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Study on the Effects of Physical and Chemical Environmental Factors on the Japanese Littleneck Clam Ruditapes Philippinarum

  • KAKINO Jun
    Chiba Prefectural Fisheries Research Center, Tokyo Bay Fisheries Laboratory

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Other Title
  • アサリの減耗に及ぼす物理化学的環境の影響に関する研究
  • 〔日本水産工学会〕学会賞受賞講演 アサリの減耗に及ぼす物理化学的環境の影響に関する研究
  • ニホン スイサン コウガッカイ ガッカイショウ ジュショウ コウエン アサリ ノ ゲンモウ ニ オヨボス ブツリ カガクテキ カンキョウ ノ エイキョウ ニ カンスル ケンキュウ

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Abstract

Production amount of Japanese littleneck clam Ruditapes philippinarum in Japan is affected by many factors, such as the fishing ground area, fishing effort depending on the density of adult clams, demand for clams and harvests abundance of other kinds of fisheries or aquaculture. These factors for the clam production are also different in each bay, and the cause of the decrease in clam production can not be discussed similarly. In consequence, the factors for the decrease of clam production should be researched in each fishing ground. The density of adult clams in fishing grounds is attributed to the recruitment of juvenile clams and the survival rate of those juvenile clams, and 17 factors affecting the survival rate have been listed up. It is well accepted that the density of clams mainly is reduced in winter, and the main factors for the reduction in winter were assumed to be low temperature, lack of food, movement of bottom sand by wave action, and prey mortality. The movement of bottom sand increased with the decrease in laver culture structures off Chiba prefecture, in Tokyo Bay and it resulted in the reduction of the clam survival rate. Retention rate of clams with 26〜28mm shell length was lower at the bottom friction velocity of over 4cm/s and Shields parameter of over 0.2 on the tidal flats of the Kisarazu area. In recent years, the bottom friction velocity was estimated to 1.2〜1.5 times and Shields parameter was 1.4〜2.3 times of those in 1984 in the Kisarazu fishing area by the hydraulic model test on the base of the decrease in laver culture structures, although those may be under estimated. Accordingly, the movement of bottom sand is considered to be an important factor for the decrease in clam density, associated with the decline in the clam resources. On the other hand, the movement of bottom sand caused higher clam mortalities in the northern area of Chiba in winter than that in the Kisarazu area, and therefore the ratio of density in February to that in October was lower than 5 percent in the northern area while that was 40〜80 percent in the Kisarazu area.

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