System and method for oxygenating a fish cage

Abstract

System for oxygenating and circulating water in a fish cage, the system is comprising a device for removal of bottom waste at the bottom of the fish cage, wherein the device comprises a collector (5) arranged at a bottom of the fish cage, a tube (6) for transporting bottom waste from the collector to a receiving section, and a pump. The system further comprises a pipe (7) for compressed gas containing oxygen fastened to the tube for removal of waste, the pipe is connected to the tube below the collector and arranged to flow the gas into the fish cage through the tube, when the tube is arranged outside of the fish cage, or the pipe is arranged to flow the gas directly into the fish cage when the tube is running inside the fish cage.

Description

[0001]The present invention relates to a system method for oxygenating and circulating water in a fish cage, according to the preamble of the independent patent claims. Background Aquaculture and farming of fish has grown to be a large industry in many countries, wherein Norway is the largest producer of farmed fish in the world, and in 2020 about 1,5 million tons fish was farmed and sold. Fish farmed along the Norwegian coast are exported to about 100 countries, and are the second largest export industry in Norway. The industry as a whole and specifically the handling of living fish is strongly regulated by the government, and the welfare of the fish is ensured from hatchery to slaughtery. Most fish are still farmed in a traditional fish cage in the sea, having a floating collar on the surface of the sea, and bag stretching downwards from the floating collar. The cage may be an open cage having a bag of net, a closed cage having a bag of waterproof material such as tarpaulin or of stiff material such as fibreglass or even concrete. The cage may further be semi-closed meaning that the bag is partly of a material preventing water to flow through, and partly of a material allowing water to flow into the cage. A cage provided with a "skirt" of a material preventing water from flowing into the cage will also be considered as a semi-closed cage in relation to the present invention. The "skirt" may either be in addition to the regular bag or be integrated in the material of the bag, and is normally arranged from the top of the fish cage to a depth where the number of parasites such as salmon lice, is considerably reduced. The bag may be referred to as "cage bag" or "fish bag" in relation to this application. When the fish cage is used for farming fish, fish feed will be added and waste i.a. from the feeding, excess feeding and faeces from the fish will occur. The faeces and excess feeding are nutrient rich waste which may disturb the environment on the seabed under the fish cage, and should be removed. During normal farming, some of the fish dies even in healthy and well operated fish plants. Dead fish, faeces, uneaten feed and other waste generated in the fish cage, will fall to the bottom of the cage due to sedimentation. This waste will in the following be referred to as "bottom waste". The bottom waste should be removed to maintain a healthy environment for the fish and to avoid disturbing the environment of the fish cage. Another problem is that dead fish may attract predators which will attack and possibly damage the net cage to get hold of both healthy and dead fish. The bottom of a fish cage is normally designed as a cone, having a centre lower than the lower part of the walls of the cage. When bottom waste as described above is falling to the bottom, it will continue moving along the bottom of the cage, and be collected at the centre. The bottom of the cage may be made of a material preventing water to flow through or a material allowing water to flow through. As bottom waste will move along the bottom, it may be an advantage that at least parts of the material of the bottom prevents waste from falling through. According to Norwegian regulations, the fish death rate must be checked and dead fish must be removed at least once a day. Whether the waste should be removed more often depends on a number of factors such as the design of the fish cage, the fish mortality, the weather and water current conditions at the location. How to calculate the optimal time period for removal of bottom waste is well known to a skilled person. Several systems are developed to remove dead fish and waste accumulated together with it, such as the Liftup system from Lift up AS. A main feature is use of an airlift pump close to a collector arranged at the bottom of the net cage. The airlift pump comprises a discharge tube running in the fish cage from the collector to the surface, and a pipe injecting compressed air into the discharge tube. As the compressed air rises to the surface in the tube it will create a flow of water and any dead fish and other waste on the collector, will be drawn into the tube and discharged with the flow of water and air. From prior art is is also known systems removing bottom waste by a discharge tube running through the bottom of the fish bag, below the fish cage and rise to the surface outside of the cage. This may also be an airlift pump as described above, and the pipe injecting compressed air into the discharge tube will be arranged at the rising part of the tube, still creating a flow of water through the whole tube. Depending on the design and management of the fish cage, oxygen may need to be added in order to ensure fish welfare. This may normally be the case for closed or semi-closed cages, but there may also be situations and times wherein addition of oxygen would be advantageous in open cages. Several solutions for addition of oxygen are well known for instance from WO2018097736 describing a circular ring equipped with a number of nozzles being supplied with air. The ring should be submersed into the fish cage to create a flow of air bubbles. In closed and semi-closed cages the water is often given a forced movement to ensure sufficiently replacement of the water, removal of any particulate matter in the water, and to improve the fish welfare. This forced movement may be caused by introducing and / or removing water at given angle, or by pumps inserted in the water in the bag. In addition to the systems mentioned above, a number of devices, cameras, sensors etc are installed in the fish cage, to monitor the quality of the water and the fish welfare. All these devices are moored with lines and ropes and supplied with cables for power etc. The design and mooring of the different parts of the fish cage itself also comprises a number of lines and ropes running inside and outside of the fish bag. When the weather is rough and / or the water current is strong, the fish cage and the bag may move and be slightly temporarily deformed, and all the devices, lines, ropes and tubes being in, and adjacent to, the bag may become temporarily displaced in relation to the bag and to each other. This may cause a problem if they are not sufficiently flexible, as they can get entangled in each other, or get in touch with the bag. A general concern related to fish cages is the risk of rupture of the fish bag with subsequent escape of fish, and it is a general understanding that the amount of devices installed in the fish bag, and the number of lines, tubes etc running in the fish bag should be reduced. The invention The problems mentioned above is solved by a system and a method according to the independent claims. Further advantageous features are stated in the corresponding dependent claims. The invention relates to a system for oxygenating and circulating water in a fish cage, the system comprises a device for removal of bottom waste at a bottom of the fish cage. The device comprises a collector arranged at the bottom of the fish cage, a tube for transporting the bottom waste from the collector to a receiving section, and a pump. The system of the invention further comprises a pipe for compressed gas containing oxygen. The pipe is arranged to flow gas into the fish cage through the tube, when the tube is arranged outside of the fish cage. Alternatively, the pipe is fastened to the tube or collector for removal of bottom waste and arranged to flow gas directly into the fish cage close to the bottom of the fish cage when the tube is arranged inside the fish cage. By "pump" it should be understood any device creating a flow in the tube. Such device may be a vacuum pump arranged at the surface of the water, an airlift pump injecting air in the tube, an ejector pump ejecting water into the tube, or any similar device. A collector may be any device suitable for collecting dead fish, debris and other waste falling to the bottom of the fish cage. The collector may be shaped as a flat or concave plate or a cone corresponding a cone of the bottom of the fish cage. The collector may be designed with a flat or concave face turning upward for collecting bottom waste and a cone turning downwards for fitting in the cone of the fish cage. The collector should preferably be made of a solid material preventing any waste from falling through the collector and out of the fish cage. The collector may comprise a housing wherein the tube for removal of bottom waste is fastened, and whereby an inlet to the tube will be correctly positioned to the collector. Further, the collector may preferably be heavy either by choice of material and design, or by comprising weights, to assist the tensioning of the fish bag. These are well known features of a device for removal of bottom waste. The collector may for instance be the bottom gyro of a fish cage. The tube for transporting the bottom waste from the collector to the receiving section may also be referred to as "discharge tube". The tube may be arranged inside the fish cage, rising continuously upwards from an inlet by the collector to the receiving section arranged at the surface. The inlet of the tube is at, or close to the collector and may be integrated in a housing of the collector. The pipe for compressed gas may be fastened to a lower part of the tube, in such a way that an outlet of the pipe will be adjacent to the collector, or fastened to the collector, such as a housing of the collector. The pipe for gas is arranged to flow the gas into the fish cage directly from an outlet of the pipe, and as the pipe is fastened to a lower part of the tube, or the collector, the pipe is arranged to flow the gas directly into the fish cage close to the collector. The expression “to flow the gas directly into the fish cage” should be understood to include directly from the outlet of the pipe whether or not a nozzle is arranged at the outlet. By fastening the pipe for compressed air to the discharge tube, and arranging the pipe to flow gas close to the bottom of the fish cage, the number of pipes and tubes which must be installed in the fish cage will be reduced. This will ensure easier installation as well as reducing the risk for damage to the equipment and fish cage. When the collector comprises a housing as described above, the pipe for compressed air may be connected to an inside or outside of the housing. When the pipe for compressed air is connected to an inside of the housing, the pipe must be arranged in such a way that the air flows out of the housing and into the fish cage. This may be performed by a nozzle or by inserting the pipe at an angle away from the inlet to the discharge tube. When the bottom of the fish cage is designed as a cone as described above, the collector should be arranged at the tip of the cone. Since the pipe is arranged to flow the gas directly from the outlet of the pipe, and the pipe is fastened to the collector and / or a lower part of the tube in such a way that the outlet of the pipe will be adjacent to the collector, the gas will be introduced close to the bottom of the fish cage. The discharge tube may be arranged outside of the fish bag, connected to the collector through the fish bag, usually through the bottom of the fish bag below the collector. The tube may have a section running downwards from the collector before it rises to the surface. In such case the pipe for compressed gas is arranged to flow the gas into the fish bag through the tube, meaning that the pipe is arranged to flow gas into the section of the tube running downwards from the fish bag. By introducing gas in this part of the tube, the gas will rise in the tube and flow out of the collector, possibly through the openings normally being used for removing bottom waste, and into the fish cage. In this case no cables, lines, ropes, pipes or the similar will be arranged inside the fish bag. By "compressed gas containing oxygen" it is herein meant any gas or mixture of gases suitable for increasing the amount of oxygen in the water, without reducing the fish welfare. An example of such gas would be air, for instance air ambient to the pump, oxygen or a mixture of these. When gas containing oxygen is introduced into the cage according to the system of the invention, it will rise from the bottom of the cage either directly from the pipe, from a housing on the collector or through the discharge tube. Water being close to the gas flow will rise together with the gas, and this will create a flow of water and gas from the bottom of the fish bag to the surface, in the centre of the fish bag. In case the fish bag is a closed bag, this flow will create a circulating flow of the water, upwards in the centre, out towards the floating collar at the top, down along the walls, and towards the centre at the bottom. The flow along the bottom will also bring along any bottom waste being collected at the bottom towards the collector being arranged at the centre. In case the bag is open or semi-closed, a similar circulating flow of water will occur, however, the uprising flow in the centre will increase a flow of water from the surroundings, through the lower part of the cage and into the cage. The replacement of water inside the cage will thereby increase. By installing a system according to the invention, bottom waste may be removed by activating the device for removal of waste, the water in the bag of the fish cage may be oxygenated by introducing gas containing oxygen, and the water will be given a forced movement. All these effects will be achieved by one system, which substantially reduces the amount of equipment and thereto belonging ropes, lines, cables etc, which needs to be installed in a fish cage. As explained above, this reduces among others the risk of rupture of the fish cage. By "bottom waste" it is herein meant dead fish, faeces, uneaten feed and any other particulate waste as defined above. Even parts of a fish, including scales and fins should be regarded as bottom waste when it falls to the bottom of the fish bag. The device for removal of bottom waste may be any known device comprising a collector for collecting the bottom waste at the bottom of the fish bag, and a tube for removal of the bottom waste. The tube runs from the collector to a receiving section, the receiving section may be on a boat or barge, possibly a barge being a common for several fish cages or plants. The receiving section is normally at the surface of the water, and the tube will thus be running upwards. The collector is arranged at the bottom of the fish bag, meaning at the lowest point. If the bottom is designed as a cone, the collector should be at the tip. Any bottom waste will be collected at the lowest point due to gravitation, and it is well known to a skilled person to arranged a collector at this point. When gas is flowing directly out of the pipe, the buoyance of the system will be increased, and the system may comprise a weight to counteract forces acting to lift the fish bag. The weight may be a part of the collector, a part of weights used to keep the fish cage tensioned, a part of the mooring of the cage, or a separate weight added to the bottom of the bag. The device for removal of bottom waste may be an airlift pump, comprising a second pipe for compressed gas used to remove the bottom waste. The second pipe for compressed gas may be referred to as airlift-pipe. An airlift-pipe used to remove bottom waste in a fish cage is well known for a skilled person and thus not described here any further. In the system according to the invention, the compressed gas containing oxygen will enter the fish cage close to or through the collector for collecting bottom waste, and the flow of gas may be stopped at a given time before the bottom waste is to be removed, in order to allow the waste to collect on the collector. The given time will depend on a number of factors such as the design of the cage bag, the type of bottom waste, and whether the gas is introduced close to or through the collector. It will be obvious to a skilled person how to calculate the optimal time for stopping the introduction of gas before the bottom waste is removed. The system may comprise a controller for operating the pump removing bottom waste from the fish cage, and the pump for adding gas to the pipe. The controller may be one or more programmable controllers, manual switches or combinations. When the device for removing bottom waste is an airlift pump, the pump for removing bottom waste and the pump for adding gas to cage via the pipe may be the same pump, and the controller may be arranged to select which pipe the air should be supplied to. The invention further relates to a method for oxygenating and circulating water in a fish cage by using a system as described above. The method comprises the following steps - stopping the pump removing bottom waste from the bottom of the fish cage, - starting the pump introducing a gas containing oxygen into the pipe, - allowing the gas to flow into the fish cage, close to the bottom of the cage, - allowing a water flow upwards in the centre of the cage to be established. When the water in the cage is sufficiently oxygenated, or bottom waste should be removed, the pump introducing gas containing oxygen into the pipe should be stopped. After the introduction of gas has been stopped, the circulation of water will still continue due to the ongoing motions of the water, but it will slowly be reduced until it stops. The time from stopping the pump until the circulation of water stops, depends on several factors, such as the design of the cage and the rate of the water flow before the pump is stopped, but also the weather conditions and water current at the localization of the fish cage. Removal of bottom waste may be performed as soon as the pump for introducing gas containing oxygen has been stopped. The removal of bottom waste may be performed in a short time, such as 1-5 minutes, and then the pump for introducing gas may be started again. This means that the circulation of water in the cage may be continuous even during removal of bottom waste. While adding gas to the bottom of the fish cage, the collecting of bottom waste at the centre of the collector may be disrupted. Removal of bottom waste may therefore be delayed 5-10 minutes after the pump for introducing gas has been stopped. Bottom waste including dead fish will then be collected at the collector and may be removed by the discharge tube before the addition of compressed gas containing oxygen is resumed. In the description relative terms such as front, top, centre, bottom, side, lower, upper, downward, upward, outward, sideward, vertical, and horizontal etc. are all related to a fish cage having a bag as described above in an installed position in the water i.e. when mounted and ready for use. Reference throughout the specification to “one embodiment” or “an embodiment”, "one aspect" or "an aspect" means that a particular feature, structure, or characteristic described in connection with an embodiment and / or aspect, is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics described may be combined in any suitable manner in one or more embodiments. The invention will in the following be described with an example given to illustrate the invention, and should not be used to interpret the invention, as it is defined in the enclosed claims, restrictive. Example The invention will in the following be described with the help of the enclosed Figures, showing preferred embodiments of a fish cage having the system according to the invention installed. The different parts of the figures are not necessarily to scale in relation to each other, as the figure is merely for illustrating the invention. The invention will in the following be described with reference to the enclosed Figures, where Fig.1 shows a fish cage having a system according to the invention, Fig.2 shows details of the system shown in Fig.1, Fig.3 shows a fish cage having another embodiment of system according to the invention, Fig.4 shows details of the system shown in Fig.3. In the figures it is shown an open fish cage having a floating collar 1 at the surface of the water, a bag 2 stretching down from the collar 1, and weights 3, sinker tube 4 are arranged to keep the bag tensioned and expanded. Moorings (not shown) may also be used to keep the bag tensioned and expanded. The fish to be farmed is contained in the bag. A system for removal of bottom waste is installed in the bag, having a collector 5 arranged at the bottom of the fish cage, a tube 6 for transporting the bottom waste to a receiving section (not shown), and a pump. The pump is an airlift pump introducing air in the discharge tube 6 though an airlift pipe 7. In Figure 1 and 2 the tube 6 is arranged on the outside of the fish bag 2, and connected to the collector 5 through the bottom of the bag. The tube 6 has a section 6a running partly downwards before it rises in a section 6b to the surface. Any bottom waste, including dead fish, being collected on the collector 5 will thus be removed vertically downwards from the collector 5 and transported to the receiving section on the surface of the water in the tube 6. In the shown embodiment the pump being used to remove the bottom waste is an air-lift pump comprising an airlift pipe 7 for compressed air. The airlift pipe 7 is arranged at the rising section 6b of the tube, as indicated in Figure 1. When bottom waste should be removed, air is injected through the airlift pipe 7, and as the air rises to the surface through the tube it will create a suction in the remaining part of the tube and thus remove the waste on the collector. Further, a first pipe 7 for compressed gas containing oxygen, such as air, is fastened to the section 6a of the tube running partly downwards, and arranged to flow the gas into the fish cage through the tube 6. The gas introduced in this part of the tube will not flow in the tube towards the surface of the water, but rather flow in the tube, through the collector 5 and into the fish bag 2 before it rises to the surface. In this way a flow of gas will rise in the fish cage, and create an upwelling of water in the centre of the cage. In the shown embodiment the gas rises through the holes of the collector being used to remove bottom waste. By introducing gas through the collector, an uprising force will act on the collector and weights 3 must be added to prevent the collector 5 and bottom of the cage to rise. In the shown embodiment, lines 8 are fastened to the bottom of the cage, guided through a collar 9 with holes on the tube 6, and attached to a bottom weight 3. The collar 9 will centre the bottom weight 3. By the embodiment shown in Figures 1 and 2, no tubes, pipes, cables or moorings are arranged inside the bag of the cage, yet the water in the cage will be oxygenated and circulated, while the bottom waste including dead fish are removed. Figure 3 and 4 shows an embodiment wherein the discharge tube 6 is arranged inside the fish bag. Any bottom waste being collected on the collector 5 will thus be removed upwards from the collector 5 and transported to the receiving section on the surface of the water (not shown). In the shown embodiment the collector 5 is the bottom gyro of the fish cage, and comprises a housing 10 arranged thereon. The inlet 12 of the tube 6 is integrated in the housing, and arranged close to the collector. The housing 10 and lower part of the tube 6 is provided with installation handles 13, to ease the installation of the system at the bottom of the fish cage. As said above, the pump to remove the bottom waste in the embodiment of Figure 3 and 4, is an air-lift pump, and in the embodiment shown in Figure 4, the airlift pipe will be fastened to a nipple 11 on the rising part of the tube 6 during installation. Any gas flowing into the tube through the nipple 11, will rise to the surface through the tube. When bottom waste should be removed, air is injected through the nipple 11, creating a suction through the tube 6 and thereby remove any waste on the collector. Further, a first pipe 7 for compressed gas containing oxygen is fastened to the housing 10 of the collector 5, by means of another nipple 14. The gas will flow into the housing and the nipple is arranged to flow the gas out of the inlet of the discharge tube and directly into the fish bag 2. The gas introduced by the pipe 7 when attached to nipple 14 will not flow through the housing and into the tube 6, but rather flow out of the housing and directly into the fish bag 2. In this way a flow of gas will rise in the fish cage, and create an upwelling of water in the centre of the cage. As the first pipe 7 is fastened to the housing 10 of the collector, an uprising force will act on the collector 5 and weights 3 must be added to prevent the collector 5 and bottom of the cage to rise.

Claims

Patent claims 1. System for oxygenating and circulating water in a fish cage, the system is comprising - a device for removal of bottom waste at the bottom of the fish cage, the device comprises - a collector (5) arranged at a bottom of the fish cage, - a tube (6) for transporting bottom waste from the collector to a receiving section, and - a pump characterized in that the system further comprises - a pipe (7) for compressed gas containing oxygen fastened to the tube for removal of waste, - the pipe (7) is connected to the tube (6) below the collector (5) and arranged to flow the gas into the fish cage through the tube (6), when the tube is arranged outside of the fish cage, or - the pipe (7) is arranged to flow the gas directly into the fish cage close to the bottom of the fish cage when the tube (6) is running inside the fish cage.

2. System according to claim 1, wherein the pipe (7) is fastened to the lower part of the tube (6) or the collector (5) when the tube (6) is running inside the fish cage.

3. System according to claim 1 or 2, wherein the pipe (7) is arranged to flow the gas directly from an outlet of the pipe into the fish cage when the tube (6) is running inside the fish cage.

4. System according to any one of the preceding claims, wherein the collector comprises a housing (10), the pipe (7) for compressed air is connected to an inside or outside of the housing, and arranged to flow gas directly into the fish cage when the tube (6) is running inside the fish cage.

5. System according any one of the preceding claims, wherein the pump is an airlift pump comprising an airlift pipe for compressed air.

6. System according to claim any one of the preceding claims, wherein the system further comprises a controller selecting which pipe the air is supplied to.

7. System according to claim 6, wherein the controller is programmable.

8. Method for oxygenating a fish cage, by using a system according to any one of claims 1-7, wherein the method comprises the following steps - stopping the pump removing bottom waste from the bottom of the fish cage, - starting the pump introducing a gas containing oxygen into the pipe, - allowing the gas to flow into the fish cage, close to the bottom of the cage, - allowing a water flow upwards in the centre of the cage to be established.

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