During the year 1991. aquaculture as an industry produced at least Ik million tonnes of produce. A percentage of this production was fin fish, cultured in floating cage structures at sea. Due to the environmental requirements of the stock, these cages are often placed in remote locations.

The stocks held in these structures usually have a long growing cycle. A large loss just prior to the fish being marketed could cause financial ruin to the enterprise. This problem is made more acute as it is very difficult to insure these fish stocks competitively. Due to these facts, losses from theft or from marine or human predators are a major concern.

It is known that some marine predators, such as seals, can be repelled from attacking fish farming cages by ultra sound at certain levels of intensity and at certain frequencies. These current devices, though, have to be in operation 24 hours a day. Consequently, there is a large power demand and predators have the opportunity to acclimatise themselves to the sound produced. Another drawback of the current devices is that they do not inform the owner of the occurrence. If a predator attack has occurred and the net has been torn, then large losses of stock may result without warning being given.

It is an object of the present invention to provide a fish farm cage security system which does not suffer from the above disadvantages .

According to the invention there is provided a fish farm cage security system which includes sound-emitting means attached to the cage for detecting tampering with one or more nets forming the cage, detector means responsive to the sound emitted by the sound-emitting means, discriminator means responsive to the output of the detector means to determine whether the sounds detected by the detector means represent a perceived threat and alarm means responsive to the output of the detector to respond in a manner appropriate to the nature of the perceived threat. According to one aspect of the invention the sound-emitting means comprise a number of bells attached to the nets forming the cage

and of a form which is not actuated by the normal movements of the cage due to tide and weather.

According to another aspect of the invention the detector means comprise sonar detector means operable to detect underwater sounds.

According to a further aspect of the invention the alarm means may comprise sound generating means operable to generate underwater sounds to deter the cause of the perceived threat. The alarm means may also include means to alert personnel to the existence of a perceived threat.

The invention will now be described with reference to the accompanying drawings, in which:-

Figure 1 is a schematic diagram of a fish farm cage incorporating one embodiment of the security system; Figure 2 illustrates in schematic form the electronic elements of the security system according to the invention.

Referring now to Figure 1, a fish farm cage 10 comprises a floating walkway 11 from which are suspended nets 12 forming an enclosure. Attached to the nets 12 forming the sides and bottom of the cage 10 are a number of underwater bells 13, generally though not exclusively attached to the nets away from the corners of the cage where the movement due to disturbance of the nets will be greatest. The sound emitted by the bells 13 is detected by one or more underwater detectors Ik . connected to cage-mounted electronics 15 which may conveniently be mounted on the walkway 11. As will be described later, the cage-mounted electronics includes means for determining the probable source of the sounds detected by the detector Ik and also communication means for sending signals to a base station, possibly by means of a radio link using an antenna 16. Also mounted on the nets 12 are further sound- emitting devices 17 which emit sounds capable of scaring away any marine predators, such as seals. The walkway may also carry one or more microphones or other devices 18 to detect the presence of people or marine creatures on the walkway 11.

Underwater bells such as those shown at 13 in Figure 1 are generally mechanical devices which are activated when the net 12 is disturbed by a predator pushing into the net. They will not be activated by water currents or other movements generated by the tides or weather.

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If a number of cages are located close together it is possible to use bells emitting sound at different frequencies on each cage so that it is possible to determine which cage is being disturbed.

The underwater detectors 14 may conveniently be hydrophones, positioned so as to detect sounds from the area around the cage 10. The detectors may be responsive not only to the sounds emitted by the bells 13 but also to other sounds generated in the area of the cages, such as those made by boats. The physical location of the detectors Ik will depend upon the area of coverage required around the cage 10. The further sound-emitting devices used to scare away marine predators will preferably emit pulses of sound at frequencies and intensities which are known to have the desired effect on the particular predators believed to constitute a threat to the stock in the cage.

It will be appreciated that the sound-emitting means 13 need not be in the form of mechanical bells as has been described. Electronic sound-emitting devices of many types will be suitable, so long as they are able to operate in the marine environment. The use of such devices will, however, normally require the addition of some form of movement detector to detect movement of the nets and cause the activation of the sound-emitting devices themselves. One advantage of using such electronic sound-emitting devices is that they may also be actuated by other effects than net movement. For example it is possible to mount detectors on the walkway 11 to detect the presence of people or marine creatures such as seals on the walkway when the system is activated, thus detecting potential poachers. The discriminator means may be arranged to use such presence of people to operate an alarm system to the base station without triggering the sound-emitting devices, so that the apprehension of the people is more likely.

The cage-mounted electronics 15 is preferably powered by an internal battery, as it is likely that the location of the cage will preclude the supply of power through a cable. Battery charging devices such as a cage-mounted wind generator may be used, as may solar cells. The electronics may simply transmit signals representing the detected sounds to the base station and receive signals from the base station representing the action to be taken, such as the actuation of the seal- scaring devices. Alternatively, the cage-mounted electronics 1 may have the ability to determine the threat posed by the sounds detected and to

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- k - cause the appropriate action to be taken, signalling the base station only when the threat is one which needs to be drawn to the attention of a base station operator. As well as transmitting signals to the base station representing action being taken, the cage-mounted electronics 15 roay also transmit to the base station the actual sounds detected by the hydrophones Ik so that a base station operator may determine whether further action is necessary. The cage-mounted electronics will usually be contained in a watertight tamper-proof container so that it is protected both from the environment and from tampering by unauthorised persons.

Figure 2 illustrates the cage-mounted electronics 15 and the base station 20 in schematic manner. The electronics 15 is represented by block 21, having connected to it one or more hydrophones Ik and one or more sound-emitting devices 17. Forming part of the electronics 15 is a radio transmitter-receiver 22 connected to the antenna 16. In most situations the radio will conveniently operate at VHF frequencies. The electronics 15 will also be connected to the battery charging system such as solar cells or wind generator already mentioned and represented schematically at 23. In addition to the sound-emitting devices 17 the electronics may also actuate flood lights 2k if the perceived threat warrants this action.

The base station 20 is also shown in Figure 2. This includes radio receiver which receives the data transmitted by the cage-mounted electronics over the radio link. In addition, the base station may include a loudspeaker 25 so that the base station operator may hear the actual sounds detected by the hydrophones Ik and a keyboard 26 through which instructions or responses may be entered into the system.

The cage-mounted electronics may conveniently include the decision-making means which determines the action to be taken in accordance with the identification of a perceived threat to the cage or the fish stock contained in it. This will require the provision of some form of data processor in the cage-mounted electronics 15 which is programmed to respond to detected sound to cause the appropriate response. Alternatively, if the purpose of the cage-mounted electronics is only to transmit the detected sound to the base station and respond to instructions received from the base station, then the data processor may be located at the base station.

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In use, the main purpose of the security system is to detect and respond to attacks on the cage by marine predators such as, but not restricted to, seals. Since such attacks will involve the seals making physical contact with the nets forming the cage, such attacks will actuate the underwater bells or other movement detectors 13- The sound emitted by the sound-emitting devices will be detected by one or more of the hydrophones 14, causing signals to be detected by the cage- mounted electronics 15. It is difficult to detect the sounds of seals or other marine predators in the water with any accuracy and hence the provision of the underwater bells or other sound-emitting devices is an essential feature of the security system. The sounds detected by the hydrophones may be transmitted to the base station 20 over the radio link but will also cause signals representing the actuation of the sound-emitting devices to be applied to the data processor, which will respond as programmed. If, for example, the signals detected by the hydrophones indicate that a single seal or a small number of seals is making unsuccessful attempts to penetrate the nets, then the system may respond only by actuating the sound-emitting devices 17 to frighten the seals away. In this ^~ situation there may be no need to alert the base station operator. If, on the other hand, the signals detected by the hydrophones indicate a mass attack, an attack by a much larger predator or the presence of people on the walkway 11, the response may be to operate the sound-emitting devices and/or to warn the base-station operator. Non-marine predators, generally human, may be able to attack the stock held in a cage without making physical contact with the nets and actuating the underwater sound-emitting devices 13- However, the hydrophones will be able to pick up the sound of their approach if by boat and these sounds may be used to actuate the security system by themselves. The data processor may be programmed to respond to such sounds by sending an alarm signal to the base station. The use of the sound-emitting devices 17 may not be necessary since it unlikely that these will affect human predators. If the incident occurs at night then the illumination of flood lights will be useful but the call-out of personnel from the base station will be the most likely response. In order to provide further information on an incident of this type the microphones 18 mounted on the floating platform will detect the sounds

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made by people on the platform, so that these sounds may be transmitted back to the base station for analysis by an operator.

The base station operator, to whom reference has been made above, may be located at the base station or may be provided with a paging device to warn when an incident occurs at the cage. Alternatively, it is possible to incorporate in the base station a telephone dialler which will call the operator by telephone and relay the signals received by the base station from the cage.

It is not necessary for the communications link between the cage and the base station to be by radio. If the cage is located close to the shore or is, for example, in an inland lake or an inlet from the sea, then cable links may be adequate. In either situation it may be advisable to provide an alarm to indicate if the communication link has been subject to physical or other interference. A single base station may be provided for a number of cages each having its own sound-emitting devices, detectors, electronics and so on. As has already been suggested it is possible to provide each cage with sound-emitting devices emitting sound of different frequencies so that the cage-mounted ^{^} electronics of one cage is not activated by sounds from emitters attached to an adjacent cage. Electronic filters may provide the necessary discrimination in this situation.

The cage-mounted parts of the security system are mounted in a hostile environment and it is advisable to include some form of self-testing program which is activated from time to time to check that the integrity of the system has not been compromised. In the event of a system failure of any type an appropriate message will be sent to the base station. In addition, other features common to security systems of other types may be incorporated where these will provide a useful output. The base station may be provided with some or all of the peripheral devices shown in Figure 2, such as a ^" printer 27, sound and signal recording devices 28, a status display 29 or other appropriate devices.

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