Southeastern Japan Sea Research Articles

Analysis of the hatching site and migratory behaviour of the swordtip squid (Uroteuthis edulis) caught in the Japan Sea and Tsushima Strait in autumn estimated by statolith analysis

ABSTRACTIn contrast to the swordtip squid (Uroteuthis edulis) caught in the spring and summer, those caught in the autumn in the Japan Sea and the Tsushima Strait are characterized by thick bodies with large clubs, long tentacles and large suckers. In addition, most females and many males are immature. However, the reasons for these characteristics, as well as the hatching site and migratory behaviour, are not well known. The empirical water temperature of juveniles was estimated at ∼17°C through strontium:calcium (Sr:Ca) ratios in statoliths and information on seawater temperature distribution. The ontogenetic variation in Sr:Ca ratios decreased from hatching to 60 days after hatching and then remained stable. The estimated water temperature was ∼21°C after 60 days. Thus, individuals of the autumn-migrating group most likely hatch in the southern East China Sea in the winter and early spring and then move north-eastwards with currents and pass through the Tsushima Strait into the southern Japan Sea during or after July. However, the squid could be present in these waters without strong currents northeast of Tsushima Island, where thermoclines occur during summer, and become confined in limited layers until vertical mixing occurs in the autumn. The distribution of the squid would thereafter extend to fishing grounds in the southern and south-eastern Japan Sea and then to the Tsushima Strait. This migratory behaviour could be the cause of the observed characteristics because bottom seawater temperatures are much lower in the Japan Sea than in the East China Sea.

Numerical simulation of the abrupt occurrence of strong current in the southeastern Japan Sea

Coastal set-net fisheries have been frequently damaged by the occurrence of sudden current (known as kyucho) in the Japan Sea. In this study, a high-resolution coastal ocean model is developed to provide a means to predict this stormy current. The 1.5km-mesh model nested in a regional ocean data assimilation system is driven by mesoscale atmospheric conditions at 1-hour intervals. The modeled results show rapid changes of the coastal current along the San-in Coast, on the eastern side of the Tango Peninsula, and around the Noto Peninsula and Sado Island, mostly associated with strong wind events. These modeled coastal water responses are consistent with in-situ velocity measurements. The simulation also shows that the vortex separated from the Tango Peninsula frequently grows to a bay-scale anticyclonic eddy in Wakasa Bay. Evidently, the coastal branch of the Tsushima Warm Current becomes unstable due to a strong meteorological disturbance resulting in the generation of this harmful eddy.

Polycyclic Aromatic Hydrocarbons in Surface Water of the Southeastern Japan Sea.

Surface water samples were collected at 15 sampling sites in the southeastern Japan Sea along the Japanese Archipelago for analysis of polycyclic aromatic hydrocarbons (PAHs). Water samples were fractionated by filtration through a glass fiber membrane (pore size 0.5 µm) and analyzed by high-performance liquid chromatography with fluorescence detection. Thirteen PAHs having 3 to 6 rings were found in the dissolved phase (DP) and 12 were found in the particulate phase (PP). The total (DP+PP) PAH concentration ranged from 6.83 to 13.81 ng/L with the mean±standard deviation (S.D.) concentration of 9.36±1.92 ng/L. The mean±S.D. PAH concentration in the DP and PP was 5.99±1.80 and 3.38±0.65 ng/L, respectively. Three-ring PAHs predominated in the DP, while the proportion of 4-ring PAHs was higher in the PP. The mean total PAH concentration in the southeastern Japan Sea was higher than the concentration in the northwestern Japan Sea (8.5 ng/L). The Tsushima Current, which originates from the East China Sea with higher PAH concentration, is considered to be responsible for this higher concentration.

Seismic constraints of the formation process on the back‐arc basin in the southeastern Japan Sea

AbstractTo clarify the formation process of the back‐arc basin in the Japan Sea, which is located next to the northwestern Pacific, a seismic survey using ocean bottom seismographs and an air gun array was undertaken in areas from the northern Yamato Basin to the coast of the northeastern Japan Island Arc off Awa‐shima Island. The crust beneath the northern Yamato Basin off Awa‐shima Island is approximately 16 km thick. The upper and lower crusts are, respectively, about 5 km thick with a steep velocity gradient and about 10 km thick with a gentle velocity gradient. In the basin, there are very few units with P wave velocity of 5.4–6.0 km/s, corresponding to the continental upper crust. The crustal structure of the northern Yamato Basin has characteristics of thicker oceanic crust. The high‐velocity lowermost crust in the northern Yamato Basin with 7.2–7.4 km/s might show melt formed by a slightly high mantle temperature during the opening of the basin. However, the crust beneath the areas from the Sado Ridge to the coast, which is approximately 25–26 km thick, is slightly thinner than that of the continental crust and island arc crust. The crustal structure beneath this area is inferred to be a rifted continental and/or a rifted island arc crust.

有孔虫群集からみた日本海暗色層堆積時の海洋環境 ― 日本海南東部MD01-2407コアの結果から ―

有孔虫群集からみた日本海暗色層堆積時の海洋環境 ― 日本海南東部MD01-2407コアの結果から ―

Vertical distribution, population structure and lifecycle of Eucalanus bungii (Copepoda: Calanoida) in the Oyashio region, with notes on its regional variations

Vertical distribution and population structure of Eucalanus bungii were investigated at site H in the Oyashio region (western subarctic Pacific) from September 1996 through October 1997 to evaluate the species’ lifecycle pattern and associated ontogenetic vertical migration. Additional temporary samplings were also made at several stations covering the entire subarctic Pacific, Okhotsk Sea and Japan Sea, as a basis for regional comparison of lifecycle features of this species. At site H, a marked phytoplankton bloom occurred from mid-March to June, and E. bungii spawned in April/May in the surface layer. Resulting nauplii and copepodite stage 1 (C1) formed a prominent abundance peak in early June. The C1 developed and reached C5 by August. The development of nauplii through C4 occurred in the surface layer. From August onwards, C5 and a small fraction of C3–C4 sank gradually deeper, and entered diapause to overwinter at >500 m depth. The C5 molted to C6 males and females in February and March, respectively. The C6 males and females mated at 250–500 m depth, and only mated C6 females ascended to the surface layer in April for spawning. Judging from the size of lipid droplets in the body, the C3–C5 specimens deposited lipids in the body through the phytoplankton bloom period, and the lipids were consumed gradually during overwintering. Taking account of sampling season, temporal changes in population structure, and vertical distribution, the data collected from the western subarctic Pacific and Okhotsk Sea are consistent with a 1-year lifecycle for the site H population, while the data from the central and eastern subarctic Pacific were consistent with a 2-year lifecycle. The populations from the southern and southeastern Japan Sea did not fit the features of either lifecycle scenario, and because of their very small population size it is suggested that they originated from the northern Japan Sea. Regional comparison of the prosome length of C6 females, including those in the Bering Sea, indicated significantly larger specimens from the Japan Sea and Okhotsk Sea, and smaller specimens in the eastern subarctic Pacific, as compared with those in the western subarctic Pacific (including site H) and Bering Sea. A possible overwintering mechanism of E. bungii is discussed.

The Japan Sea Intermediate Water; Its Characteristics and Circulation

In the southern Japan Sea there is a salinity minimum layer between the Tsushima Current Water and the Japan Sea Proper Water. Since the salinity minimum corresponds to the North Pacific Intermediate Water, it is named the Japan Sea Intermediate Water (JIW). To examine the source and circulation of JIW, the basin-wide salinity minimum distribution was investigated on the basis of hydrographic data obtained in 1969. The young JIW, showing the highest oxygen concentration and the lowest salinity, is seen in the southwestern Japan Sea west of 133°E, while another JIW with lower oxygen and higher salinity occupies the southeastern Japan Sea south of the subpolar front. Since the young JIW shows high oxygen concentrations, high temperatures and low densities, the source of the water is probably in the surface layer. It is inferred that the most probable region of subduction is the subarctic front west of 132°E with the highest oxygen and the lowest salinity at shallow salinity minimum. In addition, property distributions suggest that JIW takes two flow paths: a eastward flow along the subarctic front and an southward flow toward the Ulleung Basin. On the other hand, a different salinity minimum from JIW occupies the northern Japan Sea north of the subarctic front, which shows an apparently higher salinity and high oxygen concentration than JIW. However, this salinity minimum is considered not to be a water mass but to be a boundary between overlying and underlying water masses.

A quantitative study of seismic anisotropy in the Yamato Basin, the southeastern Japan Sea, from refraction data collected by an ocean bottom seismographic array

SUMMARY The anisotropy of the uppermost mantle beneath the Yamato Basin in the southeastern Japan Sea was studied quantitatively on the basis of data acquired during the DELP-85 Wakashio Cruise. Detailed seismic experiments were conducted along two seismic profiles, one of which was parallel to the strike of the oblong basin (Line A) and the other perpendicular to it (Line B). The inversion of the traveltime data of PmP (reflected from the Moho) provides a reliable constraint on the Moho depth. This inverted Moho and a previously reported 2-D model were used to isolate the effect of the lateral variations on the value of anisotropy. We obtained azimuthally good coverage of Pn (refracted in the uppermost mantle) data by combining shot points and observation points located on different lines. From these Pn data, the uppermost mantle velocities for each azimuth were determined by iterative ray tracing. A study of these results shows that the azimuthal anisotropy of Pn velocity is 9.1 per cent. The maximum velocity occurs at about 138° clockwise from north, which is perpendicular to the strike of the Yamato Basin (Line A). The magnitude of the anisotropy, 9.1 per cent, is nearly the largest value ever observed in the ocean. This high anisotropy may be due to the very fast spreading rate. The azimuthal dependence of the P- and S-wave velocity in the lower crust is not significant.

Sn Velocity under the Yamato Basin, Southeastern Japan Sea.

Sn Velocity under the Yamato Basin, Southeastern Japan Sea.

Two-dimensional P-and S-wave velocity structure of the Yamato Basin, the southeastern Japan Sea, from refraction data collected by an ocean bottom seismographic array.

A P- and S-wave velocity structure across the Yamato Basin, southeastern Japan Sea, was modeled on the basis of seismic data acquired during the DELP-85 Wakashio Cruise conducted by the Japanese Research Group of the International Lithosphere Project (ILP). Forward modeling of seismic wave field using the ray theoretical method was carried out to analyze the refraction data of two seismic profiles: one is a profile (230 km long) at the middle of the oblong basin along its strike (line A) and the other is perpendicular to it (line B). The result indicates that the crustal layering is twice as thick as in a normal oceanic basin but the P-wave velocity in each layer is very similar to that of the oceanic basin crust. The upper mantle has a P-wave velocity of 7.9-7.7 km/s in line A, and 8.3 km/s in line B. Moho is flat in line A with 17 km depth and is bowl-shaped in line B rising toward 14 km depth at both sides. The S-wave velocity model has a low velocity zone at depths between 12 and 16 km.

但馬のスルメイカの漁況

Taking the daily records of meteorological and oceanographical observations and those of catch per fisherman as basis, the authors investigated on the factors influencing the yield of squid off Tazima, south-eastern Japan Sea. During the whole fishing season, the rain makes the catch poor. High surface temperature of water is favourable ; the higher the better, except in July and August, when the effect is obscured.