III.i. Application of fish behaviour knowledge for development of fishing gear and methods with improved selectivity and reduced by-catch

ICES mar. Sei. Symp., 196: 155-160. 1993

Fish-herding effect of an air bubble curtain and its application to set-net fisheries

Takafumi Arimoto, Seiji Akiyama, Keiji Kikuya, and Hiromasa Kobayashi

Arimoto, T., Akiyama, S., Kikuya, K., and Kobayashi, H. 1993. Fish-herding effect of an air bubble curtain and its application to set-net fisheries. - ICES mar. Sei Symp 196: 155-160.

In order to develop a new operating system for set-net fisheries, the herding effect of an air bubble curtain was studied in laboratory and field experiments. The response of fish to a moving air bubble curtain was examined in an annular tank while air flow and speed of movement were varied. The fish continued swimming to avoid the approach­ing curtain. For practical application to a set-net operation, several air tubes were set at intervals on the bottom, and the bubbling position was switched among them in a sequence. The fish reaction was similar to that to the moving curtain; swimming speed and direction could be controlled by the valve switching operation. In a field experiment, conducted with a full-size chamber trap to confirm the application of the methods, the fish were successfully herded to the final capture area by sequencing the bubbling position. In a set-net with a final chamber trap, the fish were forced to enter through the non-return device of the slowly tapering funnel. This system can help to save time and labour during net-hauling, as well as in designing an automated capture system by continuous accumulation of fish in the non-return trap.

Takafumi Arimoto and Seiji Akiyama: Tokyo University o f Fisheries, 4-5-7, Konan, Minato, Tokyo 108, Japan. Keiji Kikuya and Hiromasa Kobayashi: Hokuriku Seimo Co., Ltd., 2-13-51, Kitayasue, Kanazawa City. Ishikawa 920, Japan.

Introduction

Control of fish behaviour by an air bubble curtain was first reported by Smith (1957, 1961). In the succeeding three decades, its barrier effect has been examined from various points of view for purposes of capturing and controlling fish (e.g., Imamura and Ogura, 1959; Igar- ashi, 1963; Kuznetsov, 1971; Lieberman and Muessig, 1978; Stewart, 1981; Patrick etal., 1985 and Sager et al., 1987).

We investigated the fish-herding effect of an air bub­ble curtain both in the laboratory and in field experi­ments, with the aim of its practical application to set-net fishing operations.

Laboratory experiments

The first step in our experiments was the basic confir­mation of the herding effect of a moving air bubble curtain in a small tank (Fig. 1) (Akiyama et al., 1991). An air pipe was set on the bottom of an annular trough, and rotated so as to create a moving barrier in the form of an air bubble curtain. The response of fish to the bubble curtain was observed in relation to variation of its moving speed (11-32 cm s-1) and air flow rate (10-40 1 min“ 1 per 1 m pipe length).

Jack mackerel ( Trachurus japonicus) (88-113 mm TL), threeline grunt (Parapristipoma trilineatum) (161- 187 mm TL), and Japanese parrotfish (Oplegnathus

155

Air pump

Drivingapparat)

Annulartrough

25

jr tube

100cm

Figure 1. Experimental apparatus for small-scale tests of the fish-herding effect of an air bubble curtain. All dimensions are in cm.

fasciatus) (101-128 mm TL) were used respectively in groups of three individuals. Each species was tested separately. Each species swam steadily, maintaining position just in front of an approaching bubble curtain. The herding effect was expressed as the time elapsed until one of the three individuals failed to keep its position and was left behind the curtain within a one hour observation period. The results are shown in Fig­ure 2. In accordance with fish species and size, fish responses varied with experimental conditions. It is noteworthy that jack mackerel and threeline grunt kept swimming to avoid an approaching bubble curtain for the whole 60 min in all nine trials at the low herding speed of 11 cm s_1 (Fig. 2A). In general, the herding effect was shown more strongly by the pelagic species than by the coastal fish. The amount of air flow and the speed could be the determining factors for the successful effect of the air bubble curtain, even in the small closed experimental area.

Similar experiments were conducted in a larger-scale tank (Fig. 3) (Akiyama et al., 1992). With a view to practical application to a set-net operation, 16 radial air tubes were set up at intervals on the bottom and the bubbling position was alternated in sequence. The re­sponse of a small school of 10 jack mackerel was exam­ined in relation to variation of the bubbling interval from 1 to 10 s, which is equivalent to 7.9-79 cm s-1 (0.5-5.4 body lengths per second). In this experiment, an air flow rate of 50 1 min-1 m ~ ', which could create an opaque barrier in the tank, was employed. The fish reaction was similar to that to the moving curtain in a smaller tank, with the swimming speed and direction being controlled by the valve switching operation.

The results are shown in Figure 4. At lower air curtain

50

40

.§30

c 20w.

1 10

403 0

(cm/s)20

Moving speed10

Moving

60 r

c 5 0

40

I 20

Air f low ( I/m in )

Figure 2. Variation of herding time in relation to (A) bubble curtain moving speed with constant air flow of 9 I m i n 1, and (B) air-flow rate with constant bubble curtain speed of 22.5 cm s . Vertical bars and symbols indicate range and average values for 9 trials of each of the following fish species: filled circles, Japanese parrotfish (Oplegnathus fasciatus); filled squares, jack mackerel ( Trachurus japonicus)\ filled triangles, threeline grunt (Parapristipoma trilineatum) (grunt data are not available for graph (B)).

speeds (10-30 cm s_ '), the fish swimming was not stable, so the average swimming position was far ahead of the bubbling position. In experiments with speeds faster than the cruising speed of 35.5 cm s-1 (observed during voluntary swimming in the tank), the typical escape reaction was obtained: fish maintained a steady swim­ming performance as a school 1-3 m ahead of the bubbling. Here, in the night-time experiment with ambi­ent luminance lower than 0.1 lux, the tendency of fish reaction was similar to that in daytime.

When an annular trough was blocked by a netting panel or an air bubble curtain, fish were successfully herded toward the stationary barrier by sequencing the bubbling position. In this way fish were confined and concentrated between the approaching bubble curtain and the stationary barrier.

156

Air tube7 in

H— 2 m — H

V

Air tube

Figure 3. Experimental arrangement of an annular trough with 16 radial air tubes arrayed on the bottom at uniform intervals. The bubbling position was alternated in sequence at different time intervals of 1 ,2 ,3 ,5 . and 10 s to control the herding speed. The response of a school of 10 jack mackerel ( Trachurus japonicus) to the rotating air bubble curtain was observed and recorded as the number of sections between the fish and the bubbling position for each bubbling instance during 10 com­plete rotations of the air bubble curtain through the trough. Air flow rate was 501 min-1 m ' 1.

Field experiments

Three different trials were conducted with model and practical set-nets. The experimental conditions are listed in Table 1. The first trial was conducted in a small- scale set-net to drive the fish toward the capture area in the chamber trap, which measured 33 m long, 24 m wide, and 18 m deep. The experimental arrangement is shown in Figure 5. The air tube (17 mm dia.) made of 20 m long porous rubber was set on the bottom of the chamber trap which was located just below an entrance funnel. Air was supplied from two sets of air tanks (50 1, 150 kgf/ cm2) on board a small fishing boat (8.0 m long, 2.0 G/T) moored at the far end of the chamber trap. The air tube was towed at a speed of 1 m m i n '1 by pulling the high- pressure hose connected to the air tank. The air curtain and fish behaviour to it were observed by divers and recorded by an underwater TV camera. Experiments were conducted with air pressure varying from 3 to 5 kgf/

E

COoS 4cn_Q_Q O

" OcCÜ

■§ 2

cCD

I 1-a>_Q<Dc °0 re

Cruising speed offish (cm/s)

20 40 60 80

Moving speed of air bubble curtain (BL/s)

Figure 4. Average distance between a school of 10 jack mack­erel (Trachurus japonicus) and bubbling position, in relation to the calibrated moving speed of an air bubble curtain in day (open circles) and night (filled circles) experiments. Circles indicate the average value for five replicate trials in each condition. The normal cruising speed of these fish was defined as 35.5 cm s ' 1 based upon voluntary movement in an annular trough. For further details, see text.

In the chamber trap, several fish species could be identified, such as jack mackerel (Trachurus japonicus), king fish ( Caranx equula), ribbon fish ( Trichiurus leptur- us), and Japanese anchovy (Engraulis japonicus).

The air bubble curtain created a perfect visual barrier at higher air pressures of 4-5 kgf/cm2. At 3 kgf/cm2, air ejection was inadequate to create an ideal barrier throughout the length of the air tube on the bottom at 18 m depth. Long-distance underwater observation inside the chamber trap was limited, owing to poor visibility and the muddy cloud generated by air ejection and the towed tube. Short-range observations revealed that fish schools which encountered the air bubble curtain, avoided it, and were driven to the capture end. When the air bubble curtain approached the final end, the fish were expected to be concentrated there. Most, however, could return to the entrance by passing through gaps between the air bubble curtain and the side wall netting. It is noteworthy that no fish passed through the air bubble curtain as observed by the limited short-range view.

In the second stage of the experiment, the response of fish to the air bubble curtain was observed in a full-scale model of a set-net, which consisted of a main net and a final trap with narrow tapered funnel (Fig. 6). Bubbling positions were switched to herd the fish through the funnel toward the final trap. Sufficient air flow was obtained with an air compressor of 870 1 m in^1 and

157

Tabic 1. Experim ental conditions used in the field test of the fish-herding effect.

Exp. no. No. 1 No. 2 No. 3

Location Takanoshima fishing ground in Chiba Prefecture

Kaiso fishing port in lshikawa Prefecture

Matsuzaki fishing ground in Shizuoka Prefecture

Date Oct 1990-Mar 1991 Jun 1991 Nov 1991Net scale 33 m long x 24 m wide

chamber trap28 m long x 10 m wide

full-scale model net120 m long x 21 m wide

set-net

Specification of air tubeMaterial Porous rubber PVC NylonDiameter (mm) 17 16 15Length (m) 10/20 5-10 16-30Numbers 1 8 19Set interval (m) - 3 4-6Hole diameter (mm) - 0.5 0.75-1.0Hole interval (cm) - 10 15-30

Air supply Air tank Compressor CompressorSpecification 501, 150 kgf/cm3 15 ps/3600 rpm 50 ps/3000 rpmAir flow (I min ') 380 Max. 870 Max. 5000Pressure (kgf/cm ) 3-5 Max. 10 Max. 7

Fish species Jack mackerel ( Trachurus japonicus)

King fish (Caranx equula)

Ribbon fish ( Trichiurus lepturus)

Jack mackerel ( Trachurus japonicus)

File fish (Navodon modeslus)

Yellowtail (Seriola quinqueradiata)

Jack mackerel(Trachurus japonicus)

Frigate mackerel (Auxis thazard)

10 kgf/cm2 at maximum, for a bottom depth of 10 m. About 100 fish of several species, such as yellowtail Seriola quinqueradiata, filefish Navodon modestus, and jack mackerel Trachurus japonicus, were introduced into the main net and adapted for 1 day or more prior to the experiment. Underwater observations and success­ive net-hauling of the final trap showed that the fish in the main net were successfully herded to the final trap by

switching the bubbling position in sequence from the first tube to the end of the funnel. Some species such as jack mackerel showed hesitation in approaching the narrow funnel, and sometimes retained their position even when the bubbling position approached, so that they were left behind when the bubbling position passed them.

As the third stage, a practical experiment was con-

Top view

Figure 5. Experimental arrangement for fish herding by a towed air bubble curtain in the chamber trap of a set-net 33 m long, 24 m wide, and 18 m deep, with a funnel entrance from a main net.

Air hose

5 m10

Air tube

►W 8 --- «■ 812

Side view

Main net

Final trapFunnelAir tube t5.5 m

IFigure 6. Experimental arrangement for herding fish by alter­nating the position of an air bubble curtain in a full-scale model set-net. All dimensions are in m.

rowing hose

158

Top view

Entrance

Air tubeAir hose Leader net

1---------------------— --------------------------------------------120 m

Side view

Main net

Final trap Funnel 20 m

V VAir tube

Figure 7. Experimental arrangement for herding fish by alternating the position of an air bubble curtain from the centre to the final trap in a set-net.

ducted in a small-scale set-net 120 m long and 20 m deep. The arrangement of the net and the air tubes is shown in Figure 7. The set-net was a demersal type with double final traps located at opposite ends of the main net. The procedure for herding the fish inside the main net was first to intercept the entrance mouth, and then to alter­nate two lines of bubbling position in sequence from the centre to the ends, toward each final trap. An air compressor capable of supplying 5000 1 m in "1 was employed. The creation of the air bubble curtain and its manual position alternation were satisfactory. The herd­ing effect was observed and video-recorded by divers for schools of jack mackerel and frigate mackerel (Auxis thazard). The problem, however, was still encountered that fish hesitated to enter the narrow tapered funnel and tended to remain in position.

We are taking these practical experiments to the final stage to confirm the herding effect of an air bubble curtain by comparing the daily catch amount with and without this herding procedure.

Discussion

The set-net fishery is one of the principal methods of coastal fishing in Japan. It is classified as passive station­ary gear consisting of a barrier system (a leader net) and a multiple trapping system (non-return devices with funnel entrance). The leader net intercepts the mi­gration of fish schools and guides them into the main net (von Brandt, 1984). The best designs of non-return entrances and funnels have been investigated (Nomura, 1980). It should be emphasized, however, that not all fish in the main net enter the trap net. This means that the majority of fish can readily escape the main net through the entrance (Inoue and Arimoto, 1988).

To harvest the fish from the trap, net-hauling is conducted every morning, and sometimes twice a day during the best fishing season by also hauling during the evening. The operational procedure is principally

manual, with the aid of machines such as net-haulers or capstans for rope hauling (Miyamoto, 1971; Honda, 1976). Air lifting systems have already been developed with the air hose bag set under the chamber trap (Fuka- hori et al. , 1988). Although such mechanization can save time and labour during net hauling, the principle of the operation remains unchanged.

The results of our experiments suggest an application of the air bubble curtain to set-net fishing operation, by herding fish toward the capture area without drying up the bottom net of the chamber trap. This kind of fish- herding system can promise the additional advantage of establishing an innovative fishing method: an automated capture system by continuous accumulation of fish from the main net into the non-return trap of a set-net. This might be done with the aid of timer-controlled operation for air valve switching, and remote control with a sensing system for fish detection. For this purpose, further prac­tical experiments will be required, especially towards designing non-return devices for preventing the escape of fish from the final trap. Final traps can then be alternated to act as keeping cages, which may lead to a stable situation of harvesting and marketing in set-net fisheries.

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