The Fish Behaviour in Relation to Gillnet
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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From the knowledge of how a lobster reacts to a net, if composing a bottom gillnet with 10.5 cm meshes and a net depth of not more than 60 cm, then it is
important not only to reduce the size of the floats and enable the buoyancy to be applied evenly in order to minimize the gap under the sinker; and it is also
important to cut surplus buoyancy as much as possible and adopt rather large shrinkage. If this is done, about half of the net depth is in the state of crawling on
the sea bottom. So, when a lobster retreats after the contact of its antennas with the net, the net entangles it if any part of its body is caught by the net. This is how
lobsters are netted. The fish behaviour also influenced the availability of fish stock. The
horizontal and vertical distribution of fish may vary seasonally and change from year to year. Such variations in fish distribution cause changes in availability of
fish to all types of fishing gear including gillnet. Seasonal changes in distribution, e.g. those caused by spawning migration, are well known for several marine fish
Ferno and Olsen, 1994. Seasonal changes in availability due to spawning and feeding migrations are
often predictable and this source of variability in stock assessment may be minimized by conducting the assessment surveys in the same areas and periods
every year. Annual changes in area and vertical distributions have been documented for cod Godo and Wespestad, 1993 cited by Ferno and Olsen, 1994
and these are a source of uncertainty that is difficult to correct for. Changes in fish availability will affect abundance estimates based on gillnet survey data in a
similar way. In feeding behaviours, some researches showed that active fish are more
likely to encounter passive fishing gear gillnet than stationary fish. In a comparison of catches of yellow perch Perca flavescens between gillnets and
trawls, gillnetted fish had significantly more food in their stomach than did trawled fish Hayward et al., 1989 cited by Ferno and Olsen, 1994. As trawl
caught fish with both high and low amounts of food in their stomachs, it concluded that fish which demonstrated a higher rate of feeding activity were
active swimmers and likely to encounter gillnets and get caught. Furthermore, the activity may be influenced by the type of food the fish feed on. The swimming
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speed of young perch Perca fluviatilis feeding upon small Daphnia was more than twice that of perch feeding upon chironomid larvae Persson, 1987 cited by
Ferno and Olsen, 1994. It is also likely that the speed of cod is lower when they are searching for benthic fauna than when feeding for active prey such as herring
Clupea harengus and capelin Mallotus villosus. External factors, such as temperature also influence the catch rate of gillnets.
Temperatures has a significant influence on swimming capabilities such as speed and duration Wardle, 1993 cited by Ferno and Olsen, 1994. He 1991 as cited
by Ferno and Olsen 1994 found that cod of 36-42 cm length had a reduction in maximum sustainable swimming speed of 56 when the water temperature was
reduced from 5 to 0
o
C, and the author suggested that temperature may have severe impact on gillnet catches. As activity falls with decreasing temperature, the
number of encounters with gillnets may be reduced. Secondly, a reduction in swimming speed may increase the ability of fish to react visually and turn away
before coming into contact with the gillnet. There may be differences in activity between sexes, especially during the
spawning season, which may affect the catch composition of passive fishing gear, including gillnet. In a comparison between catches of cod taken by gillnet,
longline and purse seine during the spawning season, differences in the sex ratio between the three types of fishing gear were obtained. The purse seine catches
assumed to give a nearly correct picture of the fish present because of the sex ratio was 50:50, while for gillnet the sex ratio was 60 males and 40 females, and
for longline the ratio was 40 males and 60 females. Rollefsen 1953 as cited by Ferno and Olsen 1994 suggested, on the basis of observations in aquaria, that
male cod are more aggressive and therefore more likely to encounter the net and be caught. Furthermore, a female fish which is just hovering about may very well
not only avoid the risk of running into a net, but also have more time to investigate edible objects. This may be the reason why females are more
frequently caught on longlines. Surveys during the spawning season should thus be avoided, in order to obtain representative samples of both sexes.
Ideally, the probability of encountering passive fishing gears should be independent of fish density. However, fishing experiments with gillnets in four
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lakes with different stock levels of brown trout Salmo trutta have shown the catchability decreased with increasing population level Borgstrom, 1992 cited by
Ferno and Olsen, 1994. The author excluded saturation as a possible cause and indicated that the results could be explained by a decline in swimming activity
with increasing density of fish as a result of reduced availability of food. Dickson 1989 as cited by Ferno and Olsen 1994 also indicated that mobility of cod
decreased as stock level rose. When stock level was low, cod were schooling in small number at dawn and not schooling at dusk, while at higher stock level there
was no distinct aggregation. The mobility due to schooling and not schooling increased the chance of encounter with gillnets.
In relation of mesh sizes of gillnet, Rudstam et al. 1984 as cited by Ferno and Olsen 1994 stated that the increase in efficiency with increasing mesh size
could be explained by the variations in swimming distances. As swimming speed increases with fish size Wardle, 1993 cited by Ferno and Olsen, 1994, the
number of encounters of fish encounters with the net increases. Borgstom 1989 as cited by Ferno and Olsen 1994 suggested that the rise in efficiency with mesh
size found for roach could not be explained only by differences in swimming speed, but also by the greater visibility of smaller meshes. Selectivity experiments
are usually carried out using different mesh sizes, but the researchers close to keep the twine diameter constant. This means that neither the open area ratio nor the
bulk of their nets could be held constant. In relation to visibility of gillnet, the studies of the reaction of mackerel
Scomber scrombus to different materials in tank experiments showed that multifilament thread was detected by mackerel at much lower light intensities than
monofilament Cui et al., 1991.