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We sought to develop and evaluate an escape vent designed for undersized swimming crabs (Portunus trituberculatus) to reduce bycatch, contributing to the preservation of marine resources. To this end, we conducted aquarium experiments and selectivity analysis to determine the appropriate size of the escape vent that would allow only undersized crabs with a carapace length of 64 mm or less to escape. The optimal dimensions for the escape vent were approximately 34.1 mm in height and 69.1 mm in width. During the sea trial, the average bycatch rates for undersized crabs per pot were 57.2%, 15.0%, and 22.3% for the control, basic, and door types, respectively. Regarding legal-sized crab catch per pot, averages of 1.40, 1.72, and 1.62 individuals were obtained for the three pot types. To our knowledge, this study is the first to assess the optimal size for an escape vent capable of reducing the bycatch of undersized crabs while maintaining legal-sized swimming crabs capture.

To investigate the age and growth characteristics of shotted halibut, Eopsetta grigorjewi, in the West Sea of Korea, the samples were collected using gill nets throughout 2019. A total of 861 specimens (693 females and 168 males) were analyzed. The BW-TL relations were BW = 0.0034TL3.3278 (R2 = 0.8716) for females and BW = 0.0031TL3.3359 (R2 = 0.8860) for males. This analysis revealed that males exhibited a larger growth rate than females (p < 0.05). Notably, the anterior and posterior sections of the otolith displayed an elongated oval shape, exceeding the radius of the dorsal and ventral regions. By examining the correlation between radii in four directions and total length, the highest correlation was observed between the total length (TL) and the ventral radius (R). The evaluation of the relationship between the fish total length and the otolith ventral radiusresulted in an equation of TL = 14.657R + 9.1601 (R2 =0.7081) for females and TL = 15.037R + 5.0265 (R2 = 0.6992) for males (p < 0.05). The formation of annuli occurred annually, consistently in January each year, as discerned through monthly changes in marginal index (MI) values. The von Bertalanffy growth equations from the relationship between the otolith annuli radius and the total length were Lt = 83.6 (1 − exp−0.046(t+4.70)) in females (n = 693, p < 0.05) and Lt = 56.8 (1 − exp−0.078(t+2.16)) in males (n = 168, p < 0.05).

In this study, the growth and age characteristics of Okamejei kenojei in the West Sea of South Korea were examined, and specimens were collected by bottom trawls from January to December 2019. The relationship between disc width (L) and body weight (W) was W = 6.1 × 10−3 L3.40. Age was determined by measuring annuli on the vertebral centrum surface, which forms around June every year. The spawning period was extrapolated from monthly changes of the gonadosomatic index in June–July. The relationship between the vertebral centrum radius and disc width, which was analyzed separately for females and males, was determined as L = 16.159R0.6145 and L = 15.543R0.6851, respectively. Finally, O. kenojei growth, that is, the disc width (L) at a certain age (t), was expressed using the von Bertalanffy growth equation as Lt = 58.70(1 − e−0.21(t+0.12)) for females and Lt = 53.94(1 − e−0.26(t+0.05)) for males. This study provides basic data on the age characteristics of O. kenojei for future research and more efficient fish stock management.

The biogeographical relationships between far-separated populations, in particular, those in the mainland and islands, remain unclear for widespread species in eastern Asia where the current distribution of plants was greatly influenced by the Quaternary climate. Deciduous Oriental oak (Quercus variabilis) is one of the most widely distributed species in eastern Asia. In this study, leaf material of 528 Q. variabilis trees from 50 populations across the whole distribution (Mainland China, Korea Peninsular as well as Japan, Zhoushan and Taiwan Islands) was collected, and three cpDNA intergenic spacer fragments were sequenced using universal primers. A total of 26 haplotypes were detected, and it showed a weak phylogeographical structure in eastern Asia populations at species level, however, in the central-eastern region of Mainland China, the populations had more haplotypes than those in other regions, with a significant phylogeographical structure (N(ST= )0.751> G(ST= )0.690, P<0.05). Q. variabilis displayed high interpopulation and low intrapopulation genetic diversity across the distribution range. Both unimodal mismatch distribution and significant negative Fu's F(S) indicated a demographic expansion of Q. variabilis populations in East Asia. A fossil calibrated phylogenetic tree showed a rapid speciation during Pleistocene, with a population augment occurred in Middle Pleistocene. Both diversity patterns and ecological niche modelling indicated there could be multiple glacial refugia and possible bottleneck or founder effects occurred in the southern Japan. We dated major spatial expansion of Q. variabilis population in eastern Asia to the last glacial cycle(s), a period with sea-level fluctuations and land bridges in East China Sea as possible dispersal corridors. This study showed that geographical heterogeneity combined with climate and sea-level changes have shaped the genetic structure of this wide-ranging tree species in East Asia.

Analysis of high-resolution seismic profiles from the SE continental shelf of Korea reveals that the shelf sequence consists of four sedimentary units, namely, Pliocene deposits (III), Lower Pleistocene deposits (IIB), Upper Pleistocene deposits (IIA), and Holocene deposits (I), which were mainly controlled by regional tectonic activity and sea-level fluctuations. Vertically, with the exception of unit I, the units form a series of superimposed prograding wedges that thicken seaward. A structural high, including several faults and folds, occurs north of Tsushima Island, extending northeastward to the Dolgorae Thrust Belt at the southern margin of the Ulleung Basin. In this study, we deduced that the deformed zone was formed due to compressional deformation associated with back-arc closure that occurred after the extension of the East Sea. Most of the faults and folds, oriented in NE–SW or NNE–SSW trends, mainly developed in the lower two units (III and IIB). The lower two units were also significantly deformed, whereas the two overlying units remained relatively undeformed. This indicated that, during the Pliocene and Lower Pleistocene (units III and IIB), sedimentation was mainly controlled by tectonic activities, whereas sedimentation after the Lower Pleistocene (units IIA and I) was mainly influenced by periodically repeated sea-level changes rather than tectonic controls.

The marine clam Meretrix petechialis is an important economic shellfish species in Northwestern Pacific, but little is known about its phylogeographical pattern. Here, we analyzed 311 samples from 22 locations along the northwestern Pacific using combined profiling of one mitochondrial gene (the first subunit of cytochrome coxidase, COI) and one nuclear DNA marker (the internal transcribed spacer region 1, ITS-1) to investigate contemporary genetic structure and reconstruct phylogenetic history of this species. The results revealed that two distinct phylogeographic lineages dominated marginal seas-the East China Sea (ECS) and the South China Sea (SCS) respectively. The estimation of divergence time between two lineages was 2.1-3.8 Ma, corresponding to a period of the early Pleistocene to late Pliocene. The vicariance of the two lineages was connected to the historical isolation of marginal seas and sea surface temperature (SST) gradient, pointing that SST might play an important role in maintaining phylogeographical patterns of M. petachialis. Significant overlaps between two lineages were observed in 23° to 29° N, located at the adjacent area of the ECS and SCS, which might be promoted by the connectivity of China Coast Current. However, the influence of ocean currents on mixings between two lineages was limited. In comparison, significant relationships were found between genetic distances and geographic distances if the North and South populations were analyzed separately, result of which might be due to some small reciprocal, rotating flows along coastal areas and special geographical conditions.

Coastal areas where submarine groundwater discharge (SGD) occurs are active mixing zones with characteristic biogeochemical and ecological functions. In this study, we investigated the microbial community associated with the changes in groundwater discharge flux at a coastal beach site on Jeju Island, South Korea. We performed water chemistry analyses, 16S rRNA gene-based pyrosequencing, and microbial community statistical analyses on coastal water samples systematically collected as functions of tidal stage and distance from the groundwater discharge point. We also carried out groundwater level monitoring and numerical simulation of the coastal aquifer to reproduce tidally induced variations in the SGD rates of the study site. Pyrosequencing and statistical analyses revealed a periodic shift in the microbial communities in the coastal waters as functions of tidal stage and SGD rates. Interestingly, the community structures in the samples collected at flood and ebb tide were markedly different, despite the similarities in their water chemistry. Groundwater simulation and canonical correspondence analyses suggest that groundwater discharging at higher velocities at ebb tide can detach and transport subsurface bacteria from the aquifer to the coastal water body, resulting in an increase in facultative anaerobes in the ebb tide samples. In addition, release of the sand-attached bacteria as a result of particle resuspension and flushing of shallow subsurface bacteria in the intertidal zone could contribute to shaping the relative abundance of the coastal microbial community. We conclude that SGD rate is an important factor influencing the dynamics of the bacterial community structure at the coastal zone of the study site.

We studied the Quaternary incised fills drilled at the northern Yangsan Fault having multiple deformation histories since Late Cretaceous or Paleogene to determine tectonic influence on development of incised valley and its sedimentation. Incised valley fills were deposited during and after the Last Glacial Maximum and are composed of fluvial lag, debris flow deposits interbedded with fluvial sediments, shallow marine sandy deposits, and fluvial sediments from bottom to top. These fills show lateral changes in sediment thickness from 44 to 11.5 m over a short distance of 230 m, implying sediment stacking in a deep and steeply inclined valley. Fluvial lag and debris flow deposits are common in the thalweg of a valley. Despite small drainage basin (195.9 km2), the development of deep incised valley is interpreted to have resulted from fluvial downcutting on erodible basement during sea level fall as a consequence of dense development and fault and fracture networks in the pre-Quaternary rocks caused by multiple movements of Yangsan Fault. With steep gradient, the damaged rocks led to frequent slope failure and forceful accumulation of debris flow deposits on the valley’s axis at the time. In addition, stacking of debris flow deposits resulted in decrease of longitudinal gradient of incised valley, promoting rapid transgression during sea level rise (9 to 7 ka). This resulted in insufficient time for the central basin mud to be accumulated, which explains why the studied fills lack central estuarine mud that is common in incised valleys fills deposited during transgression.

A 3‐D shear velocity model of the crust and uppermost mantle to a depth of 100 km is presented beneath the North China Craton (NCC), northeastern China, the Korean Peninsula, and the Sea of Japan. Ambient noise Rayleigh wave tomography is applied to data from more than 300 broadband seismic stations from Chinese provincial networks (CEArray), the Japanese F‐Net, and the IRIS Global Seismic Network. Continuous data from 2007 to 2009 are used to produce group and phase velocity maps from 8 s to 45 s periods. The model is motivated to constrain the distributed intraplate volcanism, crustal extension, cratonic rejuvenation, and lithospheric thinning that are hypothesized for the study region. Numerous robust features are observed that impose new constraints on the geometry of these processes, but discussion concentrates only on four. (1) The North‐South Gravity Lineament follows the ∼40 km contour in crustal thickness, and crustal thickness is anticorrelated with water depth beneath the Sea of Japan, consistent with crustal isostasy for a crust with laterally variable composition. (2) The lithosphere is thin (∼70 km) beneath the Songliao‐Bohai Graben but seismically fast. (3) Even thinner more attenuated lithosphere bounds three sides of the eastern NCC (in a horseshoe shape), identifying a region of particularly intense tectonothermal modification where lithospheric rejuvenation may have reached nearly to the base of the crust. (4) Low‐velocity anomalies reach upward (in a Y shape) in the mantle beneath the eastern and western borders of the Sea of Japan, extending well into continental East Asia in the west, and are separated by a ∼60 km thick lithosphere beneath the central Sea of Japan. This anomaly may reflect relatively shallow slab dehydration in the east and in the west may reflect deeper dehydration and convective circulation in the mantle wedge overlying the stagnant slab. Key Points Obtaining unprecedented abundance of data for northeast Asia Identifying and removing disturbance signals from Kyushu microseism Getting 3D shear wave structures with high resolution and precision

Phytoclasts in the form of plant debris in terrestrial sediments can be transported by water to distant areas because they are lighter than inorganic particles. The semi-enclosed East Sea, which is connected by narrow straits to other seas, is adjacent to continental shelves that are the source area of terrestrial sediment flowing into the East Sea. These shelves alternated repeatedly between terrestrial and marine environments as a result of eustatic sea-level changes during the Late Quaternary. Palynofacies analyses of the IODP Exp. 346 U1430 core, located in the Eastern South Korea Plateau (ESKP) of the East Sea, have revealed changes in the size and concentration of phytoclasts associated with glacial–interglacial cycles. These changes are generally negatively correlated with the global sea-level curve, and their anti-phase cycles with high amplitude are clearly evident during the last ca. 750 ka with the geotectonic stabilization period. In particular, several coarse-grained phytoclasts were observed during the glacial period, including the Last Glacial Maximum (LGM). These findings suggest that the concentration and size of phytoclasts flowing into the East Sea were influenced by changes in the distance of the source area, depending on the water depth of the strait and nearby shelves owing to sea-level changes in tandem with glacial–interglacial cycles and geotectonic events.