In larviculture, it is known that mass mortality occurs during larval and juvenile stages. The survival rate of larval fishes is considered to be sensitive to the flow structure in rearing tanks, which is constructed with aeration. In order to reduce the early mortality during the early phase of larviculture, it is necessary to control/understand flow field in rearing tanks. Previously, Shiotani et al. （2005） investigated the flow field in a circular rearing tank by a two dimensional single-phase simulation. However, the simulation results did not agree well with flow visualization results. Then, Sumida et al. （2013） carried out two dimensional two-phase flow simulations for circular rearing tanks, which showed good agreement with the flow visualization results. However, information on the flow field in rectangular tanks is scarce. Therefore, we investigated flow patterns in rectangular tanks by three dimensional twophase bubbly flow simulations. The rectangular tanks with three different aspect ratios AR （ratio of height to half-width of tank） are analyzed in this research. Symmetrical vortex structures are observed at right and left of the tanks at a vertical cross-section in all AR cases. Characteristics of three dimensional flows in the rectangular tanks are clarified in this research which will be beneficial for larviculture.

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The aim of this study was to compare the effectiveness of acoustic measurements between two frequencies （38 kHz, 120 kHz） for estimations of the distribution and abundance of 0+ Japanese jack mackerel, Trachurus japonicus, off the Tottori coast in the Sea of Japan. The vessel Tottori Maru No. 1 collected acoustic data off the coast of Tottori at frequencies of 38 and 120 kHz （KFC3000）. Sampling was also conducted using a midwater trawl net and an 80-cm ring net. A sound scattering layer was found to overlay the echo of the Japanese jack mackerel when sampled at 38 kHz, presenting an inherent problem to acoustic studies at this frequency. However, signals from Japanese jack mackerel could be identified on an echogram at 120 kHz. Also, the density of one individual per m3 （n） of Japanese jack mackerel, determined using the trawl net, was positively correlated with the value of the volume backscattering strength（ SV） at 120 kHz（Mean SV120 kHz ＝ 10 log n – 58.01（ r ＝ 0.68））. Therefore, it is possible to estimate the distribution and abundance of 0+ Japanese jack mackerel more accurately using 120 kHz data.

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This study has investigated the difference in wave height frequency distributions caused by the zero-up-cross and zero-down-cross methods for individual waves in a wave-by-wave algorithm. The ratio of the significant wave height to the mean wave height theoretically becomes 1.579 when the wave height frequency distribution follows the Rayleigh probability density distribution. However, some of the waves simulated in this study showed values far from the theoretical values. Therefore, in this study, a goodness of fit test was conducted based on the assumption that the wave height frequency distribution follows the Rayleigh probability density distribution. As a result, several cases showed deviations when comparing the wave height frequency distribution with the Rayleigh probability density distribution by the zero-down-cross method but not by the zero-up-cross method.

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A midwater float system with long （100m） float lines for deep setting of pelagic longline was developed to reduce sea turtle bycatch and improve fishing efficiency, focusing on the difference in swimming depth between sea turtles and tuna species. Longline setting with midwater floats which are small floats attached to the mainline allows all the hooks of one basket to be set within narrower depth range than the conventional setting. Long float lines could set the main line with all hooks in the deeper range for tuna habitat to avoid the hooks entering the sea turtle habitat. Longline operations were carried out in the Indian Ocean. One midwater float with a buoyancy of 2300gf or 2000gf was attached to each center of five baskets on the mainline, both ends of which were hung with 100m long float lines. The conventional setting with 40m float lines without any midwater float was also conducted as a control. No bycatch of sea turtles occurred in the midwater float setting while one olive ridley sea turtle was caught in the conventional setting. All hooks of the midwater float settings were set below 150m depth, while the hooks in the conventional setting were set in the wide depth range from 109m to 230m. The hook position in a basket catching bigeye tuna was recorded to estimate the depth of catching. The CPUE of bigeye tuna was higher in the hooks below 150m. The estimated catch number of bigeye tuna in longlines with midwater float setting was nearly 30% more than using the conventional setting.

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Laboratory experiments and field surveys in a set net were conducted by use of the acoustic camera, DIDSON, as a device to observe behaviours of various fish species without an influence on them. As a result of laboratory experiments, it was shown that a frame rate of images taken by DIDSON peaked out when an observation distance was shorter than 5m, while the resolution of subject improved in proportion to a shortening of an observation distance and a frame rate improved in inverse proportion to it when a distance was longer than 5m. This may cause an image deformation of the moving subject at a shorter observation distance because DIDSON draws it by the interlace scan. As a result of observations in the set net under the optimized setting of DIDSON taking above results into consideration, it was possible to discriminate images of bluefin tuna from other species.

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Broadband echo sounders are known to provide fine spatial image of each target fish in a dense group. The total number of fish in an acoustic beam can be estimated by counting the number of separated echograms. The target strength of an individual fish and beam attack angle to the fish body provided the body length of each fish. However, counting the total number of fish and body size distribution in a net enclosure or in the wild is difficult mission. Because double counting and missing are unavoidable. Here we propose the acoustic re-capture method to count the number of fish including missed ones in a net enclosure. The mark recapture is well known method for fisheries resource management. An idea is to use Sonar Bell as an individual tag. The Sonar Bell has extremely high target strength comparing with its size. Once it will be introduced in several tuna for example, high spatial resolution sonar will be used to count the number of tagged and intact individuals in the sonar beam remotely. The numbers of tagged and intact fish provide the virtual capture rate, which can be used to calculute the total number of individuals including missed fish. Multiple acoustic recaptures such as every 10 minutes will provide precise estimation of total number of fish in a net enclosure.

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We developed an effective aquaculture production management system that can measure the body length, weight, and number of cultured fishes in a tank or cage in a non-contact manner. A threedimensional measuring system that employs two commercialized video cameras was produced to measure the body size of cultured fish, including their fork length, body height, and width. The distance between the video cameras and target fish was less than five meters to reduce the error ratio. In addition, an automatic counting system for cultured fish in a tank was developed to assist in efficient aquaculture management. Finally, an algorithm for fish counting was based on estimating the mobile vectors of individual fish, in which the particle tracking velocimetry （PTV） analytical method was applied. In some experimental cases, estimated numbers by the system were coincident with actual numbers.

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Over the past decade, the farming of bluefin tuna has played an increasingly important role in the aquaculture of Japan. According to a recent report with respect to tuna farming, there are 160 tuna farms and 1432 cages of bluefin tuna in Japan. For the bluefin tuna farmer in Japan, to determine the number of farmed fish is one of the key issues to monitor the number of fish in addition to feed waste, escapement, behavior, and dead fish. However, there presently exists no reliable method to count the number of bluefin tuna in a cage. The most popular type of counter is currently the underwater stereoscopic camera system which has mainly been used by a diver for counting. However, this kind of counting method is not only labor-intensive, but its accuracy is low in dark or turbid water. The purpose of this study is to develop an accurate counting method for the farmed bluefin tuna using the multitransducer sonar and pinger. This newly developed a multi-transducer sonar system is based on counting the individual fish that has passed through “the sound curtain” consisting of 15 transducers（ 460 kHz）. In addition, a pinger was used to clarify the behavior and swimming speed of the caged fish. As a result, it was found that all of the fish regularly swim in a concentric circle in the cage space, and the lap time in each lane of 1m from cage center for one round was estimated by a linear regression equation. The number of fish in each lane for one round could be calculated by multiplying this lap time and the number of fish that passed through “the sound curtain” per unit time. The total number of fish could then be calculated by adding up the number of fish in each lane. The accuracy of this method verified by multiple actual tests was 1% or less and 1 fish. It was considered that this approach is effective for counting caged bluefin tuna with the objective of practical use.

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