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A marine raphidophyte Heterosigma akashiwo is a causative agent of harmful microalgal blooms, which often cause the massive mortality of aquacultured finfish. In the present study, the Pacific oyster Crassostrea gigas was reared with H. akashiwo, and effect of the microalga on filter-feeding behavior and microflora of the gastrointestinal tract was investigated. The intake of the raphidophyte cells inhibited the molluscan filter-feeding activities, suggesting the negative physiological effect of the microalgal cell contents. However, the bivalves ingested the H. akashiwo cells to the same extent as the diatom Chaetoceros calcitrans, a non-harmful indicator to estimate the filtration rate, showing a continuation of their non-selective ingestion of the phytoplankton. Microflora of the oyster soft tissue was dominated by bacteria affiliated with the family Rhodobacteraceae, some of which are associated with microalgae. In addition, the Bacteroidetes species, in which algicidal bacteria are included, were also found in the bivalve individuals exposed to H. akashiwo. These results suggested that the ingested phytoplankton affected the microbial flora in the gastrointestinal tracts, some constituents of which helped the mollusc assimilate the ingested red tide phytoplankton. This study will provide beneficial information to clarify mechanisms by which the oyster evades the ichthyotoxicity of harmful microalgae and the participation of the intestinal microorganisms in these processes.

A marine raphidophyte Heterosigma akashiwo is a causative agent of harmful microalgal blooms, which often cause the massive mortality of aquacultured finfish. In the present study, the Pacific oyster Crassostrea gigas was reared with H. akashiwo, and effect of the microalga on filter-feeding behavior and microflora of the gastrointestinal tract was investigated. The intake of the raphidophyte cells inhibited the molluscan filter-feeding activities, suggesting the negative physiological effect of the microalgal cell contents. However, the bivalves ingested the H. akashiwo cells to the same extent as the diatom Chaetoceros calcitrans, a non-harmful indicator to estimate the filtration rate, showing a continuation of their non-selective ingestion of the phytoplankton. Microflora of the oyster soft tissue was dominated by bacteria affiliated with the family Rhodobacteraceae, some of which are associated with microalgae. In addition, the Bacteroidetes species, in which algicidal bacteria are included, were also found in the bivalve individuals exposed to H. akashiwo. These results suggested that the ingested phytoplankton affected the microbial flora in the gastrointestinal tracts, some constituents of which helped the mollusc assimilate the ingested red tide phytoplankton. This study will provide beneficial information to clarify mechanisms by which the oyster evades the ichthyotoxicity of harmful microalgae and the participation of the intestinal microorganisms in these processes.

Environmentally sound poly-eco-aquaculture enables the preservation of aquatic environments to be compatible with that of sustainable aquaculture. With this method, not only healthy fish can be cultured in purified water, but also the productivity will increase by recycling seaweed to feed the fish. The maximum nitrogen uptake rate of each seaweed per square meter of seaweed area was 2.9 mg N/ms2/day for “Konbu” Laminaria japonica, 3.1 mg N/ms2/day for “Wakame” Undaria pinnatifida, and 3.6 mg N/ms2/day for sea lettuce Ulva pertusa. The maximum phosphate uptake rate was 0.43 mg P/m2/day, 0.54 mg P/m2/day, and 0.19 mg P/m2/day, respectively. The calculated values of nitrogen and phosphate uptake rates, obtained by integrating the nutrient concentrations, light intensity, and water temperatures, corresponded well with each observed value. The minimum seaweed cultural density necessary per unit area of yellowtail Seriola quinqueradiata farm was calculated using the values of the maximum nitrogen uptake rate. The maximal production rates were 0.75 mg O2/g wet/h for L. japonica, 0.83 mg O2/g wet/h for Un. pinnatifida, and 6.39 mg O2/g wet/h for Ul. pertusa. The minimal weight of cultured seaweeds necessary to accommodate the oxygen consumption of an individual S. quinqueradiata was calculated as 1.17 kg wet/fish, 0.83 kg wet/a fish, and 0.21 kg wet/fish.

Recirculating aquaculture systems can reduce many challenges associated with open culture systems but maintaining suitable water quality in recirculating system is difficult. Protein skimmer removes organic matter from the water before it breaks down into nitrogenous waste. Therefore, the aim of this study was to evaluate the effects of protein skimming on water quality, bacterial abundance and abalone growth in recirculating aquaculture system. Two recirculating systems utilizing artificial seawater were housed in an air-conditioned, insulated recycle frozen container (4.3×1.9×1.9 m) to maintain optimum water temperature (19.2  0.80C) for abalone growth. Each system consisted of two biofilters (100 and 200 L) and two abalone culture tanks (each 200 L) containing three plastic baskets (each 50 × 34 × 6 cm, with 12 mm mesh). One culture system incorporated a protein skimmer. Over an experimental period of 87 days, protein skimming resulted in significantly better water quality, heterotrophic bacterial abundance and abalone growth. Results indicate recirculating abalone culture systems with protein skimmer housed in an air-conditioned, insulated recycle frozen container may provide a viable alternative to current land-based, flow-through systems. More research is needed to further increase the efficiency of this system.

We used customized fish tanks as model fish ponds to observe grazing, swimming, and conspecific social behavior of common carp (Cyprinus carpio) under variable food-resource conditions to assess alterations in feeding niche. Different food and feeding situations were created by using only pond water or pond water plus pond bottom sediment or pond water plus pond bottom sediment and artificial feeding. All tanks were fertilized twice, prior to stocking and 2 weeks later after starting the experiment to stimulate natural food production. Common carp preferred artificial feed over benthic macroinvertebrates, followed by zooplankton. Common carp did not prefer any group of phytoplankton in any treatment. Common carp was mainly benthic in habitat choice, feeding on benthic macroinvertebrates when only plankton and benthic macroinvertebrates were available in the system. In the absence of benthic macroinvertebrates, their feeding niche shifted from near the bottom of the tanks to the water column where they spent 85% of the total time and fed principally on zooplankton. Common carp readily switched to artificial feed when available, which led to better growth. Common carp preferred to graze individually. Behavioral observations of common carp in tanks yielded new information that assists our understanding of their ecological niche. This knowledge could be potentially used to further the development of common carp aquaculture.

We compared growth efficiency, feeding consumption, metabolism, excretion, and energy allocation in populations of juvenile Atlantic halibut from Norway, Iceland, and Canada reared at low (8 C), medium (12 C, 15 C) and high (18 C) experimental temperatures. Our findings show that protein utilization and energy allocation in juvenile Atlantic halibut varies among different populations, as the Norwegian population displayed the highest protein efficiency ratio, protein production value, and energy conversion efficiency compared to the Canadian population, while the Icelandic population showed overall intermediate growth performance. The results do not conform to a simple thermal adaptation model but might represent an example of: 1) local adaptation of fish populations; or 2) countergradient variation where shorter growing season at higher latitudes is compensated with higher physiological efficiency. These findings have implications for halibut culture, particularly in selection work focusing on growth performance.

The combined effect of temperature and photoperiod on the growth of juvenile Atlantic halibut Hippoglossus hippoglossus (initial weight 11.6 g) was investigated under different temperature (6 °C and 12 °C) and photoperiod (continuous light and simulated natural photoperiod) combinations by studying the effects on fish weight, specific growth rate (SGR), feed consumption (F%), feed conversion (FCE) and oxygen consumption and ammonia excretion under routine and fasting conditions. SGR was greater at the higher temperature, and growth was faster under continuous light than under natural photoperiod, irrespective of temperature, SGRs being 0.83% and 0.98%, and 1.61% and 1.71% at 6 °C and 12 °C respectively. The weight data revealed a significant interaction between temperature and light, and provided evidence of a stronger growth-enhancing effect of continuous light at the lower temperature. F% increased with temperature but was independent of photoperiod, whereas FCE was higher among fish exposed to continuous light, irrespective of temperature. Independently of photoperiod, oxygen consumption and ammonia excretion increased with temperature, but the differences in these rates in relation to photoperiod became more pronounced at the lower temperature, where continuous light gave the highest rates. Higher oxygen consumption during darkness suggested that light may suppress activity in halibut. It was proposed that reduced activity and anabolic effects of photoperiod contribute to explain the increased growth and growth efficiency in fish subjected to continuous light.

This study was undertaken to quantify the rate of nitrogen excretion by the cultured Japanese flounder as a function of body weight and water temperature. Measurements of the did rate of nitrogen excretion for the cultured flounder were made at mean water temperature of about 16, 19, and 23 C. The excretion rate of nilmgen was estimated by adding the nitrogen excreted in the forms of ammonia and urea nitrogens. The diurnal pattern in the excretion rate of nitrogen was governed mainly by the feeding time. The proportions of excreted ammonia to total nitrogen were 77.9% at 16 C, 84.5% at 19 C, and 83.2% at 23 C. The average was 81.9%. The relationship between the body weight and excretion rate of nitrogen (EN, mg N/fish per h) is expressed as EN=α,.W', where α, = nitrogen excretion rate at temperature 0. The parameter α, is dependent on water temperature, such that α, =α20 Q01(0–200) where α20= nitrogen excretion rate at 20 C, and Q01, = temperature coefficient. The values of the kinetics parameters for the cultured flounder were found to be γ= 0.57, α20= 0.060, and Q01= 1.12.

As mentioned in the previous report, several types of oxygen decline belts were recorded by cruising in the Equatorial Current and adjacent seas. In this paper, the ecological significance of the belts to suspended particles and fishing rates of tunas were investigated. The observations were carried out in the same areas as in the previous survey, and the data of DO records obtained from it are cited in this paper. During the cruise, the water was sampled by a mini-pump at the rate of 15ml every 30minutes and preserved automatically in 2 or 3% formalin by modified HIRATA'S automatic solution adder. Then, the amount of suspended particles was determined using a Coulter Counter. Tunas were caught by longline gears in the Equatorial Current and Solomon Sea. High density of suspended particles was observed in areas where oxygen decline belts were recorded, 30times out of 34 recordings. It is interesting to note that a high correlation (γ=+0.84) was obtained between the frequency of sharp oxygen con caved curve and fishing rate of tunas. It might be suggested that the oxygen decline belt in the surface waters could be an important indicator for ecological studies of tuna fishing grounds.