Ariake Sound Research Articles

Reproductive cycle of the venerid clam Meretrix lusoria in Ariake Sound and Tokyo Bay, Japan

We assessed the reproductive cycle of the venerid clam Meretrix lusoria by histological analysis of the gonads. Individuals for study were collected from natural populations on the Shirakawa tidal flat, Ariake Sound, and from populations that had been transplanted from the Shirakawa flat to the Oi flat in Tokyo Bay. In both study areas, the reproductive cycle was synchronized between sexes. Gonads of the clam started to develop in early spring and matured during the summer. Mass spawning occurred in the late summer/early fall. The clam matured at a shell length of 17–20 mm, which is much smaller than previously considered. While trophic conditions and salinity differed considerably in the two study areas, water temperatures showed similar seasonal changes (12°C during the winter and around 30°C during the summer). Thus, temperature probably controlled gonadal development. The coincidence of the period of spawning with the period of frequent intrusion of hypoxic waters into the tidal flats in Tokyo Bay suggests that such hypoxic events interfere with clam recruitment and are at least partly responsible for the disappearance of the natural population at this location.

Identification of a Species of Coilia (Engraulidae) Distributed in Ariake Sound

Identification of a Species of Coilia (Engraulidae) Distributed in Ariake Sound

Distribution and Life History of Trachidermus fasciatus in Rivers around Ariake Sound, Kyushu, Japan

Distribution and Life History of Trachidermus fasciatus in Rivers around Ariake Sound, Kyushu, Japan

Estimation of Eddy viscosity and Eddy Coefficient in Internal Regions using Lagrangian Acoustic Drogue

Time-space distributions of the internal velocity were measured in Ariake Sound using Lagrangian-Acoustic Drogue for Intertidal environments (LAD-IT). The turbidity distributions were calculated using backscattering data of ADCP by solving the sonar equation. Time-space distributions of both the eddy viscosity and eddy diffusion coefficients were estimated using those of both the velocity and the turbidity. The estimated results indicated that the both coefficients changed temporally and spatially. The order of magnitude of the eddy viscosity was 10-3-10-2 and that of the eddy coefficient was 10-5-10-4. These orders were in good agreement with the conventional studies. The results indicate LAD-IT is adequate for estimating time-space distributions of eddy viscosity and eddy diffusion coefficients in the field.

Dialychone, Jasmineira and Paradialychone (Annelida: Polychaeta: Sabellidae) from Japan and adjacent waters, including four new species descriptions

The genus Chone Krøyer, 1856, and related genera of the family Sabellidae (Polychaeta) are common members of softbottom communities. Four new species from Japanese waters are described herein: Dialychone okudai n. sp., from Hokkaido, Paradialychone katsuuraensis n. sp., from Katsuura, Boso Peninsula; P. edomae n. sp., from Tokyo Bay and Jasmineira kikuchii n. sp., from Ariake Sound, Kyushu. A re-description is also provided for a poorly known species C. cincta Zachs, 1933. This species, originally described from the Peter the Great Bay, Eastern Russia was recorded in this study from Amakusa and Ariake Sound, Kyushu and transferred to Paradialychone. Jasmineira kikuchii is the first record of the genus from Japan. The Japanese species described here are compared with other conspecifics of the world and a key for the sabellid genera with a glandular girdle on chaetiger 2 and thoracic acicular uncini is provided.

Marine mucilage in Ariake Sound, Japan, is composed of transparent exopolymer particles produced by the diatom Coscinodiscus granii

Concomitant with the bloom of a diatom, Coscinodiscus sp., there was a sudden production of mucilage in the central area of Ariake Sound during the autumn of 2007. To obtain basic information about this marine mucilage and identify the causative organism, we have examined the morphological characteristics of Coscinodiscus sp. and the production of transparent exopolymer particles (TEPs) by this species. Light and electron microscopy studies revealed that the morphological characteristics of the species under investigation were consistent with previous descriptions of Coscinodiscus granii and, hence, we identified this species as C. granii. Alcian blue staining revealed that the TEPs produced by C. granii were composed of acidic polysaccharides. The properties of these polysaccharides were examined using two fluorescein isothiocyanate (FITC)-labeled lectins: FITC-concanavalin A (Con A), which is specific for d-mannose and/or d-glucose, and Ulex europaeus agglutinin-1 (UEA-1)-FITC, which is specific for l-fucose. The Con A label scarcely reacted with the surface of the marine mucilage produced by C. granii, while UEA-1-FITC showed a strong reaction. These results indicate that the TEPs produced by C. granii contain more l-fucose than d-mannose and/or d-glucose. Such TEPs with high l-fucose content are known to be highly viscous. We conclude, therefore, that the polysaccharide composition of the TEPs produced by C. granii may determine the viscosity of the marine mucilage.

Food sources of benthic animals on intertidal and subtidal bottoms in inner Ariake Sound, southern Japan, determined by stable isotopes

To evaluate the relative importance of possible food sources, including riverine particulate organic matter, reeds, benthic microalgae, seaweeds, cultured laver ( Porphyra) and coastal phytoplankton, for commercial bivalves and co-occurring benthic animals, 73 macrofaunal species were collected from intertidal and subtidal soft bottoms in the inner part of Ariake Sound, Kyushu, southern Japan, and their isotopic compositions were analyzed. The results revealed that (1) both intertidal and subtidal food webs were constituted of 3 trophic levels, (2) suspension-feeding bivalves utilize a mixture of benthic microalgae and coastal phytoplankton, and omnivores and carnivores incorporate benthic microalgae and phytoplankton through their intermediate prey, and (3) 3 bivalves ( Scapharca kagoshimensis, Modiolus metcalfei and Atrina lischkeana) inhabiting both intertidal and subtidal bottoms showed similar seasonal fluctuations, suggesting no difference in the diet composition among the species and between the 2 habitats. We conclude that a large biomass of benthic microalgae which was approximately equal to that of phytoplankton and the strong tidal currents that would resuspend benthic microalgae and transport them to subtidal bottom areas account for the benthic microalgal and phytoplankton based trophic structure in the inner part of Ariake Sound.

Reproductive biology of Ilisha elongata (Teleostei: Pristigasteridae) in Ariake Sound, Japan: Implications for estuarine fish conservation in Asia

Elongate ilisha ( Ilisha enlongata) is a commercially important species that contributes to clupeoid fisheries in Asian countries. In the present study, the reproductive biology of I. elongata in Ariake Sound, Japan is determined for the first time. Six maturity stages were described using ovarian and testicular histology throughout the annual cycle. The spawning season in Ariake Sound lasts from May to July, with peak spawning activity in May and June. Age at first maturity was estimated to be 2 years, with a few exceptions of 1 year in well-developed males. Ovaries that contained both tertiary yolk oocytes and postovulatory follicles occurred from late May to late July, indicating that I. elongata is a multiple spawner. The size–frequency distribution of oocytes provided evidence for its multiple spawning and accuracy of the fecundity estimates. The batch fecundity of this species was estimated at between 22,200 and 270,900 eggs per individual, increasing with age between two and six years. The present findings on the reproductive strategy of I. elongata in Ariake Sound are generally consistent with those in temperate or subtropical populations, but quite different from those of tropical population where first maturation occurs around 200 days and life spans are shorter, with a maximum age less than 3 years. The conservation implications of this reproductive strategy in a harsh, variable environment in Asian countries are also discussed.

Food Habits of Finless Porpoises Neophocaena phocaenoides in Western Kyushu, Japan

We examined the stomach contents of 87 finless porpoises (Neophocaena phocaenoides) in the Ariake Sound–Tachibana Bay area and Omura Bay in western Kyushu, Japan, between 1987 and 1992. Fish (Gobiidae and Atherinidae) were the most numerous and most frequently occurring prey in Omura Bay, whereas both cephalopods (Octopodidae, Sepiidae, Sepiolidae/Sepiidae, and Loliginidae) and fishes (Clupeidae, Engraulidae, and Sciaenidae) were equally important in Ariake Sound–Tachibana Bay. Species compositions in the commercial catch differed between the 2 waters, suggesting that differences in prey availability may explain the geographical variation in diet. Finless porpoises in Ariake Sound–Tachibana Bay showed ontogenetic and seasonal variations in diet. The mean length at weaning was estimated to be 101 cm, corresponding to approximately 6 months of age. Calves fed on small-sized demersal fish and cephalopods. The predominant prey species for sexually mature individuals (including lactating females) consisted of konoshiro gizzard shad (Konosirus punctatus) and cephalopods. Seasonal availability of these organisms may be related to births in the fall–winter season in Ariake Sound–Tachibana Bay. Size-dependent prey selectivity was equivocal. Day–night difference in foraging time was not indicated by the index of stomach fullness.

Spatial partitioning between species of the phytoplankton-feeding guild on an estuarine intertidal sand flat and its implication on habitat carrying capacity

The fishery yield of Manila clams, Ruditapes philippinarum, increased considerably in the 1970s but has decreased rapidly since the middle 1980s on extensive intertidal sand flats in Ariake Sound (Kyushu, Japan). A survey conducted in 2004 on a 3.4-km 2 sand flat located in the central part of the Sound (Shirakawa sand flat) revealed four dominant species: two thalassinidean shrimps ( Upogebia major and Nihonotrypaea japonica), which are deep-reaching burrow dwellers with strong bioturbation activities, and two bivalves ( Mactra veneriformis and R. philippinarum). All four species belong to a phytoplankton (diatom)-feeding guild. In the late 1970s, the Manila clam population prevailed in high densities over the entire sand flat, whereas its distribution was restricted to the lowest quarter of the shore in 2004. In contrast, the population sizes and zones of occurrence of the other phytoplankton feeders have expanded in the absence of R. philippinarum, perhaps an indication of competitive release. After establishment, effects of the thalassinidean shrimps on sediment stability appear to have further reduced clam abundances. Across the sand flat in 2004, wet weight population biomass estimates for N. japonica, U. major, M. veneriformis, and R. philippinarum (whole body for shrimps and soft tissue for bivalves) were 304, 111, 378, and 234 tonnes, respectively. Based on Manila clam fishery yield records from Shirakawa, the carrying capacity of the Shirakawa sand flat in the late 1970s was estimated to be two times greater than the sum value for the whole phytoplankton-feeding guild in 2004. It is hypothesized that (1) the amount of phytoplankton determines the carrying capacity for the benthic community on the Shirakawa sand flat, with both phytoplankton and benthic biomass at maxima in the late 1970s, and (2) the subsequent increases in competition for space have caused further declines in the Manila clam population biomass to approximately one-eighth of its past value.

有明海湾奥部における懸濁物質濃度変動特性に関する研究

Suspended sediment concentration and current velocity were measured continuously by Acoustic Doppler Current Profiler at the inner part of the Ariake Sound. Simultaneously, shear strength of the surface of the bottom mud was measured around the mooring point. Shear strength which measured by viscosity meter had finer relationships between vane shear strength. Multiple regression analysis was applied to the maximum SS concentration in every 15 hours. The main factor of the variation of the SS was the bottom shear stress. Sub-factor of the variation of SS concentration were wind velocity and the shear strength of the surface bottom mud.

Age and growth of Ilisha elongata (Teleostei: Pristigasteridae) in Ariake Sound, Japan: comparison among populations in western North Pacific Ocean

Elongate ilisha Ilisha elongata is a commercially important fishery resource in both Japan and China. Age and growth of the species were investigated by scale analysis from June 1996 to July 1997 in Ariake Sound, Japan. Scale annuli were formed once a year mainly between June and July. The age was 1–6 years for both male and female individuals. There was no significant difference in the regression of fork length (FL) on scale radius (R) between sexes by a covariance analysis. The combined sex von Bertalanffy growth equations were expressed as L t=495.4×(1-e−0.3176(t+0.4108)), where L t is FL in mm at age t. Comparison of the age and growth among different populations from coastal waters in the western North Pacific Ocean suggested that the FL of the Ariake population is the smallest at each age. The spatio-temporal variations in growth are possibly relevant to environmental conditions of the growth grounds of subadults; however, there was no gradient change in the growth of I. elongata with latitude. Water temperature may not be the main factor affecting the growth of this species. The growth of the tropical population from Sarawak differs greatly from those of temperate and subtemperate populations, which implies genetic variation or phenotypic plasticity in different climate zones.

Suspension feeding and growth of ark shell Anadara granosa: comparison with ubiquitous species Scapharca subcrenata

The ark shell Anadara granosa is a species peculiar to the Ariake Sound. To determine why distribution of A. granosa in Japan is largely confined to this area, we examined feeding and growth of A. granosa as functions of environmental and biological variables. The results were compared with those of another ark shell Scapharca subcrenata, which is ubiquitous in Japan. Feeding experiments indicated that A. granosa is eurythermal and euryhaline, as is S. subcrenata, but is adapted to temperature slightly higher than S. subcrenata. Weight-specific clearance rate (CR) of A. granosa as a function of soft-body dry-weight (w) followed the power function of w (CR=2.7×w−0.37), with coefficient and exponent very close to those for S. subcrenata. Growth rate of A. granosa increased linearly with daily ration, similar to S. subcrenata. Thus, feeding and growth characteristics of A. granosa were comparable to those of S. subcrenata and no ‘peculiarities’ of the former were detected. Therefore, the factors that make A. granosa a species restricted to the sound are probably not directly related to feeding or growth characteristics.

Food sources for three species of Nihonotrypaea (Decapoda: Thalassinidea: Callianassidae) from western Kyushu, Japan, as determined by carbon and nitrogen stable isotope analysis

Three species of the callianassid genus Nihonotrypaea occur intertidally in the Ariake Sound estuarine system, southern Japan; they consist of two sandflat species ( N. japonica; N. harmandi) and one boulder-beach species ( N. petalura). Nihonotrypaea harmandi and N. petalura are distributed along the coastline in the relatively oligotrophic sea area extending from the outermost part of the sound to the open sea, while N. japonica occurs in the more eutrophic area situated at the middle part of the sound. The trophic conditions of the two areas affect the abundance of phytoplankton in the water column relative to that of benthic microalgae in the sediment. The carbon and nitrogen stable isotope compositions of N. japonica and its potential food sources have been analysed in an earlier study, specifying phytoplankton as the exclusive food source. Potential food sources for N. harmandi and N. petalura were analysed to make interspecific comparisons of the assimilated diets in relation to the shrimp habitat characteristics. Food sources of the shrimps were assessed based on the diet-tissue isotopic fractionation of N. harmandi and N. japonica ( δ 13C = 2.0‰, δ 15N = 4.0‰), which had been determined by an earlier laboratory feeding experiment. Of several potential food sources for N. harmandi and N. petalura, riverine organic matter, sewage effluents, live/detrital terrestrial plants, and seagrass were not food sources. For N. petalura, live seaweed, in particular Sargassum spp., growing on boulders and cobbles during the seaweed high-growth season, and seaweed-derived detritus buried in the sediment and live Enteromorpha compressa during the seaweed low-growth season were the most likely food sources. For N. harmandi, phytoplankton (or fresh phytoplankton-derived detritus) and benthic microalgae constituted the most likely food sources. The δ 13C value of the estimated diet for N. harmandi was higher than that for N. japonica by 0.6‰, while the δ 15N value for N. harmandi was lower by 2.6‰. The food sources for the three species of Nihonotrypaea were species-specific, depending on each habitat characteristics.

内海性浅海域の生物とその生態的機能を探る 有明海の砂質干潟における植物プランクトン食ギルドのなかの二枚貝資源―環境収容力制限仮説の提示

On extensive intertidal sandflats in Ariake Sound, the yield of the Manila clam, Ruditapes philippinarum, considerably increased in the 1970s but has remarkably decreased since the early 1980s. A survey conducted in 2004 of the individual density and distribution of the large-sized members of the macrobenthic community on a sandflat located at the mouth of the Shirakawa River in the central part of the Sound demonstrated four dominant species: two thalassinidean shrimps (Upogebia major and Nihonotrypaea japonica), which inhabit deep-reaching burrows, and two bivalves (Mactra veneriformis and R. philippinarum). In the late 1970s the Manila clam population prevailed in high densities over the entire area, while its distribution was limited to the lowest zone in 2004. By contrast, the other three species have increased their densities and distribution ranges and may have outcompeted the Manila clam through competition for habitat space. In particular, the strong bioturbating activities of the two shrimps probably havenegatively affected the abundance of the Manila clam. The carbon and nitrogen stable isotope compositions of the four species and their potential food sources were analysed in 2003, specifying phytoplankton (mainly diatoms) as the exclusive food source and suggesting that the four species belong to the same feeding guild. The wet biomass of the populations of N. japonica, U. major, M. veneriformis, and R. philippinarum (the whole body for shrimps and the soft tissues for bivalves) over the sandflat was estimated at 315 t, 110 t, 400 t, and 237 t, respectively. In conclusion, it is hypothesized that the amount of phytoplankton determines the carrying capacity for the benthic community on the sandflat, both quantities having attained their maxima in the 1970s, and (2) the subsequent increased competition for space has caused a further decline in the biomass of the Manila clam population.

有明海におけるトラフグ Takifugu rubripes人工種苗の漁獲サイズにおよぼす放流条件，資源量指数および水温の影響

有明海において 1991～1995 および 2000～2002 年に放流した 23 群のトラフグ標識魚の 11 月 1 日の推定全長を目的変数とし，放流時期，放流サイズ，放流場所，種苗性，資源量指数および水温（7～8 月と 9～10 月）を説明変数として重回帰分析を行った。解析の結果，漁獲サイズの変動には放流サイズ，放流時期，放流場所，資源量指数および水温が影響していることが判明した。偏回帰係数から，諫早湾および湾奥において大きいサイズで早期に放流することにより，より大型の漁獲加入魚が得られることが判った。

諫早湾奥部締め切り後の有明海潮下帯ヨコエビ群集構造の変化

諫早湾奥部締め切り後の有明海潮下帯ヨコエビ群集構造の変化

Characteristics of Surface Currents around the Mouth of Isahaya Bay using DBF Ocean Radar

This study deals with the field observations on coastal surface currents by means of both Digital Beam Forming marine radar (DBF ocean radar) and Acoustic Doppler Current Profilers (ADCP) in order to realize characteristics of flow around the mouth of Isahaya Bay. In particular, the former has started since 2005. On the other hand, the latter has carried out seven times since 2001. As a result, it is found out that the feature of the M2 consituent flow detected by the DBF ocean radar was widely coincided well with other ADCP field observation data. Not only the river runoff in the head of Ariake Sound but also a flush flow from the sea-dike gate contribute to the surface currents around the mouth of Isahaya Bay significantly.

Reproductive cycle, sexual maturity and diel-reproductive periodicity of white croaker, Pennahia argentata (Sciaenidae), in Ariake Sound, Japan

The white croaker, Pennahia argentata (Houttuyn), is abundant in demersal fish assemblages in Ariake Sound and is a commercially important species. The reproductive biology of white croaker was studied by measurement of gonadosomatic indices (GSI) and histological examination of ovaries. Ovaries were classified into one of five maturity stages based on histological data: (1) immature/resting stage; (2) developing stage; (3) maturing stage; (4) ripe/spawning stage; (5) spent stage. The spawning season lasts from April to September but most activity is observed from May to August. Age at sexual maturity was assumed to be 1 year. Postovulatory follicles coexisted with yolk stage oocytes in histological sections of ovaries indicating that white croaker is a multiple spawner, spawning more than once in a single spawning season. Judging from the diel changes in the proportion of females with ovaries containing postovulatory follicles and oocytes at maturation stage, most females spawned in the evening between 19:00 and 21:30 h.

Plankton community structure and trophic interactions in a shallow and eutrophic estuarine system, Ariake Sound, Japan

Ariake Sound is a shallow, eutrophic estuarine system, located in the western part of Japan. We conducted field surveys and experiments in late autumn in 2002, 2003 and 2004 to clarify plankton community structure and trophic interactions in this system. A large photosynthetic dinofla- gellate (Akashiwo sanguinea) was persistent and the dominant phytoplankton species in all years. Diatoms and other photosynthetic flagellates were relatively minor components of the assemblage. The growth of photosynthetic flagellates such as Prorocentrum micans and cryptophycean flagellates under nutrient-enriched conditions was almost balanced with the grazing losses caused by a micro- zooplankton population dominated by protozoans, rotifers and cyclopoid copepods. Thus, P. micans and cryptophycean flagellates could not propagate their populations in the field. Diatoms had high potential growth rates and were subjected to low grazing pressure by the micro- and mesozooplank- ton. However, diatom growth was severely limited by light in this highly turbid estuarine system, and light limitation probably prevented diatoms from becoming dominant. A. sanguinea was grazed on by the ciliate Tiarina fusus, but the growth rates of A. sanguinea exceeded grazing losses by T. fusus; other zooplankton species did not graze on this dinoflagellate effectively. A. sanguinea accumulated at the surface layer in the turbid water during daytime, and thus could utilize light effectively. Low grazing pressure by zooplankton and avoidance of light limitation seem to have led to the persistent dominance of A. sanguinea, even though the potential growth rate of this species is moderate to low.