The isolating property of enclosed bays, which cuts off the connection with the open sea by the topographical constriction at the bay mouth, affects strongly the cultural environment in the bay not only positively but also negatively. In order to prevent the retrogradation of culture farm and to maintain the sound cultural environment, it needs to understand quantitatively the physical processes in enclosed bays. In this paper, the observational results in Yamada Bay of Sanriku Coast are introduced as one of examples of oyster culture farm developed in many enclosed bays. The steady process and the transitory process in the onclosed bay play respectively important roles in formation and change of the cultural environment. The tide is weak in the enclosed bay, but steadily acts to form the cultural environment over a long term. On the other hand, the actions such as an intemal Seiche (an intemal standing wave) and the Kyucho (a stomy current) tend to be overlooked because of transitory event, but their energies disturb suddenly the cultural environment which has been formed by the steady process. Further the interaction between the steady process and the transitory process is also important to form the cultural environment in enclosed bays.

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東京湾の盤洲干潟で, 1994年9月から1995年3月の期間にアサリの定位率と波浪による海底面の変動との 関係を調査した。試験には殻長26-28mmの大きさのアサリを用い,これらの生貝およびホルマリンで固 定した貝を砂中に3cmの深さに埋めた。 春期と秋期は生きアサリの方が固定アサリよりも定位率が高かったが,冬期には逆に低くなった。生貝の 累積定位率は0～40%,ホルマリン固定アサリは0～48%であった。 海底面の変動とシールズ数との関係をみると,シールズ数が0.12以下では海底面はほとんど変化せず, 0.12-0.16ではやや堆積傾向,そして0.20以上では侵食傾向となった。概括的には固定アサリや衰弱した生 きアサリの個数は海底面の侵食が2cmを越え,シールズ数で0.20以上になると減少する傾向を示した。砂 中深く自ら潜砂することによって留まることが出来る活力を持ったアサリであっても,シールズ数がしばし ば0.2を越える場所では次第に個体数が減少すると考えられた。

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波による四角柱体の局所洗堀は,四角柱体の形状特性(E/B, D/B:B,D, Eは四角柱体の幅,高さ,厚さ), クーリガン・カーペンター数(KC),砂の限界流速に対する作用波の底面流速の比(U/u。)の他に,作用波の波 向きのに関係する。 ここでは, B=6.0cm,D二9.0cmで, B/Eがそれぞれ1.0, 2.0および12である3種の四角柱体を波向きに 対して角度を変えて実験を行い,各種四角柱体の局所洗握に及ぼす波向き(θ)の影響度合を求めた。 B/E の大きな四角柱体ほど局所洗堀に及ぼすθの影響が強く現れる。これらの結果は波向きが人工魚礁の局所洗 堀に及ぼす影響を考えるうえで参考になる。

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Decline of afforested Ecklonia cava community by grazing of herbivorous fish Siganus fuscescens was studied on the coast of Hamaoka, Shizuoka Prefecture, Japan, during a period from May 1998 to November 1999. About 400 individuals of 3-years old E.cava were grazed their laminae during September to October 1999, and about 2,800 young individuals were grazed their laminae, only left their stipe from October to December. Because the fish bites basal parts of Ecklonia laminae, damage to the thallus is serious though the fish grazes small amount of plant body. Grazing by S. fuscescens was observed on and after June in 1998, while in 1999 grazing was seen on and after August. Rise of seawater temperature was earlier in 1998 than in 1999. This observation shows that grazing activity was affected by change of water temperature. Effect of grazing pressure by herbivorous fish on the growth of afforested E.cava community is clearly demonstrated from our observation.

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A macrobenthic survey was conducted around the Ishikari Bay New Port in west Hokkaido, Japan, to describe the community structure of macrobenthos associated with sedimentary conditions. A total of 70 species were collected at 29 stations using a Smith-Mcintyre grab sampler in September 1998. Dominant species were three juvenile bivalves (Mactra chinensis, Psendocardium sachalinensis, Megangulus uenulosos), six polychaetes (Goniada maculata, Spiophanes bombyx, Spio filicornis, Nephtys caeca, Magelona japonica, Glycera capitata), two echinodems (Scaphechinus griseus, Echinocardium cordatum), and four amphipods (Phoxocephalidae sp., Ampelisca brevicornis, Urothoe grimaldii, Corophium sp.). Distribution pattems of the dominant species changed with feeding type, depth and sediment particle size. Suspension feeders occurred at 2-17m depth in well-sorted mediate to fine sand. Deposit feeders occurred at 17-23m depth in well-sorted fine to very fine sand. Carnivores occurred at 7-17m depth in well-sorted mediate to fine sand and at the mouth of Ishikari River in unsorted fine to very fine sand. Tube-building deposit feeders occurred at the port in unsorted fine to very fine sand. Facultative deposit feeders occurred throughout the whole survey area, except at the river mouth. At 2-17m depth, suspension-feeding bivalves, which have a planktonic larval period of 2-4 weeks, occurred densely on the northeast side of the port, and suspension-feeding amphipods which undergo direct development, were the dominant species on the southwest side of the port. It was suggested that macrobenthic communities around this port are strongly affected by hydrodynamic conditions, which influence the food supply to the animals and their recruitment.

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