Akihide Kasai

Effect of the Earth's rotation on the circulation in regions of freshwater influence

Recent surveys in various regions of freshwater influence have shown considerably different flow patterns in each region. An analytical model including viscous effects and the Earths rotation is proposed to examine the along-channel flow pattern and to explain the differences. Model results show that the flow pattern is strongly dependent on the Ekman number E. With a large Ekman number (E > 1) the system is governed by gravitational circulation, and thus horizontal density gradients in along-channel direction are important. The whole water column is in the Ekman layer and consequent jet inflow penetrates the surface over a deep depression if the bottom topography varies in a cross-channel direction. With an intermediate Ekman number (E ∼ 0.1) this jet inflow concentrates in the lower layer in which the viscosity still plays an important role. The flow in the upper layer is, on the other hand, determined by the geostrophic balance. The contribution by the geostrophic flow becomes larger so that the cross-channel density gradients are important when die Ekman number is small (E < 0.01). Since the Ekman layer is clung to the bottom, the jet inflow exists only in the thin bottom layer. The hydrographic and acoustic Doppler current profiler surveys were conducted in Ise Bay, Japan. Both the density structure and the flow pattern were different from those observed in many drowned river valleys. A strong jet inflow existed in the lower layer over the depression while the flow in the upper layer suggested anticyclonic circulation. The estimated Ekman number is 0.07 in the bay and thus the observed pattern is consistent with the model result when E = 0.1.

Utilization of terrestrial organic matter by the bivalve Corbicula japonica estimated from stable isotope analysis

Carbon and nitrogen isotopic ratios in tissue of the bivalve corbicula (Corbicula japonica) and particulate organic matter (POM) were measured along a salinity gradient in the Kushida Estuary, Japan. The bivalve exhibited a gradual isotopic enrichment from the uppermost estuarine site (σ13C=−24.8‰ and σ15N=8.6‰) to the marine site (σ13C=−16.1‰ and σ15N=11.8‰). Using the concentration-weighted mixing model, the bivalves’ food source is estimated from the isotope values for the bivalves and POM from terrestrial plants, marine phytoplankton and benthic microalgae. The results indicated that the contributions of benthic micro algae and phytoplankton were small, while terrestrial particulate matter is significantly important for the corbicula diet, although the contribution varies among sampling sites.

Cellulose digestion by common Japanese freshwater clam Corbicula japonica

Cellulose digestion by Corbicula japonica was investigated according to the hypothesis that without any symbiotic aid, this organism can utilize cellulose as a carbon source. Enzymatic studies revealed the complete cellulase activity of this species, and molecular cloning resulted in the isolation of cDNA with an ORF encoding a 596-amino-acid protein that shares significant homology with abalone and termite cellulases with an amino acid identity of 52.2% and 50.5%, respectively. The isolated cellulase had a carbohydrate-binding module at the N-terminal region that was also reportedly present in abalone cellulase, and its mRNA were specifically expressed in the digestive gland. These findings strongly support the assumption that C. japonica has an endogenous cellulose, as well as abalones and termites. It is further believed that C. japonica plays an important roll in decomposing cellulose, and consequently contributes to the carbon-cycle in the aquatic environment, as termites do in terrestrial forests.

Entrainment of coastal water into a frontal eddy of the Kuroshio and its biological significance

The Pacific coastal areas of Japanese Island are major spawning grounds of various fishes. It is considered that large amount of eggs and larvae are dragged into the Kuroshio front so that the survival of fish larvae at the front is important for their recruitment. From this viewpoint, a low-salinity water mass, which was withdrawn from the coastal area to the Kuroshio front, was investigated by drifters, in addition to fine-scale hydrographic observations and water sampling in and around the Kuroshio frontal area off Enshu-nada. The drifters were transported to the east within the low-salinity water along the Kuroshio front in the first stage, and were thereafter entrained into an eddy, which was caused by the frontal meander. They moved closely to each other along the front, but diverged in the eddy. This movement of the drifters coincided with the deformation of low-salinity water mass; the low-salinity water concentrated at the Kuroshio front surrounded by strong salinity gradients at first, while it spread out horizontally and became vague in the shallow surface layer in the frontal eddy. Comparing temperature sections across the front, the strong upwelling was detected in the eddy. Limiting factors for primary production and growth rates were calculated in six sections using the observed temperatures and concentrations of nutrients. In the frontal area of the Kuroshio, low concentration of nutrients limited the primary production shallower than 50 m. Due to the low productivity, concentration of chlorophyll a in the low-salinity water tended to decrease, although the initial concentration was high. Once the coastal water mass was entrained into the frontal eddy, on the contrary, the concentration recovered due to the enhanced primary production in the subsurface layer supported by the upwelling of nutrient-rich water. Fish larvae in the low-salinity water are assumed to use the new production in the eddy; otherwise, they would starve. The entrainment process, which was probably caused by offshoreward movement of the Kuroshio, holds the key to successive survival and recruitment of fish larvae in the Kuroshio system.

Transport and survival processes of eggs and larvae of jack mackerel Trachurus japonicus in the East China Sea

Recent surveys showed substantial aggregation of larvae of jack mackerel in the southern East China Sea, indicating intensive spawning grounds near Taiwan. A numerical model was applied to investigate transport and survival processes of eggs and larvae of jack mackerel from the spawning area to the nurseries. The results show that: (i) the distributions of larvae simulated by the model agreed well with those obtained by field survey; (ii) the stock of jack mackerel in the Sea of Japan is composed of both groups from north of Taiwan and from the western coast of Kyushu. It takes more than two months for the former to reach the Sea of Japan, while it is within 40 days for the latter; and (iii) large proportions of the eggs and larvae spawned off the north of Taiwan are transported rapidly to the Pacific side of Kyushu by the Kuroshio Current, and the rest slowly to the east or north-east along the continental slope in the East China Sea. In contrast to the larval flux, survivors are more abundant in the northern East China Sea than in the Pacific Ocean, indicating that survival in the northern East China Sea would determine the jack mackerel stock in Japan.

Food sources for the bivalve Corbicula japonica in the foremost fishing lakes estimated from stable isotope analysis

Carbon and nitrogen stable isotope ratios in tissue of the bivalve corbicula Corbicula japonica and particulate organic matter (POM) were measured in Lake Jusan, Lake Ogawara and Lake Shinji, which are the foremost fishing grounds for the corbicula in Japan, to determine their food sources. The bivalves in Lake Ogawara and Lake Shinji showed enriched isotope composition, while those in Lake Jusan were depleted. In addition, the difference in the isotope ratios between the sampling sites was remarkable in Lake Jusan. Chlorophyll concentrations were significantly higher in Lake Ogawara and Lake Shinji than those in the inflow rivers, although that in Lake Jusan was equivalent to that in the river. Residence time of river water was estimated at 1 day, 455 days and 88 days in Lake Ogawara and Lake Shinji assimilate autochthonous phytoplankton, while those in Lake Jusan assimilate terrestrial matter in the upper reaches and marine phytoplankton in the lower reaches because of low production in the lake.

Circulation and cold dome in a gulf-type ROFI

Abstract Recent surveys in Ise Bay, which is a major gulf-type region of freshwater influence in Japan, reveal that a cold dome is often observed in the stratified season. To elucidate the formation mechanism of the cold dome, detailed hydrographic and ADCP surveys were conducted in August 1995. The results show that a cold (T

Migration ecology of juvenile temperate seabass Lateolabrax japonicus: a carbon stable-isotope approach

The migration pattern of juvenile temperate seabass Lateolabrax japonicus in the stratified estuary of the Yura River was examined using carbon stable-isotope ratios (δ(13)C). δ(13)C values of mysids, which are the most important prey items for this species, were consistently enriched in the lower estuary and surf zone (LES), while depleted in the freshwater zone (FW). δ(13)C values of juveniles in LES were enriched, while those in FW were depleted, consistent with the δ(13)C difference in prey items. The results of δ(13)C showed that many juveniles migrated upstream from April to June and most of them stayed in FW until at least July. Juveniles that stayed in FW and LES for a relatively longer period (> c. 20 days) showed higher condition factors than those that stayed in FW for a short period (several days). This indicates that residence in FW enabled juveniles to achieve as good a body condition as residence in LES.

Diet Shift In the Sand Shrimp Crangon uritai Along the Estuary-Marine Gradient

Abstract Spatial and temporal variations in the diet of Crangon uritai were examined using stable isotope and stomach content analyses. The sand shrimp and its potential prey were collected from Yura Estuary (3–5 m), shallow coast (5–10 m), and deep coast (30–60 m) stations of Tango Sea from April 2006 to February 2007. The measurements of the length (carapace and total lengths) and feeding habit (stomach content and stable isotope ratios) of C. uritai showed that this shrimp changed its diet during migrations between the estuary and marine environments. This species consumes a variety of food at the lower estuary: mysid Orientomysis japonica, small crustaceans particularly amphipod Melita hoshinoi, and bivalve Veremolpa micra. The abundant mysid O. japonica at the shallow coast and juveniles of unidentified bivalve species at the deep coast were the favored food throughout the year. The δ13C and δ15N variations in C. uritai confirmed the spatial and temporal diet shift in gut content composition as the sand shrimp grow and migrate along the estuary-marine gradient.

Partial migration of juvenile temperate seabass Lateolabrax japonicus : a versatile survival strategy

Partial migration describes intrapopulation variation in the migratory behavior, i.e. some individuals from a population migrate to low-salinity river areas, while others remain in coastal areas. This paper reviews the partial migration pattern of juvenile temperate seabass Lateolabrax japonicus, which is a migration pattern not commonly seen in Japan. Seabass spawn offshore, and eggs and larvae are transported to coastal areas. Some of these juveniles then ascend rivers, while others remain in coastal areas. Juveniles efficiently use physical structures in their habitat; they use tidal currents to ascend rivers in macrotidal estuaries, while they use the salt wedge in microtidal estuaries. Once juveniles ascend the river, they can feed on the abundant prey and attain more rapid growth than those remaining in coastal areas. As estuaries are highly productive areas, they play significant roles as nurseries for juveniles of various fishes. However, compared with coastal areas, the relative area of estuaries is considerably smaller and its environmental conditions are more variable. For example, nearly 40% of adult seabass in Tango Bay were estimated to use estuarine areas as a nursery, while the other 60% use coastal areas during their juvenile stage. Using both estuaries and coastal areas through partial migration during the juvenile stage is concluded to contribute to the stabilization and yield of seabass populations.