Factors Affecting the Efficiency of Gillnet in Relation to Fish Behaviour

Hamley 1975 and Dickson 1989 as cited by Ferno and Olsen 1994 discussed features of gillnets which influence catching efficiency and selectivity, such as colour, mesh size, twine material and thickness, and construction of the net. On this paper, the discussion will be stressed on features that relates to the visibility of gillnet. Gillnet is a stationary gear and the catch depends on the net generating a minimum of stimulation which might provoke avoidance responses. Observations of ultrasonically tagged American shad Alosa sapidisimma in the vicinity of driftnets have shown that fish can possess a remarkable ability to avoid the nets Legget and Jones, 1971 cited by Ferno and Olsen, 1994. Experimental studies in tanks strongly indicate that before fish are caught in the net, the visual stimulus of the net is the key factor that determines whether or not fish react Cui et al., 1991. Gillnets become invisible at certain threshold light intensities that are related to colour, net, material and turbidity Dickson, 1989 cited by Ferno and Olsen, 1994. Net factories produce gillnets of different colours, and fishermen choose nets with a colour which they believe will make the nets as invisible as possible for a specific season and area Ferno and Olsen, 1994. Several experiments have been carried out to investigate the effect of different colours on catch rates of fish in gillnets. The experiments by Jester 1973 as cited by Ferno and Olsen 1994 showed that gillnet colour had either a positive or negative effects on catch rates, depending on the species. However, it is not possible to draw any general conclusion about colour and visibility from these experiments, since it is the luminance of the net relative to the background which is the key factor in net visibility. Wardle et al. 1991 have pointed out that the colour of gillnet should be matched to the sea bed to make them as invisible as possible. Ferno and Olsen 1994 suggest that since water colour can vary, colour in material adds a complication that could be avoided by careful selection of a neutral grey with a density that results in a good match with the water background. In addition of colour, thread twine thickness and type of material determine net visibility; thinner twines are less visible than thicker ones. Since the fish is not expected to sense the net existence, it would be preferable to choose thin twines. 15 However, very thin twine can easily break by larger fish, so that thickness must be adapted to the species and size of fish. The efficiency of gillnets has greatly increased with the introduction of less visible synthetic fibres, especially transparent polyamide monofilaments, which have replaced natural fibres. Polyamide thread is the dominant material used in gillnets today in the western hemisphere. Four different types of polyamide thread are in use, twisted multifilament, monofilament, monotwine 3-strand monofilament and multi-monofilament. Comparative fishing experiment shown that multifilament gillnets are less efficient than nets of monofilament materials Washington, 1973; Hylen and Jacobsen, 1979 cited by Ferno and Olsen, 1994. The change in visibility due to colour, twine thickness and type of material may affect not only the efficiency but also the size selectivity of gillnets because the reaction of the fish to nets can change as contrast thresholds may improve as fish grow Anthony, 1981 cited by Ferno and Olsen, 1994. It may also have an effect on species selection since there are species differences in vision. 16 3 RESEARCH METHOD 3.1 Fish Behaviour Study 3.1.1 Time and location The fish behaviour study was conducted in Fish Behaviour Laboratory, Tokyo University of Marine Science and Technology in August – September 2005 fish behaviour towards contrast colour net panel and December 2005 – March 2006 fish behaviour study using specific white net panel. 3.1.2 Fish behaviour in relation to contrast colour of the net panel 3.1.2.1 Materials and instruments Materials and instruments used in the behavioural study towards contrast colour panel are shown in Table 1 and 2. Japanese Jack mackerels was used as test fish Figure 3. The reason of this is that these fish are a type of pelagic fish and which is target catch of millennium gillnet. Figure 3. Japanese Jack mackerels Trachurus japonicus The Japanese Jack mackerels used in the experiment was in healthy condition. The average fork length of this fish was around 13-15 cm within maximum body girth G max was around 12.7-13 cm. Three net panels used as treatment of transparency i.e. old white, new white, and black painted net as shown in Table 1. The old white net is the net which has been used for 1.5 years. The reason of the use of this net panel was that most fishermen in Cirebon use old net to catch fish instead of new white net due to minimize cost. Table 1. Instruments for the contrast colour net panel experiment No. Instruments Function 1. 2. 3. 4. 5. 6. 7. 8. 3 net panels old, new and black painted 46x53 cm framed by PVC 16 mm Figure 4. Forcing panel Experimental tank 200x100x50 cm filled with sea water 30 cm in height Figure 4. Video recording system Figure 4, consisted of: • 8 mm Sony handy-cam • underwater camera • multi-viewer • monitor • VHS video cassettes 60 minutes Thermometer Lux meter Digital camera Stopwatch Research instrument As the treatment behaviour Fish tank Recording the fish swimming and behaviour Measuring the water temperature Measuring the light intensities Documentation Counting time needed for fish on entering the net Figure 4 shows the instruments used in the contrast colour net panel experiment. It is showed that the video result of the underwater CCD camera and Sony 8 mm handy camera was displayed into the monitor by the help of multi viewer. The multi viewer is able to make the video result of both cameras displayed in the monitor at the same time. Then, after that the video player helps to record the video camera result in VHS video cassettes. 18