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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北海道松前町ではホッケ籠漁業が行われている。このホッケ寵の最適設計のために,寵の設置時の安定性 について検討した。実験には円錐台形の籠Aと四角形の籠BおよびBの寵の底面にプラスチックの網目状の 防護材を取り付けた籠Cを用いた。寵の抗力は回流水槽で測定し,寵の向きを0度から90度まで15度毎に変化 させて行い,各姿勢について流速と抗力との関係を求めた。また,寵と砂地および岩盤との摩擦力を求める ために,平面水槽内に砂および岩盤に見立てたコンクリート板を敷きこの上を寵を曳いて摩擦力を計測した。 測定結果から,寵が滑り始めるのに必要な流速は,砂地の場合には籠Bが最も大きく,岩盤の場合には籠C が最も大きくなることが判った。転倒し始める流速については円錐台形の籠Aが最も大きくなった。また, 3つの寵とも転倒し始める流速は滑勤し始める流速より大きく,寵は転倒する前に滑勤し,寵が転倒する可 能性は少ないことが推定された。

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リン栄養塩の変動が線形傾向と周期的波動からなるとの仮説に基づいてリン栄養塩変動のモデルを作った。 不等間隔時間シリーズの実測データーより周期などモデルのパラメーターを求める累進選択法を提案した。 本モデルを不等時間間隔で実測した東シナ海におけるSt.E (123°E, 30°N)に応用した結果,実測のリン栄養 塩変動はモデルで精度よく表わせた。また,モデルのパラメーターからSt.Eにおけるリン栄養塩のトレン ドが微量増加のレベルにあり,その変動を支配するのは約1年を周期とする季節的変動であることがわかっ た。リン栄養塩の変動は該当海域の基礎生産および生態系システムに重大な影響を与える可能性を示した。

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