Seto Inland Sea Research Articles

周防灘に生息するマコガレイの夏季から冬季にかけての経験水温・深度および離底行動の観測

マコガレイ成魚30個体に水温・深度ロガーを装着し，2017年7月3日に周防灘姫島地先で放流して2個体から12-1月までのデータを得た。高水温となる9月の2個体の経験水温の最頻値は24-25℃（53.9-57.6％）であった。最高経験水温は27℃に達したが26℃以上の頻度は3.9-4.5％と低かった。深度データからは離底行動が観測され，連続した離底行動後に生息水深，生息水温，分布域等が変化した。よって，天然海域では26℃未満の水温帯で生息可能であり，離底行動は移動に関連していたと考えられた。

Capsalid monogeneans of fishes from the Seto Inland Sea, Japan: Description of Benedenia kobudai n. sp. parasitic on Semicossyphus reticulatus (Perciformes: Labridae)

The Seto Inland Sea, the largest inland sea in Japan, is a rich fishery with high biodiversity and productivity. Monogeneans have been studied for more than 120 years, and 58 nominal species have been recorded in the Seto inland Sea. This study provided DNA information on five species of Benedenia sensu lato (Capsalidae) from marine fishes from the sea, and one of them, Benedenia kobudai n. sp., is described from Semicossyphus reticulatus (Perciformes: Labridae). This new species differs from the other congeners by the hooded appearance between the anterior attachment organs, the morphology of the penis, the absence of the lobe near the vaginal pore and the common genital pore, the position of the vaginal pore, the germarium lying near the slightly hexagonal testes, the morphology of the haptor, and the shape and position of the hamuli. Phylogenetic analysis showed B. kobudai n. sp. in a separate clade from the other Benedenia species, B. epinepheli, B. hoshinai, B. sekii, and B. seriolae collected from the sea. Each of the newly provided DNA sequences (28S rDNA, ITS1‐5.8S-ITS2, cox1) of the above four species are based on specimens obtained from the type hosts and/or type localities and are considered important for future taxonomic re-examination and confirmation of the reliability of the registered sequences. Furthermore, these four species are important causes of fish diseases in aquaculture, and it is expected that information on the distribution, host range, and occurrence of fish diseases for each capsalid secured by molecular identification will be accumulated.The Life Science Identifier (LSID): urn:lsid:zoobank.org:pub:26C15D17-CFD1-450D-9FCE-EFFF692D2133.

Bottom-up processes drive reproductive success of Japanese anchovy in an oligotrophic sea: A case study in the central Seto Inland Sea, Japan

Anthropogenic and/or climate-driven changes in zooplankton dynamics may serve as a bottom-up regulator of productivity of small pelagic fish. In the central Seto Inland Sea of Japan, the abundance of the recruit fish of Japanese anchovy (Engraulis japonicus) has markedly declined in the last decade under oligotrophication. However, the link between the reproductive success of anchovy and environmental variation is largely unclear. This study explored the potential association between changes in the zooplankton (copepods) community structure and the reproductive condition of anchovy in this oligotrophic sea using survey data from spring to summer of 2001–2019. The community structure of copepod species changed markedly in May–July, covering the main spawning season of anchovy, as decreases in the biomass of several copepods found in cooler water in a given season (May–June) were observed. Although relationships between chlorophyll a concentrations and the copepod biomass were unclear, the recent decline of their cooler-water copepods may be attributed to the influences of elevated sea surface temperature and decreasing levels of chlorophyll a. The latter may be associated with decreasing nitrogen levels. Among these species, Calanus sinicus and Corycaeus affinis could be important prey items for anchovy from the analyses of prey selectivity and relationships between the relative condition factor of females and the copepod biomass. The egg size–temperature relationships of anchovy appeared to be associated with the relative condition factor of females and survival rates in the early stage of life, suggesting that females with a poor body condition produced smaller eggs in recent years. Maternal food manipulation experiments supported the assumption that larvae born to females fed a restricted food ration exhibited lower starvation tolerance and slower growth than born to females provided feed ad libitum. These findings suggest that the reproductive success of anchovy could be subject to bottom-up trophic drivers mediated by maternal contributions to offspring viability, and this partly explains why the level of anchovy recruitment has rapidly declined in the last decade.

Toxification characteristics of Japanese cockles and ark shells with paralytic shellfish toxins in southwestern Harima-nada, eastern Seto Inland Sea, Japan

Toxification characteristics of Japanese cockles and ark shells with paralytic shellfish toxins in southwestern Harima-nada, eastern Seto Inland Sea, Japan

Provenance of granitic gravestones in graveyard of feudal lords evaluated by multiple non-destructive rock analyses

• The Yamauchi clan gravestones were sourced from southwestern Kochi Prefecture, Japan. • Magnetic susceptibility is a handy indicator, but not enough to identify the origin. • Fraction and grain size of colored minerals are useful for determining provenance. Magnificent graveyards were constructed for feudal lords during the Edo period throughout Japan. The large gravestones of feudal lords, which measured up to several meters high, reflected the political and economic status of the people commemorated at these sites. While the locations of the quarries and the methods used to transport the stones that were used for gravestones also provide clues about the culture of the time, few documents describing the origin of the stones remain. There are several granite gravestones of feudal lords of the Yamauchi clan in Kochi Prefecture in southwestern Japan. However, there is some debate as to their origin and whether they are made of granite from southwestern Kochi Prefecture or Sanyo granite from areas around the Seto Inland Sea. In this study, we conducted multiple non-destructive rock analyses on the granitic gravestones of feudal lords of the Yamauchi clan to clarify their origin. These analyses included magnetic susceptibility measurements, grain-size analysis of colored minerals, and color measurement of K-feldspar. The results were compared with those for granite obtained from southwestern Kochi Prefecture and Sanyo granite. The K-feldspar in the gravestone was white in color. The magnetic susceptibilities of the granite blocks for the gravestones were similar, and followed a Gaussian distribution. The grain size of colored minerals in the granite followed a log-normal distribution, and the areal fraction and grain size of colored minerals were larger than those in other granites. Similarities in color, magnetic susceptibility, and grain size were observed with granite from southwestern Kochi Prefecture. For Sanyo granite, no granite had features that matched all of the measured characteristics. The results indicate that the granite used for the Yamauchi clan gravestones was most likely sourced from southwestern Kochi Prefecture. Stone structures in Kochi Prefecture that were built in the Middle Ages before the Edo period were previously considered to be derived from Sanyo granite. The results of this study provide important insights into whether the routes used to transport stone materials in Kochi Prefecture changed before the Edo period, and if so, what triggered this change. Further, the origin of medieval stone structures in Kochi Prefecture should be reevaluated.

Ancient rivers shaped the current genetic diversity of the wood mouse (Apodemus speciosus) on the islands of the Seto Inland Sea, Japan

The current distributions of organisms have been shaped by both current and past geographical barriers. However, it remains unclear how past geographical factors—currently cryptic on the sea floor—affected the current distributions of terrestrial animals. Here, we examined the effects of currently cryptic ancient rivers on current genetic differentiation of the large Japanese wood mouse, Apodemus speciosus, which inhabits islands in the Seto Inland Sea, Japan. Genome-wide polymorphisms were identified by GRAS-Di (Genotyping by Random Amplicon Sequencing, Direct) analysis of 92 A. speciosus individuals. Maximum-likelihood analysis was performed with 94,142 single nucleotide polymorphisms (SNPs) identified by GRAS-Di analyses. Ancient rivers were visualized by Geographic Information System (GIS) channel analysis. Maximum-likelihood analysis showed strong support for the monophyly of each population in the islands in the Seto Inland Sea; it also showed close relationships between Innoshima-Ikuchijima, Ohmishima-Hakatajima-Oshima, Ohmishima-Hakatajima, Ohsakikamijima-Ohsakishimojima, Kamikamagarijima-Shimokamagarijima, and Kurahashijima-Etajima islands. The principal component analyses of the SNPs also supported these relationships. Furthermore, individuals from islands located on the east and west sides of the main stream of the ancient river were clustered on each side with strong support. These phylogenetic relationships were completely congruent with the paleogeographic relationships inferred from ancient rivers. In conclusion, the findings demonstrated that the current distribution of genetically distinct island lineages was shaped by ancient rivers that are currently submerged beneath the Seto Inland Sea, Japan.

Reconstruction of the Eukaryotic Communities in Beppu Bay Over the Past 50 Years Based on Sedimentary DNA Barcoding

AbstractComprehensive reconstruction of changes in eukaryotic communities in the recent past is useful for determining the response of the local ecosystems to global changes during the Anthropocene. We used DNA barcoding technology to reconstruct the marine eukaryotic communities of Beppu Bay, the Seto Inland Sea, Japan, over the past 50 years based on a short sediment core. Highly vulnerable DNA fragments were preserved in the sediments, possibly due to seasonally euxinic conditions. Analysis of the 18S rRNA V9 gene region indicated the temporal variability in eukaryotic communities, which consisted mainly of dinoflagellates and diatoms, in response to changes in the nutrient regime. The dominant species in the dinoflagellate genus Alexandrium changed as the water temperature increased. In addition, enhanced contributions by terrestrial plants and mosses were detected in flood sediments. Our results suggest that DNA fragments can be used as a proxy for the paleoenvironmental and paleoecological conditions in Beppu Bay.

Study on life history traits of important fishery stocks in the Seto Inland Sea

Study on life history traits of important fishery stocks in the Seto Inland Sea

Enhanced Cross-Shelf Exchange Between the Pacific Ocean and the Bungo Channel, Japan Related to a Heavy Rain Event

A heavy rain event occurred in early July 2018 near the Seto Inland Sea. Despite clear weather after the heavy rain, the water temperature at 5 m depth on the eastern coast of the Bungo Channel decreased by approximately 3°C. Postulating that the water temperature decrease was related to the heavy rain event, we analyzed observed data during and after the heavy rain and conducted numerical experiments. We found that the surface water temperature decrease was caused by vertical mixing of cold water supplied from the Pacific Ocean in the bottom layer. Numerical experiments revealed that the bottom cold water from the Pacific Ocean was not controlled by the heavy rain event but was intensified by the heavy rain event. The lower salinity (lower water density) due to the heavy rain lasted until the end of August. This lower density intensified the gravitational circulation in the Bungo Channel for 2 months, which led to a 17.5% increase in inflow from the Pacific Ocean in the bottom layer and a 20% increase in the dissolved inorganic nitrogen flux to the Bungo Channel.

Interannual variation of a bottom cold water mass in the Seto Inland Sea, Japan

Abstract. A bottom cold water mass (BCWM) is a widespread physical oceanographic phenomenon in coastal seas, and its temperature variability has an important effect on the marine ecological environment. In this study, the interannual variation of the BCWM in Iyo-Nada (INCWM), a semi-enclosed bay in the Seto Inland Sea, Japan, from 1994 to 2015 and its response to air–sea heat flux change were investigated using monthly observational data and a hydrodynamic model. Surrounded by the isotherm of 18 ∘C, the observed multi-year average water temperature inside the INCWM was 17.58 ∘C with a standard deviation of 0.27 ∘C, while the mean area of INCWM was 5.73 × 105 m2 with a standard deviation of 4.35 × 105 m2. The interannual variation of average water temperature of INCWM showed a negative correlation with its area that indicates a low temperature corresponds to a big area. In addition, the interannual variation of the average temperature inside INCWM showed positive correlations with the local water temperature from April to July and with remote water temperature below 10 m in an adjacent strait in July. Differing from previously studied BCWMs, which had interannual variations depending closely on the water temperature before the warming season, the interannual variation of INCWM is more sensitive to the air–sea heat flux during the warming season than that in the previous winter. Further, by comparing several BCWMs, we found that the BCWM size is a key factor in understanding the heat transfer process responsible for the interannual variation of BCWMs in coastal seas. These findings will help us to understand the response of bottom cold water mass in coastal seas to sea surface forcing change.

Contamination, dynamics, and health risk assessment of pesticides in seawater and marine samples from the Seto Inland Sea, Japan.

We assessed the contamination, dynamics, and health risks of the pesticides cyanazine, simetryn, fenarimol, isoprothiolane, diazinon, irgarol, fenitrothion, and diuron in marine samples (seawater, sediments, plankton, fish, and other edible organisms) at various locations in the Seto Inland Sea in Japan in 2016 and 2017. Pesticide concentrations were highest at sampling sites close to the coastline, and mean concentrations in seawater were slightly higher in surface water than in bottom water. All eight pesticides were detected in plankton. Diazinon concentrations (77-387 ng/g dw) were highest in sediments and cyanazine was the most frequently detected pesticide (88%, n = 17) in sediments. Only cyanazine (2.7-41.9 ng/g dw), simetryn (1.0-34.3 ng/g dw), and diazinon (6.3-308.8 ng/g dw) were detected in fish and other edible marine organisms. Based on the calculated bioconcentration factor, the results showed that plankton, fish, and marine animals bioaccumulated pesticides. The highest hazard quotients were calculated for diazinon in red seabream and greenling, indicating a possible risk to consumers. It is, therefore, imperative to promote strict implementation of pollution control, integrated pest management practices, and policy formulation on pesticides. Usage of diazinon must be controlled and monitored to ensure large residues do not reach aquatic ecosystems and marine coastlines.

Assessing spatiotemporal variations of AOD in Japan based on Himawari-8 L3 V31 aerosol products: Validations and applications

The next-generation geostationary meteorological Himawari-8 satellite has a high temporal resolution of 10 min and thus provides a huge advantage for monitoring aerosol optical depths (AOD) in real-time. The newly released Himawari-8 Level 3 Version 31 aerosol products include the Ångström exponent (AE) and three categories of AOD of AOD_L2_Mean, AOD_Pure, and AOD_Merged on an hourly basis. In this study, we evaluated the accuracy of these aerosol products in Japan where with low AOD using the Aerosol robotic network (AERONET), SKYNET, and moderate resolution image spectroradiometer (MODIS) AODs. The results showed that the AOD_L2_Mean had the lowest correlation coefficients and the highest RMSE with the available AODs (AERONET: R:0.558, RMSE:0.230; SKYNET: R:0.674, RMSE:0.246; MODIS: R:0.525, RMSE:0.195) while the other two had similar performances. The result also indicated that although the AOD_Pure and AOD_Merged showed good agreements with the ground observations in terms of time series, they were, unfortunately, overestimated in most cases. In comparison, the AOD_L2_Mean values were nevertheless frequently overestimated. On an hourly basis, the AOD_Merged product had the best performance. Furthermore, the spatiotemporal analysis revealed that the AOD_Merged has traced the spatial and temporal variations well in Japan, with higher loadings in spring and summer and lower values in winter. In addition, the mean AOD loading across years was high in the Seto Inland Sea in Japan. The evaluation offered in this study can contribute to understanding the error sources on retrieving AHI AOD and the spatiotemporal variations of AOD in Japan, which have frequently been ignored in current assessment studies.

Age, growth, mortality, and yield‐per‐recruit of red spotted grouper Epinephelus akaara in the central Seto Inland Sea, Japan

The red spotted grouper Epinephelus akaara is important for commercial and recreational fishing in the coastal waters of East Asia. In Japan, approximately 850,000 juveniles are released annually for stock enhancement, and the catch in several release areas has increased. Life-history traits are important for fisheries management and stocking programmers. The age, growth, and mortality of fish collected from the central Seto Inland Sea from June 2012 to November 2014 were examined using 1022 sectioned sagittal otoliths, and the yield-per-recruit (YPR) was calculated. The opaque zones in the otoliths were confirmed to be annuli, formed mainly between June and July. The oldest fish was 16 years old. The percentage of those between two and five years was approximately 80%, whereas that of more than ten years was only 0.6%. The five parameters von Bertalanffy growth equation was calculated as: TLt = 390 {1 − 0.23[(t − 3.78)2 + 1]−1}{1 − exp [−0.34 (t + 0.28)]}, where TLt is the total length (mm) at t (year). The total, natural, and fishing mortality rates were calculated to be 0.80, 0.24–0.31 and 0.49–0.56 year−1, respectively. The YPR analysis indicated that the current fishing mortality rate (Fcurrent) was higher than F0.1 and Fmax, and that increasing the age at first capture is more effective than decreasing F for increasing the YPR. These results provide important information for fisheries management and the stocking of E. akaara.

Nitidotellina hachiensis n. sp. (Bivalvia: Tellinidae) from the Seto Inland Sea, between Honshū and Shikoku, western Japan

ABSTRACT A new species of tellinid, Nitidotellina hachiensis, is described from Japan. The shell of this species is characterised by a pair of strong keels that run from the beaks to the posterior end and forms a prominent posterior notch. The recent individuals are found only in the Seto Inland Sea (Okayama, Hiroshima, Yamaguchi and Ehime prefectures), while the one fossil record so far known is from the Chibanian (middle Pleistocene) Toyohashi Formation in Aichi Prefecture, central Honshū. Living individuals are uncommon and mainly found living in sandy mud bottoms from the low tide line down to the subtidal zone in the inner parts of large bays. It can be regarded as endangered due to the recent habitat loss and water pollution of inner bay environments.

Evaluation of the effects of stock enhancement on population dynamics using a state-space production model: A case study of Japanese flounder in the Seto Inland Sea

The effect of stock enhancement on increasing biomass is not well understood and is highly controversial because released fish might have a competitive impact on wild stocks. Quantitative assessments exploring the possible impacts of stock enhancement on population dynamics are rare, especially in data-limited fisheries. We developed a state-space production model incorporating stock enhancement to evaluate the impact of stock enhancement at a population scale. Since observation and process errors impair the accuracy of parameter estimation and biomass prediction, we investigated whether prior biomass reference information could improve analytical performance of the state-space production model. We compared the performance of maximum likelihood (ML) and maximum a posterior (MAP) estimation in a special case of a state-space Pella-Tomlinson model or in a state-space Schaefer model, for three simulated levels of process and observation errors. We found that the MAP parameter estimation with valid prior biomass reference data was generally superior to the ML parameter estimation. As a case study, we applied the extended state-space Pella-Tomlinson model to disentangle the impacts of stock enhancement for the Japanese flounder (Paralichthys olivaceus) in the Seto Inland Sea using long-term fishery data. The results showed a limited contribution of stock enhancement to Japanese flounder in the Seto Inland Sea, suggesting that most released flounder replaced the wild fish niche. Given the number of fish released in 2018, future projections for the next ten years under three different fishing pressures showed that stock enhancement was more effective at augmenting biomass when fishing pressure was higher, but that total biomass decreased simultaneously. Catch increase (decrease) in 30% resulted in a 9% decrease (8% increase) in the biomass and a 28% increase (15% decrease) in the net biomass augmentation from stocking. Therefore, a balance between stock enhancement and exploitation is important to obtain sustainable production and efficient stocking, especially facing global climate change which possibly influences the carrying capacity. Our results suggest that a state-space production model can be used to evaluate the impact of stock enhancement and implement better fishery management.

Dynamics and Mass Balance of Polycyclic Aromatic Hydrocarbons in and Around the Seto Inland Sea, Japan

Polycyclic aromatic hydrocarbons (PAHs) were analyzed to elucidate the distribution, ecological risk, pathways, and fluxes of these pollutants in and around the Seto Inland Sea, an industrialized coastal region of Japan. High molecular weight PAHs (5-6 rings) were primarily found in regions close to the bay estuaries, and their proportions decreased at distances further from the estuaries (offshore areas), where low molecular weight PAHs (2-4 rings) were more ubiquitous. Screening-level risk assessments revealed that the PAHs found in the sediments should have no adverse effects on benthic communities. A mass balance for PAHs in the Seto Inland Sea, calculated based on data collected in the field and published literature findings, showed the PAH flux into the Seto Inland Sea from atmospheric deposition were ca. 6 times higher than that from riverine inflows. Comparison of the amount of the PAH mass flux between the Seto Inland Sea and the sea of the Europe and Asian countries indicated that the Seto Inland Sea is less polluted than the Gulf of Lion, the Mediterranean Sea, and the Bohai Sea, China and more polluted than the Yellow Sea. This paper is the first to determine the fluxes of PAHs in the coastal region of Japan.

Evaluation of the effect of Global Sulfur Cap 2020 on a Japanese inland sea area

The Global Sulfur Cap 2020 regulation requires a decrease in the sulfur content in the fuel oil used on ships from 3.5% to 0.5%. In this study, atmospheric sulfur dioxide (SO2), nitrogen oxide (NOx), and fine particulate matter concentrations were measured in the Seto Inland Sea, a congested sea area in Japan. A rapid decrease was observed in ship emission indices, such as SO2, vanadium (V), nickel (Ni), and the V/Ni ratio, at coastal monitoring stations, providing evidence of the implementation of this regulation. Onboard observations clearly revealed a decrease in maritime gaseous SO2 immediately after the regulation came into effect; similar changes were observed in the coastal areas from January to June in 2019–2020. Although continuous studies will be needed to distinguish the benefits of the regulation from those of other factors, these findings will be useful for policymakers when addressing atmospheric pollution issues in congested sea areas.

Satellite Tracking of Migration Routes of the Eastern Buzzard (Buteo japonicus) in Japan through Sakhalin.

We satellite-tracked the eastern buzzard (Buteo japonicus) wintering in Japan to delineate both northward and southward migration routes, destinations, and stopover behavior. Twenty-two buzzards were captured and fitted with functional platform transmitter terminals. For these buzzards that departed from the capture sites, we observed a total of 65 northward migrations during 2008-2016 and a total of 55 southward migrations during 2008-2015. In spring, the eastern buzzards migrated eastward along the Seto Inland Sea in the Chugoku region or further inland. In eastern Honshu, they followed two different routes. One was to Hokkaido via the Tsugaru Peninsula from central or northern central Honshu northward along the side of the Sea of Japan in northern Honshu. The other was to Hokkaido via the Shimokita Peninsula, mainly from the Pacific Ocean side of northern Honshu, moving eastward through central Honshu. Of the 17 birds tracked, 10 summered in Sakhalin, three in Hokkaido, three in northern Honshu, and one unknown. In autumn, the buzzards retraced their northward migration routes. Of the 14 birds that were tracked the entire southward migration, 13 (92%) returned to their respective capture sites. One juvenile wintered in an area different from the capture site. Our study contributes to a deeper understanding of the distribution of breeding and wintering grounds and the migration routes of B. japonicus. In addition, the information on migration obtained in this study can contribute toward appropriate environmental impact assessment for wind power facilities in Japan.

Observing Phylum-Level Metazoan Diversity by Environmental DNA Analysis at the Ushimado Area in the Seto Inland Sea.

The dynamics of microscopic marine plankton in coastal areas is a fundamental theme in marine biodiversity research, but studies have been limited because the only available methodology was collection of plankton using plankton-nets and microscopic observation. In recent years, environmental DNA (eDNA) analysis has exhibited potential for conducting comprehensive surveys of marine plankton diversity in water at fixed points and depths in the ocean. However, few studies have examined how eDNA analysis reflects the actual distribution and dynamics of organisms in the field, and further investigation is needed to determine whether it can detect distinct differences in plankton density in the field. To address this, we analyzed eDNA in seawater samples collected at 1 km intervals at three depths over a linear distance of approximately 3.0 km in the Seto Inland Sea. The survey area included a location with a high density of Acoela (Praesagittifera naikaiensis). However, the eDNA signal for this was little to none, and its presence would not have been noticed if we did not have this information beforehand. Meanwhile, eDNA analysis enabled us to confirm the presence of a species of Placozoa that was previously undiscovered in the area. In summary, our results suggest that the number of sequence reads generated from eDNA samples in our project was not sufficient to predict the density of a particular species. However, eDNA can be useful for detecting organisms that have been overlooked using other methods.

Philophthalmus hechingeri n. sp. (Digenea: Philophthalmidae), a Human-Infecting Eye Fluke from the Asian Mud Snail, Batillaria attramentaria

Two cases of human philophthalmosis have been reported in Japan. Gravid flukes removed from the eyes of the patients were broken, but their morphological characteristics suggest that an unknown species of the genus Philophthalmus is involved as a pathogen for humans. The mitochondrial DNA barcode of the human eye fluke enabled us to discover its larval stage from the Japanese mud snail, Batillaria attramentaria. The discovered cercaria had previously been temporarily described as "Philophthalmid sp. I." In this study, we examined the infection status of B. attramentaria with Philophthalmid sp. I found on a muddy seashore of the Seto Inland Sea, Japan, and the resulting metacercariae were experimentally administered to Japanese quails to develop them into the gravid adult stage. The complete specimens of the adult and larval stages allowed us to describe a new species. Based on morphological and molecular analyses, Philophthalmus hechingeri n. sp. is proposed for the human-infecting eye fluke in Japan. The natural definitive hosts of the new species are unknown. However, the habitat of B. attramentaria suggests that shorebirds (seagulls, sandpipers, and plovers) might be the possible candidates.