EP4618751A1

EP4618751A1 - Fish farming cage utilizing live biomass as driving force for water exchange

Info

Publication number: EP4618751A1
Application number: EP23892104.3A
Authority: EP
Inventors: Alf Reidar Sandstad
Original assignee: Sfs Group AS
Current assignee: Sfs Group AS
Priority date: 2022-11-14
Filing date: 2023-11-13
Publication date: 2025-09-24
Also published as: CL2025001396A1; WO2024107055A1; AU2023381542A1; NO20231233A1; NO348687B1; AU2023381542B2

Abstract

A fish cage (6; 7; 700) for farming of fish (2), the fish cage comprises one of a water-tight outer wall (61; 71, 781) and a close to water-tight outer wall and the outer wall is form stable, an entrance (65; 75) for water into the fish cage and an outlet (67; 77; 777) for water out of the fish cage at a lower portion (69; 79; 7819) The fish cage is adapted for an active water flow (3) from the entrance via an inlet (46) through a fish compartment (99) to an outlet (67; 77; 777), and the fish cage is adapted to keep a shoal of fish (21) with a negative buoyancy in a region within the water flow, such that the shoal of fish (21) creates the water flow. A method for exchanging water in the fish cage by utilizing the fish is also described.

Description

FISH FARMING CAGE UTILIZING LIVE BIOMASS AS DRIVING FORCE FOR WATER EXCHANGE

The present invention concerns farming of fish within a fish cage. More particularly the invention concerns a fish cage that utilize fish with a different density than the water, and fish swimming in circles, to establish and drive a water flow.

Raising fish in captivity for food is well known and has a long history. The development has been from raising fish in earth ponds and confinements made of net which has been placed in rivers, lakes, and sea water. Confinements made of net is widely used in commercial fish farming. Typically, a floating fish pen comprises a confinement made of a net to form sidewalls and a bottom and a buoyancy body which circumference the net to keep the fish pen floating in the surface. The fish pen may also comprise a walkway around the net and a handrail. The handrail may support a jumping net that extends from the water surface to the handrail to block fish from jumping out of the fish pen. An advantage with a confinement made of a net is that water may flow freely through the fish cage.

The fish need oxygen, and fresh water carries oxygen. Water may carry harmful organisms such as fish parasites and toxic algae. It is known to position skirt around a fish pen to avoid that water with harmful organisms flow through the fish pen. The skirt may be a fine meshed net or a water-tight tarpaulin. Although this may protect the fish within the fish pen from parasites and algae, it reduces the water flow and the oxygen content in the water within the pen to unacceptable low levels. Oxygen or air may be added to the water within the fish pen to compensate for this. Alternatively, oxygen rich water may be pumped into the fish pen by a propeller, an impeller or similar, or by air lift.

A semi-closed fish pen has a permanent water-tight upper wall and an open lower part formed by a net. The lower portion of the wall may be formed by the net, and the bottom is formed by a net. The water-tight upper wall may be formed by a cloth such as a tarpaulin, or the water-tight upper wall may be formed by a rigid material such as fibreglass or steel. To maintain fish health and sufficient oxygen, water is pumped into the upper part of the fish pen either laterally into the fish pen or vertically up to the upper part through one or several pipes within or on the outside of the fish pen. The pipes have their inlet below the water-tight upper wall.

A closed fish pen has a permanent water-tight wall and bottom. To maintain fish health and sufficient oxygen, sea water is pumped laterally into the upper part of the fish pen. Water is supplied through one or several external pipes with the inlet below the bottom of the cage, as not to pump parasites into the fish pen. The water-tight wall may be made of a cloth such as a tarpaulin. The hydrostatic pressure inside the water-tight confinement is slightly above the surrounding hydrostatic pressure to maintain the shape of the tarpaulin, i.e., the water level within the fish pen is slightly higher than the water level outside the fish pen. The water-tight wall may be made of a rigid material such as fibreglass or steel. Water flows out of the fish pen through openings that may be positioned on the wall or at the bottom. The amount of water flowing in equals the amount of water flowing out.

Fish require oxygen rich water to grow and thrive. Traditional pumps for this purpose comprise a propeller or impeller within a pump hose, an inlet pipe and an outlet pipe. The inlet pipe is fed with surrounding deep water and preferably with water without parasites. In the pipes or in the pen, the water is supplemented with additional oxygen. The outlet pipe may be provided with a nozzle to create a circular water flow within the fish pen as some fish species prefer to swim against the local current. The sea water pumped into the pen provide a minor portion of the oxygen the fish need. The major portion is provided by adding oxygen to the water. This requires equipment and power use, which add cost and risk in case of power or equipment failure.

Known commercial closed fish pens typically have a volume of 30,000 m³. Stocking density of fish is at a maximum 50 kg fish per m³ of water. Maximum density is rarely reached. For purpose of calculation 40 kg fish/m³ is realistic. The fish is supplied with oxygen through a combination of pumping a limited amount of fresh oxygen rich water into the pen and adding extra oxygen to the water. Commercial closed fish pens typically have four to eight inlet pipes each provided with a pump. Typical flow speed of water within the inlet pipes is of 3 m/s. Total pumping capacity is of 6-10 m³/s. At a pumping rate of 10 m³/s in a cage of 30,000 m³ stocked with 40 kg fish/m³, the amount of water pumped through the pen is typically 0.5 liter/kg fish/minute. A rule of thumb is that Atlantic salmon needs 5 liter/kg fish/minute with fresh sea water to thrive without oxygenation. If the oxygen level in the water is low due to various reasons, more than 5 liter/kg/minute is needed. The commercial closed fish pen provides around 1/10 of the needed water ex- change and compensate with adding extra oxygen to the water. This requires equipment and power use, which adds cost and risk in case of power or equipment failure.

Closed fish pens aim to collect sludge by sedimentation and pump the wastewater from the bottom of the pen to the surface for treatment and sludge deposit. Such sludge collection requires a limited water flow for the sedimentation.

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art.

The object is achieved through features, which are specified in the description below and in the claims that follow.

In the description the term fish pen and the term fish cage have the same meaning. A semi-closed fish cage as described herein, has a water-tight outer wall or close to watertight outer wall. A portion of the bottom is covered by a net. Portions of the lower portion of the outer wall may be covered by a net.

More specifically, one aim of the invention is to avoid sea lice infestation in a fish cage while also having sufficient water exchange to avoid oxygenation of the water within the fish cage in daily operation.

The invention will first be described in relation to known theory and with reference to salmonid fish (Salmo spp., Oncorhynchus spp .). Most fish at most depths will have a negative buoyancy. The invention is therefore referencing fish with different density from the water as fish with negative buoyancy. Salmonoid fish have negative buoyancy except close to the surface and with a fully filled swim bladder.

For fish, buoyancy is a fundamental driver of vertical distribution and swimming energetics. Fish have evolved several mechanisms to decrease body density, which include reduced muscle and bone mass, increased lipid levels, and the development of an internal gas filled sac, the swim bladder.

Many fish species have a swim bladder. Salmonoid fish have a swim bladder as well. The specific density of farmed Atlantic salmon (Salmo salar) with empty swim bladder is typically between 1043 and 1065 kg/m³.

Full strength sea water has a density of approximately 1025 kg/m³ (psw). The swim bladder constitutes approximately 5% of the body volume of a live farmed salmonoid fish. The pressure at the water surface is approximately 1 atm, and at a water depth of 10 m, the pressure is approximately 2 atm.

Boyle's law: P₁V₁=P₂\/₂

According to Boyle's law the volume of the swim bladder of a salmonoid fish at 10 m depth is therefore reduced to approximately 2.5% of the body volume. The loss in volume corresponds to 25 ml/dm³ body volume. In other words, a fish of 1 kg will experience a negative buoyancy of approximately 25 g at 10 m depth. The individual fish compensates for this by on average swimming with an ascending angle.

A standing stock or biomass of 100,000 kg (100 metric tons) of fish that on average swim at 10 m depth within a fish pen, will collectively experience a negative buoyancy of 2,500 kg (2.5 tons). Correspondingly, a fish biomass of 1000 metric tons will experience a negative buoyancy of 25 tons.

Fish in a fish cage typically swim in circles. An object moving in circles have a centripetal acceleration and is affected by a centripetal force. The centripetal force in turn contributes to a water flow created by the live biomass. A skilled person will understand that not every fish within the cage need to swim in circle in order to create water flow as long as the biomass in sum swim in a circle.

A fish swimming in a circle will have a centripetal acceleration of ac=v²/r. The centripetal force will be F=mac.

For a standing stock or biomass of 1,000 metric tons made up by 200,000 fish with individual weight of 5 kg, and which on average swim with a speed of 0.5 m/s at the radius of gyration, the centripetal force will be 17 kN.

Knowing depth and radii of the fish cage we translate the force to pressure by Bernoulli's equation: p + 0.5p_swv² + p_swgh = constant; (v=(2p/p_Sw)⁰'⁵).

The fish is assumed stocked in a cage according to figure 4 with fish in an inner compartment. Assuming the outer radius is 23 m, inner radius 20 m and depth is 20 m. The velocity potential will equal 0.6 m/s. The water flow will equal the velocity times the inlet area, as long as outlet area > inlet area. The water flow is then 244 m³/s. A rule of thumb says that farmed salmon needs 5 liter oxygen rich water /kg fish/min to grow. The example will yield approximately 14.6 liter/kg/min, but some of the water flow will be recirculated oxygen depleted sea water due to the proximity of the entrance and the outlet. Expecting a 20% recirculation the salmon will have 11.7 liter/kg/min. Increasing the outer radius will increase the water flow.

A water density difference between the entrance and outlet will create a pressure for the water flow created by the fish to overcome. This example includes a height between the entrance and the outlet of 2 m and a density difference of 0.1 kg/m³ of the sea water at 2 m height difference. Which gives a pressure of 1 Pa.

Ap = 0.5Ap_swAhg

For simplicity and understanding the calculations shown do not include losses and friction, so the result is not conservative. At what depth and radius the fish swim, the fish' fat percentage, swim bladder percentage, density difference, etc, are factors that make the matter complex. The velocity potential component from the negative buoyancy in the example is significantly larger than the velocity potential component from the centripetal acceleration. With the fish in the inner compartment the effect from the centripetal acceleration will detract from the effect from the negative buoyancy. With the fish in the outer compartment the effect from the centripetal acceleration will add to the effect from the negative buoyancy.

The principle has been verified by experiments in a full-scale fish cage with somewhat different parameters than in the example. Measured velocity potential, and other findings are reproduced in CFD (Computational Fluid Dynamics) analysis.

Figures 1A-E illustrates in general terms flow dynamics within a form stable container within a water column. The form stable container comprises a water-tight sidewall and a water-tight bottom. A submerged inlet is positioned at an upper part of a sidewall. An outlet is positioned at a lower part of the sidewall and below the inlet. The sidewall protrudes above a water surface and the container is open towards the air. The internal area of the container towards the air is As.

In figure 1A, a living biomass with a negative buoyancy is positioned below the outlet. The living biomass is able to maintain its vertical position within the container by their swimming action. Due to the negative buoyancy, the pressure within the lower part of the container below the outlet is increased to counteract the force due to the negative buoyancy. The water level of the water surface within the container is equal to the water level on the outside of the container. There is no flow of water through the container from the inlet towards the outlet.

In figure IB, a living biomass with a negative buoyancy is positioned above the inlet. The living biomass is able to maintain its vertical position within the container. Due to the negative buoyancy of the fish, the biomass creates a downward force FD acting on the fluid. To maintain steady state, i.e., a constant volume of water and biomass within the container, the water level within the container is lower than on the outside of the container. The height difference is AHi. FD = AHi p_sw-9-As. There is no flow of water through the container from the inlet towards the outlet. In figure 1C, a living biomass with a negative buoyancy is positioned below the inlet and above the outlet. The living biomass is able to maintain its vertical position within the container. Due to the negative buoyancy of the fish, the biomass creates a downward force FD. The force FD creates an internal overpressure forcing water out of the outlet. At the same time a sub-pressure within the form stable container creates a water flow into the container through the inlet. If assuming no friction losses, the areas formed by the container cross section, the inlet, and the outlet results in pressure variations (Bernoulli) along a flow line from the inlet to the outlet. A height difference AH2 between the water surface outside of the container and within the container is hence a result of these three areas and the driving force / pressure generated by the fish.

If, the living biomass depicted in figure 1C has a positive buoyancy it would create an upward force, and the water would flow in the opposite direction. The inlet and outlet will then swap position.

Figures 1A-F show a container that is open towards air. However, the same principles apply to a container that is closed at the top, except for at extreme conditions where a major sub-pressure is created. The whole container may therefore be submerged.

Figures 1A-F show an idealized container. The skilled person will know that there will be local effects and that the viscosity of the fluid, i.e. the water, will have an impact. E.g., the living biomass above the inlet, see figure IB, may contribute somewhat to the flow through the container.

Water-tight material means that no water will penetrate through the material. The material may be a metal or a plastic. Close to water-tight material means that the material has a pore structure or a is a woven material, but the material has sufficient resistance to a water flow through the material to create a hydrodynamic pressure difference over the material.

According to the gist of the invention fish is raised with no need for oxygenation in daily production by utilizing the fish to create the necessary water exchange. This is obtained by a fish cage comprising a large inlet cross-sectional area and a large outlet cross- sec- tional area. In this context a large area means substantially largerthan the inlet area, i.e. internal diameter of the inlet pipes, of a commercial closed pen and the outlet from the commercial pen.

The outlet and entrance are both typically positioned in the lower portion of the fish cage. A certain amount of the oxygen depleted outlet water will flow back into the fish cage through the entrance. To compensate for this the fish cage must be designed to allow for more than 5 liter water/kg fish/minute. For purpose of calculation an aim of 7 liter water/kg fish/minute is set.

The invention is defined by the independent patent claims. The dependent claims define advantageous embodiments of the invention.

In a first aspect the invention relates more particularly to a floating fish cage for farming of fish, the fish cage comprises:

- one of a water-tight outer wall and a close to water-tight outer wall, and the outer wall is form stable;

- an entrance for water into the fish cage;

- an outlet for water out of the fish cage at a lower portion of the fish cage; and

- buoyancy means.

The fish cage comprises at least two compartments, an inner compartment and an outer compartment, the at least two compartments comprise a fish compartment and at least one passage; the at least one passage comprises the entrance at a lower portion, an inlet at an upper portion, and a passage wall.

The fish compartment comprises the outlet and is in fluid communication with the at least one passage through the inlet; the fish compartment is adapted to keep a shoal of fish in a region between the inlet and the outlet.

A bottom covering net is connected to one of the outer wall and a first net positioned on an inside or on an outside of the outer wall at the lower portion. A second net is positioned inside the bottom covering net and connected to one of the passage wall. A third net is positioned on an inside or on an outside of the passage wall,

The fish compartment and the passage are circular shaped and positioned concentrically.

By circular shaped is also meant other geometries such as square, pentagonal, hexagonal, seven angled, octagonal, and decagonal. By cone shaped is also meant a tapered bottom corresponding to said geometries.

The bottom of the cage may comprise a cone shaped net. The bottom of the passage may comprise a cone shaped net.

The buoyancy means may surround the outer wall and keep the fish cage floating on a water surface. Buoyancy means may hold the passage floating on a water surface. The passage may be connected to the fish cage.

The fish cage may comprise:

- a part of the outer wall extending above the water surface;

- a form stable water-tight closed passage wall forming an upper portion, the passage wall extends from a lower portion in the fish cage, and the upper portion is positioned below a water surface within the fish cage; and

- a net fastened to the upper portion and extending above a water surface within the fish cage, the net forming an inlet; and the entrance is formed in one of: between the outer wall and the passage wall at a lower portion of the fish cage and in the lower part of the outer wall, and the outlet is formed by the bottom net, such that the fish compartment is positioned within the passage wall and the bottom and the passage is positioned in an annulus between the outer wall and the passage wall.

The fish cage may comprise:

- a part of the outer wall extending above the water surface;

- a form stable water-tight closed passage wall forming an upper portion, the passage wall extends from a lower portion in the fish cage, and the upper portion is positioned below a water surface within the fish cage; and - a net fastened to the upper portion and extending above the water surface within the fish cage, the net forming an inlet; and the entrance is formed at the bottom center of the cage, and the outlet is formed in one of: the outer portion of the bottom of the cage between the outer wall and the passage wall and in the outer wall, such that the outer compartment is the fish compartment and the passage is positioned within the passage wall.

The fish cage may comprise:

- a part of the outer wall extending above the water surface;

- an inner net forming a closed off volume in the middle of the fish cage; and the entrance is formed at the bottom in the center of the fish cage; and the outlet is formed in one of:

- between the outer wall and the inner net at the lower region, and

- in the outer wall at the lower region, such that the fish compartment is positioned in an annulus between the outer wall and the inner net and the passage is positioned within the passage wall.

The passage wall may extend beneath the outer wall. The outer wall may extend beneath the passage wall.

The entrance may be positioned in the outer wall. The outlet may be positioned in the outer wall.

A plurality of first flow generators may be positioned inside the outer cage wall and configured to create a water flow from the entrance via the inlet to the outlet. One of at least one of a first flow generator and a second flow generator may be positioned in the passage and configured to create a water flow from the entrance via the inlet to the outlet.

In a second aspect the invention relates more particularly to a method for exchanging water in a fish cage stocked with fish. The method comprises to:

- provide a fish cage as described above;

- provide a fish with one of the characteristics:

- negative buoyancy;

- swimming in circle; and

- negative buoyancy and swimming in circle,

- stock the fish cage with fish in a region between the inlet and the outlet; and

- utilize a force created by the fish to establish a water flow between the entrance via the inlet and the outlet.

The fish may be a salmonid fish.

While it is described a distinct water entrance, a distinct water inlet and a distinct water outlet, this must be understood to mean mainly an entrance, an inlet and an outlet. Some water may enter the enclosure through an area defined as a water outlet and some water may exit the cage through an area defined as a water inlet without deviating from the scope of the invention.

The outer wall and the passage wall may in use position each comprise horizontal sections. Each horizontal section may be in a net material or in a watertight material. The passage wall and the outer wall may each have a single layer in each section, and the single layer may comprise either a net material or a watertight material. The passage wall and the outer wall may comprise of a regular conventional net with a layer of watertight material positioned on the inside or the outside of the net. Thereby the watertight section of the passage wall and outer wall comprises of two layers, one of a net and one of a watertight material.

In a third aspect the invention relates more particularly to using fish with a negative buoyancy to create a water flow within a fish cage between an entrance via an inlet to an outlet where the inlet is above the outlet.

In a fourth aspect the invention relates more particularly to using flow generators in addition to fish with a negative buoyancy and the circling biomass of the fish to create a water flow within a fish cage between an entrance via an inlet to an outlet where the inlet is above the outlet.

In all embodiments as described, supplementary flow generators may be placed inside the water-tight outer wall with the flow generator's center axis along the flow lines to directly support the water flow created by the biomass from the entrance via an inlet to an outlet where the inlet is above the outlet.

In all embodiments as described, supplementary flow generators may be placed inside the water-tight outer wall configured for generating a water flow, indirectly supporting the water flow created by the biomass, by creating an under pressure in the center volume of the cage and an over pressure in the periphery volume towards the inside of the outer wall, further creating a water flow through the cage from the entrance via an inlet to an outlet where the inlet is above the outlet. In one embodiment the flow generators may be configured for generating a horizontal, circular flow. The horizontal circular flow has a similar effect as the fish swimming in circles. The centripetal force creates an under pressure in the center and an over pressure at the periphery creating the flow of water through the cage from the entrance through the inlet to the outlet, supporting the flow created by the fish. In other embodiments the flow generators are configured with their axis vertical or in an angle, i.e. not horizontal or vertical. The flow generators may be placed between the inner and outer wall. The flow generators may be placed inside the passage.

The flow generator may comprise a propeller, a waterjet, an ejector, a thruster or vertical foils. The flow generator is supplied with water from inside the water-tight outer wall.

Some examples of instances where it could be necessary or beneficial to use additional means, temporary or permanent, are:

- When there is a large difference in water density between entrance and outlet (temporary or permanent)

- Cage with a substantial height between the water inlet and water outlet (permanent)

- Shallow cage (permanent) - Periodic low oxygen level in the water entering the cage (temporary)

- Extra large biomass at the end of a production cycle (temporary)

Combining the use of flow generators and live biomass in general to create water exchange requires fewer flow generators, less energy to operate them, and it reduces the need for redundance. If the power supply or equipment fails, the biomass creates sufficient water exchange to survive.

In the description and the claims, a form stable wall is to be understood as a wall with sufficient form stability or shape stability to withstand pressure differences between one side of the wall and the opposite side of the wall. The wall may be formed by metal such as steel or aluminium. The wall may be formed by a reinforced plastic material. The wall may be formed by a cloth such as a plastic sheet or other water-tight membranes. The wall may be formed by a combination of such materials.

In the following is described examples of preferred embodiments illustrated in the accompanying drawings, wherein:

Figs. 1A-E show schematical presentations of water flow and hydrodynamics within a form stable container stocked with fish;

Fig. 2 shows schematically and simplified a fish cage in one embodiment;

Fig. 3 shows schematically and with some more details a fish cage in an alternative embodiment;

Fig. 4A shows schematically a fish cage of two concentric compartments where a central compartment is a fish compartment and where a shoal is denied access to a surrounding compartment;

Figs. 4B-C show an alternative embodiment of the fish cage shown in figure 4A;

Fig. 5A shows schematically a fish cage that comprises two concentric compartments where a central compartment is a passage and the surrounding compartment is a fish compartment, and where a shoal is denied access to the central compartment;

Fig. 5B shows the same as figure 5A, with the addition of flow generators positioned inside the watertight outer wall and configured for supporting the water flow created by the shoal;

Fig. 5C shows the same as figure 5A, with the addition of flow generators positioned within the watertight passage wall and configured for supporting the water flow created by the shoal;

Fig. 6 shows the same as figure 5A, with the addition of flow generators positioned within the watertight passage wall and configured for supporting the water flow created by the shoal;

Fig. 7 shows the same as figure 5A where a flow generator is positioned within the watertight passage wall and the flow generator generates an upward water current;

Fig. 8 shows the same as figure 5A, in an embodiment with an alternative flow generator in the passage;

Fig. 9 shows the same as figure 4A, in an embodiment with a bottom covering net;

Fig. 10 shows the same as figure 4B, in an embodiment with a bottom covering net;

Fig. 11 shows the same as figure 9, in an alternative embodiment; and

Fig. 12 shows an alternative embodiment of the fish cage.

Any positional indications refer to the position shown in the figures. In the figures, same or corresponding elements are indicated by same reference numerals. For clarity reasons, some elements may in some of the figures be with-out reference numerals. A person skilled in the art will understand that the figures are just principal drawings. Different features in the different figures may be combined so that a feature shown in one figure may apply for other figures also even when not explicitly shown or described for said figure.

The relative proportions of individual elements may also be distorted.

In figures 1A-E, the reference numeral 1 indicates a closed, form stable container. The container 1 is adapted for raising fish 2 in a fish farming operation. The container 1 is provided with a form stable wall 11, an inlet 15 and an outlet 17. The inlet 15 is above the outlet 17. The inlet 15 is shown positioned in an upper portion of the wall 11, and the outlet 17 is shown positioned in a lower portion of the wall 11. The container 1 is positioned in a water column 9, and the container 1 may be provided with floating means (not shown) to keep the container 1 floating in the water column 9 such that a part 18 of the wall 11 extends above a water surface 90. In an alternative embodiment, the container is closed at the top, and the container 1 may rest on a ground (not shown) or the container 1 may be fully submerged in a water column 9 (not shown).

The fish 2 form a living biomass 20. The fish 2 form a shoal 21. The shoal 21 freely migrate vertically within the container 1. For purpose of clarity the shoal 21 is described as composed of an upper shoal 210, a lower shoal 219, and a middle shoal 215.

As explained in the general description, the lower shoal 219 positioned below the outlet 17 has no impact on a water flow 3 through the container 1 from the inlet 15 to the outlet 17. This is schematically shown in figure 1A. The upper shoal 210 positioned above the inlet 15 has also no impact on the water flow 3 through the container 1 from the inlet 15 to the outlet 17. This is schematically shown in figure IB. However, the upper shoal 210 has an impact on the surface level within the open container 1 as explained in the general description. The water surface 91 within the container 1 becomes lower than the water surface 90 outside the container 1. The height difference is AHi.

The impact of the middle shoal 215 is shown in figure 1C. The middle shoal 215 creates a water flow 3 through the container 1 from the inlet 15 to the outlet 17 as explained in the general description. In addition, the water surface 91 within the container 1 becomes lower than the water surface 90 outside the container 1. The height difference is AH? due to the pressure drop over the inlet 15 as explained in the general description.

The combined effect of the upper shoal 210 and the middle shoal 215 is shown in figure ID. The water flow 3 is due to the middle shoal 215 only. The height difference between the water surface 91 inside the container 1 and the water surface 90 outside the container 1 is the sum of the impact of the upper shoal 210 and the middle shoal 215 and is AH1+AH2.

The effect of the whole shoal 21 is shown in figure IE. The water flow 3 and the level of the water surface 91 inside the container 1 is the same as shown in figure ID.

Figure 2 shows a simplified fish cage 4000 stocked with fish 2 where water exchange is carried out according to the principles shown in figures 1A-1E with the inlet 15 positioned above the outlet 17 with the fish in captivity in between. The fish cage 4000 comprises a form stable wall 41. The fish cage 4000 is provided with an inlet 4045 at the upper portion 40 and an outlet 47 at the lower portion 49. The inlet 4045 forms an entrance 94. The inlet 4045 and the outlet 47 is covered by a net 42 or similar to prevent that fish 2 escapes from the fish cage 4000. The negative buoyancy of the fish 2 creates a water flow 3 through the fish cage 4000 from the inlet 4045 to the outlet 47.

Figure 3 shows a schematic fish cage 5 that floats on the water surface 90. The fish cage 5 is stocked with fish 2. Water exchange is carried out according to the principles shown in figures 1A-1E. The fish cage 5 comprises a form stable wall 51. The fish cage 5 is provided with an inlet 55 at the upper portion 50 and an outlet 57 at the bottom 53 in the lower portion 59. The inlet 55 forms an entrance 94. The inlet 55 and the outlet 57 is covered by a net 52 or similar to prevent that fish 2 escapes from the fish cage 5. The negative buoyancy of the fish 2 creates a water flow 3 through the fish cage 5 from the inlet 55 to the outlet 57. A part 58 of the wall 51 extends above the water surface 91. The fish cage 5 is provided with first buoyancy means 54 to keep it floating in the water column 9. In this embodiment the fish 2 has access to the whole interior of the fish cage 5. Figure 4A shows a schematic fish cage 6 that floats on the water surface 90 by first buoyancy means 54. The buoyancy means 54 may be connected to the cage 6. The buoyancy means 54 may be integrated in the cage 6. The fish cage 6 is stocked with fish 2 which form a living biomass 20 in form of a shoal 21. Water exchange is carried out according to the principles shown in figures 1A-1E. The fish cage 6 comprises a form stable outer wall 61. A part 68 of the wall 61 extends above the water surface 91. The fish cage 6 is provided with a watertight passage wall 66. The passage wall 66 is continuous and forms a closed wall. The passage wall 66 may be form stable. The outer wall 61 and the passage wall 66 form between them a passage 4. The passage 4 may be an annulus. An upper portion 660 of the passage wall 66 is positioned below the water surface 91 within the fish cage 6. Above the upper portion 660 of the passage wall 6 is the inlet 46. A net 92 may cover the inlet 46 and extend alongside and inside the passage wall 66 as a third net 923 as shown in figures 4A and 9. The third net 923 is at a lower portion 69 of the fish cage 6 connected to a second net 82 which forms a bottom net as seen in figure 9. The net 92 may be connected to a second buoyancy means 44 at the water surface 91. The net 92 may be connected to the cage 6. The net 92 blocks the fish 2 from swimming into the passage 4 and out of the cage through the cage bottom 63. Alternatively, as shown in figure 10, the net 92 may be fastened to the upper portion 660 of the passage wall 66 and extend upwards to above the water surface 91. The net 92 may be fastened to the cage 6 or to buoyancy means 44. The second net 82 may be fastened to the passage wall 66 at the lower portion 69 of the cage 6 as shown in figure 10. The fish cage 6 is provided with an outlet 67 at the cage bottom 63. The negative buoyancy of the shoal 21 creates a water flow 3 through the fish cage 6 from the entrance 45 at the bottom of the passage 4, through the passage 4 through the inlet 46, through the fish compartment 99 and out the outlet 67. In this embodiment the fish 2 has access to a central part of the fish cage 6, but not to the passage 4. Alternatively, the net 92 may cover the inlet 46 and extend alongside and outside the passage wall 66 as a third net 923 as shown in figure 11. The third net 923 is at a lower portion 69 of the cage 6 connected to a second net 82 which forms a bottom net. The net 92 may be connected to a second buoyancy means 44 at the water surface 91. The net 92 may be connected to the cage 6. The net 92 blocks the fish 2 from swimming into the passage 4 and out of the cage through the cage bottom 63. In an alternative embodiment (not shown) the outer wall 61 may be connected to a bottom ring as shown in figure 12. The passage wall 69 may be connected to a passage ring similar to the passage ring as shown in figure 12.

In an alternative embodiment (not shown) the fish cage 6 comprises at least one first flow generator 8. The flow generator 8 may be placed with its axis horizontal, vertical or with an angle, i.e. not horizontal or vertical, anywhere in the cage 6 inside the outer wall 61. The at least one flow generator 8 is configured to enhance and support the water flow 3 created by the shoal 21.

In an alternative embodiment (not shown) the buoyancy means 54, and outer wall 61 of the fish cage 6 is replaced with the buoyancy means 784, outer wall 781, buoyancy tanks 782, bottom ring 790 and net enclosure788 as shown in figure 12.

Figures 4B-C show an alternative embodiment of the fish cage 6. The entrance 45 is in the lower portion of the outer wall 61. A bottom 43 of the passage 4 is watertight. The passage 4 may be sectioned into a plurality of vertical sections by vertical watertight divides 610 placed between the passage wall 66 and the outer wall 61.

Figure 5A shows a schematic fish cage 7 that floats on the water surface 90 by buoyancy means 54. The fish cage 7 is stocked with fish 2 which form a living biomass 20 in form of a shoal 21. Water exchange is carried out according to the principles shown in figures 1A- 1E, and the effect of the shoal 21 swimming in circle. The fish cage 7 comprises a form stable outer wall 71. A part 78 of the outer wall 71 extends above the water surface 91. The fish cage 7 is provided with a water-tight passage wall 76. The passage wall 76 is continuous and forms a closed wall. The passage wall may be form stable. A fish compartment 99 is formed between the outer wall 71, the passage wall 76, the passage bottom 43 and the cage bottom 73. The passage bottom 43 and the bottom 73 may be formed by cone shaped nets. The cage bottom 73 may be the bottom covering net 89. The bottom covering net 89 is connected to the outer wall 71 at a lower portion 79 of the fish cage 7. The lower portion of the outer wall 71 may be connected to a bottom ring (not shown) similar to the bottom ring 70 shown in figure 12. The cone shaped bottom 73 has the advantage over a horizontal bottom that dead fish sink and gathers at the bottom of the cone where dead fish can be collected by conventional means (not shown). The passage wall 76 and the passage bottom 43 form an internal passage 4. The passage bottom 43 of the passage 4 is a net. The passage bottom 43 may be the second net 82. The second net 82 blocks the fish 2 from swimming into the passage 4. The outer wall 71, the passage wall 76, the passage bottom 43 and the cage bottom 73 form between them a fish compartment 99. An upper portion 760 of the passage wall 76 is positioned below the water surface 91 within the fish cage 7. A net 720 or similar is fastened to the upper portion 760 and extends upwards to above the water surface 91. The net 720 may be connected to a second buoyancy means 44 floating at the surface. The net 720 may be connected in any way to the cage 7. The inlet 46 is above the upper portion 760. The net 720 blocks the fish 2, which is positioned in the fish compartment 99, from swimming into the passage 4. A net (not shown) may cover the inlet 46 and the passage bottom 43 and extend along the passage wall 76 on the inside of the passage wall 76 or on the outside of the passage wall 76 similar to the embodiments shown in figures 9 and 11. The passage wall 76 may be connected to at least one horizontal passage ring 787 for providing form stability to the passage 4. The passage ring 787may have buoyancy, the passage ring 787 may float neutrally and the passage ring 787 may have added weight. The lower portion of the passage wall at the lower portion 79 of the fish cage 7 or the outer edge of the passage bottom 43 may be anchored to the cage bottom 73 by e.g. ropes 1000 for providing form stability and positioning of the passage 4 within the fish cage 7 as shown in figure 5B.

The fish cage 7 is provided with an outlet 77 at the bottom 73. The outlet 77 is covered by a net 72 or similar to prevent that fish 2 escapes from the fish cage 7. The net 72 may be a bottom covering net 89. The negative buoyancy of the shoal 21 and the effect of the shoal 21 swimming in circle, creates a water flow 3 through the fish cage 7 through the entrance 75, through the passage 4, over the inlet 46, through the fish compartment 99 and out through the outlet 77.

In an alternative embodiment the outer wall 71 comprises several horizontal sections. An upper portion 710 of the outer wall 71 may be in a watertight material extending from above the water surface 90 and downwards. A lower portion 711 of the outer wall 71 is in a net material. In this embodiment the lower portion 711 of the outer wall 71 is the outlet 77. The upper portion 710 of the outer wall 71 may comprise a single layer of watertight material. The upper portion 710 of the outer wall 71 may comprise a double layer of a net and a watertight material. The watertight material may be on the inside or the outside of the net material. In this embodiment the peripheral portion of the cage bottom 73 may be covered by a watertight material.

In an alternative embodiment (not shown) the passage bottom 43 and the cage bottom 73 may be connected in a manner that a distance between the passage bottom 43 and the cage bottom 73 is sufficient to allow dead fish to slide down to the bottom of the cage bottom 73 at the centre of the fish cage 7, but the distance is so small that fish is prevented from swimming in between the passage bottom 43 and the cage bottom 73. In this embodiment the fish compartment 99 is formed as an annulus between the outer wall 71 and the passage wall 76.

In alternative embodiments (not shown) the cage 6 and 7 in figures 5A-C to 11 the net forming the bottom covering net 89 may extend from the lower portion of the cage 69, 79 up along either the inside or the outside of the watertight outer wall 61, 71 as a first net up to the buoyancy means 54 in a similar manner to how this is shown for the third net 923 on the inside of the passage wall 66 in figure 9 and on the outside of the passage wall in figure 11.

In an alternative embodiment the outer wall 71 at the lower portion 79 of the fish cage 7 and the outer edge of the cage bottom 73 may be connected to a bottom ring 790 as shown in figures 5A and 12.

Figure 5B shows the same cage 7 as in figure 5A. In this embodiment fish cage 7 may be provided with a plurality of first flow generator 8. The first flow generator 8 are positioned inside the water-tight outer wall 71. The first flow generator 8 may be positioned between the water-tight outer wall 71 and the inlet 46, i.e. in the upper portion of the fish compartment 99. The first flow generator 8 may be positioned anywhere inside the outer wall 71. The flow generators 8 are supplied with water from inside the outer wall 71. The first flow generator 8 may be fastened to the fish cage 7 by a fastening member 81. The first flow generator s may comprise a propeller, a waterjet, an ejector, or a thruster. The first flow generators 8 may be configured to create a horizontal circular water flow inside the water-tight outer wall 71. This creates a centrifugal effect similar to the shoal 21 swimming in circles. The water flow created by the first flow generators 8 creates a hydrodynamic under pressure at the center volume of the fish cage 6 and a hydrodynamic over pressure at the peripheral volume of the fish cage 7 towards the outer wall 71. This pressure difference creates a water flow 3 through the fish cage 7 from entrance 75 via the passage 4, through the inlet 46, through the fish compartment 99 and to the outlet 77. Alternatively, the first flow generators 8 may be positioned with their axis vertically or in an angle, i.e. not horizontal and not vertical, anywhere inside the cage wall 71, in order to create the pressure differences that in turn creates a water flow 3. The water flow created by the first flow generators 8 supports the water flow 3 created by the negative buoyancy of the fish and the circling of shoal 21.

Figure 5C shows the same schematic fish cage 7 as in figure 5A. The cage 7 in figure 5B may be provided with a plurality of first flow generators 8. The first flow generators 8 are positioned inside the passage 4. The first flow generators 8 may have their axis horizontal, vertical or in an angle, i.e. not horizontal or vertical. The first flow generators 8 are supplied with water from inside the outer wall 71. The first flow generators 8 are configured to create a water flow that in turn creates an under pressure in the center of the cage 7 and an over pressure at the periphery of the cage 7 which causes water to move into the cage 7 through the entrance 75, through the passage 4, via the inlet 46, through the fish compartment 99 and out the outlet 77, supporting the water flow 3 created by the shoal 21. The first flow generators 8 may be configured to directly create a water flow 3 from the entrance 75, through the passage 4, via the inlet 46, through the fish compartment and out the outlet 77.

Figure 6 shows a schematic fish cage 7 that floats on the water surface 90 by buoyancy means 54. The fish cage 7 is stocked with fish 2 which form a living biomass 20 in form of a shoal 21. Water exchange is carried out according to the principles shown in figures 1A- 1E, and the effect of the shoal 21 swimming in circle. The fish cage 7 comprises a form stable outer wall 71. A part 78 of the outer wall 71 extends above the water surface 91. The fish cage 7 is provided with a form stable, water-tight passage wall 76. The passage wall 76 is continuous and forms a closed wall. The outer wall 71, the passage wall 76, the passage bottom 43 and the cage bottom 73 form between them a fish compartment 99. An upper portion 760 of the passage wall 76 is positioned below the water surface 91 within the fish cage 7. An upper portion 760 of the passage wall 76 is positioned below the water surface 91 within the fish cage 7. A net 720 or similar is fastened to the upper portion of the passage wall 760 and extends upwards in an angle. The net 720, with the passage wall 76 and the passage bottom 43 closes off the volume in the internal passage 4 below the water surface 91. Connecting elements 741 connect the passage 4 to buoyancy means 44, or alternatively (not shown) to the cage 7. The cage 7 may be provided with a plurality of first flow generators 8 inside the passage 4. The water flow generators may have their axis horizontal, vertical or in an angle, i.e. not horizontal or vertical. The flow generators 8 are supplied with water from inside the outer wall 71. The flow generators 8 are configured to create a water flow that in turn creates an under pressure in the center of the cage 7 and an over pressure at the periphery of the cage, which creates a water flow 3 into the cage through the entrance 75, through the passage 4, through the inlet 46, through the fish compartment 99 and out the outlet 77, supporting the water flow 3 created by the shoal 21. Alternatively, the first flow generators 8 may create a water flow 3 directly supporting the water flow created by the shoal 21.

Figure 7 shows the same cage 7 as figure 5A. In this embodiment fish cage 7 may be provided with at least one first flow generator 8 inside the passage 4. The at least one first flow generator 8 is placed with its axis vertical in the middle of the passage 4. The second flow generator 8 may comprise a propeller, a water jet, an ejector, a thruster or vertical foils. The at least one first flow generator 8 may be fastened to the fish cage 7 by a fastening member (not shown). The at least one first flow generator 8 supports the water flow 3 created by the shoal 21 from the entrance 75, to the passage 4, through the inlet 46, through the fish compartment 99 to the outlet 77.

Figure 8 shows an alternative embodiment. The fish cage 7 is provided with at least one second flow generator 85 inside the passage 4. The at least one second flow generator 85 has at least one vertical foil or blade. The surface of the foil or blade may be flat or bent or any combination of flat and bent. The at least one vertical foil or blade is connected to a vertical center rod and is rotating around its vertical center axis. The second flow generator 85 is connected to the cage 7. The second flow generator 85 supports the flow 3 created by the shoal 21 from the entrance 75, via the inlet 46, to the outlet 77.

The second net 82 which covers the entrance 75 of the passage 4 is may be cone shaped with the apex facing downwards. In alternative embodiments, the second net 82 may be cone shaped with the apex facing upwards as shown in figure 8. In alternative embodiments, the second net may be horizontal as shown in figure 7.

The first flow generator 8 and the second flow generator 85 are supplementary flow generators for temporary or permanent installation, and/or for temporary or permanent operation. Examples of temporary situations where supplementary flow generators assist in maintaining sufficient water flow are:

- Large difference in water density between entrance and outlet

- Periodic low oxygen level in the water entering the fish cage

- Extra large biomass at the end of a production cycle

- Before fish is brought into the cage

Examples of permanent situations where supplementary flow generators assist in maintaining sufficient water flow are:

- Large difference in water density between entrance and outlet

- Shallow fish cage

-Inlet or outlet cross-sectional area is less than required for sufficient water flow

Figure 12 shows an alternative embodiment. The fish cage 700 is provided with buoyancy means 784 floating at the water surface 90. A walkway 780 sits on top of the buoyancy means 784 above the water surface 90. A watertight rigid outer wall 781 extends downwards from the walkway 780 and buoyancy means 784. A part 7811 of the outer wall 781 extends above the water surface 90. The outer wall 781 is divided into an upper portion 7810 and a lower portion 7819. The upper portion 7810 is made of a rigid material and extends downwards from the walkway 780 to the at least one buoyancy tank 782. The lower portion 7819 comprises of cloth material extending downwards from the at least one buoyancy tank 782 and is connected to a bottom ring 790. Some buoyancy tanks 782 may be permanently filled with air and some buoyancy tanks 782 may be connected to a ballast system (not shown) making it possible to adjust the draft of the fish cage 700. In an alternative embodiment (not shown) the fish cage 700 is not provided with buoyancy tanks 782 such that the upper edge of the lower portion 7819 is connected to the lower edge of the upper portion 7810.

A net enclosure 788 may be connected to the upper portion 7810 at the inside of the outer wall 781. The net enclosure 788 is connected to the cage 700 at the lower edge of the upper portion 7810. The net enclosure 788 is connected to the bottom ring 790. The portion of the net enclosure 788 below the bottom ring 790 comprises a cage bottom 783. The cage bottom 783 may be a bottom covering net 789. The net enclosure 788 above the bottom ring 790 may be the net 92. The lower portion 7819 may comprise the bottom ring 790.

The fish cage 700 is provided with a watertight passage wall 786. The passage wall 786 is continuous and forms a closed wall. The passage wall 786 may be form stable. The passage wall 786 may be provided with horizontal passage rings 787 providing form stability. The passage rings 787 may have buoyancy. The passage rings 787 may float neutrally. The passage rings 787 may have added weight.

A fish compartment 99 is formed between the net enclosure 788, the passage wall 786 and a passage bottom 743. The passage bottom 743 and the bottom covering net 789 may be formed by cone shaped nets. The passage wall 786 and the passage bottom 743 form an internal passage 4. The passage bottom 743 of the passage 4 is a net. The passage bottom 743 may be the second net 82. The second net 82 blocks the fish 2 from swimming into the passage 4.

The outer wall 781, the passage wall 786, the passage bottom 743 and the cage bottom 783 form between them the fish compartment 99. An upper portion 7860 of the passage wall 786 is positioned below the water surface 91 within the fish cage 700. A net 7820 or similar is fastened to the upper portion 7860 and extends upwards to above the water surface 91. The net 7820 may be connected to a second buoyancy means 44 floating at the water surface 91. The net 7820 may be connected in any way to the cage 700. The inlet 46 is above the upper portion 860. The net 7820 blocks the fish 2, which is positioned in the fish compartment 99, from swimming into the passage 4.

A net (not shown) may cover the inlet 46 and the passage bottom 743 and extend along the passage wall 786 on the inside of the passage wall 786 or on the outside of the passage wall 786 similar to the embodiments shown in figures 9 and 11.

The net 7820, the passage wall 786 and the passage bottom 743 close off the volume in the internal passage 4 below the water surface 91. Connecting elements 741 connect the passage wall 7864 to buoyancy means 44, or alternatively (not shown) to the cage 700. The net 7820 may extend upwards in an angle and close off the passage 4 volume below the water surface 91 as shown in figure 6.

The fish cage 700 is provided with an outlet 777 at the cage bottom 783. The outlet 777 is covered by a net 72 or similar to prevent that fish 2 escapes from the fish cage 700. The net 72 may be a bottom covering net 89. The negative buoyancy of the shoal 21 and the effect of the shoal 21 swimming in circle, creates a water flow 3 through the fish cage 700 through the entrance 75, through the passage 4, over the inlet 46, through the fish compartment 99 and out through the outlet 777.

In an alternative embodiment (not shown) the outlet 777 may be in the lower part of the outer wall 781.

In an alternative embodiment (not shown) the fish cage 700 may be provided with a plurality of first flow generator 8 as shown and described in figure 5B. The first flow generators 8 are positioned inside the water-tight outer wall 781. The first flow generator 8 may be positioned between the water-tight outer wall 781 and the inlet 46, i.e. in the upper portion of the fish compartment 99.

In an alternative embodiment (not shown) the fish cage 700 may be provided with a plurality of first flow generators 8 inside the passage 4 as shown and described in figures 5C and 6. In an alternative embodiment (not shown) the fish cage 700 may be provided with at least one first flow generator 8 inside the passage 4 as shown and described in figure 7.

In an alternative embodiment (not shown) the fish cage 700 may be provided with at least one second flow generator 85 inside the passage 4 as shown and described in figure 8.

The skilled person will know that the described features may be combined in a manner not shown in the figures within the gist of the present invention.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embod- iments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

C l a i m s A floating fish cage (6; 7; 700) for farming of fish (2), the fish cage (6; 7; 700) comprises:

- one of a watertight outer wall (61; 71; 781) and a close to water-tight outer wall (61; 71; 781), and the outer wall (61; 71; 781) is form stable;

- an entrance (65, 75) for water into the fish cage (6; 7; 700);

- an outlet (67; 77; 777) for water out of the fish cage (6; 7; 700) at aa lower portion (69; 79; 7819) of the fish cage (6; 7; 700); and

- buoyancy means (54), c h a r a c t e r i s e d i n that the fish cage (6; 7; 700) comprises

- at least two compartments, the at least two compartments comprise a fish compartment (99) and at least one passage (4); the at least one passage (4) comprises the entrance (65; 75), a passage wall (66;

76; 786) and an inlet (46); the fish compartment (99) comprises the outlet (67; 77; 777) and is in fluid communication with the at least one passage (4) through the inlet (46); the fish compartment (99) is adapted to keep a shoal of fish (21) in a region between the inlet (46) and the outlet (67; 77; 777);

- a bottom covering net (89) connected to one of the outer wall (61; 71; 781) and a first net positioned on an inside or on an outside of the outer wall (61; 71, 781) at the lower portion (69; 79; 7819);

- a second net (82) is positioned inside the bottom covering net (89) and connected to one of the passage wall (66; 76; 786); and

- a third net (923) is positioned on an inside or on an outside of the passage wall (66), and the fish compartment (99) and the passage (4) are circular shaped and positioned concentrically. The fish cage (6) according to claim 1, wherein the fish cage (6) comprises:

- a part (68) of the outer wall (61) extending above a water surface (90);

- a form stable water-tight closed passage wall (66) forming an upper portion (660), the passage wall (66) extends from a lower portion (69) in the fish cage (6), and the upper portion (660) is positioned below a water surface (91) within the fish cage (6); and

- a net (620) fastened to the upper portion (660) and extending above the water surface (91) within the fish cage (6), the net (620) forming an inlet (46); and the entrance (65) is formed in one of: a periphery of the bottom (63) and in the lower part of the outer wall (61), and the outlet (67) at the center of the bottom (63), such that the fish compartment (99) is positioned within the passage wall (66) and the cage bottom (63), and the passage (93) is positioned in an annulus between the outer wall (61) and the passage wall (66). The fish cage (7; 700) according to claim 1, wherein the fish cage (7; 700) comprises:

- a part (78; 7811) of the outer wall (71; 781) extending above a water surface (90);

- a form stable watertight closed passage wall (76; 786) forming an upper portion (760; 7860), the passage wall (76; 786) extends from a lower portion (79) in the fish cage (7; 700), and the upper portion (760; 7860) is positioned below a water surface (91) within the fish cage (7; 700); and

- a net (720, 7820) fastened to the upper portion (760; 7860) and extending above the water surface (91) within the fish cage (7; 700), the net (720; 7820) forming an inlet (46); and the entrance (75) is formed at the center of the cage bottom (73; 783), and the outlet (77; 777) is formed in one of: the periphery of the cage bottom (73;

783) and in the outer wall (71; 781), such that the fish compartment (99) is positioned between the outer wall (71; 781), the passage wall (76; 786), the passage bottom (43; 743) and the cage bottom (73; 783) and the cage bottom (73;

783), and the passage (4) is positioned within the passage wall (76; 786) and the passage bottom (43; 743). The fish cage (7; 700) according to claim 1, wherein the fish cage (7; 700) comprises:

- a net (720; 7820) fastened to the upper portion (760; 7860) extending upwards in an angle closing off the volume of the passage (4) below the water surface (91) within the fish cage (7; 700), the net (720; 7820) forming an inlet (46); and the entrance (75) is formed at the center of the cage bottom (73; 783) and the outlet (77; 777) is formed in one of: the periphery of the cage bottom (73;

783) and in the outer wall (71; 781), such that the fish compartment

(99) is positioned between the outer wall (71; 781), the passage wall (76; 786), the passage bottom (43; 743), and the passage (4) is positioned within the passage wall (76; 786) and the passage bottom (43; 743). The fish cage (6; 7; 700) according to claim 1, wherein the passage wall (66; 76; 786) extends beneath the outer wall (61; 71; 781). The fish cage (6; 7; 700) according to claim 1, wherein the outer wall (61; 71; 781) extends beneath the passage wall (66; 76; 786). The fish cage (6) according to claim 1, wherein the entrance (65) is positioned in the outer wall (61). The fish cage (7; 700) according to claim 1, wherein the outlet (77; 777) is positioned in the outer wall (71; 781).

9. The fish cage (6; 7; 700) according to claim 1, wherein a plurality of first flow generators (8) are positioned inside the outer cage wall (61; 71, 781) configured to create a water flow (3) from the entrance (65; 75) via the inlet (46) to the outlet (67; 77; 777).

10. The fish cage (7; 700) according to claim 1, wherein one of at least one of a first flow generator (8) and a second flow generator (85) is positioned in the passage (4) configured to create a water flow (3) from the entrance (75) via the inlet (46) to the outlet (77; 777).

11. Method for exchanging water in a fish cage (6; 7; 700) stocked with fish (2), c h a r a c t e r i s e d i n that the method comprises to:

- provide a fish cage (6; 7; 700) according to any one of the claims 1 to 10;

- provide a fish (2) with one of the characteristics:

- negative buoyancy;

- swimming in circle; and

- negative buoyancy and swimming in circle,

- stock the fish cage (6; 7; 700) with fish (2) in a region between the inlet (46) and the outlet (67; 77, 777); and

- utilize a force created by the fish (2) to establish a water flow (3) between the entrance (65; 75) via the inlet (46) and to the outlet (67; 77; 777).

12. The method for exchanging water in a fish cage (6; 7; 700) according to claim 11, wherein the fish is a salmonid fish.