BOTTOM TRAWL FOR FISH CATCHING AND ACOUSTIC EMITTER THEREFOR

Abstract: A bottom trawl (1) for fish catching comprising a trawl net having a front part and a cod-end part, and including such components usually present in a standard bottom trawl, is used in combination with an acoustic emitter (2) for producing an acoustic zone in front of the trawl net entrance. The emitter comprises transducer means mounted on the front end of a carrier towed by the trawl, and activating equipment located inside a sealed section of the carrier. The transducer means is adapted to produce a sector shaped acoustic directional zone (5, 6) oriented substantially horizontally in the direction of the trawl movement, the acoustic emitter being adjustable to set the angle of orientation of the transducer means such that an optimum level of acoustic pressure is achieved in the lower portion of said acoustic directional zone above the lop line of the trawl net.

Description: BOTTOM TRAWL FOR FISH CATCHING AND ACOUSTIC EMITTER THEREFOR Technical Field The present invention relates to fishing equipment, and in particular to a bottom trawl for fish catching having an acoustic emitter connected thereto, as well as such an emitter. Background Art Various stimuli, such as acoustic, visual, olfactory and electric stimuli, has be used to influence or control the behaviour of fish. For example, from Patent Application Publication No. WO 83/00983 it is known a method of electric fish trawling which utilizes the fright-reaction exhibited by fish exposed to an electric field, particulary an electrical field charging the water in front of the trawl net to urge bottom fish off the ocean floor. Also, such stimuli can be used for fencing off a bay or other parts of water through conditioning with electrical shocks and light pulses, for example, such as described in "Recent Progress in the Control of Fish Behavior by J.G. Balchen, IFAC Control Science and Technology, Japan, 1981. Further prior art publications include SU 1 741 696 from which it is known an arrangement for directing fish into the catch zone of a trawl using pneumatic emitters attached to ropes running from a ship to the trawl. The purpose of this dispersed arrangement of emitters is to generate among fish located in the zone between ship and trawl mouth a phenomenon of crowding (packing) and of directional movement into the trawl catch zone. Also, from RU 2 007 077 it is known a trawl fishing net with an acoustic fish attractor comprising an acoustic radiator situated at the rear of the net's cod-end to create an acoustically radiated area surrounding the net mouth perimeter. More specifically, it is known a bottom trawl for fish catching comprising a trawl net having a front part and a cod-end part, and included such components usually present in a standard bottom trawl, and an acoustic emitter for producing an acoustic zone in front of the trawl net entrance, the emitter comprising transducer means mounted on the front end of a carrier towed by the trawl, and activating equipment located inside a sealed section of the carrier. In this known arrangement, the purpose of which being to make trawling more productive, the transducer means is a narrow beam transducer, the central axis of which being oriented in the direction of the trawl movement and tilted at a certain angle relative to the longitudinal axis of the carrier body. The size of the angle is determined by the geometric dimensions of the trawl net. The carrier is towed on a rope behind the cod- end part of the trawl, and due to respective accessories, the carrier is caused to rotate around its longitudinal axis. As a result, a sounded cone is formed in the water which is intended for covering the perimeter of the trawl front and the space in front of the trawl. This acoustic cone is meant to serves as a funnel-shaped continuation of the front end of the trawl net, within which fish should gather to be caught soon later by the trawl net. Such a modified trawl, however, cannot be used for catching fish living at the bottom or near the bottom of the sea. This is because a section through the sounded cone takes the form of a circle, whereas the entrance of the trawl net, when towed at the bottom of the sea, takes a shape which is close to a rectangle typically having a side ratio of 6:1 , where 1 corresponds to the height of the trawl net entrance. Another disadvantage of the known modified trawl is the need for increased emitted acoustic power in order to compensate for power losses over the distance between the position of the acoustic emitter carrier and the front of the trawl net, which, for current trawl structures, may be as much as 100 to 500 meters, if the sounded cone is to be aligned with at least a portion of the trawl entrance. Disclosure of Invention The purpose of the invention is to increase the efficiency of bottom trawl fishing by means of artificially concentrating fish, including midwater species, in the bottom zone in front of a trawl net. Another purpose of the invention is to reduce the power demands of the acoustic emitter by moving it closer to the trawl net entrance. Thus, according to the present invention, it is provided a bottom trawl for fish catching comprising a trawl net having a front part and a cod-end part, and included such components usually present in a standard bottom trawl, and an acoustic emitter for producing an acoustic zone in front of the trawl net entrance, the emitter comprising transducer means mounted on the front end of a carrier towed by the trawl, and activating equipment located inside a sealed section of the carrier; the bottom trawi of the invention being characterized in that said transducer means of the acoustic emitter is adapted to produce a sector shaped acoustic directional zone oriented substantially horizontally in the direction of the trawl movement when being used to catch fish, the acoustic emitter being adjustable to set the angle of orientation of the transducer means such that an optimum level of acoustic pressure is achieved in the lower portion of said acoustic directional zone above the top line of the trawl net. In another aspect of the invention, it is provided an acoustic emitter to be connected to such a bottom trawl for fish catching, the acoustic emitter being adapted for producing an acoustic zone in front of the trawl net entrance when connected to said bottom trawl, the emitter comprising transducer means mounted on the front end of a carrier to be towed by the trawl, and activating equipment located inside a sealed section of the carrier; the acoustic emitter of the invention being characterized in that said transducer means is adapted to produce a sector shaped acoustic directional zone oriented substantially horizontally in the direction of the trawl movement when being used to catch fish, the acoustic emitter being adjustable to set the angle of orientation of the transducer means such that an optimum level of acoustic pressure is achieved in the lower portion of said acoustic directional zone above the top line of the trawl net. According to the invention, the angle of orientation of the transducer means producing the sector shaped acoustic directional zone is deductible from the following expression: tg(a - 2) ≥ m - 21, where: is the angle of orientation of the transducer means, m is the size of the horizontal opening of the trawl net, / is the length of a tow line for connecting the acoustic emitter to the centre of a trawl net top line. In a preferred embodiment of the invention the length of said tow line is chosen so as to exclude any influence upon fish present in the pre-entrance area of the trawl, of possible side maximums resulting from the generation of said acoustic directional zone. In another version of the invention two additional transducers are used to produce two sector shaped acoustic directional zones which are substantially vertically oriented in an outwards angled direction relative to the trawl movement when being used, each said vertically oriented acoustic directional zone acting as a side barrier to the fish present to the side of, and in front of the trawl net entrance. To enhance the desired effect, each additional transducer should be arranged to produce a vertically oriented acoustic directional zone which extends in a tapered manner towards the trawl net entrance, preferably such as to pass at some point close to respective doors of the trawl Furthermore, the vertically oriented acoustic directional zones are preferably dimensioned and oriented to coincide at some point, or along a certain length, with the horizontally oriented acoustic directional zone so as to form, in combination with the latter and the seabed, an artificial hopper or concentrator comprising acoustic side and top barriers, respectively Hence, with the present invention a bottom trawl for fish catching is provided which utilizes acoustic zones, or sounded barriers, of a more planar shape, to increase the take as compared to other devices known in the art Brief Description of Drawings Further details, features and advantages of the present invention will appear from the description below of a preferred embodiment, given as an example only, and with reference to the accompanying drawing, on which Figure 1a shows a side view of a bottom trawl provided with the acoustic emitter according to the invention, and Figure 1b shows the arrangement of Figure 1a, as seen from above Description of Preferred Embodiment In the figures, it is shown an arrangement according to the invention, comprising a bottom trawl 1 for catching fish, an acoustic emitter 2 for generating a sounded zone in the active zone of the trawl net, a rope 3 for towing the acoustic emitter, and a fishing vessel 4 towing the combination of the bottom trawl and acoustic emitter Reference numerals 5 and 6 denote an acoustic directional zone generated by a transducer mounted on the front end of the emitter 2, as seen from one side and from above, respectively Also, in the figures, possible side maximum loops 7 of the transducer are indicated, as well as the doors 9 of the trawl, and flashing lights 10 mounted on the extremes of the trawl entrance to prevent fish from escaping from the trawl catching zone Turbulence tails 8 caused by the doors 9 of the trawl, are also shown The emitter 2 comprises a transducer mounted on the front end of the emitter carrier body, and activating equipment located in a hermetic sealed section of the carrier being towed by the trawl net top line. The acoustic emitter is equipped with a transducer having a sector shaped acoustic directional characteristic, oriented substantially horizontally in the direction of trawl movement The angle of the transducer orientation is adjustable in order to choose an optimum level of acoustic pressure in the lower part of the sounded zone, over the top line of the trawl net. The angle of the sector characteristic of the transducer direction is deduced from the expression: tg(a - 2) ≥ m - 21, where is the angle of the sector characteristic of the transducer direction, m is the size of the horizontal opening of the trawl net, / is the length of the carrier tow line 3, the front end of which is chosen so as to exclude the influence upon the fish in the pre-entrance zone, of the side maximums 7 of the directional characteristic of the transducer. With the arrangement shown on the drawing, the trawler ship 4 is towing the bottom trawl 1 carrying the acoustic emitter 2 connected to the centre of the trawl net top line by means of the rope 3. The turbulence tails 8 shown in Figure 1 b, are caused by the trawl doors 9 being towed along the seabed. The sounded water zone produced by the transducer mounted on the front end of the emitter housing 2 has a shape 5 in the vertical plane as illustrated in Figure 1a, and a shape 6 in the horizontal plane as illustrated in Figure 1b. The centre portion of the zone corresponds to an acoustic pressure of 0.7 of the maximum, and the parts between the rays (drawn as straight lines) correspond to pressure levels which are reduced as a function of their distance from the centre. The "loops" 7 shown in Figure 1a represents the so-called side maximums of the acoustic direction characteristic of the acoustic emitter. The trawl operates in the following way: When the trawl net 1 is towed by the trawler ship 4, the carrier housing 2, which has a small positive buoyancy to provide for the necessary hydrodynamic and operational properties, takes an substantially horizontal position over the trawl net 1. As a result of operating the acoustic emitter, a sounded zone directed at a certain angle upwards relative to the horizontal plane, is formed, the sounded zone representing an obstacle which makes fish present in the near bottom zone, move (even) closer to the bottom, i.e. to the level which is covered by the entrance of the trawl net 1. Due to the wide horizontal sector characteristic 6 of the sounded zone (cf. Figure 1b), this influence covers the whole width of the trawl net entrance. Fish escaping to the sides, away from the opening of the trawl net, is restricted by the trawl wings with light-flashers 10 thereon, and the trawl doors 9. Having a correspondingly high emitted acoustic power, an acoustic barrier having quite a long range can be obtained. This enables concentrating fish from quite a large volume of water at a long distance, including fish living in the mid-water zone, near the bottom. In Figure 1a, this process is illustrated by separate dots (representing fish), the density of which increases with the approach to the trawl net. The length of the tow rope 3 is chosen such that the side maximums 7 of the direction characteristic of the emitter do not hit or reach the area in front of the trawl net entrance. The acoustic transducer (not shown on the drawing) is mounted on the front part of the carrier body, with the option to be tilted mechanically upward from the horizontal plane, to allow the sector directional characteristic of the emitter to be shifted by an angle providing a level of sound pressure over the top line of the trawl net, which results in the optimum influence on the behavior of the fish. In each given case, this parameter may be defined experimentally. The design of a device for tilting and/or turning the transducer should not require further explanation. As mentioned above, in another version of the invention two additional transducers can be used to produce two sector shaped acoustic directional zones which are substantially vertically oriented in an outwards angled direction relative to the trawl movement when being used. Such additional transducers (not shown) can be located in the emitter housing and constitute a part of the acoustic emitter 2 itself, or in separate housings connected individually to the trawl. Such vertically oriented sounded zones would serve as side barriers to the fish present to the side of, and in front of the trawl net entrance. To enhance the desired effect, each additional transducer should be arranged to produce a vertically sounded zone which extends in a tapered manner towards the trawl net entrance, preferably such as to pass at some point close to respective doors of the trawl. If the vertically oriented sounded zones are dimensioned and oriented to coincide at some point, or along a certain length, with the horizontally oriented sounded zone, they would, in combination with the latter and the seabed, form an artificial hopper or concentrator comprising acoustic side and top barriers, respectively, and exhibiting an approximately rectangular cross-section. As compared to prior art devices, the acoustic power generated by the transducers to produce acoustic directional zones is relatively small, and the acoustic radiation may be either constant, pulsed, or frequency modulated dependent on type of fish and local conditions. A preferred main signal frequency is in the order of 12 kHz. The fish guiding devices 10 located at the extreme points of the trawl net entrance prevent fish from escaping from the trawl catching zone. As it is known that fish may be disorientated by being exposed to bright flashes of light, these guiding devices preferably have the form light flashing units. This may have a double effect: • to disorient the fish gathered in front of the trawl and at the trawl entrance, where it tries to move in the same direction and at the same speed as the trawl, and • to prevent fish from escaping beyond the trawl wings. The range of the effect on fish in sea water from the light of such flashing sources should be about 10 to 15 m for trawls having a horizontal opening of 30 to 40 m. Whereas such light flashing units are simple in design, save power, and do not need stabilization means with respect of list and trim, it is acknowledged that the acoustic emitter according to the invention may require a specific transducer antenna, power amplifier, control circuits and hydrodynamic stabilization means to function properly.

Claims: Claims 1. A bottom trawl (1) for fish catching comprising a trawl net having a front part and a cod-end part, and included such components usually present in a standard bottom trawl, and an acoustic emitter (2) for producing an acoustic zone in front of the trawl net entrance, the emitter comprising transducer means mounted on the front end of a carrier towed by the trawl, and activating equipment located inside a sealed section of the carrier, characterized in that said transducer means of the acoustic emitter (2) is adapted to produce a sector shaped acoustic directional zone (5, 6) oriented substantially horizontally in the direction of the trawl movement when being used to catch fish, the acoustic emitter being adjustable to set the angle of orientation of the transducer means such that an optimum level of acoustic pressure is achieved in the lower portion of said acoustic directional zone above the top line of the trawl net. 2. A bottom trawl according to claim 1 , characterized in that said angle of orientation of the transducer means producing the sector shaped acoustic directional zone (5, 6) is determined by the following expression: tg(a -2) ≥m-21, where: is the angle of orientation of the transducer means, m is the size of the horizontal opening of the trawl net, / is the length of a tow line (3) for connecting the acoustic emitter (2) to the centre of a trawl net top line. 3. A bottom trawl according to claim 2, characterized in that the length / of said tow line (3) is chosen so as to exclude any influence upon fish present in the pre-entrance area of the trawl (1), of possible side maximums (7) resulting from the generation of said acoustic directional zone (5, 6). 4. A bottom trawl according to any preceding claim, characterized in that it is provided with two additional transducers each adapted to produce a sector shaped acoustic directional zone oriented substantially vertically in an outwards angled direction relative to the trawl movement when being used, each said vertically oriented acoustic directional zone acting as a side barrier to the fish present to the side of, and in front of the trawl net entrance. 5. A bottom trawl according to claim 4, characterized in that each said additional transducer is arranged to produce a vertically oriented acoustic directional zone, such that the zones extend in a tapered manner towards the trawl net entrance, preferably such as to pass at some point close to respective doors (9) of the trawl. 6. A bottom trawl according to claim 4 or 5, characterized in that said vertically oriented acoustic directional zones are dimensioned and oriented to coincide at some point, preferably along a certain length, with said horizontally oriented acoustic directional zone (5, 6) so as to form, in combination with the latter and the seabed, an artificial hopper or concentrator comprising acoustic side and top barriers, respectively. 7. A bottom trawl according to any of claims 4 to 6, characterized in that said transducers are adapted to produce acoustic directional zones of a relatively small acoustic power, the acoustic radiation being either constant, pulsed, or frequency modulated dependent on type of fish and local conditions, a preferred main signal frequency being in the order of 12 kHz. 8. A bottom trawl according to any preceding claim, characterized in that at the extreme points of the trawl net entrance the trawl is provided with fish guiding devices, such as light flashing units, to prevent fish from escaping from the trawl catching zone, the range of such light effects on fish in sea water preferably being about 10 to 15 m for trawls having a horizontal opening of 30 to 40 m. 9. An acoustic emitter (2) to be connected to a bottom trawl (1) for fish catching comprising a trawl net having a front part and a cod-end part, and included such components usually present in a standard bottom trawl, the acoustic emitter (2) being adapted for producing an acoustic zone in front of the trawl net entrance when connected to said bottom trawl, the emitter comprising transducer means mounted on the front end of a carrier to be towed by the trawl, and activating equipment located inside a sealed section of the carrier, characterized in that said transducer means is adapted to produce a sector shaped acoustic directional zone (5, 6) oriented substantially horizontally in the direction of the trawl movement when being used to catch fish, the acoustic emitter being adjustable to set the angle of orientation of the transducer means such that an optimum level of acoustic pressure is achieved in the lower portion of said acoustic directional zone above the top line of the trawl net. 10. An acoustic emitter according to claim 9, characterized in that said angle of orientation of the transducer means producing the sector shaped acoustic directional zone (5, 6) is determined by the following expression: tg( -2) ≥m-21, where: a is the angle of orientation of the transducer means, m is the size of the horizontal opening of the trawl net, / is the length of a tow line (3) for connecting the acoustic emitter (2) to the centre of a trawl net top line. 11. An acoustic emitter according to claim 9 or 10, characterized in that it further comprises two additional transducers each adapted to produce a sector shaped acoustic directional zone oriented substantially vertically in an outwards angled direction relative to the trawl movement when being used, preferably such that said vertically oriented acoustic directional zones coincide at some point, or along a certain length, with said horizontally oriented acoustic directional zone (5, 6) generated by the acoustic emitter.

Inventors: ANDERSEN IVAR, ANDERSEN, IVAR
Applicants: ANDERSEN IVAR, SKIPPER ELECTRONICS A S, ANDERSEN, IVAR, SKIPPER ELECTRONICS A/S
Application year: 1997
Main classification: A01K73
IPC codes: A01K73, A01K75, A01K79