On the Relation Between the "Horizontal Tuna-Long-Line Form" and the Spot Foretelling of the Fishing Condition in the Celebes Sea

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  • セレベス海におけるマグロ延縄の水平的形状と漁況とについて
  • セレベス海におけるマグロ延繩の水平的形状と漁況とについて
  • セレベスカイ ニ オケル マグロ ハエナワ ノ スイヘイテキ ケイジョウ ト ギョキョウ ト ニ ツイテ

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Some informations obtained by the author as the result of tuna-long-line fishing training carried out near the Maratua Island in the Celebes Sea, with the training-ship Keiten-Maru (269 ton) of our Faculty shall be reported. Up to this day a lot of studies have been done about the vertical tuna-long-line form, but as to horizontal form few studies have been done, as the attentions have been paid chiefly to the fixing of the letting-go-line and heaving-up-line positions and to the counting of the shift and the distance between the both ends of the long-line. Accordingly the track of the horizontal tuna-long-line form was ascertained, by clearly plotting, with the interval of half an hour, the ship-position, namely the long-line position. Provided the technical conditions of the letting-go-line, the heaving-up-line, and seasonal conditions remain the same, the author divided the horizontal forms into three kinds, A, B, and C: Here kind A shows good catch (hooked rate of tuna 5.26%), kind B shows bad catch (hooked rate of tuna 1.30%), C shows the one which belong neither of these two. As to these two kinds, the difference of the form was investigated with the ascertainment that concerning the shift of the line-end, the shifting distance is small, in case of A, showing the shift of 0.68 mile per hour, in case of B, 1.97 mile. Some difference can be seen in the figures and types of the line. The relation between the line-shift and the fishing conditions was studied oceanographically, with the result that in A kind eddies, convergence is to be observed, which prevent the fish from swimming away in B kind, the tide current showing a fixed direction, the shift of the line is apt to be accelerated.

The observation of the horizontal "long-line form" led the author of the following conclusion: 1). Good catch can be expected when the long-line form shows A form, provided that water temperature in 150 m. layer 21~23°C. 2). Bad catch is to be expected when it shows B form in this case the water temperature in 150 m. layer is 19°C. 3). when C form is got, this may be fixed to be belonging to A or B form on the second operation. As reported above, the ascertainment of the relation between the "horizontal tuna-long-line form" and the shifting of the line, enables us to foretell the catch on the spot, giving us much benifit in establishing the fishing schedule here after.

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