf habitats. Predicted spatial distributions from the GAMs were broadly consistent with multivariate assemblage patterns, providing a complementary perspective on species distributions within the same dataset. These findings highlight the value of integrating assemblage‐level analyses with species‐specific occurrence‐based habitat analyses to understand climate‐driven changes in pelagic fish communities and to support ecosystem‐based, climate‐responsive fisheries management.

ABSTRACTJapanese jack mackerel (Trachurus japonicus) is subject to substantial and escalating commercial harvesting activities, yet its habitat structure is poorly understood. We analyzed the carbon and nitrogen stable isotope ratios (δ13C and δ15N) of this species to evaluate geographical fluctuations in these ratios, allowing a delineation of the jack mackerel feeding habits. Fish specimens were collected across the East China Sea (ECS) and Sea of Japan (JS), and isotopic data were retrieved from two previous studies. Six subareas were considered: the southern, central, and northern ECS; Tsushima Strait; and eastern and western JS. The residuals of isotope ratios predicted from fork length showed significant geographical variation in the ratios (a lower δ13C in the JS than in the ECS and Tsushima Strait; a lower δ15N in the eastern JS and southern ECS than in other areas). Comparisons of stable isotope ratios between jack mackerel and prey candidates also revealed geographical differences in potential prey selection. The potential prey was identified as anchovy larvae and larger Copepoda in the western and eastern JS, respectively, whereas various prey species contributed to the diet in the other areas. Both δ13C and δ15N increased with fork length, suggesting ontogenetic changes in foraging habitat, associated with habit shifts into demersal layers of jack mackerel during their growth. Collectively, our results suggest that stable isotope ratio values—and consequently, the feeding habits—of jack mackerel differed between the ECS and JS but that they possess a limited migration range after an ontogenetic shift to demersal layers.

Stable isotopes facilitate the comparison of isotope niches across populations, allow for the estimation of community metrics related to dietary preferences, enable the integration of diet over various time scales, and assist in identifying functional groups as well as isotopic niche overlap among populations. Tagging programs are commonly used to study the habitats and migration patterns of fish. However, the diversity of demersal fish species is high, and archival tagging instruments are expensive. Isoscapes of stable isotope ratios of carbon (delta C-13) and nitrogen (delta N-15) in demersal species were generated using 1802 measured values of demersal species in the East China Sea. We then used generalized additive models (GAMs) to assess explanatory variables, such as feeding habits, body form, and season. The high values of delta C-13 and delta N-15 areas were observed in the central East China Sea due to the Changjiang River plume. These isoscapes can be used to estimate habitats and migration of demersal species. High trophic niche overlap ratios (> 0.5) were observed among crustaceans, fish, benthos, gelatinous, and plankton feeders, which were separated from other groups. The species with the highest trophic position was the Longtooth grouper (TP = 4.4; Epinephelus bruneus), and the lowest was Okamejei acutispina and Psenes maculatus (TP = 3.2). Such trophic niche overlap ratios and trophic positions are useful for the construction of ecosystem-based fishery management.

Geographical and species differences in fatty acid composition were evaluated among 546 specimens of small pelagic fish, micronekton, and squid in the Northwestern Pacific. For all species except for four micronekton species, the dominant fatty acid was docosahexaenoic acid (DHA). The ratios of ω3 to ω6 fatty acids and DHA to eicosapentaenoic acid (EPA) differed among species and organisms. ω3:ω6 and DHA:EPA were generally lower in micronekton than in squids and small pelagic fishes. The ω3:ω6 values in the northeastern area was higher than that in the southwestern area, meanwhile the DHA:EPA values in the northeastern area was lower than that in the southwestern area. Geographical differences in fatty acid components, DHA:EPA, and ω3:ω6 could have been caused by environmental factors as well as by regional differences in the quality and quantity of phytoplankton-produced organic matter. Non-metric multidimensional scaling of the main fatty acid components showed clear species and geographical differences. A permutational multivariate analysis of variance revealed significant differences among small pelagic fishes, micronekton, and squids. These results indicate that fatty acid analysis can be used to understand the ecology and ecosystem roles of various species.

In fisheries management, accurate estimates of fish stock abundances are crucial for sustainable harvesting practices. Traditional methods often rely on catch-per-unit-effort (CPUE) data, assuming fishing effort is uniformly distributed across the stock range. However, this assumption is often violated, leading to potential biases in CPUE-based abundance indices (AI). In the present study, we focused on chub mackerel stock in the East Asian Marginal Seas (EAMS), where shifting fishing grounds and ocean warming have raised concerns regarding the reliability of the nominal CPUE trend. We developed a spatiotemporal machine learning approach to predict the CPUE values while taking into consideration environmental variables and changes in fish distribution. Our model accounts for unexploited areas, thereby addressing the sampling biases inherent to traditional CPUE analyses. The results suggest that recent declines in the nominal CPUE observed in Japan do not reflect the actual stock declines but instead reflect biases due to shrinking fishing areas. These findings highlight the need for more sophisticated methods in fisheries management to ensure sustainable practices and highlight the importance of considering environmental and distributional changes in fish stock assessments.

Red seabream (Pagrus major) is an important fisheries species in Japan, where it has been managed since 2025. Accurate stock assessment is necessary for rational fisheries management, and catch per unit effort (CPUE) is one of the key indexes used for this purpose. In the present study, the CPUE of red seabream in the East China Sea and Sea of Japan was standardized by incorporating seawater temperature and salinity based on fishing data obtained from bottom trawlers operating in key fishing grounds in these regions. We used data from operation reports available from 1982 to 2022, including fishing positions and efforts during each cruise and incorporated temperature and salinity data into the model from 1993. We applied a delta log-normal model for CPUE standardization and revealed an increase in this index since the early 1990s. We examined the horizontal distribution patterns of red seabream in each fishery grid that is 30 x 30 min latitude and longitude, and observed that the high-CPUE area extended northeastward from Taiwan to Japan along the shelf edge, which corresponds to the main fishing grounds covered by Japanese fleets after 2000. The probability of presence of red seabream increased drastically when water temperature at a depth of 50 m exceeded 15 degrees C. The reasons for the observed stock fluctuations may be a reduction in fishing pressure and warming seawater temperature.

Trace element analysis of otoliths from core to edge was used to reveal differences in habitats of larvae and early juveniles of Japanese jack mackerel (Trachurus japonicus) caught in the East China Sea (ECS), Pacific Ocean, and Sea of Japan. Multi-element signatures (Sr:Ca, Mg:Ca, Na:Ca, K:Ca, and Ba:Ca) were analyzed with multivariate statistics to determine whether these element signatures provide insight into regional population structure. The median values of elements: Ca ratios in otolith core region differed significantly among areas, with most elements between the ECS and the Pacific Ocean or Sea of Japan showing significant differences. The Sr:Ca ratios exhibited a V-shaped trend from the otolith core to the edge, which was likely related to the vertical habitat layer shift of larvae and early juveniles of T. japonicus. Canonical analysis of principal coordinates using the element: Ca ratios and the hatching day were re-classified according to the sampling area with higher accuracy in the ECS (88-95%) and the Sea of Japan (76-83%) compared to the Pacific Ocean (69-72%). These results indicate that the proportion of eggs, larvae, and early juveniles transported by the Kuroshio Current from the southern ECS-the main spawning ground-to the Pacific Ocean or the Sea of Japan is low and that juveniles distributed in the coastal waters off southwestern Japan may have originated from local spawners. These results suggest that determining the elemental composition of larvae and juveniles of T. japonicus is effective in determining the habitat differences of this species in the three regions. These findings will help to understand population structure and recruitment process of this species around Japan.

Pacific bluefin tuna (PBF) Thunnus orientalis have 2 major spawning grounds and migrate widely in the North Pacific Ocean. To understand the population structure and migratory ecology, we analyzed vertebral samples collected from a wide range of ages and areas and then estimated their natal ground from the first annulus in the vertebra, which does not change significantly after formation. Both spawning groups of PBF, including fish that originated from the Sea of Japan (assigned as group SJ) and from the waters around the Ryukyu Archipelago and Taiwan (group RT), were observed in all sampling areas and age classes. In younger age classes, the percentages of group SJ were higher around Japan, whereas those of group RT were higher in the eastern Pacific Ocean (EPO). The percentage of group RT decreases around Japan as they migrate to the EPO and then increases when they return. These results suggest a tendency toward different migration patterns depending on the natal area. Interestingly, the results suggest that fish from the EPO rarely migrate to the Sea of Japan. The percentages of group RT for age 10+ were similar and higher in all sampling areas, and these are considered to be the final percentages of the relative contribution of the 2 natal grounds. This is a useful approach that enables us to easily examine the relative contribution of the 2 spawning grounds across time and space, providing insights into the dynamics of movement around the Pacific based on variations in the population composition.

Vertical distribution of zooplankton abundance is essential for understanding the linking primary production to higher-trophic marine organisms. In this study, we investigated the zooplankton abundance at the near-surface layer (0-20 m depth; ZA20) and those at 0-200 m depth (ZA200) from 240 stations in the Sea of Japan (SOJ) and the East China Sea-Kuroshio (ECSK) area during summer to evaluate the environmental parameters controlling zooplankton accumulation in the near-surface layer. The total numerical ZA20 was 22 +/- 15% (mean +/- standard deviation) and 15 +/- 11% of ZA200 in the SOJ and ECSK, respectively. Temperature and chlorophyll a concen-tration at 20 m depth, as well as the sampling time, had a significant impact on the degree of surface accu-mulation (ZA20: ZA200 ratio) in a generalized additive model (GAM); the ZA20: ZA200 ratio increased in cold and eutrophic conditions and during the nighttime. The difference between SOJ and ECSK was insignificant, while the zooplankton communities were significantly different between the areas. Therefore, the vertical distribution of zooplankton is seamlessly explained by environmental parameters when setting the zooplankton as one ecological functional group. Members belonging to six orders, Calanoida, Harpacticoida, Cyclopoida, Ctenopoda (Penilia avirostris), Onychopoda (family Podonidae), and Copelata (appendicularian), were dominant based on microscopic observations. When the ZA20: ZA200 ratio was estimated at the order level, the responses to tem-perature differed between copepods and non-copepods groups: the copepods stay in the subsurface water when the surface water is warm.

The size and age structure of a population are crucial aspects for understanding population dynamics. Many marine fish species have experienced long-term reductions in size and age owing to overfishing and/or global warming; however, these aspects remain unexplored. The present study elucidated the spatiotemporal variations in chub mackerel size and age structure based on the operational reports of large- and medium-sized Japanese purse seine fisheries for nearly half a century. Our results revealed size- and age-dependent segregation at a fine spatial scale, which may indicate ontogenetic niche shifts in response to thermal and feeding conditions or intercohort competition. On a long-term temporal scale, a significant reduction in size and age was observed in the East China Sea, whereas a contrasting increase in size and age was observed in the northern Sea of Japan. This may indicate that large fish have greater sensitivity to global warming and shift their distribution more rapidly northward than do small fish. In addition, the reduction in size and age during the spawning season in the East China Sea may result in a bias towards younger spawners, potentially rendering population dynamics more unstable. This study highlights the importance of investigating the spatiotemporal variation in the size structure of exploited fish stocks.

The preferred habitats of the round scad Decapterus maruadsi and five other scads (amberstripe scad D. muroadsi, mackerel scad D. macarellus, red scad D. akaadsi, roughear scad D. tabl, and shortfin scad D. macrosoma) in the East China Sea (ECS) and the Sea of Japan were examined based on catch records of Japanese large- and medium-type purse seine fisheries from 1993 to 2020. Delta-lognormal generalized additive models indicated that the habitats of the round scad and the other five scads were clearly segregated. A high round scad catch per unit effort (CPUE) was observed in the southwestern ECS on the continental shelf and coastal waters around Kyusyu, whereas for the other five scads, a high CPUE was observed around the shelf break and Kuroshio area. The standardized annual CPUEs of the scads decreased after 2010. Based on the proportion of round scad to all six scads calculated by spatial standardized CPUEs for each fishery grid, the total catch amount of round and the other five scads including coastal fisheries was approximately 2,071-13,382 tons and 3,496-24,617 tons, respectively. These estimates are important for scad stock assessment and developing effective management strategies.

Diamond squid Thysanoteuthis rhombus (Troschel, 1857), which can grow to a dorsal mantle length of similar to 100 cm in 1 year, is an important fisheries resource in subtropical and temperate waters worldwide. Around Japan, it is an important resource in the East China Sea, Sea of Japan, and western North Pacific Subtropical Gyre, including the Kuroshio area. It is not known if linkages occur between these habitats. To clarify the species distribution, we estimated the habitat of young squid using a generalized additive model based on trawl surveys and analyzed carbon and nitrogen stable isotope ratios (delta C-13 and delta N-15) in tissue samples. Based on trawl observations, young squid were distributed around northeastern Taiwan and the Okinawa Islands in June-July and in the Sea of Japan and the Kuroshio extension areas in August-September. We observed ontogenetic changes in the stable isotope ratios. The delta N-15 values in large diamond squid (dorsal mantle length > 400 mm) were significantly lower in the Pacific than those in other areas. Considering that the delta N-15 of forage fish is low in the Pacific and high in the East China Sea and Sea of Japan, large diamond squid in the northwest Pacific presumably have two distinct habitats.

Catch per unit effort (CPUE) of commercial fisheries is recommended to be standardized because various factors can affect catch rates. Recently, the spatio-temporal analysis has been required to standardize fishery data, because catch is spatially distributed in proportion to fishery distribution and not in proportion to fish abundance. We developed the spatio-temporal models for the CPUE of large- and medium-sized purse seine fisheries for the different age groups of Japanese jack mackerel (Trachurus japonicus) to evaluate the potential spatial distribution and estimate the annual trends in abundance index. Japanese jack mackerel is widely distributed in the East Asian seas, including the East China Sea and Sea of Japan, and is an important fishery resource in Japan. Our models were delta-lognormal generalized additive mixed models that incorporated the temporal components of year and month, and size of fishing vessels as categorical variables, and the identity of the fishing vessels as a random variable. The spatial component represented by the latitude and longitude points of the fishing location was smoothed using an isotropic bivariate function. The water temperature at a depth of 50 m was incorporated to detect the effects of environmental factors. After all combinations of explanatory variables were evaluated, the model with the lowest Bayesian information criterion score was selected as the best model. All the best models among both binomial models and lognormal models contained all explanatory variables, although the best binomial model for age 2 exclude the vessel size and the best lognormal model for age 0 exclude the water temperature at a depth of 50 m. Our results indicate that the potential distribution areas for all ages of Japanese jack mackerel exist on the continental shelf in the southern part of the East China Sea, the western side of Kyushu Island, and along Honshu Island in the western part of the Sea of Japan. In contrast, the main distribution areas with higher density for age 0 were segregated into two: the southwestern East China Sea and the coastal area off Kyushu Island, although fish over age 1 were widely distributed. The standardized CPUE was higher than the nominal CPUE for age 0 from 2015 to 2020, and the standardized CPUE was also higher for age 1 in 2020. These results were derived from spatial models that compensated for the lack of spatial data due to the recent decrease in fishing operations in the Southeast China Sea. The yearly trends of standardized CPUE for each age could be used for the age-structured stock assessment model such as a tuned virtual population analysis. Improving the accuracy of the abundance index helps to further improve the reliability of stock assessments.

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.

This study elucidated the age–growth relationship, maturation body size, and stable isotope ratios of kawakawa Euthynnus affinis in the East China Sea. A total of 396 individuals were collected, of which 266 were captured off the west coast of Kyushu and 130 around Okinawa. Ages were determined from otolith analysis, and the estimated von Bertalanffy growth functions were FLt = 1007{1 – exp[–0.107(t + 3.835)]} near Kyushu and FLt = 640{1 – exp[–0.381(t + 1.111)]} around Okinawa, where FLt is fork length (mm) at age t for ages 0.25 to 5. The estimated fork lengths of the fish near Kyushu were larger than those around Okinawa. The smallest fork length of a fish in the spawning stages was about 440mm. Mature females and males were observed from May to October near Kyushu and from March to July around Okinawa. Females in the spawning stages were observed from July to September near Kyushu and from April to June around Okinawa. The δ15N of fish captured near Kyushu (12.39‰ ± 0.55‰, mean ± standard deviation) was higher than that of those captured around Okinawa (9.94‰ ± 1.39‰). This suggests that the kawakawa caught in each area occupy distinct habitats, although some mixing of individuals between the two habitats might occur.

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.