The number of red tides reached a peak of 299 in 1976 and decreased to about 160 in proportion to the decrease in nutrient concentrations. Thus, the frequecy of red tide occurrences is closely related to eutrophication. The total damage amounted ¥41.6 billion between 1971 and 1995 (about ¥1.7 billion per year) . Most of the fish damage has been caused by red tides of Gymnodinium mikimotoi and the genus Chattonella. However, in the second half of the 1980s red tides of Heterocapsa circularisquama and Gymnodinium sp. (type-Imari) began to occur.
Predicting techniques of red tides caused by the genus Chattonella and G. mikimotoi were developed in Harima-Nada, Suo-Nada, Gokasho Bay, and Tanabe Bay.
Direct preventing techniques include the removal of red tide plankton by spraying non-biological agents such as the active clay containing acidified montmorillonite or using organisms such as bacteria, the move of culture cages, and fasting. On the other hand, indirect prevention techniques are mostly those for improving the bottom environment such as spraying of lime and clay.

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Matsushima Bay, one of the richest oyster-culture areas in Japan, was subject to heavy artificial eutrophication, mainly from the inflow of city and factory sewage. The physiological activity in the oyster, Crassostrea gigas, in hanging cultures in this embayment declined markedly with progressive development of the gonads; this decline in activity coincided with eutrophication-induced accumulation of fatty material in the epithelia of the digestive organs; the oyster was thereby forced to depend on these accumulated fats for respiratory substrates in order to maintain its increased physiological needs. However, the fats were inefficient energy sources for the oyster and hence, during each spawning season, 50% mortality occurred. In addition, such phenomena as overmaturation of the gonad and disturbance of the lipid and steroid metabolism seemed to accelerate this mass mortality.
The progress of eutrophication in recent years is due not only to an increase in the inflow of city and factory sewage, but also to an expansion of aquaculture areas in coastal waters. Dense cultivation can markedly accelerate eutrophication, especially in a small bay or inlet, leading to deterioration or self-pollution of culture areas. In a eutrophic embayment or inlet, the inflow of organic matter and the sedimentation of dead organisms often lead to an increase in organic content of the sea-bottom sediments. In summer, oxidative decomposition of these organic substances results in decrease of dissolved oxygen of both the water layers near the bottom and the intermediate stratum, with liberation of free hydrogen sulphide. The oysters naturally fail to survive in such an environment.
A high density that is artificially created can result in symptoms of intraspecific physiological stress. Dense oyster culture in an area often facilitates the introduction and propagation of new strains of bacteria or parasites that were never present before. This sometimes causes disruption in the structure and function of the ecosystem.
Hence, one must take precautions against artificial eutrophication of coastal waters and dense oyster culture in order to maintain steady production of oysters by means of hanging cultures.

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Lake Nakaumi is a brackish lake located on the Shimane Peninsula in Shimane Pref. and connected to the Sea of Japan through the Sakai Channel.
Eutrophycation adversely affects both water and sediment conditions, causing a drop in fishery production in lakes such as Nakaumi.
To examine the effects of an artificial sand cover on sediment, changes in the cover of macro benthos were examined from February 1993 to December 1994 in Nakaumi.
Our results indicate that sediment covered with sand was a suitable environment for macro benthos, and in summer, low oxygen levels in deep water cause adverse effects on the lake's macro benthos.

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Horizontal starch gel electrophoresis was used to analyze the genetic variation and relationship between the mud crab genus Scylla and four portunid crabs, Portunus trituberculatus, Charybdis japonica, Thalamita pelsarti, and T. sima. Seventeen allozymic loci of 11 enzymes were examined using muscle. The reaction of AAT and EST enzymes were not found in those crabs except for Scylla. Gene replacement was found at 12 loci, FH-1, FH-2, GPI, HK-1, HK-2, IDHP-1, IDHP-2, LAP-1, LAP-2, MDH-1, PGDH, and SOD of all 14 loci among 7 species. Genetic analyses showed that Nei's genetic distance was 0.643 between the genus Scylla and P, trituberculatus and 0.628 between the genus Thalamita and C. japonica.

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The diurnal fluctuaton, body length and area of hatching ground of ayu, Plecoglossus altivelis, larvae during seaward migration were investigated in the Shou River from 1992 to 1996. The fluctuation of larvae collected at a site 5.5 km from the rivermouth showed a single peak, either at 20 : 00 or 22 : 00. Individuals of ayu larvae collected at peak hour comprised 46.5-56.2% of the total in a day. The standard length ranged from 3.7 mm to 7.6 mm, with a mean of 5.8 mm. That of ayu larvae collected in late September or in early October was significantly shorter than those collected from early or late October to early January. The area of hatching ground of ayu larvae in the Shou River was considered to be restricted between 5.5 km and 11 km from the rivermouth.

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The maturity stages of gonads and the fecundity of the Japanese whiting, Sillago japonica, were investigated from October 1993 to January 1995 in Tateyama Bay using samples collected by encircling trammel net and hand line. Sex ratio (M/F) of the fish varied from 0.68 to 1.67. Both male and female had a peak CF (condition factor) value in June (0.86 in female and 0.83 in male) . The peak value for GSI (gonad somatic index) was observed in August (4.19 in female and 1.62 in male) . The maturing and mature stages of gonads were observed from June to October, whereas the immature gonad was found during November to May. Fecundity of S. japonica varied from 13, 600 to 68, 900. The GSI, CF, gonad maturity stages and size frequency of the ova-diameter indicated that the spawning period of the S. japonica was also quite long.

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Sampling surveys were carried out in order to investigate the time spent for ayu larvae to move from the spawning area to the mouth of a river. The surveys were conducted from 1994 to 1996 in the spawning area of the Shou River and the rivermouth. Each survey lasted 24 hours with the sampling interval of two hours. The specimens were randomly chosen from the collected samples for measuring the standard length. The time of the peak occurrence of ayu larvae in the spawning area was either 20: 00 or 22: 00, whereas the time of the peak occurrence in the mouth of the river, appeared between 01: 00 and 07: 00. The frequency distributions of standard length in the two areas were very close to each other. The statistical difference of the mean standard length between the two areas was within the range of the increase of individual growth for the time difference. These results suggest that most ayu larvae hatched in the spawning area at night migrated to the mouth of the Shou River by the next morning.

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As a part of a resources-increase project of nigorobuna, Carassius auratus gyandoculis, a marking method for the otolith with alizarin complexone (ALC) was examined.
At every developmental stage, red fluorescent marks were visible under UV light with a G-filter in otoliths (lapilli) of ngorobuna immersed in ALC solution. The optimum immersion conditions at 20°C were: 16 mg/l for 24h for eggs (after eyed-stage), larvae and early juveniles (<16 mm BL) : 32 mg/l for 24h for juveniles (16-35 mm) . The fluorescent marks remained visible for at least 807 days after immersion. From a rearing experiment with the marked fish and unmarked fish in the same tank, it was confirmed that this marking did not exhibit negative effect on fish growth and survival.

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The relationship between the stocked juvenile and returning adult kokanee, Oncorhynchus nerka, in Lake Chuzenji was examined using the statistical data collected from 1968 to 1998 by Lake Chuzenji Fisheries Co-operative Association. According to the Ricker's reproduction model, approximately 6, 000 adult fishes were expected to be caught after 2 years if 1.0 to 1.2 million juvenile kokanees were stocked initially. The annual amount of the influent to the lake and the number of the adult fish caught 2 years after stocking were positively correlated; two regression lines with different slops were obtained. The result suggested the presence of two possible groups.

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Electrocardiogram was recorded from Halocynthia roretzi. Increase in the heart rate (IHR) as well as squirting was elicited by somatosensory stimulation of the atrial siphon. IHR was abolished by extirpation of the ganglion, but squirting was still evoked. Stimulation which increased the intrabranchial pressure did not cause IHR. These results suggested that heart in Halocynthia roretzi is controlled neurally.

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To evaluate the effect of ozone-produced oxidants on respiration of the Pearl Oyster, Pinctada fucata, the amount of oxygen uptake (Vo₂), the ventilation volume (Vg), the oxygen utilization (U) and the gill cilia movement (SP) were examined before, during and after exposing to ozoned seawater at various concentrations of the oxidants for 2 hours. The exposure decreased all the four criteria markly or to zero. And when the oxidant concentration reached to 5.80 mgO₃/l, SP did not recover even 20 hours after the shell being returned to normal seawater. However Vo₂, Vg and U recovered to the normal levels 3 hours after being returned to normal seawater when the oxidant concentration were equal to or less than 6.12 mgO₃/l. The results suggested that the 2 hours exposure of the Pearl Oyster to ozoned seawater at equal to or less than 6.12 mgO₃/l would not likely to cause the damage of the tissues of the shell.

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To forecast the result of mass culture of the rotifer, Byachionus plicatilis, physiological activities of individual animals were evaluated. Indices Criteria were percentage of feeding individuals, extent of digestive organs in the body, frequency distribution of lorica length and tolerance to hyper-saline environment.
The percentage of feeding individuals decreased with time, and the lowest value was observed near the peak of the growth curve. The size distribution of feeding individuals and the extent of digestive organs changed only when the culture condition deteriorated sharply. As for the non-feeding individuals, a specified size fraction increased in the course of culture. Hyper-saline tolerance for individuals in 70ppt for 3 hours suddenly dropped when the growth was close to the peak. It was more sensitive in egg-bearing females. The culture started with rotifers of low tolerance showed a stagnant growth.
From the results, the percentage of feeding individuals and the size distribution of non-feeding individuals seemed to be useful for forecasting the change of mass culture. And, hyper-saline tolerance was considered to be a suitable predictor of rotifer growth and production in the cultures.

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We observed the development of denuded eggs of common carp, Cyrinus carpio. The embryos grew straight without the chorion. They developed normally and reached the hatching stage about 62 h after insemination at 24-27°C, as same as the normal eggs. The dechorionation and cultivation method for denuded eggs enables us to observe the early development clearly. It may also make easier to observe the development of internal organs by fixing and sectioning the straightened embryonic body for tissue observation.

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The leukocyte suspension, obtained by hemolyzing carp blood with distilled water, was screened on a flow cytometer. The flow cytometric analysis of carp leukocytes in FSC vs SSC dot plots mainly yielded 3 fractions: neutrophils, basophils and a mixture of thrombocytes and lymphocytes. Since fractions of neutrophils and basophils could be easily distinguished from other leukocytes, counts of neutrophils, basophils and total leukocytes were easily obtained and compared with manual counts. The correlation coefficients between FACS and manual counts of neutrophils, basophils and total leukocytes were r=0.86 (n=66), r=0.93 (n=33), and r=0.98 (n=33), respectively.

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To obtain basic information on artificial seed production for aquaculture of bluefin tuna, Thunnus thynnus, the ontogenetic change in oxygen consumption of artificially hatched bluefin tuna was investigated from egg to 132 days after hatching. Individual oxygen consumption (M, μl/min) of eggs was almost constant from morla to the appearance of Kupper's vesicle stage. Then, M rapidly increased from the heart beating stage (HB), at just before hatching it was about 8 times the volume before HB. After hatching, ontogenetic change of M showed a four-phasic allometry in relation to body mass (g) . Flection points were at the transitional period from endogenous to exogenous feeding, postflexion stage, and juvenile stage. During the endogenous feeding stage, M was almost constant. From the exogenous feeding to juvenile stage, M rapidly increased with the development of body weight. From the juvenile to young stage, the increase of M slowed down. It was shown that oxygen consumption of bluefin tuna rapidly increases at an early stage.

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In order to develop a food chain (phytoplankton-fish) in a closed ecological recirculating aquaculture system (CERAS), tilapia, Oreochromis niloticus, weighing about 2.8 g (body length, 4.4cm) were cultured with commercial diet, dried Spirulina diet, and dried Spirulina diet supplemented with vitamin and mineral mixtures for 6 weeks.
The results showed that tilapia can be cultured for a 6-week by dried Spirulina diets with or without vitamin and mineral mixtures, although the fish fed the commercial diet grew better. Fish fed both Spirulina diets had higher protein content and n-6 fatty acids composition in body compared with the fish fed commercial diet. Especially, tilapia fed Spirulina diet supplemented with vitamin and mineral mixtures showed high calcium content. On the other hand, vitamin A, E, and C contents decreased in the fish fed dried Spirulina diet, suggesting that vitamin supplementation is required for a long term feeding.

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In order to investigate the growth performance of formula feed (mash type) on the growth, feed efficiency, and protein digestibility of yellowtail, Seriola quinqueradiata, in low temperature period, the fish were reared on three different diets, 1) a single moist pellet diet (SMP), 2) a protease-treated single-moist-pellet fish meal (E-SMP), and 3) a mainly-raw-fish moist pellet diet (RF-MP) for 130 days from October to February.
Growth, feed efficiency, and protein efficiency ratio were the highest in fish fed RF-MP; E-SMP gave higher values than SMP. Although pepsin-like and trypsin-like activities were the lowest in fish fed RF-MP, protein digestibility was the highest in fish fed RF-MP, and it was slightly higher in fish fed E-SMP than SMP. These results indicated that the growth performance of the fish fed the formula feed was lower than that fed RF-MP in low temperature period. This was thought that lower water tempareture directly decreased protein digestibility of formula feed although fish increased their digestive enzyme activities to adopt the formula feed.

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Pejerrey, Odonthestes bonayiensis, was reared in the recirculation system for one year. The system consisted of two particle-trap equipped octagonal tanks (0.75 m³) for the rapid collection of feces and uneaten feed, a mechanical filter, a UV unit, a biological filter which provided nitrification, and a hollow-fiber oxygen injection system. Total system volume was about 2.1m³. The flow rate was fixed at 2.1 m³/h. Three hundred and eighty three pejerrey (avg. wt. 1.6 g) were stocked in the system with salinity of 7‰ and temperature was maintained at 24°C. After one year, survival rate was 92%. An average of 8.5lof saltwater and quantity of evaporation water was supplemented to the system daily. At maximum load, just before expiry (avg. wt. 109.1 g), water quality was very good with NH₄-N at 0.2 mg/land NO₂-N at 0.1 mg/l. During the experiment, Nitrates peaked at 900 mg/l, but dropped to 150 mg/lafter a denitrification system was installed. Of the total nitrogen produced in the system, 60% was oxidized to nitrate in the nitrification filter and another 14% in pipes/ tank walls, and both were eventually removed as nitrogen gas in the denitrification system. The remaining 26% was removed as solids of which 19% from the particle trap and 7% from the drum filter.

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The present study attempted to investigate Penaeid rod-shaped DNA virus (PRDV) infection in the wild kuruma prawn, Marsupenaeus japonicus, in Lake Hamana and out-lake region of Ensyunada and other crustaceans. PRDV was detected in male (1.5%), female (2.7%), adult (2.1%), puberty (2.3%), post-larvae (5.3%) of kuruma prawn, and other crustaceans (0%) by using PCR. Annual changes of PRDV detection rate between adult and puberty size were highly correlated. These results suggested that PRDV in Lake Hamana was transmitted vertically.

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Coded wire tagging on external ligament of brackish water clam, Corbicula japonica, was evaluated. There was no effect on survival by the tagging and also no significant differences in body weight, shell length and condition factor between the tagged clams and nontagged clams under 275 days of laboratory experiment. Moreover, the tag retention rate was high (93%) .

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The multiple linear regression equations of attraction activity (MLREAA) on the amino acid contents in the 12 fishing baits were estimated, for seeking probable feeding attractive amino acids for yellowtail, Seriola quinqueradiata. The MLREAA on the contents of 17 amino acids and taurine, and the number of fish used in the respective experiments revealed that the attraction index (1) increased with the increase of the content of histidine, (2) had no relation to the contents of cystine, methionine, aspartic acid, and arginine and (3) might be related to the contents of the rest of 11 amino acids and taurine.

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