Coast Of Japan Research Articles

Temporal trends of community and climate changes in the anthropocene: 21-year dynamics of four major functional groups in a rocky intertidal habitat along the Pacific coast of Japan

The influence of climate change on marine organism abundance has rarely been assessed (1) at the functional-group level; (2) simultaneously in major functional groups within the same ecosystem; (3) for >10 years; and (4) at metapopulation/community scales. A study simultaneously addressing these gaps would greatly enhance our understanding of the influence of climate change on marine ecosystems. Here, we analyzed 21 years of abundance data at the functional-group and species levels on a regional scale for four major functional groups (benthic algae, sessile animals, herbivorous benthos, and carnivorous benthos) in a rocky intertidal habitat along the northeastern Pacific coast of Japan. We aimed to examine the 21-year trends in regional abundance at both functional-group and species levels, plus their driving mechanisms and their dependence on species properties (thermal niche, calcification status, and vertical niche). Significant temporal trends in abundance were detected at functional-group levels for benthic algae (increasing) and herbivores and carnivores (both decreasing); they followed the temporal population trends of the dominant species. At species level, the metapopulation size of 12 of 31 species were increasing and 4 of those were decreasing, depending on the thermal niche and species calcification status. At both functional-group and species levels, temporal trends in abundance are caused by the direct or indirect influence of warming and ocean acidification. Comparing these results with community responses to marine heat waves in the same study area offered two implications: (1) long-term ecosystem changes associated with global warming will be unpredictable from the community response to marine heat waves, possibly owing to a lack of knowledge of the influence of calcifying status on species’ responses to climate change; and (2) thermal niches contribute greatly to predictions of the influence of warming on population size, regardless of the time scale.

Variability of nutrient transport and associated upper ocean primary production induced by Kuroshio meanders in the Enshu-nada Sea, Japan

Towards the desirable preservation of the marine environment under a changing global climate, this study aims to investigate the coastal biological response to varying oceanic conditions, namely the Kuroshio meander, and associated nitrate transport on the Pacific coast of Japan around the Enshu-nada Sea, using a coupled three-dimensional regional ocean circulation model with a nitrogen-based ecosystem model. A prominent difference was found in nitrate transport processes between for a period when the Kuroshio took a meandering path and for a non-meandering period, leading to about 1.5 times higher surface primary production measured by chlorophyl-a concentration during the non-meandering period than during the meandering period. The upper ocean nitrate flux budget analysis showed that the subsurface nitrate was transported upward as a vertical diffusive flux around the north (shoreward) of the Kuroshio path and as a mean vertical advective flux in the Kuroshio downstream region around the Izu-Ogasawara Ridge. In contrast, high-frequency eddy vertical advective fluxes caused downward transport, but to a degree about 20% smaller than the other fluxes. After nitrate was supplied to the upper layer, it was transported horizontally by the counterclockwise rotating cyclonic eddy formed between the coast and the Kuroshio and supplied to phytoplankton in the Enshu-nada Sea. These results suggest that carbon sequestration due to biological pump may also vary in response to the Kuroshio that is influenced by basin-scale oceanic conditions.

Morphology and Molecular Characterisation of Karenia mikimotoi (Dinophyceae) from Sabah Malaysian Borneo, with a Focus on the Second Internal Transcribed Spacer (ITS2) of Ribosomal RNA gene

The first recorded bloom of Karenia mikimotoi (initially Gymnodinium mikimotoi) occurred off the coast of Japan in 1934, causing mass mortality of shellfish and fish. This event highlighted the devastating impact of K. mikimotoi blooms and marked a turning point in harmful algal bloom (HAB) research, driving studies on its identification, biology, toxicology, and effects on marine life and ecosystems. The past reported bloom events in Southeast Asia have raised public concerns, leading to further investigation into the occurrence and geographical distribution of K. mikimotoi in the region. As of yet, there is no recorded evidence of K. mikimotoi blooms in Malaysian waters. This prompt the investigation of the occurrence and distribution of K. mikimotoi in Malaysia, and this study represent the first record of K. mikimotoi in Malaysian waters. In this study, clonal cultures of K. mikimotoi isolated from Sepanggar Bay, Sabah were examined using light microscopy (LM) and scanning electron microscopy (SEM) to observe its morphological features. Cells of K. mikimotoi from Malaysian Borneo exhibited a typical dorso-ventrally flattened body with bi-lobed and linear apical grooves on the cell apex. Molecular characterisation of the strains based on the internal transcribed spacer (ITS) region and large-subunit (LSU) ribosomal DNA revealed close phylogenetic relationships with other strains of K. mikimotoi from other regions, forming a monophyletic clade that positioned as sister to K. brevis, supporting the species identity of K. mikimotoi. The secondary structure of the ITS2 RNA transcript revealed a universal structure with four major helices. Structural comparison between K. mikimotoi and its relatives revealed four to six hemi-compensatory base changes. The results demonstrated the efficacy of ITS2 secondary structure information in delimiting species in Karenia. The detailed morphology and molecular characteristics of K. mikimotoi were revealed, for the first time, from the coastal waters of Malaysian Borneo.

Subtropical specialists dominate a coral range expansion front

AbstractPotential range expansion of scleractinian corals in high-latitude reefs is critically dependent on the coral host-symbiont relationship that determines coral growth and survival. Although increases in coral cover have been observed at higher latitudes, the identities of habitat-building reef corals and their symbionts are underreported. Here, we examine how scleractinian host and symbiont Symbiodiniaceae diversity changes along a tropical–temperate environmental gradient. We use Pocillopora spp. and associated symbiont communities as a model to understand whether they are expanding their range poleward and the role of symbionts in this process. Along the Kuroshio Current, which carries warm equatorial waters northward along the Pacific coast of Japan, we collected coral tissues from 23 (sub)tropical-to-temperate reefs, from southern Iriomote in the Ryukyu Islands (24°N) to northernmost Kushimoto on mainland Japan (33°N). We examined host identities through direct sequencing of the mitochondrial open reading frame (mtORF) and symbiont identities with next-generation sequencing of the internal transcribed spacer 2 (ITS2) region of the ribosomal DNA. Our results show a dramatic reduction of Pocillopora haplotypes and a marked change in dominant symbiont types northward (poleward) from Cape Sata (30°N), Kagoshima. ‘Tropical’ Pocillopora haplotypes were absent from mainland Japan sites. We also demonstrate high host specificity between the subtropical Pocillopora haplotype and Cladocopium symbiont types. Our findings question how common ‘coral tropicalisation’ is, and the location of the coral range expansion front. The specificity of hosts and symbionts in high-latitude corals suggests that high-latitude reefs are unlikely to support the persistence of tropical zooxanthellate corals.

Segmented Trapdoor Fault in Kita‐Ioto Caldera, Japan: Insights From Millimeter Tsunami Waves Captured by an Array Network of Ocean Bottom Pressure Gauges

AbstractSubmarine calderas with active magma supply have recently been identified as potential sources of volcanic tsunamis due to sudden meter‐scale uplift by trapdoor faulting, occurring every few years to a decade. These trapdoor uplifts are seismically recorded as non‐double‐couple earthquakes with magnitudes M > 5. Kita‐Ioto Caldera, a submarine caldera in the Izu‐Bonin arc, caused such earthquakes every 2–5 years. Our previous study (Sandanbata & Saito, 2024, https://doi.org/10.1029/2023jb027917) analyzed data from a single ocean bottom pressure (OBP) gauge in the Philippine Sea, confirming trapdoor uplifts during the earthquakes in 2008 and 2015. However, high temporal‐resolution data for the earthquakes in 2017 and 2019 were lost, preventing source mechanism investigation. To address this, we examine OBP data of the two recent earthquakes from the array network of Dense Oceanfloor Network system for Earthquakes and Tsunamis, deployed off the southwestern coast of Japan. Despite the poor signal‐to‐noise ratio in each record, we successfully detect clear tsunami signals associated with the earthquakes using a waveform stacking method, with sea‐surface wave amplitudes of only 1–2 mm. By analyzing the data, we propose source models that represent trapdoor uplifts in the submarine caldera and accurately reproduce the detected tsunami waveforms, confirming the recurrence of trapdoor uplifts. Notably, differences in the tsunami waveforms between the 2017 and 2019 earthquakes suggest that different segments of the intra‐caldera fault system were activated. This segmentation likely influences the recurrence characteristics of the inflation cycle in calderas, which would be a key to understanding the magma accumulation process and assessing the sizes and timings of future trapdoor uplifts.

Regional range extension of two xanthid crab species (Decapoda, Brachyura) to the mid-Sanriku Coast, the coldest waters off Honshu, Japan

Abstract This paper reports the first occurrences of two brachyuran crab species of the family Xanthidae MacLeay, 1838, viz., Actaea semblatae Guinot, 1976 and Gaillardiellus orientalis (Odhner, 1925), from the mid-Sanriku Coast, along the Pacific coast of northeastern Japan. While these records do not represent the northern limit of distribution of these species, they do show a range extension into the coldest waters of Honshu, the main island of Japan, probably due to global warming and changes in flow patterns of the local sea currents.

Restricted recovery of Eisenia bicyclis kelp forest following sea urchin removal on the Pacific coast of Tohoku, Japan

Little information is available on the destruction of Eisenia bicyclis kelp forests (Phaeophyceae: Laminariales) due to overgrazing by sea urchins. In Shizugawa Bay, on the northeastern Pacific coast of Japan, overgrazing by the sea urchin Mesocentrotus nudus caused the destruction of E. bicyclis forests between 2014 and 2015. This study investigated the recovery of an adult kelp forest in relation to sea urchin densities through the continuous removal of sea urchins at 3 permanent experimental transects spanning 100 m2 for 3.8 yr from September 2015 to June 2019. The relationship between the densities of sea urchins and kelp juveniles was also analyzed. Estimates of the threshold density and biomass of sea urchins required to trigger the phase shift from an E. bicyclis forest to a barren were 15.5 ind. m-2 and 712.0 g m-2, respectively. The removal of sea urchins expanded the offshore kelp growth area. However, an increase in adult kelp following the return of juveniles was restricted to the nearshore half of the transects. The successful recruitment of kelp juveniles, followed by the recovery of adult kelp forest, requires reduction of the density of M. nudus of 40-50 mm diameter to 0 or 1 ind. m-2. Reducing sea urchin densities in autumn, when many move from barrens to invade kelp forests, is crucial to ensure the survival of kelp juveniles. This study highlights the presence of an intensified positive feedback mechanism of the barren state due to high grazing intensity and a weakened Oyashio Current.

Influence of Spawning and Nursery Ground Environmental Changes on Walleye Pollock Catches Along the Eastern Coasts of Korea and Japan After the Late-1980s Climate Regime Shift

The eastern coasts of Korea (ECK) and Japan (ECJ) are located at the southernmost limit of walleye pollock distribution in the Northwest Pacific. Following the climate regime shift (CRS) in the late 1980s, pollock catches in these regions have declined sharply, with different trends emerging. This study examined the relationship between environmental factors, such as sea surface temperature (SST) and habitat suitability, and changes in pollock catches from the late 1980s to 2022. From the late 1980s to the late 1990s, El Niño and positive Pacific decadal oscillation (PDO) phases dominated, increasing SST in the ECK and ECJ habitats and rapidly decreasing catches. Although spawning grounds (SGs) have maintained high habitat suitability, nursery ground (NG) suitability has declined. From the late 1990s to 2022, La Niña and negative PDO phases prevailed, with SST continuing to rise along the ECK, further reducing catches. SG suitability remained high, but NG suitability declined. Along the ECJ, SST decreased after the late-1990s CRS, stabilizing catches. After the mid-2010s, the SST increased along the ECJ, reducing pollock catches, although SG suitability remained high. This study elucidates SST changes during early life stages and their effects on pollock catch, habitat, and resources in future marine environments.

The calculated voyage: benchmarking optimal strategies and consumptions in the Japanese eel’s spawning migration

Eels migrate along largely unknown routes to their spawning ground. By coupling Zermelo’s navigation solution and data from the Japan Coastal Ocean Predictability Experiment 2 (JCOPE2M), we simulated a range of seasonal scenarios, swimming speeds, and swimming depths to predict paths that minimize migration duration and energy cost. Our simulations predict a trade-off between migration duration and energy cost. Given that eels do not refuel during their migration, our simulations suggest eels should travel at speeds of 0.4–0.6 body-length per second to retain enough energy reserves for reproduction. For real eels without full information of the ocean currents, they cannot optimize their migration in strong surface currents, thus when swimming at slow swimming speeds, they should swim at depths of 200 m or greater. Eels swimming near the surface are also influenced by seasonal factors, however, migrating at greater depths mitigates these effects. While greater depths present more favorable flow conditions, water temperature may become increasingly unfavorable, dropping near or below 5 °C. Our results serve as a benchmark, demonstrating the complex interplay between swimming speed, depth, seasonal factors, migration time, and energy consumption, to comprehend the migratory behaviors of Japanese eels and other migratory fish.

Prediction of an enigmatic tsunami in October 2023 at Kii Peninsula, Japan

On October 8, 2023, a tsunami was observed along the Pacific coast of Japan, but its cause was complicated. We applied tsunami data assimilation, a forecast method that does not consider the source, to predict coastal tsunami waveforms in the Kii Peninsula. We assimilated offshore observations recorded by pressure gauges (DONET) and reconstructed the wavefield before the tsunami arrived at the coast. The waveforms at Kumano, Kushimoto, and Shirahama were accurately forecasted at least 20 min before the tsunami arrival, indicating that data assimilation is applicable for tsunami early warning. Quantitative analysis showed that the score and accuracy index generally increased since 21:00 (UTC). Meanwhile, we investigated the tsunami decay process for tsunami warning cancellation by analysing the moving root mean squared (MRMS) amplitude. As data assimilation progressed, the prediction of tsunami later phase became increasingly accurate, and the errors between observed and predicted MRMS amplitudes decreased. Overall, the tsunami later phase was satisfactorily predicted at approximately 22:00 at coastal tide gauges. Hence, data assimilation approach contributes to a comprehensive tsunami early warning process, from the issuance to cancellation.

Assessing the trophic interactions between two pelagic fishes in the northwestern Pacific based on stable isotopes

Scomber japonicus (S. japonicus) and Sardinops melanostictus (S. melanostictus) are widely distributed off the coast of Japan and in the northwestern Pacific. To investigate the trophic interactions between these two species, samples of S. japonicus and S. melanostictus were taken from the high seas of the northwestern Pacific Ocean (38°59′N–43°00′N, 150°30′E–161°48′E) from June to July 2021. We utilized stable isotope analysis to study the intraspecific and interspecific differences of them. For intraspecific variability, there was no significant difference in δ13C or δ15N values between females and males for either S. japonicus or S. melanostictus (P > 0.05). Significant differences in the δ13C and δ15N values of S. melanostictus were found among the body length groups (P < 0.01). The δ13C values of S. japonicus significantly differed among body length groups (P < 0.05). However, there was no significant difference in the δ15N values of S. japonicus (P > 0.05). For interspecific variability, there were significant differences in the δ13C and δ15N values between S. japonicus and S. melanostictus (P < 0.01). According to the GAM results, both the δ13C and δ15N values of S. japonicus gradually increased with body length, whereas both the δ13C and δ15N values of S. melanostictus tended to decrease and then increase. The niche overlap between S. japonicus and S. melanostictus decreased with increasing growth. Compared to those of S. melanostictus, the δ15N levels of S. japonicus were greater, while the niche width was smaller. The intraspecific niche overlap and width for both S. melanostictus and S. japonicus tended to increase and then decrease with growth. We suggest that the average trophic level of S. japonicus is greater than that of S. melanostictus. However, S. melanostictus has a greater diversity of prey items, and these two species coexist by reducing inter- and intraspecific competitive pressures through dietary differentiation.

Lightning strike characteristics of wind turbines in Japan

In recent years, as global warming advances, countermeasures for reducing gases of various greenhouse are needed. One of the countermeasures is a power generation using renewable energy that does not emit greenhouse gases. In particular, wind power generation is attracting attention due to its low cost compared to power generation using other renewable energies. However, as wind turbines become larger and the number of wind turbine increases, the probability of lightning strikes increases, it is important to establish more reliable lightning damage countermeasures. In response to this situation, the "Interpretation of Technical Standards for wind turbines" was revised in 2015 in Japan, installation of lightning detection systems to accurately detect lightning strikes and automatically shut down the wind turbine have been obligated in wind turbines in winter lightning regions in coastal regions in Japan. In addition, the performance of lightning detection systems has been improved by the results of recent research, and the systems with high detection accuracy are widely used in the market. In this study, we report the results of investigating the characteristics of lightning strikes using lightning data on approximately 450 wind turbines along the coast of Japan.

Population connectivity of the swordtip squid Uroteuthis edulis between southern Japan and northern Taiwan using statolith trace elemental analysis

Uroteuthis edulis (Hoyle, 1885) is an Indo-Pacific squid species widely distributing in the western Pacific, and commercially important especially in Japan and Taiwan. It has been suggested that some individuals are possibly transported from the spawning ground in north Taiwan to the coasts of Japan, however, the strength of population connectivity between those areas and its influence on U. edulis population dynamics were unveiled. To understand the U. edulis population connectivity in this area, the correlations between statolith trace elements and abiotic/biotic factors were examined first, and then squid experienced environments were postulated throughout their entire life cycle. Sr/Ca ratio showed a strongly negative correlation with ambient water temperature but no correlation with individual growth rate, suggesting that Sr/Ca ratio can be used to reflect squid experienced temperatures. Most squid caught in the Sea of Japan hatched in the areas having similar water temperature with where Taiwanese squid hatched, that would be off the north Taiwan or even warmer area. Statolith trace elements successfully distinguished the catch locations but not the hatching grounds, implying that hatching grounds of Japan and Taiwan squid were largely overlapped. Thus, we suggest that there is strong population connectivity of U. edulis population between southern Japan and northern Taiwan. As there was no clear evidence for existence of local population hatched in the Sea of Japan in this study, U. edulis population might display a source-sink population dynamics, that is, population in Taiwanese waters and/or further south as the source, and the one in the Sea of Japan as a sink population. As U. edulis should be considered as a metapopulation, collaboration among countries in the northwestern Pacific is required for sustainable fishery management of this species.

Successive Tsunamigenic Events Near Sofu Seamount Inferred From High‐Frequency Teleseismic P and Regional T Waves

AbstractAn unexpected major tsunami from the region near Sofu Seamount was observed on 8 October 2023. Sofu Seamount is located approximately 600 km from the coast of Japan. Due to far epicentral distances and the successive occurrence of seismic events, the conventional seismic analysis to reveal the accompanying seismic sequence cannot work well. We investigated high‐frequency teleseismic P and regional T waves from the accompanying seismic sequence during the tsunamigenic events near Sofu Seamount. Envelope shapes of teleseismic P and regional T waves were similar, indicating that T‐wave envelopes also reflected source properties of seismic sequence. During seismic events near Sofu Seamount, observed regional envelopes were characterized by weak body waves and large amplitude T waves with durations of 39–68 s. According to numerical simulations of seismic wave propagation using a realistic topography model, characteristics of T waves exhibit weak slope‐angle and strong source‐depth dependencies. Strong T waves with durations less than 60 s only appeared in results with sources at depths ≤0.5 km below the seafloor. We concluded that high‐frequency radiation of the accompanying seismic sequence during the tsunamigenic events near Sofu Seamount possibly occurred at shallower depths just below the seafloor. If seismic and tsunami sources coincide, shallower source depths might cause tsunamigenic uplifts. The observed peak seafloor uplifts and T‐wave amplitudes during tsunamigenic events were scaled. This result suggests the possibility of tsunami forecasting based on T‐wave amplitudes from submarine volcanoes.

The Sofu Seamount Submarine Volcano Present in the Source Area of the October 2023 Earthquakes and Tsunamis in Japan

AbstractOn 8 October 2023 (UTC), unique earthquakes occurred in the Izu‐Ogasawara Arc, Japan, in which the P‐ and S‐phases were barely visible and only the T‐phases were evident, followed by tsunamis that reached islands in the Izu‐Ogasawara Arc and a wide area of the Pacific coast of southwest Japan. Our estimated T‐phase source area coincides with the Sofu Seamount, which was previously unrecognized as an active submarine volcano. A bathymetric survey of the seamount conducted 1 month after the event revealed characteristics of the seamount with a caldera and a central cone. Compared to the bathymetry in 1987, the topography in the caldera had changed significantly such as a crater forming in the central cone. This seamount is likely to be an active volcano. The topographic changes on the caldera‐sized scale that occurred at the caldera can be explained as a source of the October tsunami.

Relationship between Newly Fallen Snow Density and Polarimetric Parameters Obtained from X-Band Radar Observations along the Sea of Japan Coast in January 2021

Abstract The density of newly fallen snow ρN is an important parameter for assessing accumulated snowfall depth. We examined the relationships between polarimetric parameters of X-band radar and the ρN in dry snow cases with ground temperatures less than 0°C. Our study was based on observations at Niigata Prefecture, Japan, along the coastal region of the Sea of Japan. This region is subjected primarily to sea-effect snow during the winter monsoon season, and convective clouds and rimed snow are common. We assumed that snow particles that accumulated on the ground originated from altitudes above an altitude with a temperature of −15°C, and we focused on the ratio of the differential phase KDP to radar reflectivity Zh, which is influenced by both aspect ratio and inverse particle size. We found that KDP/Zh at an altitude with a temperature of −15°C exhibited a greater magnitude for lower ρN values. Its correlation coefficient was the best among the polarimetric parameters that we examined. The difference in ice crystal flatness is highlighted rather than the difference in size because aggregation growth has not progressed at this altitude. On the basis of this result, we propose an empirical relationship between KDP/Zh at an altitude with a temperature of −15°C and ρN on the ground, thereby facilitating the estimation of snowfall depth by combining the estimated ρN with the liquid equivalent snowfall rate from, for example, Zh or KDP. Significance Statement This study aims to estimate the density of newly fallen (just-accumulated) snow from polarimetric radar observations. Understanding the newly fallen snow density will help to determine the exact snowfall depth. Focusing on polarimetric parameters at an altitude with a temperature of −15°C, we conducted radar and ground-based observations of snow particles and found that the newly fallen snow density of dry snow can be estimated. We were able to highlight the difference in ice crystal flatness before aggregation growth progressed by focusing on higher altitudes.

Impact of the 2011 Tohoku Earthquake on the species diversity of rocky intertidal sessile assemblages.

The impacts of large-scale disturbance events on the species diversity of rocky intertidal sessile assemblages across multiple spatial scales are not well understood. To evaluate the influence of the 2011 Tohoku Earthquake on alpha and beta diversities of rocky intertidal sessile assemblages, we surveyed sessile assemblages in the mid-shore zone from 2011 to 2019 and compared the data with those collected from 2003 to 2010 before the earthquake at the same region. The census was conducted across 22 study plots on five rocky shores along 30 km of the Sanriku Coast of Japan, which is located 150-160 km north-northwest of the earthquake epicenter. Alpha diversity was measured with three Hill numbers (H 0, H 1, and H 2), which represent the number of equally common species that would exist in a community with the same diversity as the sampled community, with higher values of the subscript indicating more weight placed on abundant species. Beta diversity was measured with two metrics (BD total at two spatial scales). Values were compared between the post-earthquake period (2011-2019) and the pre-earthquake period (2003-2010). The results show that the Tohoku Earthquake significantly altered the species diversity of intertidal sessile assemblages across multiple spatial scales. All diversity metrics obtained at multiple spatial scales (i.e., alpha diversities: H 0, H 1, and H 2; beta diversities: BD total at the shore and regional scales) decreased immediately after the earthquake and then increased in subsequent years. At 2 years after the earthquake, H 0 recovered to within the range of pre-earthquake values and H 1 and H 2 became significantly higher than pre-earthquake values. Most metrics of alpha and beta diversities recovered to pre-earthquake levels after several years, but regional BD total remained low for a longer period.

Partial abundances of three fishery-targeted species of Delphinidae cetaceans (false filler whale, striped dolphin, and Pacific white-sided dolphin) off the Pacific coast of Japan, estimated by the conventional line-transect approach

Partial abundances of three fishery-targeted species of Delphinidae cetaceans (false filler whale, striped dolphin, and Pacific white-sided dolphin) off the Pacific coast of Japan, estimated by the conventional line-transect approach

Feeding Habits of Scomber japonicus Inferred by Stable Isotope and Fatty Acid Analyses

Scomber japonicus is widely distributed off the coast of Japan and in the northwestern Pacific. It is an important target for fisheries. To reveal the differences in diet shifts and niche changes of S. japonicus, we collected samples in the high seas of the northwest Pacific (38°59′ N–43°00′ N, 150°30′ E–161°48′ E) from June to August 2021. We utilized stable isotope and fatty acid analyses to study the differences in body length and sex of S. japonicus. The results showed no significant differences in stable isotope values and fatty acid composition between male and female individuals (p &gt; 0.05). Differences in δ13C and δ15N values among different body length groups were also not significant (p &gt; 0.05). Both δ13C and δ15N values showed a trend of increasing and then decreasing across different body length groups. The niche width of S. japonicus in different body length groups first increased and then decreased, with niche overlap among the groups exceeding 60%. Principal component analysis (PCA) results showed that the main fatty acids in S. japonicus were C14:0, C16:0, C18:0, C16:1n-7, C18:1n-9, C18:3n-6, C20:5n-3, C20:4n-6 and 22:6n-3. Except for C18:0 and C20:4n-6, the content of the other fatty acids showed significant differences among different body length groups (p &lt; 0.05). The results of the similarity analysis (ANOSIM) indicated that the fatty acid compositions of the 100–130 mm length group were significantly different from those of the 131–160 mm and 161–190 mm length groups (p &lt; 0.05). However, there were no significant differences among the other size groups (p &gt; 0.05). During the growth and development of S. japonicus, the proportion of krill in their diet gradually decreased. Meanwhile, their consumption of zooplankton, diatoms and fish significantly increased. Additionally, S. japonicus also consumed crustaceans, but their intakes of planktonic bacteria and green algae were relatively low. We suggested that there were no significant differences between male and female individuals of S. japonicus. As they grew and developed, the ecological niche and feeding habits of S. japonicus continuously changed.

Role of aggregates in the forming process of tephric loess dunes distributed along the Pacific coast of eastern Japan

The Pacific coast of eastern Japan contains specific aeolian landforms of geomorphological interest in terms of their formation processes in humid climates and Holocene paleoenvironments. Wind erosion landforms, characterised by blowouts, were observed at the heads of coastal cliffs which consisted of marine gravel sediments, weathered volcanic ash layers (WVALs) and black soil layers (BSLs). The blowouts were accompanied by downwind tephric loess dunes. We analysed the physicochemical properties of the WVAL, BSL and dune sediment samples. The results revealed that clay and silt contents in dune sediments were 5–48% and 20–35%, respectively. The fine particles were in the form of aggregates in the WVALs, BSLs and dune sediments, with contributions from andic properties. WVAL and BSL aggregates were prone to movement in saltation, depositing into the dunes in a formation process similar to that of clay dunes in semiarid zones. The tephric loess dunes were characterised as clay dunes based on their high clay and silt contents. In addition, the dunes were characterised by durable aggregates derived from short-range-order minerals, such as allophane. The prevailing northeasterly winds caused by intense low pressures were responsible for dune formation in the study areas. Radiocarbon dating revealed two active wind periods. The first was approximately 1005–895 cal. BC to 358–281 cal. BC, and the second was 771–886 cal. AD to 1021–1155 cal. AD.