イワシとその食餌プランクトンとの関係-I

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タイトル別名
  • The Relation between the Sardine and the Food Plankton-I
  • イワシ ト ソノ ショクジ プランクトン ト ノ カンケイ 1
  • On the Food Intake by <i>Sardinops melanosticta</i>
  • マイワシの摂餌量について

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We have studied both quantitatively and qualitatively the digestive-tract contents and the seasonal changes of plankton production in relation to the food intake. The materials and data were collcted from several fishing grounds on the coast of Kyoto Prefecture. The results obtained are:<br> 1) The amount of the stomach and intestine contents was found small in the early morning and great in the evening. The plankton taken as food during the daytime might be digested until the next morning. It has been suggested that the feeding intensity attained to a great extent in the daytime but the feeding almost ceased in the night. Thus it may be concluded that the sardine follow a feeding rhythm during the course of a day.<br> 2) Examining the stomach and intestine contents of the sardine caught in the Bay of Maizuru on Dec. 18 (1953), we found that the important plankton components were Skeletonema costatum and Thalassionema nitzschioides. Especially the hard-bodied frustules of the latter might little decompose in the stomach and intestine, and hence they were comparatively easy to evaluate. Using these two diatoms, estimations were made of the number of cells reserved in the digestivetracts and also of the plankton density in the water. From these investigations, how much water should be used each day for the straining of plankton was calcuated. A sardine of 12.0cm. long should take water of ca. 270 litres per day, and the fish could strain the plankton of ca. 270•P (P: number of plankton per litre) from it. In comparison with the data obtained from the sardine of different length, it was deduced that the volume of strained water as well as the food intake was nearly proportional to the square of body length.<br> 3) The analyses of the digestive-tract contents of the sardine and the investigation of the plankton composition in the waters showed a considerable confluence. The sardine indicated little selectivity of food. So far as the food selectivity is concerned, it will be further discussed in the next paper from the standpoint of the feeding mechanism.

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