The aim of this study is to ascertain the behaviour of Net fishing gear submerged in the wateras well as that of the fish schools against the gear, by means of diving and camera snapshotting.Basing on the 12 times repeated experiments on board the training ship of our Faculty, namedNANSEI MARU (83 ton, 400 P.S) the present study was carried out in October, 1984 and inMay, 1985 at the sea-front which is 38~62m deep and situated on the ocean lying west of Taneisland.The behaviours of Net fishing gear under the trawling condition and those of the fish schoolson the point of entering the gear were snapped by the low illumination diving television camerasattatched both at the otter board and at the Net mouth. The results obtained were as follow :(1) The figure of the otter board at the commencement of the towing, showed a slightslanting one, turned a little inwardly at the horizontal view and ackwardly at the vertical view,being influenced by the drawing force coming from the upward and insidely directed workingsof the towing line ; the Net trawling velocity being 2 knots, without any amount of waveringmotion, being stable as thoroughly as possible. Net mouth was kept open regularly, waveringwas reduced to almost zero. The hanging chains attatched the fishing line became detatched 5~10cm from the sea bottom, showing the figure expected to appear on the design.A big amount of smoking mud was brought up out of the hanging chains, the iron ballsattatched both at the fishing line and otter board, which prevented the fish schools fromescaping out of the net-mouth, thus promoting the catching effect of the trawl net. (2) In the experiments carried out in May of 1985, the number of the fish observed was ninenear the otter board, and fifty in the vicinity of the net mouth, respectively. Almost all the fishkept themselves swimming 30~50cm in front of the fishing line of the net mouth, showing thesame direction as that of the Net, for a long time. Of these fish, such big sized ones with forklength about 70cm as Thick lip bream Plectorhynchus pictus and Rock bream Oplegnathus fasciatusmade tumbling motions just above the fishing line of the net mouth, and then transferredthemselves into the Net while the small sized fish were threatened by the tail movements of thebig sized ones to be induced into the interior of the Net.

1984年10月と1985年5月,種子島西方海域の水深38~62mの海域において,本学練習船南星丸(83トン,400P.S)により計12回の曳網実験を行った。実験方法は低照度用水中テレビカメラをオッターボード及び網口付近に装着し,曳網中における漁具の動態と魚群行動を撮影した。その結果は次の通りである。(1) オッターボードの姿勢は,曳網速力2ノットで内方と後方に傾斜し,動揺も少なく最も安定した状態となる。網口は正常に拡網しほとんど動揺しない。沈子綱に装着した垂下用チェーンは約10cm離底し,設計通りの値を示した。オッターボード,鉄球及び垂下用チェーンからは大量の砂煙が発生し,魚群の網口からの逸出を防止し,その入網効果を発揮している。(2)1985年5月の実験において観察出来た魚群は,オッターボード付近で9尾,網口付近で約50尾であった。ほとんどの魚群は網口の沈子綱の前方30~50cm付近において網と同方向に長時間に亘って遊泳する。このうち体長70cmのコロダイとイシダイは網口の沈子綱の直上において反転入網する。また小型魚は前方を泳ぐ中型魚の尾部の動きに威嚇され入網した。