Seawater exchange and mixing between the inner and outer side of a bay are important factors controlling its marine environment and productivity. Studies on the hydraulics of stratified culture grounds are very few, and many things still remain unknown. A study on seawater exchange due to intemal tide and intrusion of density front has been carried out on the basis of observed oceanographical conditions from yellowtail culture grounds in Nomi Bay and the scallop culture farm at Lake Saroma in the season when stratification occurs. Moreover, a density current ditch method that can promote the seawater exchange of fishing ground utilizing density current has been proposed. Its effectiveness was examined using a hydraulic model experiment.

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Seawater in fishing ports should be clean not only because it is used for washing the seafood but also because it is used to stock live fishes. Additionally, the interest in seawater quality increases due to the use of the anchorage as a caring place of the fry. On the other hand, to ensure tranquil waters in a small fishing port so that the fishing boat can be moored safely requires the insta11ation of breakwaters over and over again, which makes a great effort for a small fishing port. But, such an effort results in fact to the deterioration of water quality in the basin. To improve on this, an exchange hole in the breakwater is often installed, which brings fresh open sea water into the interior. This however can create some problems : these exchange holes tend to decrease the tranquility of inside waters and, for low waves the the open sea water introduction is hardly effectual. The authors have proposed a new structure that combines the open-hole breakwater with a submerged mound that is installed in front of the breakwater. The fresh seawater is driven into the basin by raising the mean water level in the pool that is surrounded by the submerged mound. They proposed also : a technique for calculating the amount of flow ; a design method of this facility system that combines the submerged mound, open holes and outlets ; and facility specification that is based on scaled model tests and site investigation. This system was already adopted and constructed in many fishing ports, such as, the Shikanosima fishing port in Fukuoka Prefecture, Yokata fishing port in Ishikawa Prefecture etc.

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Geophysical upwelling is a important marine physical movement for creating a fishing zone. When current along coast encounters cape, its current makes a boundary layer developed along the cape. Vortices are generated in the boundary layer and separate at the tip of the cape. These separated vortices make its diameter enlarged and vanish. Geophysical upwelling is generated in this process at the same time. In this paper, it is theorized that the boundary layer thickness is proportional to the cape length and inverse proportional to the Strauhal number of vortices. While, it is theorized that this thickness is proportional to the vortex diameter and inverse proportional to the progressing time of the enlargement of the vortex diameter. The separate periods of vortices and the diameter after separation were measured in the hydraulic model and in site photograph. It was showed that two theories were agreed well.

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Barren ground, so-called Isoyake in Japanese, exists along the southwest coasts of Hokkaido, Japan. In the Isoyake area, the sea urchin, Storongylocentrotus nudus, dominates the bed rock, and occurrence of the kelp, Laminaria religiosa, was limited in the refuge at shallower zones. To develop the kelp forest technology in the Isoyake area, seasonal changes of some environmental factors, water movement, water temperature, vegetation, and grazers, were observed continuously during 1998 to 1999 in Oshoro Bay. Environments conducive to kelp forest were characterized ; sea urchins took refuge from strong water movement in deeper zones and.kelp buds were prptected from overgrazing during winter. We propose two methods to increase the kelp forest in Isoyake area : 1) seasonal control of excess grazing pressure using fences and, 2) adjustment of the depth with block and stone to remove urchins by increasing wave velocity.

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Marine fish larvae are fragile against physical stress. However, few studies have been conducted to evaluate the flow field in rearing tanks. The flow field is generated by aerators, which are commonly used to provide oxygen as well as aid in the even distribution of live food. The procedure of fluid dynamics was used to analyze the flow field in rearing tank (1m³ polyethylene tank) of seven band grouper larvae. Among fish species, grouper larvae are highly sensitive to physical stress ; mass mortality caused by flotation has been commonly reported. When aerating rate exceeded 500ml/min, strong vertical circulating flow was observed, and production of air bubbles at faster rate will have increased the chance of direct physical damage to larvae. The fast water flow can also have caused a decrease in encounter rate between larvae and prey organisms. When aerating rate was below 200ml/min, there was no water movement in the central body of the vertical circulation flow. This area probably did not receive direct supply of oxygen from the aerator, which may have affected physiological status of larvae distributed in the area. The estimation based on fluid dynamics analysis was consistent with experimental data of larval growth, feeding incidence, and mortality obtained from larval rearing trials. Aerating rate at 200ml/min gave highest survival and growth of grouper larvae.

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The investigation was conducted in 'Isoyake area' Oshoro Bay, Otaru City, Hokkaido, during October 1999 to April 2000. Four experimental sites were established to observe the efficiency of benthic velocity for afforestation of the kelp bed. All sites were enclosed by fence to control the grazing pressure of the sea urchin. Biomass of the kelp increased with the mean benthic velocity while angulations and corrugation decreased with the velocity. For the afforestation of the kelp, it was prefer the yelocity was more than about 0.3m/s. To examine the optimum timing of fence establishment, settlement of zoospore and survival of gametophyte of the kelp were investigated. New substratum was embedded regularly at inside of the fence. Monthly collection was carried out both of stomach contents of the sea urchin and natural substratum at outside of the fence. The contents and the substratum were cultured under laboratory condition. Occurrence of the kelp was restricted to the new substratum, which was embedded before early November, the stomach contents, which were collected before early December, and the natural substratum, which was collected before middle December. For afforestation of the kelp, we should remove the sea urchin before early November.

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For the optimum design of an arabesque greenling trap, experiments were carried out to study the trap stability. The traps have two shapes (conical and rectangular) and dimensions. The traps also have two opposite fumel-shaped entrances sloping up from the bottom・ The bait is hung, usually in a netting-bag, midway between the entrances. To estimate the trap stability, drags and static frictional force were measured in experiments. The drags of each trap were measured in a flume tank. The relation between the current speed and drag is presented. Static frictional forces, on the sand surface and rock surface, were measured respectively. The results showed that, when the seabed was in the case of sand, it was harder for Trap B (rectangular trap) to slide than Trap A (conical trap) and C (for which a plastic fender is made on Trap B), while in the case of the rock, it was harder for Trap C to slide. On a flat seabed, a rectangular trap was easier than a conical trap to tum over. The possibility of tuming over was little, since before tuming over the trap should slide first.

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Based on the hypothesis that phosphorus nutrient variation consists of linear trend and periodical fluctuation, a mid and long-period variation model of phosphorus nutrient (PVM) is developed. The stepwise choice method (SCM) is adopted to estimate model parameters, such as period parameters based on unequal time interval series. The satisfactory results have been obtained by using the PVM model in fitting the observed data of phosphorous nutrient and testing the phosphorus nutrient variation as a forecast measure. It is shown that the mean value of phosphate increase during the mid and long-period variation has been kept at a level of minute increase at survey station E (123°E, 30°N), and that amual seasonal change is the main factor affecting the variation of phosphate concentration. The variation of phosphate may have a significant influence on marine primary production and ecosystem balance in the sea area.

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