サケ・マス流網の投揚網時刻と魚群の羅網との関係

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For enlightening the efficiency of drift net operation in salmon fishery, some available data of catches of red salmon, chum salmon and pink salmon were schematized as stated below and looked at from new angles independent of the extreme divergency of their actual catches: 1. According to the number of fish individuals actually caught in every one of 12 sections constituting a stretch of drift net and differing inevitably from section to section as regards the times of laying out and hauling up, daily ranks of catches were decided for each one of these sections of net as well as for each one of said kinds of salmons. From the daily ranks the corresponding averages of daily ranks, the average daily ranks so to speak, were further calculated in proportion to the days on which the drift net was actually operated and divided into 2 groups, one higher and the other lower than the mean of ranks 12-1, viz., 6.5. These groups were marked with ?? and ??, respectively, and entered in panes of a rectangular chart on the co-ordinates of which the times of day of laying out and hauling up of net-sections were graduated. Similar types of charts were prepared for the avearge daily ranks for (i) the whole interval of fishing season from June to August, (ii) the catches in June and July, respectively, (iii) the cases in which the sum of caught individuals of red salmon and chum salmon was greater and smaller, respectively, as compared with those of pink salmon and (iv) the cases in which big catches and small catches of said three kinds of salmon were respectively experienced (Figs. 1, 2 and 3). 2. Comparing between the charts the kind of marks of said two groups, or in other words, examining whether the marks in corresponding panes are concordant or discordant with each other, efficiency of catching chum salmon may be said to be likely bound up with the times of day at which a drift net is laid out and hauled up (Figs. 2, b)-g) and Table 1), while any positive rule can hardly be found as for efficient catches of red salmon and pink salmon (Figs. 1, b)-g), 3, b)-g) and Table 1). 3. According as the average daily ranks above and below 6.5 assigned to the panes in Figs. 1-3, b)-g) were predominant at least 3 times over each other, three charts carrying the panes marked with ?? and ?? were prepared for visualizing the times of day of effective catches of chum salmon (Fig. 4, a)), red salmon (Fig. 4, b)) and pink salmon (Fig. 4. c)). On the basis of Figs. 4, a)-c), it may be said that chum salmon and pink salmon are caught most effectively by keeping the net laid out rather for a long and a short time, respectively, while any positive comment is difficult to be made upon the time of effective catch of red salmon. The fact that effective duration of drift net operation varies according to the species of salmon to be caught seems to be explained to some degree from the ecology of the fish in question, Thus pink salmon, being small in size, may perhaps be liable to get out of the net in which it became once entangled so that the apparently strange situation is met with that an effective catch of this fish is realized with a net kept laid out rather for a short time. Actually observed body portion by which the different kinds of salmons are held entrapped (Table 2) suggests that these fishes differ likely from one another also in regard to the tendency of getting out of the net. 4. From many assumed types of catch in which a fish may visit a drift net and fall entangled therein, the types indicated in Figs. 5, a) and b) were picked up as bearing closest resemblances to those in Fig. 2, a) and Fig. 1, a) respectively.
社団法人日本水産学会の論文

著者

小池篤
東京水産大学

サケ・マス流網の投揚網時刻と魚群の羅網との関係

スポンサーリンク

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