A submersible fish farm with a fish rearing tank adapted to separate sludge from water and a method of operating such a farm

Abstract

A submersible fish rearing tank 1 includes an exterior enclosure forming a fish habitat and a vertical, central axis. One or several flow machines 15 pump water into the submersible fish rearing tank 1 to provide a pressure inside tank 1 exceeding a pressure acting on the outside of the tank 1 and a circulating and spiralling water flow inside the tank. A funnel shaped bottom portion of the tank is adapted to receive sludge. A tubular sludge barrier 14 extends upwards from a bottom of the funnel shaped bottom portion and forms a flow path from a location at a height h at a level above a bottom of the funnel shaped tank portion 8. The level h is determined by a level of expected sludge concentration. A sludge suction port 45, connected to a sludge receiving facility 19, is in the vicinity of the bottom of the funnel shaped tank portion 8, outside the tubular sludge barrier 14. An angle α between the vertical axis and the funnel shaped tank portion 8 is sufficiently small to allow gravity to transport the sludge towards the centre of the tank 1.

Description

[0001] A submersible fish farm with a fish rearing tank adapted to separate sludge from water and a method of operating such a farm

[0002] Field of the invention

[0003] The present invention relates to fish farming and how to reduce the discharge of sludge from closed fish farms with a forced spiral shaped water flow and an outlet into the sea. Furthermore, the present invention seeks to reduce volume of the sludge containing water for treatment in a sludge treating facility.

[0004] Background

[0005] Environmentally sustainable development is one of the goals of the aquaculture industry. The industry therefore strives to develop solutions that are energy efficient, that reduce consumption of fossil fuels, and that reduce the environmental footprint.

[0006] Preventing diseases, infections and spread of salmon lice and other parasites, is a major concern for the aquaculture industry. Escaping fish is also a problem especially for the wild salmon stock - and is often due to technical failures, incorrect use of equipment, or storms.

[0007] In addition, emissions from the aquaculture industry have increased, and the industry accounts for large amounts of seabed waste in coastal areas. The waste / sludge largely consists of waste from feed and faeces. The local environmental impact from the waste is concentrated below or in the immediate vicinity of the fish farms, and the discharges can affect life on the seabed and the environment near the sites.

[0008] The above-mentioned issues create a need for closed fish farms that reduce the environmental problems while ensuring growth and sustainability. To increase production, there is also a need for new locations in more weather-exposed areas at sea. Closed and semi-closed fish farms have been deployed to remedy the above problems. Land-based facilities, require considerable land areas, results in increased energy requirements and relies on a high output water supply. Handling of sludge production yields significant costs.

[0009] Lice and pathogens from which the fish is sought to be separated from in closed fish farms, are mainly a problem in the upper water layers. Waste is however released to the sea as in traditional cages unless measures are taken.

[0010] It is an object of the invention to provide an easy to transport, energy efficient, closed, submersible fish farm with low weight and that is cost efficient, easy to deploy and easy to maintain. It is also an object of the invention to provide a facility that is adapted to be submerged below the upper water layers to avoid sealice, harsh weather conditions and floating debris. Finally, it is an object of the invention to provide a facility allowing controlled water treatment, evenly distributed water flow within the facility, controlled temperature and salinity, and controlled waste discharge to obtain ideal fish rearing conditions, fish welfare and environmentally friendly production.

[0011] Closed fish farms have been developed to alleviate some of the above-mentioned problems with open fish farms and can be divided into two groups: Recirculating Aquaculture Systems (RAS), where the water is treated and recirculated, and closed systems where water is taken from the surroundings, flows through the fish farm and is returned to the surroundings. Both systems have their advantages and disadvantages.

[0012] It is a challenge to prevent the water returning to the environment from containing sludge / waist in systems where water is taken from- and returned to the environment to create the required water current through the fish farm.

[0013] Norwegian patent 160753 discloses a system for cleaning the water in containers for fish. Vertical inlet pipes create a spiral shaped waterflow towards the middle of the fish tank. A centrally located tower shaped water outlet leads water out of the container, and a secondary water current at a funnel shaped bottom leads sludge from the fish towards a centre where water with a high concentration of sludge is pumped out and into a cleaning system. Water exits at the centre of the tank.

[0014] Systems such as the one described in Norwegian patent 160753 relies on a secondary water current to enable aggregation of sludge at the middle of a fish tank. This secondary water current may be produced by an interaction between a direction of inlet nozzles and the centrifugal force acting on the water during circulation that will force the water flow towards the bottom and towards the centre. It is suggested that sludge containing water that is pumped out, constitutes 30% of the total amount of water to produce the secondary water current and thus to achieve the intended effect. In fish rearing systems of the above type, it has been found to be a problem that faeces from fish located close to the centre at the upper portions of the tank is entrained in the water exiting through the centrally located tower shaped water outlet. Accordingly do an undesirably high proportion of the faeces I sludge exit through the main water outlet without being cleaned.

[0015] Systems of the above-described type rely on a secondary water current that is produced in bodies of water with a large water surface. The centrifugal force of the circulating water tends to give a higher water level at the periphery of the tank than in the middle, thus in a difference of head pressure and a secondary water current in addition to the primary, circulating water current. This effect is not predictably applicable in submerged water tanks that are substantially full of water and fails to include a water surface resulting in the intended effect.

[0016] A secondary water current may also upset the sludge in the lower portions of the tank, mix the sludge with the cleaner water and thus reduce the intended settling effect in the tank. It is thus a purpose of the present invention to reduce the disadvantages with the abovementioned systems, to provide a system with a less negative impact on the surrounding environment and that can be implemented in subsea tanks.

[0017] The present invention is developed to provide a system with a lower impact on the environment in enclosed fish farms of the type that act as precipitation tanks for the sludge to reduce the amount of sludge escaping through the outlet, while omitting the use of traditional filters that would require frequent attention to operate satisfactory and that can be implemented in subsea tanks. Submerged tanks cannot readily be filled with air to include a water surface to replicate surfacebased solutions as pressurized air inside submerged tanks has unfavorable consequences including providing unwanted buoyancy and issues related to compressive fluids. It is also a purpose of the invention to provide a system that will collect a high proportion of the sludge for further processing as the sludge can be treated and used for various purposes and is valuable. Furthermore, it is a purpose to provide a sludge with a high concentration of solids to drastically reduce the volume that must be treated in a sludge preparation facility compared to the prior art solutions discussed above. The proportion of sludge containing fluid to the total amount of fluid through the tank can be reduced from in the order of 30% as discussed in the prior art to less than 1 % with the solution of the present invention. This results in a considerable reduction of the dimensioning of the sludge transport pump and treatment facilities.

[0018] The present invention thus relates to a submersible fish farm with a fish rearing tank adapted to separate sludge from water. The submersible fish rearing tank includes an exterior enclosure forming a closed fish habitat and a fish residence area FR, including a vertical, longitudinal, central axis. A utility transition provides a transition between an exterior and an interior of the exterior enclosure, for at least one of a water inlet, a water outlet, a gas outlet, an air inlet, and connections for instrumentation. At least one flow machine is adapted to pump water into the submersible fish rearing tank to provide a pressure inside the submersible fish rearing tank exceeding a pressure acting on the outside of the submersible fish rearing tank and a circulating and spiraling water flow substantially from a perimeter of the exterior enclosure and towards a central area along the vertical, longitudinal, central axis. A funnel shaped tank portion is adapted to receive sludge at a bottom end portion of the exterior enclosure. A tubular sludge barrier forms a flow path for water from a location at a height h at a level above a bottom of the funnel shaped tank portion. At least one sludge suction port is connected to a sludge receiving facility in the vicinity of the bottom of the funnel shaped tank portion, outside the tubular sludge barrier. An angle a between the vertical, longitudinal, central axis and the funnel shaped tank portion is sufficiently small to allow gravity to transport the sludge towards the centre of the tank.

[0019] The terms sludge and water are used to distinguish the fluid above a certain level with a water quality suitable for the fish and with a low content of sludge, from the sludge at the bottom where sludge precipitates at the bottom of the tank. Clearly both the water and the sludge include some level of sludge and water.

[0020] A water penetrable fish restrictor may be cone shaped and the first diameter D1 may represent the minimum diameter of the cone and the second diameter D2 may represent a maximum diameter of the cone.

[0021] The water penetrable fish restrictor may be made of a flexible fish barrier such as a fish net.

[0022] The water penetrable fish restrictor may be made with a rigid, permeable, fish barrier such as a metal mesh.

[0023] The submersible fish farm with a fish rearing tank may be a closed, submergible fish rearing tank with a top portion, and the penetrable fish restrictor may provide a tension element between the top portion and the funnel shaped tank portion. The penetrable fish restrictor may be a rigid element between the top portion and the funnel shaped tank portion.

[0024] The angle a between the vertical, longitudinal, central axis and the funnel shaped tank portion may be in the range 30°-70°.

[0025] The angle a between the vertical, longitudinal, central axis and the funnel shaped tank portion may be in the range 40°-60°.

[0026] A level h may be determined by a level of expected sludge concentration, whereby sludge is prevented from entering the flow path formed by the tubular sludge barrier.

[0027] A method of operating the submersible fish farm with a fish rearing tank as described above includes sucking sludge out through the sludge duct with a flowrate less than 1 % of a total flowrate of water flowing through the submersible fish rearing tank.

[0028] Brief description of drawings

[0029] Fig. 1a is a cross-sectional side view of the underwater fish farm showing a typical application of the invention;

[0030] Fig. 1b corresponds to fig. 1a with the addition of a bottom element;

[0031] Fig. 2 is a cross-sectional side view of the underwater fish farm of the invention, including a fish barrier;

[0032] Fig. 3 is a transparent, perspective view of a portion of the underwater fish farm showing in a typical application for the invention;

[0033] Fig. 4 is a top view of the underwater fish farm of the invention, including a fish barrier; and

[0034] Fig. 5 is a perspective view of a lower support plate from an inside of the fish farm.

[0035] Detailed description of an embodiment of the invention with reference to the enclosed drawings. In the following description are similar reference numerals used to describe similar components throughout the specification.

[0036] Figs. 1a and 1 b are a cross-sectional side views of an underwater fish farm with a fish rearing tank 1 . The submersible fish rearing tank 1 includes an enclosure formed as a cylindrical portion 12 forming enclosure sides and a funnel shaped tank portion 8 at a bottom end of the cylindrical portion 12. The funnel shaped tank portion 8 is typically conical or may include slightly rounded walls. The funnel shaped portion 8 leads sludge and dead fish towards the middle for removal through a sludge duct 10 and a dead fish duct 11 . In fig. 1 a, the sludge duct 10 is connected to a sludge receiving facility 19 and a sludge inlet 45. The sludge receiving facility 19 typically includes a sludge pump (not shown). The volume sucked out through the sludge duct is typically less than 1 % of the total volume of water flowing through the tank. The dead fish duct 11 is connected to a dead fish inlet 46.

[0037] The angle of the inner wall of the funnel shaped tank portion 8 is sufficiently steep to allow the sludge inside the fish rearing tank 1 to accumulate at the centre due to gravity only and does not rely on any further elements for accumulation. The inclination of the inner wall that is required for the sludge to slide down and produce a sludge transport towards the centre of the funnel shaped tank portion 8 due to gravity, will depend on the material of the inner wall and the consistency of the sludge. The size of the tank, the water flow, the rate of sludge deposition may also affect the required angle.

[0038] A simple experiment may be set up to provide information about this angle and an experiment set up to determine the required angle may include providing a down scaled mock-up of a tank of the material in question and replicate the conditions of the full-scale tank / fish farm. Alternatively, may the angle be calculated based on parameters including wall friction and sludge properties. No additional or secondary flows are required to force the sludge towards the centre as long as the inclination of the funnel shaped tank portion 8 is sufficient to allows the gravity to transport the sludge downwards along the inclined funnel wall.

[0039] The tank 1 includes an exterior enclosure 32 and a utility transition element 33 forming a closed habitat for fish. The utility transition 33 provide a transition between an exterior and an interior of the exterior enclosure, for at least one of a water inlet, a water outlet, a gas outlet, an air inlet, and connections for instrumentation. In fig. 1 b the thank additionally includes a lower support plate 7. The habitat must be sufficiently closed to allow a pressure to build up inside thank to maintain the shape of the tank and to keep unwanted elements out. Unwanted elements include parasites, jellyfish, plankton and algae. Although the tank is closed, the tank 1 receives seawater (fresh or saline), fluids such as air and oxygen, and feed. Used water, gas and waste are discharged. Intake and discharge may be automatically controlled by a controller connected to a plurality of sensors and cameras installed inside the tank, thereby allowing controlled water treatment and flow for achieving optimized fish rearing conditions and power consumption.

[0040] Incoming water may be filtered to prevent sea lice from entering the tank.

[0041] A feed distributor 9 distribute feed in the tank.

[0042] The exterior enclosure 32 is preferably a membrane made of a flexible material such as PE, PVC, latex, nylon or any water impermeable or substantially water impermeable and flexible plastic or fabric material. The exterior enclosure 32 may also be made of a rigid material forming a rigid tank structure. The exterior enclosure 32 is fixed to the utility transition element 33 and in fig 1b, to a lower support plate 7. The lower support plate 7 and the transition element 33 are rigid and preferably made of metal, plastic, or composite materials. The material of the external enclosure 32 does not need to be totally impermeable. Furthermore, figs. 1a and 1b show that the underwater fish rearing tank 1 includes two water supply columns 5 attached to the exterior enclosure 32 on diametrically opposite sides. Each water supply column 5 is a soft or rigid tubular and elongated structure, typically cylindrical that extends vertically along the exterior of the tank 1 in parallel with the vertical central axis of the tank 1 . The water supply columns 5 attached to the exterior enclosure 32 may be curved to accommodate the shape of the submersible fish rearing tank 1 and may be integrated into the wall of the exterior enclosure 32.

[0043] A variable throttle 13 is typically coordinated with the pumps to maintain the pressure inside the tank while accommodating for variations in waterflow. The flow may be adjusted to accommodate for varying fish size as larger fish require higher flow velocities.

[0044] An angle a between the vertical axis, typically coinciding with a central duct, and the funnel shaped portion 8 forms a funnel angle a of typically 55°, i.e. , 35° steep - off horizontal. The angle a is typically in the range 30° to 60°. The angle must be sufficient to allow the sludge I faeces to move towards the centre to be removed from the tank 1 .

[0045] The angle is adapted to the specific materials and conditions to ensure that the angle is sufficiently steep to allow the sludge to be collected at the centre of the tank 1 without using additional elements or waterflows. The angle can be calculated based on friction coefficients and other factors. The angle can also be determined experimentally by setting up a suitable model.

[0046] The steep angle results in a considerable loss of effective volume that otherwise could be used to contain fish, but this disadvantage is compensated for by the advantages of the low volume of the produced sludge, the purity of the water that exits the tank and the quality of the water for the fish.

[0047] Water enters two inlets 3 with flow machines 15 creating a water flow into the two water supply columns 5. Inlet nozzles 4 are angled to create a swirling waterflow towards the middle of the tank 1 at the centre and out of an outlet 2 at a bottom portion of the tank 1 . The inlet nozzles 4 are arranged along at least a part of the height of the two water supply columns 5

[0048] A central outlet duct 30 with perforations 31 allows water to exit out of the outlet 2 while preventing fish from escaping. The pressure loss in the central outlet duct 30 may contribute to maintain the shape of the tank 1 by maintaining the internal pressure during water circulation.

[0049] The central outlet duct 30 is connected to and extending between the utility transition element 33 and a tubular sludge barrier 14. The central outlet duct 30 is cylindrical and oriented vertically along the vertical centre axis of the tank 1 which also acts as a support column between the utility transition element 33 and the lower support plate 7. The tubular sludge barrier 14 is adapted to lead water out of the tank 1 .

[0050] In fig 1a, the lower support plate 7 is omitted and the tubular sludge barrier 14 is fixed directly to the funnel shaped tank portion 8.

[0051] The pressure difference between outside and inside of the tank, presses water through the central outlet duct 30 and out through the water supply column outlet 2. Water flows in a horizontal direction between the nozzles 4 of the water supply columns 5 and nozzles / ports 31 of the central outlet duct to create an evenly distributed water flow inside the tank while reducing the water current at the bottom of the tank, facilitating water replacement while allowing sedimentation and settling of dead fish and faeces at the bottom.

[0052] The utility transition element 33 includes a gas pocket collecting gas from the inside of the tank.

[0053] The lower part of the tank does not include water supply nozzles to reduce flow and to allow faeces and dead fish to settle at the bottom for removal. The bottom element 7, fig. 1 b is adapted to collect sludge and dead fish and includes one or several dead fish inlets and one or several sludge inlets. The sludge and the dead fish are sucked into the dead fish duct 11 and the sludge duct 10 respectively. The flowrate through the sludge duct 10 and the dead fish duct 11 can be considered negligible compared to the total flowrate through the tank 1 and will not affect the overall flow pattern at the bottom of the tank.

[0054] The central duct forms a flow path to the tubular sludge barrier 14, extending vertically from the bottom element.

[0055] The tubular sludge barrier 14 is without perforations and extends a height h up to a portion where water is allowed to access the duct. The tubular sludge barrier 14 prevents water with a high concentration of sludge from exiting through the outlet 2 and reduces currents that could cause stirring up of the sludge.

[0056] The arrangement described above allows the tank 1 to act as a settling tank to provide sedimentation of the sludge.

[0057] Fig. 2 corresponds to fig. 1 with the exception that the central outlet duct 30 is substituted or supplemented with a fish restrictor 6.

[0058] The fish restrictor 6 prevents fish from swimming to a position to close to the central axis of the tank 1 , and thus too close to the central duct lower portion 31 (Figs. 1a, 1b) above the outlet 2. The purpose of preventing fish from swimming to positions to close to the vertical central axis of the tank 1 , is to prevent faeces from the fish to be entrained in the water that exits through the outlet 2. The sludge should rather sink onto the funnel shaped bottom 8 to be led towards the inlet for sludge and then to be brought out of the tank 1 through the sludge duct 10. The exact shape of the fish restrictor will depend on the flow pattern inside of the tank 1 . The flow pattern will depend on the geometry of the tank, the geometry of the inlet nozzles 4, the volume flow through the system, etc. Faeces 41 schematically represented as black dots from the fish 40 will sink at a given velocity while at the same time it will be influenced by the rotating, spiral shaped water current from the inlet nozzles 4 and towards the centre. The fish restrictor 6 prevent the faeces 41 from moving all the way into the centre and thus out of the tubular sludge barrier 14 and out of the outlet 2.

[0059] The water penetrable fish restrictor 6 is schematically represented by five lines that is intended to represent a structure that allows water to flow through while preventing the fish 40 from moving closer to the centre. The fish restrictor 6 may be a fish net or a rigid, perforated structure. A net is in most cases the most practical solution. The fish restrictor 6 is shown substantially conical I funnel shaped with the wide portion upwards. The shape may however vary, and flow tests may be used to determine the actual shape.

[0060] The water penetrable fish restrictor 6 has a first diameter D1 at a bottom portion that increases to a diameter D2 towards the top of the fish residence portion FR of the tank 1 . The fish residence portion FR of the tank 1 is considered to be the portion of the tank that the fish stays in. Water quality typically affects the fish residence portion FR and thus the effective volume of the tank. The funnel shaped bottom portion is typically not a fish residence portion FR as the water circulation and the water quality is inferior compared to the water quality above. Fish will not stay in this area due to the low water quality.

[0061] The fish restrictor 6 may also form a structural element in the same way as the central flow duct 30 (Figs. 1a, 1 b), and the central flow duct 30 may form a fish restrictor. The central flow duct 30 may be rigid or flexible, just as the fish restrictor 6.

[0062] The upper portion of the fish restrictor 6 may be secured to the upper transition element at the centre of the top portion of the tank 1 . A ring (not shown) secured to the top of the fish restrictor 6 may be used to keep its shape. The dimensions of the fish restrictor 6 are typically dependent on the current, magnitude, current direction, allowable amount of sludge in the water flowing out of the outlet 2, the amount of biomass / fish in the tank 1 , the size of the fish 40, the feed, etc.

[0063] The sedimentation process allows the expelled water to be substantially free from sludge, and the fish farm thus provides a solution with low impact on the surrounding environment. Furthermore, the sludge that is pumped out may have a relatively low content of water, and this allows a size of the components to be used in the sludge handling system to be reduced as the total volume of fluid to be handled is lower. This is also energy efficient as less energy will be used for pumping.

[0064] The height h of the tubular sludge barrier 14 is important as experience has shown that the fish will not move far into the volume below the portion without perforations as the water quality is reduced. The rotating current flowing in a spiral from the inlet nozzles 4 and towards the centre is reduced towards the bottom and the water quality and hence the fish density is reduced. The reduced current also facilitates the sedimentation at the bottom.

[0065] The height h of the central tubular sludge barrier 14 also affects the effective volume of the tank 1 as the water quality below the inlet of the central tubular sludge barrier 14 deteriorates rapidly towards the bottom. The lowermost inlet nozzles 16 are located close to the same height as the inlet of the tubular sludge barrier 14 to prevent water current from the lowermost nozzle from stirring up the sludge in the lower funnel shaped part of the tank 1 .

[0066] Fig. 3 is a perspective view, partly transparent, showing an embodiment where the central element 26 is indicated as several ropes secured to the top of the central tubular sludge barrier 14 forming the main outlet pipe. An outlet pipe extension 25 is secured to the bottom part of the bottom element 7 forming the sludge receiving element. The outlet pipe extension 25 may ensure that oxygen depleted water from the tubular sludge barrier 14 not is drawn into the tank 1. The top of the tubular sludge barrier 14 is approximately level with the top of the funnel shaped area of the tank to prevent water with a high content of sludge from the bottom of the tank to be drawn out through the tubular sludge barrier 14. The outlet pipe extension 25 as shown in fig. 3 may of course be used for all the shown fish farms.

[0067] Fig. 4 is a top view of the tank 1 shown in fig. 2, showing the water supply columns 5 providing directed water currents 43 to provide the circular I spiral shaped water flow from the perimeter of the cylindrical portion 12 of the tank 1 and towards the central portion 44 of the tank along its vertical central axis. The conical fish restrictor 6 prevents the fish from moving too close to the central portion 44 to ensure that faeces from the fish lands on the bottom of the tank and not is entrained in the water that exits through the outlet of the tank at the centre.

[0068] Fig. 5 shows an embodiment of a bottom element 7 forming the sludge receiving element surrounding the tubular sludge barrier 14. Sludge may be sucked in through sludge inlet 45, and dead fish may be sucked in through the dead fish inlet 46. An air supply may be used to provide air-lift and suction for the sludge and the dead fish. An emergency oxygen supply ring 47 surrounds the outlet pipe. The sludge duct 10 and the dead fish duct 11 are shown extending along the inside of the outlet water channel 48 formed by the tubular sludge barrier 14. A cut-out part of the funnel shaped portion 8 is shown secured to the bottom element 7. A funnel- shaped part 18 of the bottom element 7 provides a joint between the flexible tank and the bottom element.

[0069] It should be noted that the carious components and features are exchangeable between the embodiments shown in the figures. All the embodiments may for instance be with or without the lower support plate. Likewise, is the sludge receiving facility 19 (Fig. 1a) a part of the system for all the shown embodiments.

Claims

AMENDED CLAIMS received by the International Bureau on 5 April 2023 (05.04.2023)1 . A submersible fish farm adapted to separate sludge from water comprising: a submersible fish rearing tank (1 ) with an exterior enclosure (32) with a closed fish habitat and a fish residence area FR, including a vertical, longitudinal, central axis; a utility transition providing a transition between an exterior and an interior of the exterior enclosure, for at least one of a water inlet, a water outlet, a gas outlet, an air inlet, and connections for instrumentation; at least one flow machine (15) adapted to pump water into the submersible fish rearing tank (1 ) to provide a pressure inside the submersible fish rearing tank (1) exceeding a pressure acting on the outside of the submersible fish rearing tank (1) and a circulating and spiralling water flow substantially from a perimeter of the exterior enclosure (32) and towards a central area along the vertical, longitudinal, central axis; a funnel shaped tank portion (8) adapted to receive sludge at a bottom end portion of the exterior enclosure (32); a tubular sludge barrier (14), forming a flow path for water from a location at a height h above a bottom of the funnel shaped tank portion (8); at least one sludge suction port (45) in the vicinity of the bottom of the funnel shaped tank portion (8), outside the tubular sludge barrier (14), and wherein the at least one sludge suction port (45) is connected to a sludge receiving facility (19); and wherein an angle a between the vertical, longitudinal, central axis and the funnel shaped tank portion (8) is sufficiently small to allow gravity to transport the sludge towards the centre of the tank (1 ).

2. The submersible fish farm of claim 1 , further including a water penetrable fish restrictor (6) extending vertically upwards from the tubular sludge barrier (14), wherein the fish restrictor (6) has an outer diameter that increases from a first diameter D1 at the tubular sludge barrier (14), towards a larger second diameter D2 at a location at a top of the fish residence area FR.AMENDED SHEET (ARTICLE 19)3. The submersible fish farm of claim 2, wherein the water penetrable fish restrictor (6) is cone shaped and wherein the first diameter D1 represents the minimum diameter of the cone and the second diameter D2 represent a maximum diameter of the cone.

4. The submersible fish farm of claim 2 or 3, wherein the water penetrable fish restrictor (6) is made of a flexible fish barrier.

5. The submersible fish farm of one of the claims 2 or 3, wherein the water penetrable fish restrictor (6) is made with a rigid fish barrier.

6. The submersible fish farm of one of the preceding claims, wherein the fish rearing tank (1) is a closed, submergible fish rearing tank with a top portion, and wherein the penetrable fish restrictor (6) provides a tension element between the top portion and the funnel shaped tank portion (8).

7. The submersible fish farm of claim 6, wherein the penetrable fish restrictor (6) is a rigid element between the top portion and the funnel shaped tank portion (8).

8. The submersible fish farm of one of the preceding claims, wherein the angle a between the vertical, longitudinal, central axis and the funnel shaped tank portion (8) is in the range 30°-70°.

9. The submersible fish rearing farm of claim 8, wherein the angle a between the vertical, longitudinal, central axis and the funnel shaped tank portion (8) is in the range 40°-60°.

10. The submersible fish farm of one of the preceding claims, wherein the level h is determined by a level of expected sludge concentration, whereby sludge is prevented from entering the flow path formed by the tubular sludge barrier (14).AMENDED SHEET (ARTICLE 19)11 . A method of operating the submersible fish farm of one of the preceding claims, including sucking sludge out through a sludge duct (10) at a flowrate less than 1 % of a total flowrate of water flowing through the submersible fish tank (1 ).AMENDED SHEET (ARTICLE 19)

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