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 7 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 8 catch. He assumed the morphology of catch compressed or not compressed affected the result. It also proved that hanging ratio affecting the selectivity.

2.2 Capture Conditions of Gillnet

The capture conditions of gillnet were defined according to the ways of fish being retained in the net. The fish can be captured in gillnet by being: 1 Gilled, when the fish is meshed immediately behind the gill cover Figure 2. 2 Wedged, when the fish is meshed around the body somewhere behind the gill cover. Wedging is hardly distinguishable from gilled when the maximal girth is found at a position close to the gill cover Figure 2. 3 Snagged, when the fish is attached to the netting at the head region. This catch process is the most common for species with protruding maxilla or pre opercula Figure 2. 4 Entangled, when the fish is wrapped into the netting, held by pockets of netting or attached to the net by teeth, fins, spines or other projections. Fish that are already caught by other catch processes may subsequently be wrapped into the netting while struggling to free themselves Figure 2. Baranov 1914 as cited by Ferno and Olsen 1994 has distinguished three different ways of fish being retained by gillnet, which are: 1 Gilled, if the fish entering a mesh and being unable to back out because the mesh is behind the gill cover. 2 Wedged, if the fish is being held tightly by a mesh around the body. 3 Tangled, if the fish does not penetrate a mesh but is caught in the net by teeth, maxillaries or other projections. 9 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 10