The present invention relates to a method and a fish farm for supplying air to fish with a physostome swim bladder, or physostomes such as e.g. salmonids, and to allow the fish to fill the swim bladder with air by swallowing air bubbles.

Salmon lice (Lepeophtheirus salmonis) are located in the upper water layer or lice belt in the sea, and lowering fish under this lice belt, will prevent infestation of salmon lice on the fish.

Fish with a physostome swim bladder, depend on refilling air to the swim bladder, which it naturally does by swallowing air at the water surface. Fish with a physostome swim bladder have an air channel connecting the swim bladder to the throat. This enables the fish to take up air to the swim bladder via the mouth and to release excess air the same way. The swim bladder is an air-filled bag allowing the fish control its buoyancy. The fish control the amount of air in the swim bladder by releasing or swallowing air. Allowing air from the swimming bladder to escape, enables the fish to rise rapidly towards the sea surface without the pressure drop causing the bladder to expand and harm the fish. Fish with a physostome swim bladder must thus have access to the water surface to swallow air to the bladder in order to maintain buoyancy. If the fish is unable to fill the swim bladder with air, then the swimming speed I and upwards tilt if the fish is increased (the fish swims with the head high), the appetite and growth is reduced, wounds and fin wear is more prominent, and the general fish welfare is reduced.

Submerged fish must refill the swim bladder by regularly accessing the water surface, and this may lead to exposure to lice or pathogens that are present in the upper water masses. Rearing fish in a way that exposes the fish to lice or pathogens is not ideal to ensure good fish welfare. The present invention thus seeks to supply air to the fish without having to provide access to air at the water surface, thus reducing or eliminating exposure to lice or pathogens that are present in the upper water masses.

Various submersible cage systems for use in the aquaculture industry have been developed. Common to these is that they try to alleviate the problem of salmon lice infestation and other environmental problems in the sea. There are also welfare challenges associated with submerged raring concepts.

Air access is a prerequisite for all submersible concepts as explained above. Today's submerged cage concepts solve this challenge by creating an artificial airfilled water surface, often in the form of a dome or another device. An artificial water surface filled with air has a considerable buoyancy and is therefore dependent on a heavy ballast for balance.

A device supplying compressed air, creating an ascending water flow has been practiced for many years and is thus well known as shown in patent NO343181 , aiming to move a large amount of water towards the water surface by releasing compressed air at depth.

Another technology described as “Midt Norsk Ringen” to improve the water quality in cages with skirts includes a pressurized aeriation ring where air bubbles are released from nozzles. The bubbles causes a waterflow and the water inside the cage circulates.

JP H03154698 suggests increasing the amount of dissolved oxygen in a water. An air diffusion apparatus performs aeration operation by injecting O2-containing gas. A rising stream is generated by the rising force of air bubbles, and 02 contained in air bubbles is dissolved in the rising stream to increase the amount of dissolved oxygen. The present invention is based on research made by the inventors showing that physostomes may be accustomed to fill their swim bladder from air bubbles of sufficient size.

The object of the invention is to allow physostomes to refill their swim bladder by means of air bubbles.

The method is intended to allow aquaculture cages I pens to be submerged without giving the physostomes access to the water surface. The method involves releasing air bubbles in a lower part of the pen or below the pen, and to allow the fish to swallow air from the bubbles to replenish air as needed. Through this simplified method, the invention aims to enable the fish to refill the swim bladder. Past solutions to solve the problem of allowing physostomes to replenish air to the swim bladder in submerged pens only involves providing av artificial water surface. This is opposed to solutions designed to provide air bubbles to create water circulation in fish farms. The object of the present solution is not to supply water at a different temperature, or to supply more oxygen-rich and lice-free water to the upper part of a breeding pen, but to provide air to the swimming bladder when the fish is not given access to a natural sea surface.

When the fish is located in a fish cage of a fish net, the bubbles may be generated below the cage. In closed fish pens, clearly the bubbles must be generated inside the fish pen.

Air bubble systems designed to release air that should carry with it as much water as possible to create a circulating flow are different from air bubble systems designed to release air for filling the swimming bladder.

The present invention differs significantly from these in that the method is based on making air bubbles which fish can swallow in order to be supplied with sufficient air in the swim bladder when the fish is not given access to a natural sea surface.

In a normal situation, physostomes fill the swim bladder by breaking the water surface with their head and back in a curved motion. However, physostomes from the smolt stage that go into air bubbles as the patent describes, will learn to replenish the swim bladder by swallowing supplied air bubbles under water. The fish will take this lesson with them in further stages. This is an approach that is based on completely new knowledge and is thus an innovation in the industry. As a result, the invention will provide increased value creation, and be able to benefit the entire aquaculture industry. Accordingly, the solution of the present invention utilizes this knowledge to provide the claimed system and method.

The method is used in fish farms where the fish does not have access to the water surface and in situations where the fish also have access to an artificial water surface I air pocket. The most important use of such a method and such a system is in submerged fish farms.

Accordingly, the present invention relates to a fish farming pen for rearing fish with a physostome swim bladder. The pen includes fish pen walls forming a confined submersible fish residence volume adapted to allow the submersible fish residence area to be submerged below a sea surface, whereby the fish is prevented from accessing the sea surface. An air releasing bubble generating element is adapted to provide bubbles to the confined submersible fish residence volume.

The air releasing bubble generating element may be located inside the confined submersible fish residence volume.

In the event the fish farming pen is an net cage, the air releasing bubble generating element may be located outside, typically below, the confined submersible fish residence volume.

The air releasing bubble generating element may be adapted to generate bubbles with a diameter of at least 20mm when they reach the fish to allow the fish with a physostome swim bladder to ingest air to its air bladder. The air releasing bubble generating element may be adapted to generate bubbles with a diameter of at least 100mm when they reach the fish to allow the fish with a physostome swim bladder to ingest air to its air bladder.

The fish pen walls forming a confined submersible fish residence volume may be formed as a net allowing circulation of water.

The fish pen walls may include a separate, releasable net portion forming a separate physical barrier preventing the fish from accessing the sea surface.

The fish farming pen may further include a releasable joint, securing the separate, releasable net portion forming the separate physical barrier.

The air releasing bubble generating element may be adapted to be supplied with compressed air from an air source through an air line.

The fish farming pen as described above may further including an air dome adapted to form an artificial water surface below the sea surface.

The fish farming pen may further include a monitoring system adapted to monitor parameters indicating if the swim bladders are sufficiently filled with air, a control unit connected to the monitoring system, and an air source controlled by the control unit, the air source being connected to an air releasing bubble generating element to generate bubbles if the swim bladders are insufficiently filled with air.

The monitoring system may include a camera monitoring swimming speed and fish tilt when swimming and thus if the fish requires air to their swim bladder if the fish at least one of tilts and swims with their heads high and swims at a higher velocity.

The monitoring system may include an echosounder indicating if the swimming bladder is filled with air. Furthermore, the present invention relates to a method of rearing fish with a physostome swim bladder in a submerged fish pen without access to a natural sea surface. The method includes the steps of generating air bubbles at a lower part of the submerged fish pen or below a submerged fish cage, whereby the fish with a physostome swim bladder is enabled to replenish air to their swim bladder from said air bubbles.

The air bubbles may be generated only 1 ,5-3 hours a day.

The method may include monitoring the fish with monitoring system to monitor if the fish requires air to their swim bladder, feeding information from the monitoring system to a control unit, and controlling an air source connected an air releasing bubble generating element to generate bubbles if the monitoring system indicated that the fish requires air.

The method may further include generating bubbles if the fish at least one of tilts and swims with their heads high and swims at a higher velocity, and the monitoring system includes a camera monitoring swimming speed and fish tilt when swimming.

The method may further include generating bubbles if an echosounder indicate that the swimming bladders are insufficiently is filled with air, wherein the monitoring system includes the echosounder indicating if the swimming bladder is filled with air.

The method may further include the step of intermittently providing a surface access in addition to the air bubbles for up to three days to make the salmon accustomed to interact with air-bubbles while the air releasing bubble generating element creates air bubbles in the lower part of a submerged farming pen or below the submerged fish cage, and where a specified amount of compressed air is sufficient for the fish to learn to refill the swim bladder at the same time as it prevents the fish from getting too little air by immersing the farming pen removing access to a natural water surface. The surface access may be to an artificial water surface formed in an air pocket of the submerged farming pen.

Short description og the enclosed figures:

Fig. 1 is a cross section of a fish with a physostome swim bladder, in a tilted position;

Fig. 2 is schematic representation, in cross section, of a fish pen of the invention in a first embodiment; and

Fig. 3 is schematic representation, in cross section, of a fish pen of the invention in a second embodiment.

Detailed description of the invention with reference to the enclosed figures: Figure 1 is an illustration of the internal anatomy of a fish 1 (type of trout) with a physostome swim bladder 4 in the family of physostomes. The figure shows fish 1 with an air channel 3 which connects the swim bladder 4 to the pharynx 2. This enables the fish 1 to swallow and ingest air into the swim bladder 4 via the mouth. Fish 1 with a physostome swim bladder will thus be able to supply a sufficient amount of air to the swim bladder 4 by swallowing air bubbles 5 created by air releasing bubble generating element 9. The fish 1 is shown with a centerline T tilted at an angle with the head high in relation to a horizontal line H to indicate a swimming angle of the fish 1 when the swimming bladder 4 is insufficiently filled with air. The swimming angle and swimming speed can be monitored to indicate if the swimming bladder 4 is sufficiently filled with air. An echosounder can also be used to indicate if the swimming bladder 4 is sufficiently filled with air.

As the fish 1 is shut off from the water surface with a physical barrier, it will over time learns to swallow air bubbles 5 created by an air releasing bubble generating element through compressed air in a submerged farming pen, without access to a natural water surface for refilling air in the swim bladder 4.

To learn the fish to swallow air bubbles 5 created by an air releasing bubble generating element in a submerged farming pen, it is also possible to provide access to an artificial water surface I air pocket inside the pen to allow the fish to refill air in the swim bladder 4 until it is accustomed to refill air in the swim bladder 4 from the air bubbles. Three days have been found to be sufficient for this purpose.

Figure 2 shows a submerged breeding pen or cage 6. Compressed air through air line 7 from air source 8 will be released through a hose into an air releasing bubble generating element 9. The air line 7 is typically a flexible hose or a rigid pipe. The air releasing bubble generating element 9 typically includes a perforated hose or pipe with air releasing bores typically with an 8mm diameter. The air releasing bubble generating element 9 may also include a mechanical element allowing air to be trapped into a volume before the volume of air is released, thus intermittently generating large bubbles 1 . The air source 8 is typically an air compressor compressing ambient air. This air releasing bubble generating element 9 creates air bubbles 5 with air that will rise towards the water surface. The air bubbles 5 are of a size that are sufficient to enable the fish 1 to swallow air for the swimming bladder. Such air bubbles are larger than air bubbles normally used to cause circulation to maintain the water quality within certain limits. The fish 1 will swim into the ascending bubble stream and swallow air bubbles 5. The fish will then be able to replenish air in the swim bladder without surfacing as otherwise normal. The fish 1 is prevented from reaching the water surface with a physical barrier 10 located at a depth sufficient to keep the fish away from parasites, particularly salmon lice. The fish are thus not exposed to the upper water masses which contain larger amounts of lice larvae and any other unfavorable environmental conditions that can be harmful to the fish. The air bubbles 5 released through the air releasing bubble generating element 9, enables the fish to maintain good health, growth and fish welfare without having to surface. This means that the fish 1 can be submerged during the production period in environmentally favorable conditions.

The physical barrier 10 at least include a portion that allows the compressed air through air line 7 from air source 8 to be released through the hose into the air releasing bubble generating element 9 to escape to the surface to prevent creation of an air pocket under the physical barrier 10.

A fish monitoring system 20 with a control unit 21 and a monitoring system 22 such as a camera system is connected to the air source 8 to monitor the fish and to ensure that air bubbles are generated if the fish behavior indicates that the fish 1 requires fish bubbles. Submergence of physostomes without access to air results in negative buoyancy over time, amongst other things leading to increased swimming speed and tilted swimming with the head upwards to compensate negative buoyancy. This behavior can be monitored, and necessary action may be taken by supplying air. The control unit may automatically monitor the fish and supply air as required. The monitoring system 22 can alternatively or additionally include an echosounder as echosounders can indicate the amount of air in the swim bladder and provide appropriate signals to the control unit.

Investigations have shown that submergence without swim-bladder filling extending three weeks have resulted in negative effects of reduced appetite and growth, fin and snout damage and vertebra compression from tilted swimming, and it is concluded that submerged salmon should be provided access to air for swim bladder re-filling. To ensure fish welfare, tests have shown that supplying air at an interval of 1 ,5-3 hours a day is sufficient.

Figure 3 shows a submerged farming pen 6 with an artificial water surface 16 formed in an air pocket 11 . In the same way as the submerged cage in figure 2, the fish 1 is closed off from the water surface with a physical barrier 10. The air pocket 11 forming artificial water surface 16 below the sea surface 15 is supplied with air through a separate air line 17 from the air source 8. and air bubbles 5 through air releasing bubble generating element 9. The artificial water surface I air pocket 11 is what is traditionally used in submerged farming pens 6. But this is a more challenging construction compared to compressed air 4 which through an air releasing bubble generating element 9 will create air bubbles 5.

The size and number of air releasing bubble generating elements 9 will vary depending on the size of the cage 6, the amount of biomass and the number of fish 1 . The location should be towards the bottom of the cage so that fish 1 will have plenty of space to swallow air bubbles 5 that will rise upwards due to the buoyancy of air in water. Compressed air comes through air line 7 from air source 8 and the air is transported by hose down to the air releasing bubble generating element 9. It is important to position the air releasing bubble generating element 9 so that the air bubbles 5 do not come into conflict with other elements in the cage 6 such as elements for feeding.

The air bubble diameter when passing the fish should preferably be greater than 20mm, even more preferably greater than 50mm and is typically up to 250mm.