Tsushima Strait Research Articles

Influence of the Changjiang River on the light absorption properties of phytoplankton from the East China Sea

Abstract. Phytoplankton light absorption properties were investigated at the surface and subsurface chlorophyll a maximum (SCM) layer in the East China Sea (ECS), a marginal sea which is strongly influenced by the Changjiang discharge in summer. Results from ECS were compared with those from the Tsushima Strait (TS) where the influence of Changjiang discharge is less. The probable controlling factors, packaging effect (cell size) and pigment composition of total chlorophyll a (Tchl a)-specific absorption coefficient (aph*(λ)) were examined by the corresponding measurements of pigments identified by high-performance liquid chromatography. We observed distinct phytoplankton size structure and thereby absorption properties between ECS and TS. At the surface, mixed populations of micro-, nano- and pico-phytoplankton were recorded in ECS while pico-phytoplankton dominated in TS, generating a lower average aph*(λ) in ECS than in TS. Within SCM, average aph*(λ) was higher in ECS than in TS because of the dominance of nano- and micro-phytoplankton in ECS and TS, respectively. By pooling surface and SCM samples, we found regular trends in phytoplankton size-fraction versus Tchl a; and correlations between aph*(λ) and Tchl a consistent with previous observations for the global ocean in TS but not in ECS. In ECS phytoplankton size-fraction was not correlated with Tchl a, which consequently caused poor relationships between aph*(λ) and Tchl a. The abnormal values mainly originated from the surface low-salinity waters and SCM waters beneath them. At high Tchl a, aph*(λ) of these samples was substantially higher compared to the values in TS and from the global regressions, which was attributable to the lower micro-phytoplankton fraction, and higher nano- and/or pico-phytoplankton fractions in ECS. These observations indicated that the distinct light absorption properties of phytoplankton in ECS were possibly influenced by the Changjiang discharge. Our findings imply that general bio-optical algorithms proposed based on the correlations between aph*(λ) and Tchl a or the patterns in size-fraction versus Tchl a are not applicable in ECS, and need to be carefully considered when using these general algorithms in river-influenced regions.

Effect of spawning ground location on the transport and growth of chub mackerel (Scomber japonicus) eggs and larvae in the East China Sea

Chub mackerel (Scomber japonicus) are an important pelagic fish species within the China Sea. Annual recruitment of this species is determined primarily by survival in the early life history stages. Minor changes in the physical marine environment can have a significant effect on the growth and survival of eggs and larvae, thereby affecting recruitment of population. To model this interaction, we constructed a bio-physical dynamic model of the early life history of chub mackerel in the East China Sea (ECS). The physical model was based on the unstructured grid Finite Volume Coast and Ocean Model (FVCOM) and simulated the 3-D physical fields. The biological model was based on individual-based models (IBMs) in which the early life stages of chub mackerel were divided into five stages based on age or length. The model was parameterized using functions describing spawning, growth, and survival for chub mackerel in the ECS. Using this coupled physical and biological model, driven by the March–July climatological forcing, we tracked super individuals from spawning grounds to the nursery grounds to evaluate the influence of the physical environment at each of the spawning locations (western, normal, eastern) on the transport and survival of chub mackerel. The model suggests that spawning location has a significant effect on larval transport, although the larvae were generally advected northeastward to enter the Japan/East Sea through the Tsushima/East Strait or southeastward with the Kuroshio Current which then flows along the eastern Japanese coast. Spawning to the west was highly influenced by the Taiwan Warm Current (TWC) during early transport when the larvae were advected northward and then northeastward. The speed of drifting during this period was relatively slow. The model predicted that a large number of eggs and larvae would enter and transit through China’s coastal waters (Changjiang River Estuary, Hangzhou Bay, and the Zhoushan Islands). Under this scenario, the majority of larvae were transported to the northern nursery grounds, 79% to the nursery at Jeju Island and 10% to the nursery at Tsushima Strait. In contrast, only 11% were transported to the southern nursery grounds in the Pacific Ocean and Kyushu. Larvae spawned at the eastern spawning ground were primarily influenced by the Kuroshio Current which transported the larvae southeastward. Kuroshio acts as a barrier, restricting larvae from being advected to the interior of the western Pacific Ocean. Under such circumstances, almost no eggs and larvae were retained in the coastal waters of China. Instead, the larvae were rapidly transported northeastward from the Chinese shelf towards the coast of Japan. The model predicted that a large number of larvae would be transported to the southern nursery grounds in the Pacific Ocean and Kyushu, before entering the Pacific Ocean and Japan Sea. In total, 36% of larvae were transported to the Pacific Ocean nursery, 45% to the northern nursery grounds of Jeju Island and Tsushima Strait, and 27% to the Jeju Island nursery. The three simulations assumed the same number of eggs were spawned (2.17×1012) and the survival of larvae at the western, normal, and eastern spawning grounds was 0.0306%, 0.0353%, and 0.0234%, respectively. The average length was 123.7, 126.0, and 123.5mm, respectively. Our results suggest that larvae spawned in different regions encountered different physical environments and were subject to different transport processes. These differences explain the changes in survival and growth observed between larvae from the different areas. Survival and growth was highest for chub mackerel that were spawned at the normal spawning location and subject to suitable water depths and temperatures during transport.

Paleoceanographic changes of surface and deep water based on oxygen and carbon isotope records during the last 130 kyr identified in MD179 cores, off Joetsu, Japan Sea

We reconstructed the paleoenvironmental history of surface and deep water over the last 130kyr from oxygen and carbon isotope ratios of planktonic and benthic foraminifera in two cores (MD179-3312 and MD179-3304) from the Joetsu Basin, eastern margin of the Japan Sea. Our data showed that paleoceanographic changes such as influx of surface currents and vertical circulation were associated with global glacial–interglacial sea level change. Surface water conditions were influenced by the influx of Tsushima Current, East China Sea coastal or off-shore waters through the Tsushima Strait during interglacial or interstadial stages, and strongly affected by freshwater input during the glacial maximum. During interglacial maximums such as Marine Isotope Stages 1 and 5e, development of well-oxygenated bottom water was indicated. A density-stratified ocean with weak ventilation was inferred from the isotopic records of benthic foraminifera during the Last Glacial Maximum. Local negative excursions in carbon isotopes during deglacial or interglacial periods may suggest the dissolution of gas hydrates or methane seep activities.

A paleobathymetric transition during the mid-Pliocene warm period: Ostracode evidence from Japan

We reconstructed a paleoenvironment reflecting the mid-Pliocene warm period (MPWP) off the coast of southwest Japan on the basis of high-resolution statistical analysis of fossil ostracodes. A total of 107 ostracode taxa were obtained from the upper part of the mid-Pliocene Sadowara Formation (~3.25–3.12Ma), Shintomi Town, Miyazaki Prefecture, southwest Japan. Most taxa such as Acanthocythereis munechikai, Argilloecia spp., Bradleya albatrossia, Cytheropteron mediotumidum, and Krithe spp. are extant species distributed in the lower sublittoral to upper bathyal zones. These species have been reported in the Pacific coast of southwest Japan, the Tsushima Strait, and off the coast of China. We estimated the paleobathymetry of the Sadowara Formation using Q-mode factor analysis. The results of the factor analysis showed that a regression occurred during deposition of the upper part of the Sadowara Formation. The lower part of the studied sequence was deposited at water depths greater than 300m. The water depth decreased upward and reached around 250m in the upper parts of the formation. The timing of the regression corresponds with the peak of the marine isotope stage, KM2. The large regression estimated to have occurred during deposition of the upper part of the Sadowara Formation could have been related to global cooling events during the MPWP.

Tsushima Warm Current paths in the southwestern part of the Japan Sea

We compiled a long-term record of monthly mean sea surface currents in the Japan Sea from satellite altimetry and satellite-tracked drifter data. The record covers the entire Japan Sea for the period from May 1995 through January 2009. Using these data, we examined the path of the Tsushima Warm Current in the southwestern part of the Japan Sea. The mean sea surface current field revealed a “triple-branch” pattern. The first branch through the eastern channel of the Tsushima Strait flows along the coast of Japan with a mean current velocity of 20cms−1. The second branch flows along the continental shelf edge at around the 200-m isobath with a mean current velocity of 12cms−1. The third branch flows along the east coast of the Korean peninsula with a mean current velocity of 25cms−1. We investigated seasonal variation in the path of each branch. The first branch flowing along the Japanese coast throughout the year is highly stable. The second branch is stable throughout the year except in November. The third branch flowing along the east coast of Korea is highly stable until 38°N. Downstream of the third branch around 38°N, the current bifurcates into northeastward and southeastward currents. The northeastward current exhibits stable flow from June to February except in January. This current joins an eastward current originating around 39°N, 128.5°E. The southeastward current flows stably in June, September, and December. There were two current paths east of 131°E over the continental shelf off the San’in coast of Japan. The nearshore current flows along the Japanese coast with a mean current velocity of 23cms−1, and the offshore current flows around the continental shelf area with a mean current velocity of 19cms−1. These two currents are highly stable throughout the year. A current flows to the west of Oki Island to around 38.5°N and is highly stable in all months except March to July. These results provide a schematic view of the paths of the Tsushima Warm Current in the southwestern part of the Japan Sea.

Aseismic deep subduction of the Philippine Sea plate and slab window

We have made great efforts to collect and combine a large number of high-quality data from local earthquakes and teleseismic events recorded by the dense seismic networks in both South Korea and West Japan. This is the first time that a large number of Korean and Japanese seismic data sets are analyzed jointly. As a result, a high-resolution 3-D P-wave velocity model down to 700-km depth is determined, which clearly shows that the Philippine Sea (PHS) plate has subducted aseismically down to ∼460km depth under the Japan Sea, Tsushima Strait and East China Sea. The aseismic PHS slab is visible in two areas: one is under the Japan Sea off western Honshu, and the other is under East China Sea off western Kyushu. However, the aseismic PHS slab is not visible between the two areas, where a slab window has formed. The slab window is located beneath the center of the present study region where many teleseismic rays crisscross. Detailed synthetic tests were conducted, which indicate that both the aseismic PHS slab and the slab window are robust features. Using the teleseismic data recorded by the Japanese stations alone, the aseismic PHS slab and the slab window were also revealed (Zhao et al., 2012), though the ray paths in the Japanese data set crisscross less well offshore. The slab window may be caused by the subducted Kyushu-Palau Ridge and Kinan Seamount Chain where the PHS slab may be segmented. Hot mantle upwelling is revealed in the big mantle wedge above the Pacific slab under the present study region, which may have facilitated the formation of the PHS slab window. These novel findings may shed new light on the subduction history of the PHS plate and the dynamic evolution of the Japan subduction zone.

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.

Holocene evolution of depositional processes off southwest Japan: Response to the Tsushima Warm Current and sea-level rise

An understanding of the characteristics and evolution of shelf sediments is crucial for reconstructing ancient shelves from stratigraphic records. Oceanic currents have a major influence on shelf sediments. However, the evolution of sediments in narrow strait/seaway settings influenced by oceanic currents, especially in response to sea-level change, is still unclear compared with open shelf settings. The Tsushima Strait is the only gateway for the Tsushima Warm Current (TWC) into the semi-enclosed Sea of Japan. The evolution of depositional processes in the strait during the Holocene is not well documented, and this makes it difficult to develop a depositional model for this strait environment. To remedy this, we investigated marine sediments off Fukuoka, southwest Japan, adjacent to the eastern channel of the Tsushima Strait. A vibrocorer allowed us to collect 1.5–4.9m long cores from 11 sites on the coarse-grained sandy shelf (38–66m water depth). As a result, 10 lithofacies are recognized, and radiocarbon dating results indicate that these deposits formed mainly at two times, from 11.6 to 8.4calkyrBP and since 6.6calkyrBP. The main temporal changes recorded in the depositional environment off Fukuoka include (1) coastal and shallow-marine environments before the Last Glacial Maximum, (2) an embayment during the period 11.6–8.4calkyrBP, (3) a period of non-deposition from 8.4 to 6.6calkyrBP (apart from a few tidal channel deposits), and (4) TWC-influenced shelf deposition after 6.6calkyrBP. The formation of the TWC-influenced deposits shifted towards shallower water regions during postglacial sea-level rise. Overall, the evolution of the TWC-influenced deposits suggests that the strait/seaway environment was controlled mainly by water depth, but also by sediment supply from the proximal region in the strait.

Mean field and annual variation of surface flow in the East China Sea as revealed by combining satellite altimeter and drifter data

An analysis of a surface flow field in the East China Sea (ECS) obtained by combining satellite altimeter and drifter data provides comprehensive information of its mean field and annual variation. The Taiwan Warm Current, the onshore Kuroshio intrusion, the Kuroshio, the northeastward Kuroshio branch, the northward current west of Kyushu, the Cheju Warm Current, and the Tsushima Warm Current are regarded as persistent currents. Annual variation is a dominant signal in the ECS and is a major component of the seasonal variation. The contribution of the annual variation to the seasonal variation is estimated as 50% over the ECS, and exceeds 70% for the Kuroshio northeast of Taiwan, the Taiwan Warm Current, the Taiwan Strait Warm Current, the onshore Kuroshio intrusion, the northeastward Kuroshio branch, the northward current west of Kyushu, and the Tsushima Warm Current. The annual variation is characterized by propagation of vorticity anomalies from northeast of the Taiwan Strait and in the Tsushima Strait. The vorticity anomaly from northeast of the Taiwan Strait propagates downstream in the northeastward Kuroshio branch along isobaths of about 100m on the shelf edge. In contrast, the vorticity anomaly in the Tsushima Strait propagates upstream of the Tsushima Warm Current along isobaths of 100–200m. The propagation of the vorticity anomalies causes the annual variation with a phase difference in the northeastward Kuroshio branch and the northward current west of Kyushu, and the behavior of the vorticity anomalies can be well explained as a first approximation by a dispersion relation of a quasigeostrophic topographic Rossby wave propagating in a mean flow. Generation of the vorticity anomalies from northeast of the Taiwan Strait and in the Tsushima Strait appears to be closely related to the annual variation of the Kuroshio’s current speed northeast of Taiwan and of volume transport through the Tsushima Strait, respectively.

Sea level at the last glacial maximum, constrained by oxygen isotopic curves of planktonic foraminifera in the Japan Sea

AbstractThe Japan Sea is connected with the Okhotsk Sea and Northwest Pacific through three straits, viz. Mamiya (Tartarskiy), Soya and Tsugaru, and with the East China Sea through Tsushima Strait, all having sill depths of <140 m. Therefore, the Japan Sea was almost isolated from the surrounding seas during the Last Glacial Maximum (LGM; ca. 20 ka). As a result of nearly broken communication with the surrounding seas and strengthened surface stratification due to locked fresh water in the basin during the LGM, it became an anoxic basin. This major shift in the surface water properties of the basin left its imprint in the oxygen isotopic composition (δ18OPF) of the planktonic foraminifera. The δ18OPF exhibits extremely low values (∼0.5‰ vs. Belemnite from Pedee formation, South Carolina: PDB) during the last two glacial maxima, ca. 20 ka (Marine Isotope Stage (MIS) 2.2) and ca. 140 ka (MIS 6.2). These decreases are in contrast to the expected generally high glacial values of >3‰ recorded in the MIS 3–4 sections of the studied cores. This observation indicates nearly similar surface water conditions in the Japan Sea during both the previous glacial maxima, and hence suggests negligible effect of strait tectonics on the sea level since the last 140 ka. Low δ18OPF values with small amplitude are also observed at the cold stage MIS 6.4, but not observed at another cold stage MIS 4.2. This observation suggests that the low‐salinity surface water in the Japan Sea started to develop at the sea level between MIS 4.2 and 6.4. Judging from the cross‐section of the Tsushima Strait and the sea floor topography of the East China Sea, the sea level between MIS 4.2 and 6.4 must have been between 90 m and 100 m below the present. From a proportional relationship between the standard δ18O curve (LR04) in deep‐sea cores and globally averaged sea‐level change, the eustatic sea level obtained for the LGM is 120 ± 7 m below the present. Thus the Japan Sea, in a far‐field location from the continental ice sheets, provides an important clue for estimation of the eustatic sea level at the LGM. Copyright © 2012 John Wiley & Sons, Ltd.

Genetic Distinctness and Variation in the Tsushima Islands Population of the Japanese Marten,Martes melampus(Carnivora: Mustelidae), Revealed by Microsatellite Analysis

A carnivoran mammal endemic to Japan, the Japanese marten (Martes melampus) is native in forested regions on Honshu, Shikoku, Kyushu (main islands of Japan), and the Tsushima Islands. The Tsushima population is classified as a different subspecies (M. m. tsuensis) from populations on the main islands (M. m. melampus). To elucidate the genetic structure of the Tsushima population, we genotyped 101 individuals from the Tsushima Islands and 43 individuals from Honshu and Kyushu using 10 microsatellite loci, and performed population genetic analyses on the genotype data. Genetic diversity was lower in the Tsushima population than in three geographic populations on the main islands: heterozygosity was 0.189-0.364 in the former, compared to 0.457-0.747 in the latter. In addition, high pairwise Fst values (0.485-0.682) and Nei's standard distance (0.550-1.183) between the Tsushima and main-island populations indicated a high degree of genetic differentiation. Finally, a Bayesian clustering analysis showed that the Tsushima population is apparently differentiated from the main-island populations and comprises two genetic clusters. A factorial correspondence analysis corroborated these results. Our results suggest that restricted gene flow or inbreeding may have reduced genetic diversity in the Tsushima population, which has been geographically isolated from the main-island populations since the formation of Tsushima Strait.

Synergistic surface current mapping by spaceborne stereo imaging and coastal HF radar

Well validated optical and radar methods of surface current measurement at high spatial resolution (nominally <100 m) from space can greatly advance our ability to monitor earth's oceans, coastal zones, lakes and rivers. With interest growing in optical along‐track stereo techniques for surface current and wave motion determinations, questions of how to interpret such data and how to relate them to measurements made by better validated techniques arise. Here we make the first systematic appraisal of surface currents derived from along‐track stereo Sun glitter (ATSSG) imagery through comparisons with simultaneous synoptic flows observed by coastal HF radars working at frequencies of 13.9 and 24.5 MHz, which return averaged currents within surface layers of roughly 1 m and 2 m depth respectively. At our Tsushima Strait (Japan) test site, we found that these two techniques provided largely compatible surface current patterns, with the main difference apparent in current strength. Within the northwest (southern) comparison region, the magnitudes of the ATSSG current vectors derived for 13 August 2006 were on average 22% (40%) higher than the corresponding vectors for the 1‐m (2‐m) depth radar. These results reflect near‐surface vertical current structure, differences in the flow components sensed by the two techniques and disparities in instrumental performance. The vertical profile constructed here from ATSSG, HF radar and ADCP data is the first to resolve downwind drift in the upper 2 m of the open ocean. The profile e‐folding depth suggests Stokes drift from waves of 10‐m wavelength visible in the images.

Quaternary palaeoenvironmental oscillations drove the evolution of the Eurasian Carassius auratus complex (Cypriniformes, Cyprinidae)

AbstractAim We sought to reconstruct the spatio‐temporal genetic diversification in goldfish of the Carassius auratus complex, which is widely distributed in Eurasia, to test whether vicariance events or human‐mediated translocations best explained lineage divergence and genogeographical history.Location East Asia and the Oriental islands including Japan, the Ryukyus and Taiwan, and Europe, including Russia and the Czech Republic.Methods We reconstructed the matrilineal history of Eurasian goldfish using 1876 sequences from the partial mitochondrial DNA control region (426 bp) and 191 complete sequences of cytochrome b (1140 bp) from 67 localities representing most of the range of the species. Divergence times were estimated using a Bayesian Markov chain Monte Carlo approach based either on molecular clock data or on the fossil record. Genetic structure and the historical demography of populations were analysed using partial correlation tests and analyses of molecular variance.Results Three lineages had high levels of regional specificity. Lineages A and B from the main islands of Japan differed greatly from lineage C, which occurred on the mainland, Taiwan and the Ryukyus. Lineages A and B had late Pliocene origins. Six geographically constrained sublineages within lineage C had near‐simultaneous mid‐Pleistocene divergences.Main conclusions Genetic structure in the C. auratus complex appears to have been driven by palaeoenvironmental perturbations rather than human translocations. The disappearance of a land bridge in the Tsushima Strait around 3.0 Ma is responsible for the separation of Japanese and continental lineages; the estimated divergence time is 2.75–2.32 Ma. Fujian, China and Vietnam appear to have provided important refugia for the C. auratus complex during glaciation. After warm, moist summer monsoons intensified during the mid‐Pleistocene, goldfish are likely to have dispersed north‐eastwards to recolonize the Ryukyus via Taiwan and northwards to recolonize mainland China.

Fluctuations in the late Quaternary East Asian winter monsoon recorded in sediment records of surface water cooling in the northern Japan Sea

AbstractThe East Asian monsoon climate system involves interactions between land and ocean and is composed of summer and winter monsoons. Cooling of surface water by the East Asian winter monsoon (EAWM) promotes the formation of sea ice and deep water in the northern Japan Sea. Modern observations suggest close relationships between intensification of the EAWM and both sea ice formation and sea‐surface temperature (SST) decrease in the northeastern Japan Sea. Records of ice‐rafted debris (IRD) and alkenone‐based SST in the northeastern Japan Sea are examined to clarify the temporal variability of the EAWM and its controlling factors. Close relationships with oxygen isotope records of Chinese stalagmites and Northern Hemisphere summer insolation are observed in the records of IRD occurrence and SST during Marine Isotope Stages (MIS) 3–5. The co‐occurrence of maxima in the abundance of IRD and minima in SST in the northeastern Japan Sea indicates that the intensity of the EAWM was enhanced during periods of low summer insolation and high oxygen isotope ratios in Chinese stalagmites. Decoupling between the IRD and SST records during MIS 2 and MIS 6 might be the result of differences in surface water conditions caused by low sea levels and the resultant closure of the Tsushima Strait. During MIS 3 and 4, some small but clear maxima in IRD and minima in SST occurred at relatively low summer insolation periods. Some of these peaks co‐occur with North Atlantic Heinrich events, suggesting a possible climate linkage between the North Atlantic and the Asian monsoon. The presence of finer fluctuations in the occurrence of IRD than in summer insolation suggests finer millennial–centurial timescale fluctuations in the intensity of the EAWM. Furthermore, some IRD peaks occurred during periods of high insolation, such as 39, 86, 103, 128, 145, and 153 ka. Some of these IRD peaks correlate with minima in reconstructed SST and maxima in oxygen isotope ratios of Chinese stalagmites. This result suggests that extremely cold winters occur even during periods of enhanced East Asian summer monsoon intensity, and indicates that solar insolation is not the only factor that controls variations in the intensity of the EAWM at millennial–centurial timescales. Copyright © 2012 John Wiley & Sons, Ltd.

Effect of a stepwise lighting method termed “stage reduced lighting” using LED and metal halide fishing lamps in the Japanese common squid jigging fishery

Lighting systems combining light-emitting diodes (LEDs) and metal halide lamps (MHs) are expected to be energy-saving tools in Japan’s squid jigging fishery. Previous research has shown the need for light stronger than LEDs (9 kW) and 36 MHs (108 kW) to catch the Japanese common squid Todarodes pacificus. We tested a stepwise lighting method termed “stage reduced lighting” in the Tsushima Strait in January and February 2010 using nine fishing boats. LEDs (9 kW) and 50 MHs (150 kW) were lit for 3.9 h on average, and then the number of MHs was reduced to either 30 or 36 until the end of fishing (7.3 h on average). This method reduced fuel consumption by 22–25 % compared to the continuous use of all fishing lamps (159 kW). We carried out a catch analysis of nine experimental boats and 21 commercial boats during the experimental period. Generalized linear modeling analysis suggested that the squid catch can be explained by the illuminated fraction of the moon and monthly changes in squid abundance, and the lighting method. The stage reduced lighting using LEDs and MHs has the potential to reduce fuel consumption while maintaining the squid catch.

Seasonal variation of counterclockwise eddies downstream of the Tsushima Islands

The seasonal variation of counterclockwise eddies downstream of the Tsushima Islands in the Tsushima Straits was investigated from 2004 to 2008 using long-term acoustic Doppler current profiler (ADCP) data. Eddy activity was high from summer to autumn (July–November). The eddies were formed in short-term intervals of 5–8days, almost corresponding to the formation cycle of Karman vortices. The eddy activity varied on a longer time period (about 13days). The mean kinetic energy (MKE) converted to eddy kinetic energy (EKE) at current shears between the eastern and western channels of the Tsushima Straits. The counterclockwise eddies that formed downstream of the Tsushima Islands were intensified and maintained by energy conversion from MKE to EKE at the northern shear of the current core in the eastern channel. The countercurrent east of the Tsushima Islands from summer to autumn was intensified by energy conversion from EKE to MKE.

Inter-annual and decadal sea level variations in the north-western Pacific marginal seas

Long term sea level changes in the Okhotsk, Japan/East, East China and Yellow Seas have been explored based on mean monthly values of sea level from tide gauge and altimetry measurements. The analysis of low frequency sea level variability reveals clearly differentiated areas: the Okhotsk Sea and the northern sector of the Japan/East Sea display lower sea level variances and no sea level rise. The southern Japan/East Sea presents larger sea level variability associated with the circulation regime of the warm current entering through the Tsushima Strait and inter-annual sea level variations that are driven by steric and atmospheric changes. The largest sea level variances are found in the Yellow Sea due to the effect of atmospheric forcing over the continental shelf. Inter-annual variability is spatially varying within the Yellow Sea and the East China Sea and is mainly related to the steric sea level changes. Regional mean sea level indices have been synthesized for each area using the longest tide gauge records and have revealed correlations between the southern Japan/East and Yellow Seas with PDO and NP climatic indices. Linear trends at coastal sites in the Japan/East, East China and Yellow Seas show a rather heterogeneous pattern, with values between −1.5 and 5.5mm/yr for the period 1960–2000. During the period 1993–2008 linear trends derived from coastal tide gauges and from altimetry observations are coherent and reveal a southwards increasing pattern with maximum averaged values reached at the Yellow Sea (4.9±1.9mm/yr) followed by the Japan/East Sea (3.8±0.9mm/yr). Decadal rates of sea level change show distinct behaviour among basins as well as with the global average. The southern Japan/East Sea, the East China Sea and the Yellow Sea display decadal variability which is out of phase with respect to the global values.

On meteotsunamis around Tsushima Strait generated by the Baiu front

On July 15, 2009, a meteotsunami occurred over Tsushima Strait and flood damage was recorded on the west coast of Tsushima Island. This study investigated the meteorological systems related to that meteotsunami event using surface observation data, grid-point value data, and a mesoscale meteorological model. The mid-latitude trough in the mid-troposphere approached the moist air of the Baiu precipitation system, and a cold downdraft occurred over a wide area of western Japan. In the western Tsushima Strait, the warm, moist air of the lower atmosphere kept the front of the cold downdraft stationary. Around this area, an atmospheric gravity wave was generated continuously with the wave-conditional instability of the second kind mechanism and propagated toward Tsushima Island. The atmospheric gravity wave generated a pressure disturbance at sea level, which produced the meteotsunami. The frequency of the pressure disturbance using the numerical model had a peak in the 5- to 20-min period range, which corresponded with the peak frequency range of the observed tidal records.

Palaeogeographic reconstruction of the 1.55 Ma synchronous isolation of the Ryukyu Islands, Japan, and Taiwan and inflow of the Kuroshio warm current

In the Quaternary, the Ryukyu Islands evolved from a continental margin arc to an island arc by backarc spreading of the Okinawa Trough, accompanied by subsidence and isolation of the islands, a process that has continued to the present. Trough-parallel half grabens were filled with marine siltstone. Similar sediments filling orthogonal fault-controlled and west-draining non-tectonic valleys record island separation. New Quaternary nannofossil biostratigraphic data date the deposition of the marine siltstone at 1.552 ± 0.154 Ma. At that time, the entire 1000 km-long island chain comprising the Ryukyu Islands separated from the Asian continent by rifting extending from the Okinawa Trough to the Tsushima Strait. The Tokara, Kerama, and Yonaguni gaps, branched or transverse rifts of the Okinawa Trough, separate the island chain into subgroups of the Osumi, Amami, Okinawa, and Yaeyama islands, and Taiwan. The shallow Taiwan Strait separated Taiwan from the Chinese mainland. The Kuroshio warm current that previously ran offshore of the continental margin arc began to enter the opening backarc basin through the Yonaguni gap and to exit through the Tokara gap, flowing along the axis of the Okinawa Trough. Under influence of the warm current and because of entrapment of continentally sourced detrital sediments by the Okinawa Trough, coral reefs formed around each island. These reefs make up a unit called the Ryukyu limestone. Subsidence continued through the deposition of this limestone, resulting in further isolation of each island. Some islands did not separate from the mainland but emerged above sea level later as a result of volcanic edifice construction or forearc uplift. Following initial isolation, the Japanese islands and Taiwan may have been connected to the mainland by land bridges during some sea level low stands related to glacial periods, whereas the other islands remained isolated. Based on ages of isolation of each island, a Quaternary palaeogeographic map and ‘phylogenetic tree’ of the islands can be drawn showing the separation time of each island from the mainland and from each other. This information should be useful for phylogenetic molecular biologists studying evolution of Ryukyu endemic species and vicariant speciation and could facilitate analysis of the DNA substitution rate.

Temporal Changes of Plio—Pleistocene Ostracoda from the Takanabe Formation, Miyazaki Group, Southwest Japan

A total of 145 ostracode taxa were obtained from the upper part of the Plio—Pleistocene Takanabe Formation, Miyazaki Group, in the northern part of the Miyazaki Plain, Southwest Japan. Most of the assemblages are extant temperate and subtropical water species that have been reported from the recent western North Pacific Ocean around Southwest Japan, the Tsushima Strait, and the shelves off China. Four ostracode bioassociations (UA, MA, LA, and DA) and two biofacies (SF and DF) were discriminated on the basis of Rand Q-mode cluster analyses. Biofacies SF is divided into six sub-biofacies (SF1–6). The results revealed at least two transgressions and one regression during the deposition of the upper part of the Takanabe Formation. The sea-level fluctuation at approximately 2.6 Ma inferred in this study is extremely large compared with the glacio-eustatic sea-level fluctuations during the late Pliocene to early Pleistocene, suggesting that local tectonic movement in the study site during the deposition of the upper part of the studied interval at ∼2.6 Ma may have led to the large sea-level fluctuation. This paper also includes descriptions of two new species, Acanthocythereis abei sp. nov. and Acanthocythereis takanabensis sp. nov.