The Description of the Gillnet
In North Sulawesi, Indonesia several types of gillnets are operated i.e. drift gillnet, bottom gillnet, semi-encircling gillnet and shark gillnet in artisanal
fisheries. Drift gillnet mostly used nylon monofilament. The mesh size was ordinarily 1.75 inches; length of float line was about 40 to 80 m and 70 number of
mesh depth. The catch species were flying fish Cypsolurus sp., needle fish Tylosurus sp., barracuda Sphyraena sp. and sardine Sardinella sp. Reppie
and Lalamentik, 1999. Bottom gillnet is almost the same as the drift gillnet, the difference between
them just on the sinking force and buoyancy ratio, where the sinking power of the bottom gillnet is much bigger than its buoyancy, because it should laid stable on
the bottom. The catch consist of Acanthurus sp., yellow tail Caesio sp., black pomfret Formio sp., grouper Epinephelus sp., Chaetodon sp., emperor
Lethrinus sp., slip mouth Leigonathidae and trevally Caranx sp. Reppie and Lalamentik, 1999.
Semi encircling gillnets utilizing the onshore migration behaviour of fish in the tidal waters. The net material was nylon monofilament 1.75 inches, float line
about 90 m in length and 23 number mesh depth, sinking force much larger than its buoyancy. The catch species were slip mouth Leigonathidae, Lutjanus sp.,
Caranx sp., Lates sp., Lethrinus sp., Tylosurus sp., Pomadasys sp., Sphyraena sp. and Mugil sp Reppie and Lalamentik, 1999.
Shark gillnet was just a simple deep bottom gillnet for demersal species, included shark. This gillnet used to catch two specimen of “living fossil”
coleacanth Latimeria chalumnae in Menado Tua Dua. The mesh size limited to 5, 5.5 and 8 inches since only this size was available in the local market Reppie
and Lalamentik, 1999. Most of gillnet experiments in Indonesia studied the selectivity of types of
gillnet, such as drift gillnet, bottom gillnet and also trammel net. One of the examples of gillnet experiment in Indonesia was conducted in Pelabuhan Ratu,
West Java to determine selectivity of sweeping trammel net for banana prawn Penaeus merguensis using five types of trammel net which different in their
nominal inner mesh i.e. 38.1, 44.5, 50.8, 57.2, and 63 mm Yokota et al., 2003. The curve of selectivity resulted to a peak of 3.8 prawn length to mesh size lm
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with 50 selection range of the relative efficiency of 3.7. The right side of the selectivity curve was skewed i.e. the right side of curve decline slightly,
indicating that the selectivity of sweeping trammel net for banana prawn was not high, especially for large individual. However, the reliability of the result was
considered low due to large residual squares between the experimental and the estimated values.
Research in selectivity and escape mechanism of tiger prawn was carried out through simulated capture using trammel net at shrimp pond in Lampung
District Purbayanto et al., 2007. The size selectivity research was conducted to compare performance of catching efficiency and size selectivity of black tiger
shrimp caught by trammel net inner net of monofilament nylon 1.25, 1.50, 1.75, 2.00 inch mesh size. The result showed that trammel net with inner net of
monofilament nylon has relatively high catching efficiency especially for catching large size shrimp compared with multifilament nylon. The size selectivity for
monofilament nylon net was lower than multifilament nylon. Black tiger shrimp escapes through the mesh net when encountering the net and finding a space on
the inner net which is larger than the shrimp body for escaping. Some other gillnet studies was conduct in Cirebon to investigate the effect
of differences of hanging ratio on catching swimming crab Portunus pelagicus in Bondet Waters, Cirebon Ansharullah, 2004; Firmansyah, 2004. They used 5
different size of hanging ratio 0.42, 0.44, 0.46, 0.50, and 0.48. The result showed no differences of hanging ratio towards the swimming crab, but they give
recommendation of using hanging ratio of 0.42 since it had less mesh size than the others.
Some examples of gillnet studies in Indonesia was done by Rengi 2002 which investigated the effect of hanging ratio against drift gillnet selectivity in
Bengkalis Water Regency, Riau Province. Six level of hanging ratio i.e. 0.45; 0.55; 0.6; 0.65; and 0.7 was used to analyze the catch parameter number of catch
and catch size and gillnet selectivity of dorab wolf herring Chirocentrus dorab and barred Spanish mackerels Scomberomorus spp.. Anova statistical analysis
showed that the hanging ratio gave significant effect on number and weight of
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catch. He assumed the morphology of catch compressed or not compressed affected the result. It also proved that hanging ratio affecting the selectivity.