Short-necked Clam Research Articles

Inactivation of 3-strain cocktail pathogens inoculated into Bajirak jeotkal, salted, seasoned, and fermented short-necked clam ( Tapes pilippinarum), by gamma and electron beam irradiation

The objective of this study was to determine the efficacy of gamma and electron beam irradiation of the food-borne pathogens including 3-strain cocktail of Listeria monocytogenes (ATCC 19114, 19115, and 19111), Staphylococcus aureus (ATCC 6538, 25923, and 29213), and Vibrio parahaemolyticus (ATCC 17802, 33844, and 27969) in Bajirak jeotkal (8% salt), salted, seasoned and fermented short-necked clam, commercially available in the market. Irradiation (0.5, 1, 2, and 5 kGy) significantly reduced the initial microbial level not only immediately after irradiation but also during storage at 10 °C for 4 weeks ( P < 0.05). No viable cells were detected at 5 kGy of irradiation at a detection limit of 10 1 CFU/g. Gamma irradiation was more effective than electron beam irradiation, and yielded D 10 values of 0.64, 0.63, and 0.29 kGy for L. monocytogenes, S. aureus, and V. parahaemolyticus, and those of electron beam irradiation were 0.79, 0.81, and 0.36 kGy, respectively. Results suggest that a low dose irradiation can improve the microbial quality and reduce the risk by the food-borne pathogens of Bajirak jeotkal, which has limited alternative sterilization methods due to the temperature characteristics of the products. Furthermore, in practical application, the irradiation source should be considered to obtain an effective dose for decontamination.

Pathology survey of the short-neck clam Ruditapes philippinarum occurring on sandy tidal flats along the coast of Ariake Bay, Kyushu, Japan

The pathological condition of the short-neck clam Ruditapes philippinarum was surveyed along the coast of Kumamoto, Japan, in June 2004. DNA sequences of the non-transcribed spacer region and internal transcribed spacer region flanking 5.8S rRNA identified Perkinsus olseni among the clams. Ray’s fluid thioglycollate medium assay indicated that 96.7% of the clams surveyed from the Kiguchi River tidal flat (native clams, Stn KR-N) and 96.7% of the clams surveyed from the Midori River tidal flat (Stn MR) were infected with P. olseni with an infection intensity of 464,278 and 199,937 Perkinsus cells/gram tissue wet weight (gWW), respectively. In contrast, 66.7% of the clams imported from China and stored along the Kiguchi River tidal flat (Stn KR-I) and 20.2% of clams from the Arao tidal flat (Stn AT) were infected with P. olseni with an infection intensity of 37,547 and 3382 Perkinsus cells/gWW, respectively. Brown ring disease was detected in the clam population from Stn KR-I at a prevalence of 90.0%. Polymerase chain reaction and the 16S rRNA sequence suggested that the agents of brown ring disease observed at Stn KR-I were Vibrio tapetis-like bacteria. Sporocysts and metacercariae of unidentified trematodes were also observed in the gonads and mantle of the clams from Stn KR-I, Stn MR, and Stn AT, at prevalences of 7.1–42.9%. Metacestodes (larval tapeworms) were found in the foot and digestive gland at a prevalence of 52.5%, 30.0%, and 14.3% in clams from Stns MR, AT, and KR-N, respectively. Histology also showed massive hemocyte infiltration and inflammation among clams heavily infected with P. olseni. Castration of the follicle was typical among clams infected with the trematode. The data indicate that most of the clams along the coast of Kumamoto are infected with various pathogens at various rates of infection, and these pathogens could have negative effects on the clam population in the long term.

THIS ARTICLE HAS BEEN RETRACTED: Effects of diet, stocking density and environmental factors on growth, survival and metamorphosis of clam, Paphia malabarica (Chemnitz) larvae

A series of experiments were conducted to evaluate the effects of diet, stocking density and environmental factors on the growth, survival and metamorphosis of short neck clam Paphia malabarica larvae. These experiments examined the following factors: diet [Isochrysis galbana, Nannochloropsis salina and a mixture of I. galbana and N. salina (1:1 w/w)], stocking density (1, 3, 5 and 7 larvae mL−1), light intensity (unshaded, partially shaded and fully shaded) and water filtration (unfiltered and sand filtered). Results indicated that N. salina could replace 50% of I. galbana as a food source for the clam larvae with an increase in growth, survival (47.2%), metamorphosis (33.5%) and early settlement. Larval growth decreased significantly with increasing stocking density. A density of 1–3 larvae mL−1 appeared to be optimal for normal growth of clam larvae. Neither diet nor stocking density used in the study had a significant effect on larval survival. Under partially shaded (light intensity=1000–5000 lx) and fully shaded (light intensity <1000 lx) conditions, larval growth was significantly faster than under direct sunlight (unshaded). Larvae grew significantly faster in the unfiltered water than in the filtered water.

The first detection of okadaic acid and its derivatives in Korean bivalves by liquid chromatography-mass spectrometry

Lipophilic toxin profiles associated with diarrhetic shellfish poisoning in Korean bivalves were investigated by liquid chromatography-mass spectrometry (LC-MS). Okadaic acid (OA), dinophysistoxins (DTX), pectenotoxins (PTX) and yessotoxin (YTX) in bivalves were quantified by LC-MS. Thirty-eight bivalve samples were collected from five different areas on the south coast of Korea during January to June 2006. Dinophysistoxin-1 (DTX1) and OA were found in some samples of both mussel Mytilus edulis and oyster Crassostrea gigas. The levels of DTX1 and OA found in the hepatopancreas of the samples were 0.04–0.23 μg/g and 0.03–0.08 μg/g, respectively. Dinophysistoxin-3 was found in the hepatopancreas of the short-necked clam Ruditapes philippinarum at a concentration of 0.06 μg/g. PTX and YTX were not detected from all of the samples examined. This is the first report of the unambiguous identification of OA and its derivatives, by LC-MS, in Korean bivalves.

山口県瀬戸内海側における浅場造成適地の評価方法の検討

This study was carried out to investigate the evaluation method for shallow waters, as suitable area for the short necked clam breeding fields and the eelgrass beds in the area of Seto Inland Sea of Yamaguchi Prefecture. To evaluate the sites comprehensively, we utilize GIS on the analysis of the environmental and social conditions, combined HEP on habitat suitability. We observed the distributions of short necked clam and eelgrass in recent years along with important environmental factors for them. Comparison of our findings with the data obtained from the model analysis indicates that our evaluation method can be applied for the site selection in this area. Consequently, it is considered to be effective GIS with HEP for evaluating habitat suitability for the shallow-water sites.

Environmental Evaluation Methodology for Tidal Flats and Shallows Focusing on Short-Necked Clams and Decomposition Rate of Organic Matter

An environmental evaluation model system for tidal flats and shallows focusing on population dynamics of short-necked clams and organic carbon decomposition rates is developed on the basis of integration of existing models. The evaluation model is applied to three tidal flats and shallows in Tokyo Bay, forcing with realistic boundary conditions and parameters. After verifying the model through the comparison with field data, feathers for each water along with their mechanisn are analyzed. The proposed model system would be useful to understand the mechanism for the appearance of differences among those waters.

Genetic characteristics of three recently discovered populations of the tideland snail Cerithidea djadjariensis (Martin) (Mollusca, Gastropoda) from the Pacific coast of the eastern Japan

The genetic characteristics of three populations of the tideland snail Cerithidea djadjariensis (Martin) that were recently discovered on the Pacific coast of the eastern Japan were compared with other Japanese conspecific popu- lations of this snail on the basis of the nucleotide sequences of mitochondrial DNA. Two populations from the Pacific coast of northeastern Honshu, the Japanese mainland, showed lower genetic diversity than and no significant differentia- tion from populations in western Honshu and Kyushu. The foundation of these populations might be attributed to range expansion along the coast, probably due to global warming. On the other hand, the population in Tokyo Bay, which was rediscovered in 2001 on an artificial tideland, was shown to be genetically different from all other Japanese populations whose genetic structures have been analyzed. It is suggested that this population was founded by human-mediated intro- duction such as transportation with short-necked clams, ballast water, or hull fouling.

橋梁耐震補強における鋼製函体を用いた気中補修工法の採用による環境影響低減の試み

This field is located in the estuary. The tidal flat is formed at low tide, and many water birds come flying here. In addition, short-neck clams and corbiculas lives in the riverbed. Therefore, it was necessary to attempt the decrease of the environmental impact by shortening of construction period or construction range reduction in this river. When we constructed earthquake resistant reinforcement of new Yahata river bridge, we used dry repair industrial method by temporary steel caissons these problems. As a result, we contributed to shorten the construction period.

Field and laboratory observations on the hypoxic impact on survival and distribution of short-necked clam Ruditapes philippinarum larvae in Tokyo Bay, central Japan

There are few descriptions of the impact of hypoxic conditions on short-necked clam Ruditapes philippinarum larvae in the natural environment, while a number of reports describe catastrophic mortality of benthic clams caused by coastal upwelling of hypoxic water in the eutrophicated bays in Japan. We conducted seasonal observations of larval density in the water column along with measurement of DO at coastal and off-shore stations in Tokyo Bay during 2001–2003, and also experimental exposure of the clam larvae to low DO to examine the impact of hypoxic water on larval survival. Under hypoxic conditions (<1.0 mgO2 L−1) in the bay, a marked decline in larval density was consistently detected in the bottom layer at coastal stations adjacent to adult habitats and at off-shore stations of the central bay. In the low DO exposure experiment, the ratio of swimming larvae decreased with exposure period. Larval survival declined within 12 h, and most of the larvae at early developmental stage (shell length <170 μm) died within 24 h under severe hypoxic conditions (<0.2 mgO2 L−1) at 25°C. These results indicate that larvae being transported to the hypoxic zone may have reduced swimming ability and may inevitably sink to the lower layer and die shortly thereafter. In Tokyo Bay, anoxic or hypoxic conditions in the lower layer during May–October are continuous, and this time period overlaps with the entire spawning period of the short-necked clam. Hypoxic water exerts critical stress on larval survival, and may result in decline of juvenile recruitment to the benthic populations of short-necked clam in Tokyo Bay.

Search for sea sand alternatives suitable for growth of Ruditapes philippinarum and characterization of the factors affecting the larval settlement

This research was carried out to search for sea sand alternatives suitable for larval settlement of the short-neck clam (Ruditapes philippinarum), as well as growth of its young shells. Among the tested alternatives, melting slag derived from municipal waste incineration (MSI-slag) showed the higher number of settlers than the mountain and real sea sands (MS, SS), whereas the number of settlers on the granulated ashes derived from industrial waste combustion (IWC-A) was comparable to that on MS and SS. The addition of MSI-slag to SS accelerated the larval settlement, indicating that MSI-slag attracts the short-necked clam larvae. In the growth test of young shells, MSI-slag and IWC-A were excellent livability than MS.

Impact of high concentrations of manganese on the survival of short neck clam Ruditapes philippinarum juveniles in sandy tidal flat sediment in Ariake Bay, Kyushu, Japan

Ruditapes philippinarum is a popular edible clam that occurs densely on the sandy tidal flats on the Japanese coast. However, on the tidal flats in Ariake Bay in Kumamoto Prefecture, western Japan, the clam suffered from extremely high mortality just after settlement on the sediment and this mortality seriously affected the population persistence of the clam. In this study, we focused on the negative impact of manganese in the sediment on the juvenile clam. We surveyed manganese ion concentration in the interstitial water of the sediment on three different sandy tidal flats facing Ariake Bay in Kumamoto Prefecture and one in the west coast of Korea. We conducted laboratory experiments exposing juvenile clam just after settlement to different contents of manganese dioxide in the sediment or manganese ion in the water: with sand containing high content of manganese dioxide (2,300 mg kg−1 dry sediment; henceforth Experiment 1); with seawater containing 5.4 mg L−1 of manganese ion (Experiment 2); and with natural sand and seawater containing 5.4 mg L−1 of manganese ion (Experiment 3). Significant mortality was not noted in Experiment 1 or in Experiment 2, while high mortality was seen in Experiment 3. In the field survey, we detected manganese ion concentration in the interstitial water at only 2.78 mg L−1 on the Arao Tidal Flat. However, we confirmed that manganese ion concentration in the interstitial water became more than 40 mg L−1 in the extraction test with the sediment on the Arao Tidal Flat. These results indicate that manganese ion liquated from the sediment into the interstitial water, due to reduction of manganese oxide, may influence seriously the physiology of juvenile clams on the tidal flats.

マルチエージェントモデルによる潮干狩り行動の数値シミュレーションに関する研究

When we use ocean space for human activities, we should consider sustainability and allowable human impacts.After the 1960's, we have lost 90 percent or more of the tideland of Tokyo-bay by the reclamation. "Uminokouen" is one of the artificial seashore parks developed in 1988 at the southeast of Yokohama and is well known as a relaxation place for the citizens. Over 20,000 people visit there for seashell-digging (SHIO-HIGARI) in a holiday in the spring.It is easy to understand that the human impact is quite large to an ecosystem of living bivalves in the park. Actually,the most part of living bivalves which are grew up after the last summer or autumn disappeared during the season. In this paper, we propose a computational model which predicts the human action of seashell-digging at this park using a multi-agent-algorithm. Comparing the simulation result with the actual data, the staying time of the visitors agrees well. Then, we investigate the relation among the initial spatial distributions of the bivalves, the spatial distributions of the amount of the caught short-neck clam and the trace of the visitors. Then we predict the human impact in the best weekend for the seashell-digging and examine the sustainability of the relaxation park.

설사성 패류독의 LC-MS/MS에 의한 분석

LC-MS/MS법에 의하여 설사성 패류독소 7종(okadaic acid, dinophysistoxin-1, -3, pectebotoxin-1, -2, -6, yessotoxin) 의 동시 분석 조건을 확립하였고, 2006년 3월부터 9월까지 매월 5종의 이매패(굴, 진주담치, 반지락, 가리비, 개조개, 6월부터 9월까지는 가리비 대신 피조개)를 정기적으로 통영시장에서 구입하여 분석하였으며, 기존의 마우스 검사법과 비교하였다. 마우스 검사법으로는 총 35종 시료 중 3월의 굴과 진주담치에서 0.05～0.1 MU/g의 독성을 나타내었으나, 이미 알려진 설사성 패류독소는 전혀 검출되지 않아 다른 성분에 의한 것으로 판단되었다. 한편, LC-MS/MS법에 의하여서는 6월 진주담치에서 dinophysistoxin-1(DTX1)이 0.05 ㎍/g 검출되었으며, 8월의 모든 시료에서 okadaic acid(OA)가 0.01～0.02 ㎍/g 검출되었다. 이들은 외국의 기준치[0.05 MU/g(일본), OAs(OA＋DTX1) 0.16 ㎍/g(EU)] 보다 매우 낮은 함량으로 식품 위생상 안전하였다.

Seasonal distribution of the shellfish-killing dinoflagellate Heterocapsa circularisquama in Ago Bay monitored by an indirect fluorescent antibody technique using monoclonal antibodies

Heterocapsa circularisquama is the most noxious dinoflagellate to bivalves such as oysters, short-necked clams and pearl oysters because its red tides have caused major mass mortalities. In order to reduce the negative impacts of H. circularisquama, predictions of red tide occurrences are essential, and hence it is important to grasp its population dynamics. The population dynamics of H. circularisquama, however, have not yet been clarified throughout the year. To precisely monitor population dynamics, the fluorescent antibody technique is effective. We here report on H. circularisquama population dynamics monitored with the indirect fluorescent antibody technique (IFAT) using monoclonal antibodies. Samplings were carried out once a week in summer and twice a month in other seasons at 4 points in Ago Bay, Mie Prefecture, Japan, from April 2001 to March 2005. Direct counting of the cells was also performed using a normal optical microscope. Vegetative cells of H. circularisquama monitored with IFAT were generally detected from late spring (May) to late autumn, and the cell density increased (maximum 2.33×106 cells L-1) during summer. The lowest detection level was 1.33 cells L-1 with IFAT. In contrast, monitoring by counting with a common optical microscope often failed to detect H. circularisquama cells even at a density of 103 cells L-1 or more. Water temperature was significantly correlated to the abundances of H. circularisquama, and the cells were almost always observed at a temperature of 25°C or higher (summer season), and never detected at 10°C or lower (winter and early spring) in Ago Bay. In the waters of the bottom layer, H. circularisquama cells tended to be more abundant with lower dissolved oxygen (DO) values during summer. Thus, the seasonal distribution of H. circularisquama was clarified almost perfectly together with environmental factors in Ago Bay between spring and early winter.

Accumulation and Elimination Profiles of Paralytic Shellfish Poison in the Short-necked Clam Tapes japonica Fed with the Toxic Dinoflagellate Gymnodinium catenatum

The paralytic shellfish poison (PSP)-producing dinoflagellate Gymnodinium catenatum (Gc) was fed to the short-necked clam Tapes japonica, and the accumulation, transformation and elimination profiles of PSP were investigated by means of high-performance liquid chromatography with postcolumn fluorescence derivatization (HPLC-FLD). The short-necked clams ingested most of the Gc cells (4 x 10(6) cells) supplied as a bolus at the beginning of the experiment, and accumulated a maximal amount of toxin (181 nmol/10 clams) after 12 hr. The rate of toxin accumulation at that time was 16%, which rapidly decreased thereafter. During the rearing period, a variation in toxin composition, derived presumably from the transformation of toxin analogues in the clams, was observed, including a reversal of the ratio of C2 to C1, and the appearance of carbamate (gonyautoxin (GTX) 2, 3) and decarbamoyl (dc) derivatives (decarbamoylsaxitoxin (dcSTX) and dcGTX2, 3), which were undetectable in Gc cells. The total amount of toxin contained in clams and residue (remaining Gc cells and/or excrement in the rearing tank) gradually declined, and only about 1% of the supplied toxin was detected at the end of the experiment.

耐熱性溶血毒 (TDH) または耐熱性溶血毒類似毒素 (TRH) 産生腸炎ビブリオの島根県沿岸における分布状況および市販貝類の汚染状況

We studied distribution of thermostable direct hemolysin (TDH)- and TDH-related hemolysin (TRH)-producing Vibrio parahaemolyticus in coastal sea water, sediment, and shellfish and related retail shellfish contamination in Shimane Prefecture, Japan, between 2002 and 2004. V. parahaemolyticus was isolated from > 80% of sea water, sediment, and shellfish. The detection of TDH gene (tdh) and TRH gene (trh)-positive V parahaemolyticus in sea water was 11%, in sediment 16%, and in shellfish 26%. The number of genes and gene-related in seawater was 23 MPN/L, in sediment 29 MPN/100 g, and in shellfish 460 MPN/10 g. TDH- and TRH-producing V. parahaemolyticus detected in seawater was 5%, in sediment 11% and in shellfish 14%. The continuous distribution of TDH-producing O2:K28, O4:K88, O4:K37, and O4:KUT organisms on the western coast and TRH2-producing O5:k30, O5:K43, O10:K19, O10:KUT, O11:K40, O11:KUT, and OUT:KUT organisms on the Oki Island coast suggested the settlement of these organisms in these coastal environments. From 7 (12%) of 59 retail short-necked clam samples, we isolated TDH-producing O 1:KUT, O3:K6 (2 strains from 2 samples imported from Korea), O4:K12, OUT:K8, and TRH2-producing OUT:K40 and OUT:K51 organisms. These findings suggested that TDH- and TRH-producing V. parahaemolyticus are widely distributed along the coast of this prefecture and are transported by contaminated retail shellfish from other areas.

Risk assessment of TBT in the Japanese short-neck clam ( Ruditapes philippinarum) of Tokyo Bay using a chemical fate model

A risk assessment of Tributyltin (TBT) in Tokyo Bay was conducted using the Margin of Exposure (MOE) method at the species level using the Japanese short-neck clam, Ruditapes philippinarum. The assessment endpoint was defined to protect R. philippinarum in Tokyo Bay from TBT (growth effects). A No Observed Effect Concentration (NOEC) for this species with respect to growth reduction induced by TBT was estimated from experimental results published in the scientific literature. Sources of TBT in this study were assumed to be commercial vessels in harbors and navigation routes. Concentrations of TBT in Tokyo Bay were estimated using a three-dimensional hydrodynamic model, an ecosystem model and a chemical fate model. MOEs for this species were estimated for the years 1990, 2000, and 2007. Estimated MOEs for R. philippinarum for 1990, 2000, and 2007 were approximately 1–3, 10, and 100, respectively, indicating a declining temporal trend in the probability of adverse growth effects. A simplified software package called RAMTB was developed by incorporating the chemical fate model and the databases of seasonal flow fields and distributions of organic substances (phytoplankton and detritus) in Tokyo Bay, simulated by the hydrodynamic and ecological model, respectively.

Accumulation and depuration profiles of PSP toxins in the short-necked clam Tapes japonica fed with the toxic dinoflagellate Alexandrium catenella

A toxic dinoflagellate responsible for paralytic shellfish poisoning (PSP), Alexandrium catenella ( Ac) was fed to the short-necked clam Tapes japonica, and the accumulation and depuration profiles of PSP toxins were investigated by means of high-performance liquid chromatography with postcolumn fluorescence derivatization (HPLC-FLD). The short-necked clams ingested more than 99% of the Ac cells (4×10 7 cells) supplied once at the beginning of experiment, and accumulated a maximal amount of toxin (185 nmol/10 clams) after 12 h. The rate of toxin accumulation at that time was 23%, which rapidly decreased thereafter. Composition of the PSP toxin accumulated in the clams obviously different from that of Ac even 0.5 h after the cell supply, the proportion of C1+2 being much higher than in Ac, although the reason remains to be elucidated. In contrast, a higher ratio of gonyautoxin (GTX)1+4 than in Ac was detected in the toxin profiles of clam excrements. The variation in toxin composition derived presumably from the transformation of toxin analogues in clams was observed from 0.5 h, such as reversal of the ratio of C1 to C2, and appearance of carbamate (saxitoxin (STX), neoSTX and GTX2, 3) and decarbamoyl (dc) derivatives (dcSTX and dcGTX2, 3), which were undetectable in Ac cells. The total amount of toxin distributed over Ac cells, clams and their excrements gradually declined, and only 1% of supplied toxin was detected at the end of experiment.

Critical events in the Ariake Bay ecosystem: Clam population collapse, red tides, and hypoxic bottom water

Ariake Bay is located off the west coast of Kyushu, western Japan, and is a typical enclosed coastal sea of approximately 1,700 km2 in area. The tidal amplitude reaches approximately 6 m in the inner parts of the bay at the spring tide, and consequently approximately 20,000 ha of tidal flats appear along the coast of the bay at ebb tide. The tidal flats and the shallow waters offshore from the tidal flats are extremely productive areas, and utilized for various fisheries. However, the ecosystem of Ariake Bay is now in critical condition. The abundance of benthic animals has decreased markedly on the sandy tidal flats in the bay. This phenomenon is represented by the drastic decrease in the harvest of the most popular short neck clam, Ruditapes philippinarum, on the sandy tidal flats of Kumatoto Prefecture facing Ariake Bay. The annual harvest on the sandy tidal flats exceeded 50,000 tons until 1980, but decreased to only around 400 tons between 1995 to 1997. On the sandy tidal flats where the dense clam population collapsed, very few macro-benthic animals occurred. Since the 1960s, a large amount of sand and gravel has been collected from the rivers that flow into Ariake Bay. It is apparent that the marked decrease in newly deposited sand on the sandy tidal flats is one of the main causes of the collapse of the dense population of the short neck clam and marked decrease of other macro-benthic animals on the sandy tidal flats, and thus it appears that the only effective measure to recover the benthic communities on the sandy tidal flats is creating sand covers with natural sands collected from the sea floor offshore of the bay.In the subtidal areas of the bay, the scale of the autumn bloom of phytoplankton has increased markedly and red tides have covered almost the entire inner bay area since 1998, although the nutrient input from the land to the bay through the major rivers has fluctuated stably over the past three decades. The nori culture was seriously damaged by the occurrence of a large-scale autumn red tide on two occasions (2000 and 2002). In early summer, hypoxic bottom water has occurred in the wide areas of the inner parts of the bay since at least 2001, causing a serious environmental disturbance to the benthic ecosystem. Both of these phenomena commonly occurred when the water was stratified mainly due to the decrease of salinity in the surface layers. These facts indicate that the stratification of the water was enforced in the inner parts of the bay from the end of the 1990s. In Isahaya Bay, an inner bay of the west side of Ariake Bay, a big reclamation project of the tidal flats to create land and a fresh water pond of approximately 3,500 ha in total area has been conducted since 1986, and the gates of the dike were closed to lower the water levels inside the dike for reclamation in April 1997. A decrease in the tidal current was recently observed in some parts of the west coast of the bay. It is very likely that the changes of the hydrographical conditions caused by the modification of the topography of Isahaya Bay are responsible for the development of the water stratification in the inner parts of Ariake Bay, following the occurrence of the large-scale autumn red tides without any increase in the nutrient input from the land and the hypoxic bottom water in the summer.

Eutrophication and occurrences of harmful algal blooms in the Seto Inland Sea, Japan

The Seto Inland is the largest enclosed coastal sea in Japan and is also a major fishing ground includ- ing aquacultures of fish, bivalves and seaweeds. The incidents of red tides dramatically increased in frequency and scale in the Seto Inland along with serious eutrophication in the 1960s and 1970s. The maximum incident of 299 was recorded in 1976, but the incident has since shown a clear decreasing trend, reaching about 100 per year in the late 1980s by virtue of regulation by law, and this level has been maintained thereafter with the level of nutrients supporting red tide occurrences. The Law Concerning Special Measures for Conservation of the Environment of the Seto Inland Sea was legislated in 1973 and industrial loading was decreased to half the level of 1972. The important red tide or- ganisms causing huge fishery damages by fish-kill are Chattonella antiqua, C. marina, C. ovata and Heterosigma akashiwo (Raphidophyceae), and Karenia mikimotoi and Cochlodinium polykrikoides (Dinophyceae). The maximum fishery damage (death of 14.2 million yellowtails) was 7.1 billion yen (about US $60 million) caused by C. antiqua in Harima-Nada in 1972. In 1988, a novel red-tide dinoflagellate species Heterocapsa circularisquama appeared for the first time, and has repeatedly killed both natural and aquacultured bivalves, with the highest damage of 3.9 billion yen to cultured oysters in Hiroshima Bay in 1998. Among the important red-tide organisms, C. antiqua, H. circularisquama and C. polykrikoides are rated as extremely harmful species that can easily reach the warning level of fishery damage by consuming only small amounts of nutrients. In toxic blooms, the dinoflagellate Alexandrium tamarense has become dominant in the Seto Inland in the spring season, causing toxicity in short-necked clams and cultured oysters al- most every year. Many countermeasures have been applied for harmful algal blooms in Japan. Laws for the regulation of water quality have been most effective in decreasing red-tide occurrences. No physical and chemical controls have been successful except for clay treatments. Clay spraying has been investigated and implemented in Kyushu and Korea for the removal of C. polykrikoides red tides. As environment-friendly mitigation strategies for red tides, biological con- trols using algicidal bacteria and viruses are proposed. A new finding of the abundant existence of algicidal bacteria on the surface of seaweeds suggests that co-culturing fish and seaweed is a prevention strategy for harmful algal blooms by virtue of the continuous release of many algicidal bacteria to the surrounding seawater. The artificial development of seaweed beds would also be effective as a prevention strategy for red tides.