Southwestern Japan Sea Research Articles

Subsurface nutrient maximum and submesoscale structures in the southwestern Japan Sea

From summer to autumn, in the southwestern Japan Sea, the maximum subsurface nutrient (DIN: nitrate + nitrite) concentration and minimum dissolved oxygen (DO) concentration layers lie at depths of approximately 75–125 dbar. The subsurface nutrient-rich water originates from the East China Sea through the lower layer (50–100-dbar depths) of the Tsushima Straits. On a smaller scale, in the subsurface nutrient-rich layer, submesoscale structures (~10 km) with high DO and chlorophyll-a (Chl-a) concentrations were observed below the surface euphotic zone and the minimum Chl-a layer in late October 2008. Considering that photosynthesis cannot occur in the minimum Chl-a layer, the submesoscale Chl-a maxima did not coincide with the normal maximum subsurface chlorophyll layer, where phytoplankton can mature, in the seasonal pycnocline in summer. The subsurface submesoscale water masses were zonally distributed from west to east downstream of the Tsushima Warm Current. In late November 2008, high DO and Chl-a water subducted from the surface density front to the subsurface layers along the isopycnal surface. In the large-scale frontal region of the Tsushima Warm Current, the subsurface isopycnals were convex upward both in October and November 2008, suggesting meso- and submesoscale upwellings.

Distribution of Nemopilema nomurai in the southwestern Sea of Japan related to meandering of the Tsushima Warm Current

We investigated the horizontal distribution of Nemopilema nomurai medusae using a midwater trawl in the southwestern Sea of Japan from September to October of each year from 2006 to 2012. Numerous medusae of this species found in 2006, 2007, and 2009 were mainly distributed far (>40 km) from the mainland of Japan in the western part of the survey area, but were distributed in the stations closest (<25 km) to the mainland in the eastern part, particularly in 2006 and 2009. These distribution patterns were associated with the path of the second branch of the Tsushima Warm Current (TWC) that flows offshore to the west of Oki Islands and usually approaches the mainland of Japan to the east of Oki Islands. Differences in jellyfish distribution across the survey periods were related to the distribution of the second branch being associated with types, position and intensity of eddies in the eastern part of the survey area. Hence, the formations of eddies and consequent variation in the path of the TWC would be responsible for transporting medusae from offshore to near the mainland to the east of Oki Islands. Individuals with large bells accumulated in the northwestern part of the survey area, where a cyclonic eddy was present downstream of the third branch of the TWC. This variation in bell size distribution may be ascribed to differences in the physical and/or biological factors among the three branches of the TWC.

Migratory movements of rhinoceros auklets in the northwestern Pacific: connecting seasonal productivities

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 525:229-243 (2015) - DOI: https://doi.org/10.3354/meps11179 Migratory movements of rhinoceros auklets in the northwestern Pacific: connecting seasonal productivities Akinori Takahashi1,2,*, Motohiro Ito1, Yuuya Suzuki3, Yutaka Watanuki3, Jean-Baptiste Thiebot1, Takashi Yamamoto1,3, Takahiro Iida1,2, Phil Trathan4, Yasuaki Niizuma5, Tomohiro Kuwae6 1National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan 2Department of Polar Science, The Graduate University for Advanced Studies (SOKENDAI), Tachikawa, Tokyo 190-8518, Japan 3Graduate School of Fisheries Sciences, Hokkaido University, 3-1-1 Minato-cho, Hakodate 041-8611, Japan 4British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK 5Faculty of Agriculture, Meijo University, 1-501, Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan 6Port and Airport Research Institute, 3-1-1, Nagase, Yokosuka 239-0826, Japan *Corresponding author: atak@nipr.ac.jp ABSTRACT: Spatial and temporal variability in marine biological productivity may drive heterogeneity in seasonal resources available for marine animals in temperate waters. Migratory seabirds are expected to adjust their annual cycle of breeding activities and migratory movements to exploit seasonally available resources efficiently. We studied the movement and trophic position of rhinoceros auklets Cerorhinca monocerata breeding at Teuri Island, Japan Sea, during the nonbreeding and early breeding periods over 2 yr. After breeding, the auklets moved northward from the colony to the Sea of Okhotsk, where phytoplankton blooms enhanced biological productivity in autumn. The birds then moved southward to the southwestern Japan Sea (~1470 km from the colony), where major epipelagic fish and squid concentrations have been reported in winter. Stable isotope analyses suggest that the auklets fed on higher-trophic level prey, including fish and/or squid during the autumn and winter nonbreeding periods. The auklets moved northward and returned to the colony in mid-March. During the early breeding period, the birds foraged close to the colony (~380 km) on lower-trophic level prey including fish and/or krill, which were available during the spring phytoplankton bloom. The timing of the return migration does not match with the northward migration of warm-water anchovy, a profitable prey during summer, but may be related to timing the chick-rearing period to correspond with anchovy arrival. We suggest that rhinoceros auklets follow spatial and seasonal changes in prey availability by a distinctive ‘3-step’ migration (first northward, second southward, third northward) in the temperate marine system of the northwestern Pacific. KEY WORDS: Seabird foraging · Light-based geolocation · Productivity · Seasonal movements · Stable isotope analysis · Japan Sea Full text in pdf format PreviousNextCite this article as: Takahashi A, Ito M, Suzuki Y, Watanuki Y and others (2015) Migratory movements of rhinoceros auklets in the northwestern Pacific: connecting seasonal productivities. Mar Ecol Prog Ser 525:229-243. https://doi.org/10.3354/meps11179 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 525. Online publication date: April 09, 2015 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2015 Inter-Research.

First records of small-sized young giant squid Architeuthis dux from the coasts of Kyushu Island and the south-western Sea of Japan

Three individuals of small, young giant squidArchiteuthis duxwere caught in or near Japanese coastal waters during April–June 2013. The first occurrence of a young individual of 140.8 mm dorsal mantle length with a total weight of 44.81 g was recorded on the coast off Uchinoura, Kagoshima, Kyushu Island, southern Japan. The remaining two young individuals, both measuring 332.0 mm in dorsal mantle length with total weights of 390.63 and 356.95 g respectively, were caught on the coast off Hamada, Shimane, south-western Sea of Japan. Most morphological traits of all the specimens were consistent with those ofA. dux, which was supported by further genetic analysis. Here we present some morphological traits of young giant squids with morphometric data and photographs.

Age Composition of Japanese Spanish Mackerel Scomberomorus niphonius (Cuvier 1832) Caught off Hyogo Prefecture, South-western Sea of Japan, as Determined by the Otolith Cross-section Method

Age Composition of Japanese Spanish Mackerel Scomberomorus niphonius (Cuvier 1832) Caught off Hyogo Prefecture, South-western Sea of Japan, as Determined by the Otolith Cross-section Method

Age of the Taishu Group, southwestern Japan, and implications for the origin and evolution of the Japan Sea

AbstractThe Taishu Group, a marine formation with a thickness of &gt;5400 m, crops out on Tsushima Island, located in the southwestern Japan Sea. The group, which is generally regarded as early Eocene to early Miocene in age, provides important information about the tectonic setting of the Japan Sea. In this study, we present new SHRIMP U–Pb dates for igneous zircons from the Kunehama Tuff, which is in the basal part of the Taishu Group, and the Oobaura Tuff, which is in the uppermost part of the group. Results show that the Taishu Group was deposited rapidly, during the short interval of 17.9–15.9 Ma (early–middle Miocene), and is equivalent to other early–middle Miocene strata found in the Japan Sea region. Our results provide new constraints on the geological history of the Japan Sea and its islands.

日本海東部西津軽沖, 久六島東方中層域における第四紀後期の有孔虫群集と珪藻群集

We analysed assemblages of diatoms and foraminifera in a late Quaternary sequence from the core MD179-3326G (core length; 5.53 m) recovered at the seabed 325 m depth, east of Kyuroku Island, eastern part of the Japan Sea. We recognized six foraminferal bio-zones which are comparable to those from two sediment cores at the upper slope (around 300 m depth) in the southwestern Japan Sea (off Tottori). The occurrence of Elphidium excavatum suggests that the “Oyashio” cold surface water would have flowed into the Japan Sea through the Tsugaru Strait during the early part of the last deglacial stage. The distribution of Verneuilinulla advena in the Japan Sea was restricted to the northern shallow to intermediate area, which is narrower than that of E. excavatum. Furthermore, the bottom environments of the intermediate part in the Japan Sea could have been different between the east (east of Kyuroku Island) and the northwest (off Tottori) during the early part of the last deglaciation.

Wood Sheds with Stone Roof in Tsushima Islands, Southwestern Japan Sea, Have the Merit of an Earth Heritage

Wood Sheds with Stone Roof in Tsushima Islands, Southwestern Japan Sea, Have the Merit of an Earth Heritage

Regional spatial distribution of multiple elements in the surface sediments of the eastern Tsushima Strait (southwestern Sea of Japan)

A total of 402 coastal sea-sediment samples were collected from the continental shelf, slope, and basin off Tsushima Island in the western Sea of Japan, and were analyzed for 51 elements as part of a nationwide marine geochemical mapping project. The samples were compared to potentially related sample sets, and the results were considered from the viewpoint of the origin of marine surface sediments in the western Sea of Japan. The spatial distribution of elemental concentrations in the coastal sea sediments correspond to texture, grain size, the presence of shells and foraminifera, and the mud content of surface marine sediments. Most elemental concentrations increased with increasing mud content. Some samples located in littoral areas included sediment particles apparently supplied from nearby rivers, but their contribution was limited. Overall, the mean chemical compositions of clastic material in coastal sea sediments appear to differ from those of stream sediments in adjacent terrestrial areas. In addition, the geochemistry of the coastal sea sediments cannot be fully explained by the mixing of the material supplied from Korean and Chinese stream sediments, which are the most feasible sources. Coastal sea sediments in the study area are well mixed by transportation processes; therefore, elemental abundances in these marine sediments may be homogenized to such an extent that it is not possible to determine their origin. Alternatively, most of the clastics in the sediment may actually represent relict reworked material, originally formed in the Yellow Sea and Tsushima Strait during the last glacial stage and subsequent transgression.

Lateral variation of 134Cs and 137Cs concentrations in surface seawater in and around the Japan Sea after the Fukushima Dai-ichi Nuclear Power Plant accident

A total of 82 surface seawater samples was collected in the Japan Sea and the southwestern Okhotsk Sea before and after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Analysis of 134Cs and 137Cs concentrations using low-background γ-spectrometry revealed that the 137Cs concentration of the samples collected in June 2011 was 1.5–2.8mBq/L, which is approximately 1–2 times higher than the pre-accident 137Cs level, while the 134Cs concentration was less than detectable to 1mBq/L. In addition to 134Cs being clearly detected (∼1mBq/L), 137Cs concentration in water samples from the northeastern Japan Sea (2–2.8mBq/L) was also higher than that from the coast in the southwestern Japan Sea (∼1.5mBq/L). These higher concentrations in the northeastern Japan Sea could be ascribed to the atmospheric transport of nuclides from the FDNPP as aerosols and subsequent transport and dilution after delivery to the sea surface.

Size Structure and Reproduction of Violet Shell Janthina prolongata Stranded on the Coast of Iwami-cho in Tottori Prefecture, Southwestern Sea of Japan

Size Structure and Reproduction of Violet Shell <i>Janthina prolongata</i> Stranded on the Coast of Iwami-cho in Tottori Prefecture, Southwestern Sea of Japan

Spatial and Temporal Variation of a Cyclonic Eddy Detected Downstream of the Tsushima Islands in November 2007

Horizontal, vertical and temporal distribution of a cyclonic (counterclockwise) eddy, where biological productivity is high, downstream of the Tsushima Islands in the eastern channel of the Tsushima Straits in November 2007 was revealed using conductivity–temperature–depth and acoustic Doppler current profiler data. The eddy had a horizontal scale of approximately 40–60 km, and the accompanying baroclinic current was more than 15 cm s−1 at the edge of the eddy. The island-induced cyclonic eddy moved east-northeastward at about 10 km day−1 (∼10 cm s−1) along the Tsushima Warm Current and was intensified by the barotropic instability in the current shear. The cyclonic eddy with high surface chlorophyll a concentrations intensified in the vicinity of the Tsushima Islands and was advected by the Tsushima Warm Current towards the southwestern Japan Sea.

Interannual–interdecadal variations of spear squid Loligo bleekeri abundance in the southwestern Japan Sea during 1975–2006: Impact of the trawl fishing and recommendations for management under the different climate regimes

A climatic regime shift, characterized by an abrupt change from cool to warm conditions, occurred in the Tsushima Warm Current (TWC) region in the Japan Sea in the late 1980s. The abundance of spear squid Loligo bleekeri in the southwestern part of the Japan Sea responded strongly to the changing thermal regime. Spear squid are widely distributed in Japanese coastal waters and form one of the most important target species of pair trawlers in the southwestern Japan Sea. Catch from the southwestern Japan Sea has fluctuated from a minimum of 16 tons in 2003 to a maximum of 13,700 tons in 1977. Catch has decreased to less than 100 tons in recent years, and consequently the stock is at the point of collapse. The abundance index of spear squid shows a decadal pattern of variation with a step change from positive to negative annual anomalies that occurred around 1990; this pattern corresponds closely with the changing regime in water temperature in the TWC, strongly indicating that the decadal variability in spear squid was largely affected by the climatic regime shift in the late 1980s. Significant negative correlations between water temperature and an abundance index of spear squid ( R 2 = 0.39, p < 0.01) indicate that increasing ambient water temperature reduced spear squid abundances during the 1990s. Monthly CPUE (catch per unit effort) decreases sharply during the fishing season from autumn through winter, a patterns that was well described by a DeLury model. DeLury estimates of apparent mortality showed that the fishing mortality coefficient was relatively stable during 1975–1987, but has increased greatly since the late 1980s. The average fishing mortality coefficient averaged during the warmer 1990s was about 2.1 times higher than during the colder late 1970s and 1980s, indicating that the fishing mortality has intensified during the 1990s and has accelerated the collapse of the stock. This suggests that the decadal pattern of variation of spear squid was largely forced by the late 1980s climatic regime shift of the TWC, while high fishing magnified interannual variation of spear squid and attributed to the subsequent collapse of the stock. Reducing fishing pressure on juvenile squid during the early part of the fishing season in autumn is considered to be particularly important for the recovery of this stock under the current unfavorable thermal regime.

Seasonal variation in Tsushima Warm Current paths over the shelf off the San’in coast, Japan

Using long-term sea surface temperature (SST) and acoustic Doppler current profiler (ADCP) data, we examined variations in the current axis of the Tsushima Warm Current (TWC) off the San’in coast of Japan, near the entrance to the Japan Sea. There were large horizontal temperature gradients along the shelf edge in the southwestern Japan Sea from October to May, suggesting that the second branch of the TWC appears not only in spring and autumn but also in winter. From the ADCP data analysis, we found that currents with speeds of approximately 20 cm s −1 and greater appeared around the shelf edge off San’in coast in all seasons. The SST and ADCP data analyses suggested that the second branch of the TWC exists around the shelf edge off the San’in coast throughout the year. This finding differed from those of previous studies. A relatively strong current (speed greater than 15 cm s −1) appeared on the shore side in all seasons, except at line W in winter. This current might be the first branch of the TWC. The first branch seemed to occur around in 100 m isobaths, but shifted northward and southward because the bottom topography around lines W and M was relatively flat and the shelf was broad. The first branch was very obscure, and it was difficult to define the two branches of the TWC off the San’in coast from the seasonally averaged vectors. However, snapshots of current distribution derived from the ADCP data clearly showed these branches. Hence, both the first and second branches might occur throughout the year off the San’in coast.

Characteristics of water mass under the surface mixed layer in Tsushima Straits and the southwestern Japan Sea in autumn

Two different cold waters were found under the surface mixed layer in Tsushima Straits and the southwestern Japan Sea in autumn 2004. One is cold saline water with a low concentration of dissolved oxygen, and the other is cold less saline water with a high concentration of dissolved oxygen. The older saline water originates from the bottom of the East China Sea, strongly influenced by the Kuroshio water with high salinity. The bottom density in the eastern channel of the Tsushima Straits is coincident with that of the East China Sea in autumn, corresponding to the season when the cold saline water was frequently found in the Tsushima Straits. The newer less saline water originates from the front of Tsushima Warm Current between the Tsushima Warm Current water and the surface cold water in the Japan Sea. This water is formed by subduction above the isopycnal surface from the front of the Tsushima Warm Current.

Recent benthonic ostracod assemblages as indicators of the Tsushima warm current in the southwestern sea of Japan

One hundred and sixty-seven species of podocopid ostracods are reported from 26 recent surface sediment samples collected on the shelf of the Tsushima Strait in the southwestern Sea of Japan. Four biofacies (A–D) are recognised based on Q-mode cluster analysis. Among these, biofacies B is distributed along the Tsushima Warm Current and includes typical East China Sea species. This fact suggests that the migration of the shelf-living ostracods between the open sea and the adjacent semi-enclosed marginal sea mainly occurs in association with the inflow of this sea current. The distribution of the recent ostracod assemblages from the Tsushima Strait are expected to be a sensitive indicator of past and future changes in the Tsushima Warm Current. One new ostracod species of the family Trachyleberididae, Trachyleberis ikeyai, is described and illustrated herein.

Vertical distribution of Todarodes pacificus (Cephalopoda: Ommastrephidae) paralarvae near the Oki Islands, southwestern Sea of Japan

The diel vertical distribution patterns of Japanese common squid, Todarodes pacificus, paralarvae were examined using a Multiple Opening Closing Net and Environmental Sensing System (MOCNESS) in the southwest Sea of Japan near the Oki Islands (Japan) during five late-autumn surveys in 1998–2002. A total of 1,511 paralarvae ranging in mantle length (ML) from 0.7 to 7.3 mm were collected at 63 of the 68 stations surveyed. Most (84%) were collected above 75 m depth and in the mixed layer. The vertical distribution patterns varied little between day and night. Hatchling-sized (<1.0 mm ML) paralarvae were abundant at 0–25 m depth, and paralarval ML increased with increasing sampling depth. Our results suggest that T. pacificus paralarvae do not exhibit large diel vertical migration patterns, but as they increase in size, paralarvae gradually descend in the water column and the variability in depth increases with ontogeny.

A numerical study on nutrient sources in the surface layer of the Japan Sea using a coupled physical‐ecosystem model

A nitrogen‐based four‐compartment ecosystem model coupled with a physical model is applied to the Japan Sea. Seasonal variability in ecosystem dynamics simulated by the coupled model is in good agreement with the observations. A set of artificial tracers labeled according to specified regions is introduced to identify nutrient sources in the surface layer of the Japan Sea. The results of the labeled tracer experiment show that one of the large nutrient sources in the southern Japan Sea is located along the east coast of Korea. The upward flux of dissolved inorganic nitrogen along the east coast of Korea and the nitrogen flux through the western channel of the Tsushima/Korea Straits (TKS) sustain high primary productivity in the southwestern Japan Sea. The surface nutrient conditions in the nearshore region along the Japanese coast depend on the nitrogen flux through the eastern channel of the TKS throughout the year. Since materials transported through the TKS have significant impact on the biological productivity of the southern Japan Sea, field observations of chemical and biological data, in addition to physical ones, should be continued in the TKS.

Millennial-scale fluctuations in seasonal sea-ice and deep-water formation in the Japan Sea during the late Quaternary

The East Asian winter monsoon is characterized by cold, dry air blowing from the Siberian high to the surrounding lows. This cold wind cools the surface water along the Russian coast of the Japan Sea. Modern oceanographic observations indicate that sea-ice formation along this coast during an extremely cold winter is an important process for deep-water formation. Thus, formation of both seasonal sea ice and deep-water masses in the Japan Sea are closely related to the East Asian winter monsoon. Two sediment cores were analyzed to reconstruct the history of sea-ice expansion and deep-water formation. Seasonal sea-ice history was reconstructed from the occurrence of ice-rafted debris (IRD) in a core from the northeastern Japan Sea, whereas the deep-water ventilation history was estimated on the basis of records of a deep-dwelling radiolarian species, Cycladophora davisiana, in a core from the southwestern Japan Sea. In the records of sea-ice expansion and deep-water ventilation for the last 160 ky, clear fluctuations on a millennial timescale were observed during marine oxygen isotope stages 3–5. Some of the peaks in sea-ice formation and C. davisiana abundance occurred contemporaneously, indicating strong surface cooling along the Russian coast by a strong winter monsoon. Decoupling of the two records might have occurred because of surface-water stratification caused by the freshening of surface water in relation to high precipitation in summer. Synchronicity of high IRD peaks with North Atlantic Heinrich events suggests that high-latitude climate changes related to the Bond cycle also occurred in the Asian monsoon region. On shorter timescales, IRD occurrence was not clearly related to Dansgaard–Oeschger cycles, which are revealed in the alternating dark and light layers in the late Quaternary Japan Sea sediments. Therefore, during the studied period, strong East Asian winter monsoons did not have a simple see-saw-like relationship with strong summer monsoons.

P-wave velocity structure of the margin of the southeastern Tsushima Basin in the Japan Sea using ocean bottom seismometers and airguns

The Tsushima Basin is located in the southwestern Japan Sea, which is a back-arc basin in the northwestern Pacific. Although some geophysical surveys had been conducted to investigate the formation process of the Tsushima Basin, it remains unclear. In 2000, to clarify the formation process of the Tsushima Basin, the seismic velocity structure survey with ocean bottom seismometers and airguns was carried out at the southeastern Tsushima Basin and its margin, which are presumed to be the transition zone of the crustal structure of the southwestern Japan Island Arc. The crustal thickness under the southeastern Tsushima Basin is about 17 km including a 5 km thick sedimentary layer, and 20 km including a 1.5 km thick sedimentary layer under its margin. The whole crustal thickness and thickness of the upper part of the crust increase towards the southwestern Japan Island Arc. On the other hand, thickness of the lower part of the crust seems more uniform than that of the upper part. The crust in the southeastern Tsushima Basin has about 6 km/s layer with the large velocity gradient. Shallow structures of the continental bank show that the accumulation of the sediments started from lower Miocene in the southeastern Tsushima Basin. The crustal structure in southeastern Tsushima Basin is not the oceanic crust, which is formed ocean floor spreading or affected by mantle plume, but the rifted/extended island arc crust because magnitudes of the whole crustal and the upper part of the crustal thickening are larger than that of the lower part of the crustal thickening towards the southwestern Japan Island Arc. In the margin of the southeastern Tsushima Basin, high velocity material does not exist in the lowermost crust. For that reason, the margin is inferred to be a non-volcanic rifted margin. The asymmetric structure in the both margins of the southeastern and Korean Peninsula of the Tsushima Basin indicates that the formation process of the Tsushima Basin may be simple shear style rather than pure shear style.