Figure 2. Capture conditions of a same sized fish by gillnets of different mesh size Sparre and Venema, 1999
On sweeping trammel net, the fish captured by snagged, gilled, entangled and pocketed. Snagged, when the fish was held by a mesh along the head part
from the operculum to the front part of the dorsal fin. Entangled, when one part of the fish body teeth, maxillaries, fins and other projections was held by a mesh or
entirely the fish body was wrapped tightly by the nets. And pocketed, when the fish was entrapped in the pocket formed by loose inner net without necessarily
entangling or penetrating the net Purbayanto et al., 1999. From the result of capture conditions of trammel net in Tateyama Bay,
Chiba Prefecture, Japan in June to September 1995 showed that a small Japanese whiting was mostly gilled, while medium and large fish were mostly pocketed.
The by-catch species were mostly entangled in the net Purbayanto et al., 1999.
2.3 The Fish Behaviour in Relation to Gillnet
The knowledge of the basic behaviours of fish can help improve the efficiency of nets Nomura, 1991. For fish, fishing gear is the strongest of all
external stimuli and they naturally react to it in various manners. Fish behaviour
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towards gear is become the interesting objects to investigate by researchers. Not only fish being the experiment objects but also some of species of crustaceans,
such as lobsters, crab, and shrimps Nomura, 1991. Some observe that if a fish encounters a gillnet, it stops its movement
temporarily, but in shallow water, if the net swing because of waves and tilts to the front, the fish follow the net and, when the net swings back, the fish gets its
head caught in a mesh of the net. Generally, fish seem to lack the faculty of judgement but it is known that there are certain patterns in the behaviours of fish
that react reflexively to the gears Nomura, 1991. A survey on the distribution of spot lined sardines enmeshed in midwater
gillnets shows that they were more often caught on the sinker side regardless of the length of the barrel ropes by which the net is suspended from the sea surface.
This seems to indicate that when the sardines encountered the net, they had been panicked into starting a downward escape immediately before they were
enmeshed. Therefore, fishermen adjust their ropes so that the centre of the width of the net will be somewhat deeper than the water depth in the centre of the fish
group detected by the fish finder Nomura, 1991. In laying the net, the direction in which the net extends is at an acute angle
to the direction of movement of the school which is believed to be the opposite to the direction of the tidal current and the net is placed at a suitable distance from
the school. It is desirable to time net casting so that the school will encounter the net immediately on completion the net’s descent after proceeded straight ahead
during the length of time the net takes to sink to a depth of 40-50 m after it is cast. This exemplifies the technique of operating nets according to the behaviours of
fish Nomura, 1991. Lobsters get entangled in bottom gillnet as they leave their reefs on dark
nights to feed. They are not caught on moonlit nights because they are timid. If a lobsters contact gillnet, it cleverly passed through the gap under the sinker and
escapes to the opposite side. If the surplus buoyancy is too great, the meshes open up and the lobster passes through the mesh by bringing its antennas down on its
back Nomura, 1991.
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