gillnet has various dark colour, that includes dark blue and dark green. Fishes did the reaction behaviour when they recognized the existence net panel on their
swimming movement, they stop first, and tried to pass the net panel, either way they avoiding the net panel and just swimming in circling. Large mesh perimeter
and hanging ratio made the fish easily to pass the mesh. The result of this study demonstrates the natural behaviour of fish was to
keep clear the netting panel. To keep clear means to swim not too close with unknown object, in this case the net panel. This natural avoidance response is
emphasized by the fact that both netting panel could be recognized by fish. Jack mackerels as pelagic fish, is a typical of obligate schooling fishes.
After they passed through the mesh, they tend to return to its school. This fish was always tried to swim constantly, even though sometimes they seemed less active.
These responses were similar to the result to the herding and avoidance responses shown in the laboratory with school of mackerels Scomber scrombus
and haddock Melanogrammus aeglefinus as they passed into the area of funnel. This experiment was described in Glass et al. 1993, in order to study the
principles underlying mesh penetration. The millennium gillnet twine was diamond shaped mesh, as same as the
shape used in other types of gillnet. The experiment using different types of twine consequently gave different kinds of responses. The square mesh panel elicit the
strongest avoidance response. As the square mesh is split into its constituent horizontal and vertical components, the avoidance response is reduced, but
significant differences also apparent in the reactions of the fish to each component Glass et al., 1993.
White net old and new in each monofile, showed structures acting like vertically arranged mirror. Thus structures helping them less visible to predators
or fish target. Most fishing applications of monofilament lines involve a fish first seeing the material from relatively close up Wardle et al., 1991. In midwater, the
least visible lines are those with a colour which reduces the amount of green illumination as it passes into and out of the transparent material, resulting in a
better match with the background Wardle et. al., 1991. Suppose this was happen
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also in the multi-monofilament net, as they way that fish could be able see its material in clear vision.
5.1.2 Behaviour experiment using specific white net panels
In order to minimize the cost, Cirebon fishermen usually bought used net or second hand net, instead of buying the new gear. The reducing of transparency of
used old net was not affecting the catch efficiency of millennium gillnet. This concluded from the result that the effect of transparency on the number of fish
passing through net panel old white, new white and white dyed was not significant. This probably because of the visibility of the net still considered
tolerable by the school of fish. But whether the material damages caused by the usage of net affecting the catch efficiency still needed further research.
In Cirebon, the water visibility is quite clear in the deep water but not too good in the shallow water. While in the laboratory, the water visibility in the
laboratory was being controlled in clear condition. The millennium gillnet operates in the deep water as well as in the shallow water.
Gray et al. 2000 studied the use of transparent netting to improve size selectivity in fish seine nets demonstrated that the insertion of transparent panels
in the haul net was effective in allowing the escapement of small sand whiting Sillago ciliata and therefore improved the selectivity for this species. Surface
and underwater observations of the modified net with and without cover confirmed that the majority of sand whiting were escaping through the transparent
panels. The function of transparent colour of millennium gillnet is not for the
escaping panels, like the studied did by Gray et al. 2000, but to make more fish attempt to pass the net. But what can be inferred from this was that the transparent
net affected the fish to pass through the net. It means that the more transparent the net, the more likely fish to pass through it.
Wardle et al. 1991 stated that coloured line in shallow water can be more visible to some fish species, particularly pelagic fish with good colour vision..
Near the sea bed, it may be more important to match the line to the sea bed. Because water colour can vary, colour in material adds a complication that could
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be avoided by careful selection of a neutral grey with density that results in a good match with the water background when vertical, as well as attenuating the glint
when horizontal. As the result of the experiment, most fish tried to keep clear of the netting
panel. The number of fish passing through the net panel also considering fluctuated. Herding panel helped to create stimulation to fish to approach and
penetrate meshes more readily. This was very important to understand their passing behaviour.
The hanging ratio of mesh panel used in the experiment 0.65 is bigger than the average size of hanging ratio mostly used by fishermen, which is around 0.4-
0.5. But there is a consideration that could be interfered from this situation, which as soon as fish reached the mesh gate, they will reduced the swimming speed then
tried to enter the mesh panel without any contact to the mesh. This was called the contact and non contact reaction. Contact, is when fish
passed the mesh, their body incidentally contact the mesh of net panel, and non- contact is when fish able to passed through the mesh without any body contact.
The body includes all of the physical appearance of the fish, such as: jaws, head, fins and skin.
The result demonstrates that most fish able to pass the mesh without any contact. The result also shown that from the three panels used on the experiment,
old white mesh has the largest number proportion on non-contact reaction on passing. This probably caused by old white net was more invisible than 2 others
net new white and white dyed. As stated by Glass et al. 1993 that the less visible twine arrangements and the weaker the contrast of the twine with the
background, the more likely fish are to approach and penetrate the mesh. Cui et al. 1991 studied the fish behaviour on light level threshold for
visual reaction of mackerel Scomber scrombus L. to coloured monofilament nylon gillnet materials. The result of this experiment is almost similar with that on
Japanese Jack mackerels, which are: 1 When the mackerel can clearly see the lines i.e. at a light intensity well
above threshold intensity, they turn back at points between 0.3 and 1.0 m before reaching the test frame. This behaviour was almost similar with the
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behaviour in this experiment when the fish reached the gate of the net panel, some fish try turn back to another direction or either way try to keep clear.
2 When they see the lines from a distance of only 10 cm from the test frame, they start to swim erratically, alter course to find a gap and some pass
through the panels and others turn back. If some fish pass through the lines, then those fish had turn back, turn again and pass directly through to reform
a single school rather than swimming back separately. In this study, mostly fish as if one fish passed the mesh panel, then its fish will try to return to its
school. 3 When the fish do not see the line, they show no response to them before
passing through or colliding with them. Occasionally, a mackerel collided with a line between its jaws, maneuvered briefly, freed itself and then
passed through. The threshold light level was taken as the highest light level increment in
which the fish showed no reaction to the barrier Cui et al., 1991. In this study, mostly fish could recognize the existence of mesh panel, but if they
were not able to see, they show none of any response.
In the gillnet fishery, fish enter the net since they could not recognize the existence of the net, so then they accidentally bumped and get caught. If the result
of the experiment will be applied in the real situation, this means that the net with the less visible colour or transparency has the greater of possibility of fish being
caught, in this case the old white net. The more transparent netting panels used in this study show potential as a means of improving the daytime effectiveness of
this gillnet. Baranov 1976 cited by Tweddle and Bodington 1988 suggested: 1 that
nets of the same colour and shade as the backs of the exploited fish species were most effective; and 2 that some medium shade which conceals a net to an
acceptable extent is necessary, therefore, nets conforming to the background shade are the best.
The ability of fish recognizing the existence of an object, in this case a net is related to the visual acuity of fish. The visual acuity of an animal is a measure of
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its capacity to resolve fine details in the view Muntz, 1974 cited by Purbayanto, 1999, and it can be expressed as the minimum separable angle in discriminating
nearest two visual targets which is measured through histological examination. The minimum visible angle can be also measured as the reactive distance to visual
target by behavioural method. For diurnal fish, which active at daytime, the retina’s cone cell is presented in mosaic. Cone cell is visual cell layer that could
be found in the retinas. It consists of single cone and twin cone. The cone cell is colour vision receptors and influencing the visual acuity. The cone cell could be
presented in line shape or in rectangular shape. Commonly, the fish that has mosaic’s cone cell is predatory fish. The cone cell intensity in fish is lasting as
long as they still alive. The growth of the lenses develops the ability of their visual acuity. Japanese Jack mackerels, as the object of this behaviour experiment are
pelagic fishes that have a good visual acuity. This was because this fish is a type of which active at daytime.
5.2 Capture Process Experiment
Millennium gillnet fisheries in Cirebon Water Regency dominated by small scale artisanal fishing. The operation was one day fishing trip with the size of
fishing boat was less than 5 GT. The number of fishermen operates this gillnet was 3 persons. One person works as the owner and skipper, and two others work
as crew members. In Indramayu, the western part of Cirebon, most of fishermen operates bigger scale of millennium gillnet within the size of fishing boat around
15-20 GT. The operation method of millennium gillnet classified as anchored set
gillnet that was set as a barrier in a straight line across the tidal flow. The one side of the net panel anchored on the sea bottom with a rock, while the other side is
towed by the boat set the net in straight course with respect to the anchor point. This net operation method was different than the sweeping trammel net operation
which usually operates in full circular course or sweeping operation Purbayanto et al., 1999.
The duration of the millennium gillnet operation process in Cirebon required 15 min, 2-3 hours, and 30 minutes for respectively setting, one set
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operation of 12 pieces gillnet and hauling process. For one day fishing trip, there were 2-3 times setting-hauling process, depends on the weather and current
condition. The catch number of millennium gillnet was dominated by threadfins
Polynemus spp. for 71.2871 of catch. While by-catch consists of striped catfish eel Plotosus lineatus, spotted catfish Arius maculatus, great barracuda
Sphyraena barracuda, triple tail Labotes surinamensis, barramundi Lates calcarifer, blue spotted snapper Lutjanus rivulatus, silver pomfret Pampus
argentus, stingray Dasyatis spp., garfish Hemirhampus spp. and shark Charcarinus spp..
The millennium gillnet, mostly caught threadfins. The slim and long-shape figure and lack of spines on its body makes this fish caught mostly caught on its
gill or caught on its maximum body girth and not to be entangled. There are some theories of how fish encountered gillnet. 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 follows the
net and, when the net swings back, the fish get its head caught in a mesh of the net. Nomura, 1991.
Captured conditions depend greatly on the length size and struggling effort. The small fish, which body girth, were slightly smaller than the mesh perimeter
could penetrate the mesh by their head and become finally snagged or gilled. Meanwhile, the medium or large fish whose body girth exceeds the mesh size
could result into the complicated entanglement in the net by their violent struggling. This was happening also by fish that caught in sweeping trammel net
Purbayanto et al., 1999. Struggling effort usually caused injuries on fish body. Purbayanto 1999
described six patterns of damaged body of the dead Japanese whiting Sillago japonica caught by sweeping trammel net in Tateyama Bay, Japan. The patterns
are: 1 minor scale loss on the upper parts of the fish head in front of dorsal fin; 2 scar injuries and minor scale loss around the circumference of the maximum
body girth; 3 scale loss on several parts of the fish body; 4 moderate scale loss
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