サケ・マス罹網行動に影響する2•3の要因について

概要

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In the waters of western North Pacific, it has been known that salmon usually gill the mesh of gill-net in the night and it is rare to gill the mesh in the day time. As to the factors effect to this phenomenon, it can be considered that day light and the vertical movement of plankton as the food of salmon is most important. With above idea the following surveys were carried out by the scouting boats in 1962 fishing season. 1. Hourly variation of the intensity of sun light. 2. Vertical movement of plankton. 3. Hourly variation of the density of the plankton in the surface layer. The hourly variation of the intensity of light between 17 oclock, gill net will be set, and 5 oclock, gill net will be retrieved, are shown in table 1. The vertical movement of plankton is shown in Fig. 1 and 2. The hourly variation of the density of the plankton in the surface layer is shown in Fig. 3. With the results of above surveys, it is considered that if the salmon possess the some nature as that of plankton, or if the behavior of salmon follow the vertical movement of plankton to take their food, the hourly variation of the numbers of salmon gill the mesh may show the same curve as shown in Fig. 3 which shows the hourly variation of the density of plankton in the surface layer.
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