章节大纲

  • Otter trawling is a fishing gear and fishing method that is expected to produce a large catch, and the operation procedure is simple: (1) cast the net, (2) tow the net, and (3) retrieve the net, but the process is extensive and dangerous. Safe and efficient trawl operations can be achieved only through the cooperation of skilled on-site workers, winch operators, and boat operators. The following is a video showing the operation sequence and a video of the operation.


    • Figure 2 Cast netting operation (surface trawl)


    • ①Casting net operation

       First, the trawl net is cast into the sea through the slipway at the stern while the ship is moving forward. Next, the trawl winch is turned to unroll the towline that had been wound onto the drum (Figure 3). At this point, the otter board is still suspended from the top roller at the stern (Figure 4).

      Figure 3 Casting net operation 1 (casting a net)

      ハンドロープ・網ペンネント: Hand ropes and net pennents オッターペンネント ・先付ワイヤー :Otter pennent  and Tipped wire トロールウィンチ: Troll winches トップローラー: Top roller オッターボード: Otter board

       The trawl winch is wound with play wire, hand rope, net pennent, tailsleeve rope, and trawl net in this order. The trawl net is pulled out by turning the winch, and is then fed into the sea from the tail section. The otter pennent and tip-sleeve wire are preconnected to the otter board.



      Fig. 4 Top roller

      舌状ストッパー: tonguelike stoper オッターボード: otter board

      The top roller serves not only as a pulley for warping, but also to secure the seine chain and suspend the otter board aft. There is a movable stopper that works like a check valve, and the seine chain cannot be unrolled unless the stopper is opened (orange: closed state, blue: open state). To release the stopper, the seine chain must be pulled at warp so that there is no load on the stopper.

      A pre-tied wire is connected to the connection between the play wire and the hand rope, and the winch is further unloaded. Eventually, the otter pennent is tensioned and the play wire and warp are slackened (Figure 5).

      Figure 5 Casting net operation 2 (connection of otter board and net)

      ワープ: Warp 遊びワイヤー: Play wire

      A three-link chain is incorporated at the connection (blue circle) between the hand rope (yellow wire) and the play wire (green wire), and the end of the tip wire (blue wire) is connected to the center link. As the winch is unrolled, the net falls backward and tension is applied to the otter pennent. The tension in the warp (black wire) is released so that the connection to the play wire (red circle) can be disconnected.

      Once the tip of the warp has extended to the point where it reaches the seine chain, the winch is stopped and the play wire is disconnected. The tip of the warp is connected to the end of the towline, and the end of the play wire (so called because it is loose during the tow. In Japanese, the state in which a wire is loose is sometimes described as "playing"(日本語特有の表現だと思ったので補足説明を入れました)) is connected to the middle of the seine chain (Figure 6).

      Figure 6 Casting net operation 3 (connection of otter board and warp)

      ワープ: warp 曳網鎖: seine chain 枝ワイヤ: branch wire 遊びワイヤ: play wire

       Connect the end of the warp to the end of the seine chain. The end of the play wire should be tethered to the middle of the seine chain via a branch wire.

      Once the winch is wound in and the warp is tensioned, the top roller stopper is released. As soon as the stopper is released, the warp is immediately unrolled and the otter board is placed in the water (Fig. 7). Once the otter board is placed in the water, the warp continues to be unrolled until the net reaches the desired depth.


      Figure 7 Casting net operation 4 (putting in the otter board)

      外向きの揚力: outward lift　流速大: high flow velocity　流速小: low flow velocity

      　Once in the water, the outward lift force causes the left and right otter boards to separate. As the otter boards separate, the opening of the net widens laterally.



    • (2) Seining the net

       When the trawl net reaches the desired depth, the warp is stopped and the net continues to be towed for the desired time (or distance). During the tow, the depth and shape of the net and changes in the tension applied to the warp are monitored, and the ship is steered and the warp length is adjusted appropriately. Especially in the case of bottom trawling, the net must be towed carefully because it may get caught on the unevenness of the seafloor and damage the net.



    • The trawl net is pulled onto the vessel by reeling the warp into the winch, and the otter board is detached from the towline in the reverse procedure to cast the net. The trawl net is pulled onto the ship by reeling the warp into the winch, and the otter board is detached from the seine in the reverse procedure to cast the net.


      Figure 8 Cod end of trawl net

      The tail of the trawl net is sewn shut with a rope. If the rope is untied with the tail suspended, the catch is swept out of the net.


    • LASBOS YouTube　～Biological collection using otter trawl (bottom trawl )～

      Photo by Shunsuke Yamashita (Faculty of Fisheries Sciences, Hokkaido University)

      Editing: Teruki Tomita (Training Ship Oshoro-Maru, Faculty of Fisheries Sciences, Hokkaido University)