A sledge net is a bottom trawl used to
collect benthic organisms such as starfish, sea cucumbers, shellfish, and
bottom fish that live on or near the seafloor surface. Since samples are
obtained by dragging it over the seafloor surface, the underside of the frame
is shaped like a sledge so that it can move smoothly over the uneven seafloor.
Unlike fishing gear such as otter trawls, in which the shape of the net opening
changes during towing, the net opening is fixed by the frame, making it
possible to calculate the towed net area and thus collect biological samples in
a more quantitative manner. The population density of benthic organisms (number
of organisms per unit area) can be estimated from the number of organisms
collected and the tow net area.
Fig. 1 Sledge net to be put into the sea
Figure 2 shows the configuration (schematic
diagram) of a sledge net. A sledge net consists of a bag-shaped mesh mouth
fixed to a sledge-shaped frame. The frame that holds the opening of the net is
made up of a pair of frames connected by metal rods (beams), and the frames and
beams can be disassembled for easy transportation and storage. An inner mesh
with a smaller mesh size than the outer mesh is attached at the tail end
(cod-end) of the mesh where the collected samples are gathered to prevent the
loss of samples collected inside the outer mesh.
The sledge net is towed by
cables (bridle wires) connected to the left and right frames. Weights are
suspended from the tail of the net to prevent the net from floating up and
getting entangled in the sledge (frame) sections and ropes while being lowered
toward the seafloor before being towed, or to keep the net in proper shape
during towing. A steel chain is connected to the end of the fuse wire to
prevent the main wire from rubbing against the seabed and damaging it. The weight
of the chain also keeps the frame from lifting during towing.
Fig. 2 Illustration of sledge net structure
重錘: weight ビーム: beams チェーン: chain メインワイヤ: main wire
When towing a sledge net, it may get caught
on a reef or other obstacle on the seafloor. If the net continues to be towed
while caught, not only will the net be wrecked, but in the worst case, the main
wire of the winch may break, causing the entire net to be lost.
Therefore, in actual observations, a device
is used to avoid the risk as shown in Fig. 3. A fuse wire is attached between
the bridle wire and the chain. The fuse wire is a wire rope thinner than the
main wire, and it breaks before the main wire when excessive tension is likely
to be applied. The net is then towed by the “lifeline” connected to the tail of
the net, and at the same time the direction of the sledge changes, releasing
the sledge net from the obstacle and allowing it to be retrieved.
If the net contains a larger-than-expected
amount of collected material, there is a risk that the fuse wire may be loaded
to the point of breaking when the net is pulled on board. To avoid such a risk,
a “squeeze rope” is installed in the middle of the net to squeeze the cod-end
and lift it up at two places to collect it on board.
Fig.3 Manrope of sledge net
絞り綱: squeeze rope 命綱: lifeline ヒューズワイヤ: fuse wire
Summary of collecting operations
Figure 4 illustrates the flow of the
collection process using a sledge net. The sledge net is placed in the sea, and
the main wire of the winch is unrolled and sunk into the sea (casting). When
the net reaches the seafloor (landing), the vessel tows the net over the
seafloor surface (towing). The length of wire required to reach the bottom is
predicted in advance based on the water depth in the research area, and the
final determination of landing is based on the change in tension applied to the
When the net reaches the bottom, the
tension on the main wire decreases, and the tension increases or decreases in
small increments due to drag resistance on the seafloor surface. If the main
wire is stopped immediately after landing the bottom, the net may float up, so
it should be stopped when it has been extended for a certain length longer than
the length of the wire at the time of reaching the bottom or continued to be
extended at a slower speed after reaching the bottom. The tension is carefully
monitored throughout the tow to respond immediately in the event of a large
load, such as being caught on an obstacle or a large amount of material being
caught in the net.
After towing a desired distance or after elapsed
a desired amount of time, the main wire begins to wind up. Until the net leaves
the seafloor (leaving), it is wound up at a slow speed while continuing to
monitor changes in tension. When the net leaves the seabed, the hoisting speed
is increased and the net is retrieved on board (hauling). Once the sledge net
is recovered on board, the cod-end is opened and the material is removed from
It is not easy to estimate the condition of
a net submerged at the bottom of deep water by relying only on tension.
Especially when the water depth is great, the weight of the wire rope is much
greater than the weight of the net, and the slack and stretch of the wire rope
have a large effect, so it is difficult to determine whether the net has
reached or left the bottom only by changes in tension. Therefore, acoustic
communication devices such as fishing gear shape measuring devicesare sometimes used to determine the position and posture of the net during towing in more detail.