Tsugaru Strait Research Articles

Seasonal changes in the protist communities of Hakodate Bay, southern Hokkaido, from 2020 to 2022

Hakodate Bay at the southern end of Hokkaido experiences physical and chemical environmental changes in the adjacent Tsugaru Strait. Harmful algal blooms have occurred since 2015. However, information on the response of protist community composition, photosynthetic activity to environmental variables or the mechanisms causing harmful blooms in the Hakodate Bay is lacking. We investigated the seasonal changes in the protist community using weekly sampling to reveal the relationship between the protist community and environmental variables in the Hakodate Bay. Based on cluster analysis, six groups were identified: Winter, Spring, Late Spring, Summer, Benthic, and Karenia. High Fv/Fm was seen in the Summer, Benthic and Spring groups supported by high nutrients, warm temperature and sufficient irradiance. From the Winter to Spring group, a species transition from low-temperature-adapting Thalassiosira spp. to Chaetoceros spp. was found. The Summer group comprised high salinity-tolerant species (the diatom Leptocylindrus danicus and dinoflagellate Prorocentrum spp.) because of high fluctuations in salinity due to precipitation. The other groups (Late Spring, Benthic, and Karenia) randomly filled the spaces between the three groups. The dinoflagellate Scrippsiella spp. was dominant in the Late Spring group because of accumulation by wind-driven currents. Because a strong relationship between benthic species and wind speed was exhibited, the Benthic group comprised benthic diatoms released into the water by mixing. The Karenia group was observed only from late August 2021 to early January 2022, when higher ammonium and lower light conditions (i.e., cloudy conditions) were detected. Owing to environmental conditions, diatom growth was restricted by low light intensity, whereas that of Karenia mikimotoi increased by taking advantage of the abundant nutrients. Seasonal changes in the protist community of the Hakodate Bay can generally be explained by temperature; however, in the short term (1–2 weeks), changes were also influenced by meteorological conditions (solar radiation, wind, and precipitation).

Scale-dependent influences of environmental, historical, and spatial processes on taxonomic and functional beta diversity of Japanese bat assemblages.

This study investigated the relative influences of environmental, spatial, and historical factors, including the island-specific history of land connectivity, on bat assemblages in the Japanese Archipelago. We collected bat distribution data from 1408 studies and assigned them to Japan's First Standard Grid (approximately 6400 km2). Japanese bat assemblages were analyzed at two scales: the entire Japanese Archipelago comprised 16 islands and exclusively the four main islands. At first, we calculated taxonomic and functional total beta diversity (β total) by Jaccard pairwise dissimilarity and then divided this into turnover (β repl) and richness-difference (β rich) components. We conducted hierarchical clustering of taxonomic beta diversity to examine the influence of the two representative sea straits, Tsugaru and Tokara, which are considered biogeographical borders. Variation partitioning was conducted to evaluate the relative effects of the three factors on the beta diversity. Clustering revealed that the Tokara Strait bordered the two major clades; however, the Tsugaru Strait did not act as a biogeographical border for bats. In the variation partitioning, shared fraction between spatial and historical factors significantly explained taxonomic and functional β total and taxonomic β repl at the entire archipelago scale, but not at the four main islands scale extending only Tsugaru Strait but not Tokara Strait. Pure environmental factors significantly explained functional β total at both scales and taxonomic β total only at the four main islands scale. These results suggest that spatial and historical factors are more pronounced in biogeographical borders, primarily structuring assemblage composition at the entire archipelago scale, especially in taxonomic dimension. However, current environmental factors primarily shape the assemblage composition of Japanese bats at the main island scale. The difference in results between the two scales highlights that the primary processes governing assemblages of both dimensions depend on the quality of the dispersal barriers between terrestrial and aquatic barriers for bats.

Изменение окружающей среды северо-западного сектора Японского моря в период последней дегляциации

Environment changes in the northwestern part of the Sea of Japan and adjacent land during last 21.0 kyr was reconstructed based on the analysis of pollen and diatom data with a high temporal resolution. Relatively dry climate caused a wide distribution of forest-steppe landscapes in the region between 21.0 to 16.6 ka. Boreal forests covered mountain slopes. Broad-leaved trees survived in refugia located in lower mountain belts. The low sea level during the Last Glacial Maximum (LGM) changed the general patterns in the sea circulation and longshore current directions, which transported the waters formed in the shallow areas of the northwestern and northern parts of the Sea of Japan. A slight increase in sublittoral benthic diatoms indicates the lowered sea level during the LGM. Occurrence of Betula indicates the start of warming after 18.5 ka; it was interrupted by the cold event Heinrich 1 and marked by the spread of coniferous Larix, Picea, and sphagnum bogs. The beginning of late glacial climatic amelioration Bølling-Allerød (BA) is contributed to dispersal of Quercus and Polypodiaceae ferns. The distribution of conifers Larix and Picea as well as of sphagnum bogs is correlated with the Younger Dryas (YD) cooling. Significant changes in vegetation occurred in the Holocene. The high percentage of sublittoral planktonic diatoms reflects a gradual rise in sea level during the BA and YD. A greater component of oceanic diatom species in the Holocene indicates the inflow of the Pacific waters through both the Tsushima and Tsugaru straits. Some increase in the participance and species diversity of relatively warm-water diatoms was noticed in the warming periods. The study of pollen and diatom records demonstrates a clear response to climatic fluctuations and changes in the Sea of Japan level in the Late Pleistocene and Holocene.

Diurnal coastal trapped waves propagating along the east coast of the Shimokita Peninsula, Japan

In this study, we investigated diurnal coastal trapped waves in the eastern coastal area of the Shimokita Peninsula near the Tsugaru Strait. The coastal trapped waves in this area have not yet been observed. We observed current velocities at three sites on the coast to clarify the propagation and seasonal features. We also used an ocean general circulation model to determine the detailed structure and the causes underlying the seasonal characteristics of the waves. The coastal trapped waves propagated southward along the coast from the strait, where significant tidal currents exist. Coastal trapped waves depend on cross-shelf length and stratification strength. The coastal trapped waves propagate as internal Kelvin waves from summer to early winter in the northern part of the peninsula (where the shelf is narrow) and as shelf waves in the southern part (where the shelf is wide). From late winter to spring, the coastal trapped waves practically disappear in the northern part of the peninsula owing to the vertical uniform density off the east coast and the small cross-shelf width in the northern part. In autumn, the tidal current flows north of the sill near Cape Shiriya at the eastern mouth of the straits owing to the northward shift of the Tsugaru Warm Current in the strait; thus, the coastal trapped waves along the Shimokita coast weaken.

Protist dynamics in the eastern Tsugaru Strait, Japan from 2010 to 2018: Implications for the relationship between decadal climatology and aquaculture production

Environmental changes such as seasonality, decadal oscillation, and anthropogenic forcing may shape the dynamics of lower trophic-level organisms. In this study, 9-years (2010–2018) of monitoring data on microscopic protists such as diatoms and dinoflagellates, and environmental variables were analyzed to clarify the relationships between plankton and local/synoptic environmental changes. We found that time-series temperature increased in May, whereas it decreased in August and November. Nutrients (e.g., phosphate) decreased in May, remained unchanged in August, and increased in November from 2010 to 2018. The partial pressure of CO2 increased in May, August, and November over time. It is notable that the change in seawater temperature (−0.54 to 0.32 °C per year) and CO2 levels (3.6–5.7 μatm CO2 per year) in the latest decade in the eastern Tsugaru Strait were highly dynamic than the projected anthropogenic climate change. Protist abundance generally increased or stayed unchanged during the examined period. In August and November, when cooling and decreases in pH occurred, diatoms such as Chaetoceros subgenus Hyalochaete spp. and Rhizosoleniaceae temporally increased from 2010 to 2018. During the study period, we found that locally aquacultured scallops elevated soft tissue mass relative to the total weight as diatom abundance increased, and the relative scallop soft tissue mass was positively related to the Pacific Decadal Oscillation index. These results indicate that decadal climatic forcing in the ocean modifies the local physical and chemical environment, which strongly affects phytoplankton dynamics rather than the effect of anthropogenic climate change in the eastern Tsugaru Strait.

Impact of satellite and regional in-situ profile data assimilation on a high-resolution ocean prediction system in the Northwest Pacific

The impacts of observation data sets on the high-resolution (1/24°) Northwest Pacific prediction system were investigated with the model sensitivity tests. We compared the model experiments assimilating the different combinations of the observation data sets, such as the sea surface height derived from satellite altimetry, sea surface temperature, and in-situ profiles, based on the Ensemble Optimal Interpolation. Pseudo-profiles constructed by the method of Cooper and Haines (1996, CH96) were assimilated into the model to assimilate sea surface height data. CH96 applied a conservation principle to derive pseudo-profiles by rearranging preexisting profiles. The comparison of the model experiments suggests that each observation data set enhances the model performance. Especially, the assimilation of the sea surface height reduces the model error by more than 9.81% and 6.44%, respectively, in terms of the root-mean-square error of the ocean temperature and salinity in the subsurface layer. It is interesting that the assimilation of the in-situ temperature profiles in the Korean marginal seas contributes to improving the reproducibility of the subsurface temperature and salinity in the East/Japan Sea (EJS) as well as Kuroshio-Kuroshio Extension (K-KE) regions. The improvement in the K-KE region seems to be related to the reproducibility of the Kuroshio axis. As the water mass in the EJS flows into the Pacific Ocean through the Tsugaru Strait, it affects the front of the Sanriku confluence, and it seems to eventually control the Oyashio Current and Kuroshio axis. In conclusion, this study evaluated the contribution of each observation component to ocean analysis in the KOOS-OPEM and confirmed the role of the existing observation networks. This study also suggests that greater attention should be paid to the role of regional ocean observation networks to improve the forecast skill of the ocean prediction system not only in the region but also in the open ocean, such as the Pacific Ocean.

Data Assimilation Using High‐Frequency Radar for Tsunami Early Warning: A Case Study of the 2022 Tonga Volcanic Tsunami

AbstractHigh‐frequency (HF) radar monitors the sea surface current velocity and provides information for tsunami early warning. SeaSondeR, an HF ocean radar system in the eastern Tsugaru Strait, Japan, measured the tsunami‐induced current velocity during the 2022 Tonga volcanic tsunami. As an air‐coupled tsunami, the generating mechanism was complex, making it difficult to predict coastal tsunamis using traditional early warning methods. We adopted the tsunami data assimilation (DA) approach, which reconstructs the tsunami wavefield using offshore data and does not require source information, to forecast the coastal tsunami waveforms. Observations from the HF radar and offshore bottom pressure gauges (OBPGs) were used as the input for tsunami DA. The assimilation process started at 09:00 (UTC, hereafter) and forecasts were made at 14:00 and 15:00. The surface current velocity recorded by the HF radar reached the maximum (∼0.25 m/s) at 13:00, which corresponded to a negative phase of ∼2 cm sea level variation observed by OBPGs. We compared the forecasted waveforms with the observed waveforms at Hakodate and Shimokita tide gauges. The assimilation results obtained using HF radar showed a better performance in tsunami forecasting than those using OBPG in this case study. At 14:00, the forecasting accuracy indices were 91% and 67% for the next 2 and 6 hr, respectively. At 15:00, it was 63% and 70% for the next 2 and 6 hr, respectively. We suggest that HF radar could be a good supplement to OBPG for tsunami early warning purposes.

Predicting catch of Giant Pacific octopus Enteroctopus dofleini in the Tsugaru Strait using a machine learning approach

We attempted to forecast catches of giant Pacific octopus Enteroctopus dofleini using past catch data and the machine learning approach. The algorithms tested were the Seasonal Autoregressive Integrated Moving Average (SARIMA), Light Gradient Boosting Machine (LightGBM), a type of Gradient Boosting Decision Tree (GBDT), and the Recurrent Neural Network (RNN), the latter of which is represented by Long Short-Term Memory (LSTM) and Gated Recurrent Unit (GRU). The catch of giant Pacific octopus on the southern coast of the Tsugaru Strait was backcasted for 2001–2019, using data of at least 20 years (1981–2018) for training. In this study, a longer training period led to better performance of SARIMA; the model trained from all past years performed better than models trained from the past 20 or 5 years. Moreover, for LightGBM, fishing-season accuracy of the basic L-GBM's was equal to that of L-GBM-T, which adds environmental factors to the explanatory variables. As a result of 19 backcasting trials, the SARIMA model obtained the best agreement with the actual catch data (R2 = 0.848) followed by L-GBM and LSTM (both R2 = 0.847), and GRU (R2 = 0.839). The root mean square error (RMSE) was within a narrow range for all four models (28.5, 28.0, 27.9, and 28.2 metric tons [t] for SARIMA, L-GBM, LSTM, and GRU, respectively). The SARIMA model was unstable when performance was assessed by the mean absolute percentage error (MAPE), which was standardized by average catch per year, whereas LSTM and GRU remained relatively stable. However, the latter two models were difficult to interpret due to their black box-like structures. By contrast, the SARIMA model was advantageous for interpreting the results. LightGBM showed good predictive performance for time-series data, despite being a static model. Thus, each model has advantages and disadvantages, so it is necessary to choose the best model depending on the purpose.

Intrusion of Coastal Oyashio water to Funka Bay and Tsugaru Strait occasionally disturbed by Kuroshio-originating warm core ring

Intrusion of Coastal Oyashio water to Funka Bay and Tsugaru Strait occasionally disturbed by Kuroshio-originating warm core ring

First Satellite Tagging of the Northern Fur Seals (Callorhinus ursinus) on the Tyuleniy Island, the Sea of Okhotsk.

In October 2018, three Northern fur seals (two adult females and one juvenile male) were deployed with satellite tags on the Tyuleniy Island in the Sea of Okhotsk. The operational time of the tags ranged from 33 to 203 days. The adult females started their winter migration in the first half of November; the initial stage of their winter migration occurred in the Japan/East Sea, which they entered through the La Perouse Strait. The juvenile male left the rookery in mid-October, crossed the Sea of Okhotsk in a north-western direction and returned to the south. The male had reached the coastal areas of Hokkaido Island, Japan by the end of November. From the Sea of Okhotsk, the male entered the Pacific Ocean through the Yekaterina Strait and subsequently entered the Japan/East Sea via the Tsugaru Strait. The winter foraging of the male occurred within the north-eastern part of the Japan/East Sea just off the Tsugaru Strait. After 3 months, the male returned to the Pacific and remained off the Sanriku Coast (Honshu Island).

Subinertial frequency variations in the axis of the Tsugaru Warm Current east of the Tsugaru Strait

The eastern part of the Tsugaru Strait (located at ~ 41.5° N) is a remarkable region of the coastal flow system around Japan due to the Tsugaru Warm Current (TgWC), an intensive eastward current that flows over its complex topography and exhibits marked tidal variation. The TgWC has a jetlike structure and demonstrates drastic seasonal changes in its path and vertical current structure. Using a high-resolution dataset obtained from a high-frequency radar system (HFR) and a coastal data assimilation experiment (JCOPE-T DA), we focused on the frontal variation, which is shorter than the seasonal timescale, along the axis of the TgWC in relation to the meanders of the jet. We found variations in the axis with multiple timescales between 1 and 14 days, especially in summer and autumn. The ~ 14-day variation, a remarkable occurrence on the west side of the eastern Tsugaru Strait, was consistent with the tidal variation in the strait. The zonal scale of the axis meander estimated by the HFR matched a scale several times larger than the internal deformation radius, especially in summer and autumn. JCOPE-T DA outputs showed that the crest of such a meander at greater depths progresses in relation to that at the surface. The outputs also suggested instabilities, indicating potential and kinetic energy conversion from the mean to the eddy field. The propagation speed of the several-day variation in the axis estimated from the HFR data was consistent with that of the two-layer baroclinic instability. The several-day variation also showed relatively high coherence with wind variation from summer to autumn, during which time the seasonal wind over the strait switched from easterly to westerly. These variations with multiple timescales imply an effect on north–south water mass mixing in the eastern part of the strait, including rapid acidification in the strait and intensification of stratification.

Genetic evidence for multiple mating and mixing of sperm from multiple males in the seminal receptacles of female Japanese flying squid (Todarodes pacificus)

ABSTRACT The genetic diversity of sperm and spermatangia stored by mated Japanese flying squid (Todarodes pacificus) was examined to better understand how this ommastrephid squid mates and stores sperm. Analyses were conducted using spermatangia implanted in the lips of immature squid collected in Tsugaru Strait, northern Japan, in July–August 2019, spermatangia implanted in the lips of mature squid collected in the East China Sea in April 2019, and sperm stored in the seminal receptacles of squid collected off Tosashimizu, Kochi Prefecture, Japan, in February 2019. DNA was extracted, microsatellite loci were amplified using five PCR primers, and allele peaks were analysed. Analysis of the spermatangia confirmed that females in the field mate with multiple males, suggesting that males transfer from 1 to at least 14 spermatangia in each mating. Analysis of the seminal receptacles showed that sperm from multiple males are mixed within individual sperm chambers. Spermatangia were also found implanted in the lips of males, providing evidence of same-sex sexual behaviour. To date, studies of cephalopod mating systems have focused on coastal species, and the results of this study add to our growing knowledge of more oceanic species.

Host preference of the tree‐killing bark beetle Polygraphus proximus across a geographic boundary separating host species

AbstractThe life cycles of bark beetles are intimately linked to their host species. Therefore, bark beetle species are expected to show traits that vary among host species and across geographic ranges. The taxonomic proximity of host tree species can also influence host selection. Abies species (Pinaceae) native to Japan are genetically classified into three phylogenetic groups. Their natural distributions are separated by a distinct biogeographic boundary in Japan (i.e., the Tsugaru Strait, also referred to as Blakiston's Line). Therefore, I investigated the host preference of Polygraphus proximus Blandford (Coleoptera: Curculionidae, Scolytinae) within four allopatrically distributed Abies species across north and south of the Tsugaru Strait through field surveys and field choice experiments. Abies sachalinensis (F. Schmidt) Mast. is the only species native to Hokkaido, north of the Tsugaru Strait. Field observations and field host‐choice bioassays showed that A. sachalinensis was less severely attacked by P. proximus than was Abies veitchii Lindl. on both sides of the Tsugaru Strait. The beetles also preferred to attack Abies firma Siebold & Zucc. – which belongs to a genetically distinct group (i.e., section Momi) from that of A. veitchii and A. sachalinensis (i.e., section Balsamea) – over A. sachalinensis in both Hokkaido and Honshu. Although Abies homolepis Siebold & Zucc. also belongs to the section Momi, A. firma and A. veitchii had more attacks than A. homolepis. The results provide evidence that P. proximus does not show specialization across the various parts of their geographic range where the host species are different, and there is no positive correlation between taxonomic proximity and host preference for P. proximus.

Security risk assessment and visualization study of key nodes of sea lanes: case studies on the Tsugaru Strait and the Makassar Strait.

Key nodes of sea lanes are important hubs for global trade and cargo transportation and play important roles in ensuring the safety of maritime transportation and maintaining the stability of the global supply chain. The safety guarantee of key nodes of sea lanes is facing more risks and higher requirements currently because the global shipping industry is gradually recovering. This paper focuses on key nodes of sea lanes, conducting regional security risk assessment and risk spatial scale visualization. A set of security risk assessment and visualization study methods for key nodes of sea lanes is constructed, which includes constructing a security risk assessment index system of key nodes of sea lanes with 25 indicators selected from three risk categories (hazard, vulnerability and exposure, and mitigation capacity) and using geospatial analysis to form the multi-criteria spatial mapping layers and then creating comprehensive risk layers to realize the risk visualization in the strait area by weighted overlaying based on the combined weights calculated by Analytic Hierarchy Process and Grey Relational Analysis. After taking the Tsugaru Strait and Makassar Strait as case studies, the results show that the comprehensive risk layers can effectively present the spatial distribution of security risks of key nodes of sea lanes, reflecting the spatial changes of risk levels (i.e., very low, low, medium, high and very high) and the methods can precisely identify and analyze crucial factors affecting the security risk of key nodes. These findings may strengthen the risk prevention and improve the safety of the navigation environment in the strait to ensure the safety and stability of maritime trade.

Forecasting design values of tidal/ocean power generator in the strait with unidirectional flow by deep learning

• Design values of a tidal/ocean power generator are forecasted by deep learning. • The power generator is operated in the strait with unidirectional flow. • Flaring Flanged Diffuser is used as a part of the tidal/ocean power generator. • The shape of the FFD can be automatically decided based on the stream regime. • The forecasting method can be applied in the design of power generators. Renewable energy is an essential factor in guaranteeing the sustainability of society. In Japan, there have been developments to harness energy from the ocean. The Tsugaru strait, in the northern region of Japan, is an area that has attracted attention for this purpose. We propose a tidal/ocean power generator utilizing a Flaring Flanged Diffuser (FFD) to harness the power. However, for the power generators utilizing FFD to generate power at the optimal condition, design values based on the stream regimes need to be determined. In this paper, the objective is to forecast the design values of tidal/ocean power generators utilizing FFD. We are especially interested in the dimensions of the diffuser shape that relate to effective factors for increasing flow velocity. Fluid field data around FFD is obtained by experimentation. The fluid field data is measured by particle image velocimetry (PIV). The trained deep neural network can forecast design values from a given fluid field. Moreover, we can recognize correlations between the changes in design values and the increase of fluid velocity.

Environmental changes clarified by pollen and diatom proxy records in the sedimentary archive of the northwestern Japan Sea during last 21.0 kyr

High-temporal resolution pollen and diatom records from the northwestern Japan Sea document environmental changes over the last 21.0 kyr. Vegetation and climate changes were evaluated using the forest-steppe (SFI) and the pollen temperature (Tp) indices. The highest SFI occurred between ca. 21.0 ka and 16.6 ka and indicates the presence of a relatively dry climate and large areas of forest-steppe in the region. Boreal forests covered the mountain slopes. The broad-leaved trees survived in refugia located in lower mountain belts. Beginning ca. 16.6 ka, SFI decreases, while Tp increases, especially between ca. 11.0 ka and 5.7 ka. However, variations in these indices indicate fluctuating climates during the deglaciation. Significant vegetation changes occurred in the Holocene. Mixed deciduous forests with a predominance of Quercus became widely present beginning ca. 9.8 ka. The area of forest-steppe was reduced occurring only in local habitats. Pinus became one of the most common taxa in the forest ca. 5.3 ka. The geographic features of the coring area have determined in the past the persistence throughout the record of cold-water diatom flora. Cold-water diatoms have dominated in both cool and warm intervals; however, a small increase in the percentage and species diversity of warm-water diatoms was noted for warming. Low sea level during the Last Glacial Maximum (LGM) and early Holocene changed the general patterns in the sea circulation and in the vectors of the longshore currents, which transported the waters formed in the shallow areas of the northeastern and northern parts of the Japan Sea. A slight increase in sublittoral benthic diatoms indicates a lowered sea level during the LGM. The increase in sublittoral planktonic diatoms reflects a gradual rise in sea level during the Bølling–Allerød (BA) and Younger Dryas (YD). A greater component of oceanic diatom species at Holocene indicates the inflow of Pacific waters across both the Tsushima and Tsugaru straits. This study of pollen and diatom records demonstrates a clear response to climatic fluctuations and sea level changes in the Japan Sea during the Late Pleistocene and Holocene.

北海道津軽海峡沿岸域で認められたイベント堆積物

北海道津軽海峡沿岸域で認められたイベント堆積物

Mesoscale Dynamics and Eddy Heat Transport in the Japan/East Sea from 1990 to 2010: A Model-Based Analysis

The driving mechanisms of mesoscale processes and associated heat transport in the Japan/East Sea (JES) from 1990 to 2010 were examined using eddy-resolving ocean model simulations. The simulated circulation showed correctly reproduced JES major basin-scale currents and mesoscale dynamics features. We show that mesoscale eddies can deepen isotherms/isohalines up to several hundred meters and transport warm and low salinity waters along the western and eastern JES boundaries. The analysis of eddy kinetic energy (EKE) showed that the mesoscale dynamics reaches a maximum intensity in the upper 300 m layer. Throughout the year, the EKE maximum is observed in the southeastern JES, and a pronounced seasonal variability is observed in the southwestern and northwestern JES. The comparison of the EKE budget components confirmed that various mechanisms can be responsible for the generation of mesoscale dynamics during the year. From winter to spring, the baroclinic instability of basin-scale currents is the leading mechanism of the JES mesoscale dynamics’ generation. In summer, the leading role in the generation of the mesoscale dynamics is played by the barotropic instability of basin-scale currents, which are responsible for the emergence of mesoscale eddies, and in autumn, the leading role is played by instabilities and the eddy wind work. We show that the meridional heat transport (MHT) is mainly polewards. Furthermore, we reveal two paths of eddy heat transport across the Subpolar Front: along the western and eastern (along 138∘ E) JES boundaries. Near the Tsugaru Strait, we describe the detected intensive westward eddy heat transport reaching its maximum in the first half of the year and decreasing to the minimum by summer.

The Role of an Intense Jet in the Tsugaru Strait in the Formation of the Outflow Gyre Revealed Using High‐Frequency Radar Data

AbstractThe seasonal features of the eastward jet through the Tsugaru Strait was investigated using surface velocity data between 2014 and 2019 obtained from high‐frequency radar located in the eastern part of the strait, as well as data obtained from repeated shipboard observations from 2009 to 2019. The potential‐vorticity‐front‐model was used, and the volume transport of low potential vorticity (PV) water and the intensity of the PV gap at the front in the eastern part of the strait were estimated from these data. The influence of these parameters on the seasonal flow pattern of the Tsugaru Warm Current out to the Pacific Ocean (the summer/autumn “gyre mode” or winter/spring “coastal mode”) was examined. The distribution of the parameters was consistent with the theoretical understanding, showing that in addition to the volume transport, an increase in the local PV gap can also contribute to the development of the gyre.

The phylogeographic puzzle of Pseudoacanthocephalus toshimai, an amphibian acanthocephalan in northern Japan.

The amphibian acanthocephalan, Pseudoacanthocephalus toshimai, was considered to be an island-endemic species in Hokkaido, Japan. However, the parasite was found from Rana ornativentris, Rana tagoi, Zhangixalus arboreus, and Bufo japonicus formosus in northern Honshu (Aomori and Iwate Prefectures), which is separated from Hokkaido by the Tsugaru Strait. The mitochondrial DNA-based phylogenetic and population genetic analyses of P. toshimai showed that the northern Honshu isolates are far distantly related to the Hokkaido isolates, and that a demographic population expansion occurred in Hokkaido during the recent geological past. The rich genetic diversity of P. toshimai in northern Honshu suggests a scenario that anuran hosts invaded Hokkaido together with P. toshimai via the land bridge of the Tsugaru Strait. However, the evolutionary history of Rana pirica, a main definitive host for P. toshimai in Hokkaido, is contradictory to the introduction scenario inferred from the parasite. The finding of several geographically mismatched isolates of P. toshimai from both northern Honshu and Hokkaido suggests a possibility that the migration of the parasite infrequently occurred between the two areas even after the land bridge disappeared. More detailed information on the evolutionary history of anurans is needed to resolve the biogeographical enigma of P. toshimai.