A foil structure and system for lice skirts for fish farm cages for improving isolating of the fish farm cage against undesirable organisms and increase exchange of water using water currents.

Field of the invention

[0001] The present invention relates to a device and system to improve isolating of the fish farm cage against undesirable organisms, preferably sea lice, and increase exchange of water using water currents

Background of the invention

[0002] The parasitic salmon louse (Lepeophtheirus salmoise), commonly known as sea lice, is member of the Caligidae family of copepods within the order Siphonostomatoida. The sea lice are a parasite living on aquatic organisms, mostly on salmon, particularly on Pacific salmon, Atlantic salmon, pink salmon chum salmon and sea trout, but is also sometimes found on other fish. It feeds on the mucus, skin and blood of the fish. Once detached, they can be blown by wind across the surface of the sea, like plankton. When they encounter a suitable marine fish host, they adhere themselves to the skin, fins, or gills of the fish, and feed on the mucus or skin.

[0003] The sea lice are one of the major threats to salmon farmers as almon farms are an ideal environment for the sea lice to breed. One sea louse cause little damage to a fish although if populations increase on a fish, the parasites can cause physical damage to the fish's fins, skin erosion, constant bleeding, and open wounds creating pathways for other Pathogens, and often leads to death. Not only does the sea louse have a grave effect on the lives and wellbeing of the fish inside the fish cages, but the infestations of sea lice speeds to the surrounding water dramatically. Sea lice will also attack salmons and smolt migrating from the rivers to the ocean when they pass by fish farms. The sea lice may also act as a vector for diseases between wild and farmed salmon.

[0004] The damaged caused by parasites on aquatic organisms in farms leads to large financial losses, great suffering in the farm animals and great risk of infection from the farm to the surroundings outside. To counteract or avoid attack by parasites aquatic farms, it is known to place a lice skirt on the outside and around fish cages, both around the entire cage and its upper part, which is impermeable to parasites, such as salmon lice. The mesh width of such a lice skirt should be well below 1mm which reflects the size of the salmon lice, tight membranes are also used, but it reduces the flow of water and thus the oxygen level. These lice skirts may be non-preamble to water, semi-permeable or fully permeable to water, but not to sea lice. The smaller the mesh width, the more efficient the lice skit is to protect against sea lice; however, the less water is exchanged inside the sea cage.

[0005] The sea lice cannot swim directionally against the water current, but drift passively, and are found in largest concentrations in the upper water layers. Therefore, common lice skirts are commonly only covering the top half of the depth of the fish cage. Lice skirts depths normally varies between 3 and 15 meters, while the depth of the fish cage may be much deeper. Currents in the water and changes in the water column moves the sea lice around and may force the lice underneath the lice skirt if the lice skirt is too shallow. A deeper lice skirt, on the other hand, reduced the through current of water, and thereby reduces the oxygen levels inside the farm cage. In other words; the more efficient the lice skirt i.e. deeper and smaller mesh width is, the worse the oxygen levels and living conditions of the aquatic animals inside the farm cage becomes. The salmon should have at least 70% oxygen saturation in the water, and there have been examples of widespread fish deaths due to lack of oxygen in cages with lice skirts.

[0006] To counteract the downside of this, inventions such as disclosed in Norwegian patent 342541B teaches a device to hoist up parts of the lice skirt during periods of low oxygen productions. This solution is expensive and requires machineries, such as vices, that do not thrive in the harsh environments of the sea. Solutions with water preamble nets with mesh width between 100 - 1000 pm may be efficient in letting oxygen rich water through the skirt when first employed, but will rapidly clog due to marine fouling and growing organisms covering the mesh, hereby rendering the skirt material none- permeable after a certain period. Norwegian patent 343181 Bl discloses a device and method for supplying pressurized air to the bottom part of the farm cage to form a ricing current of oxygen rich water when using a non-permeable lice skirt. This device is expensive and cumbersome in use. Norwegian patent 335947 Bl discloses a lice skirt slanted in 45 degrees to prevent passing lice-infected-waters of the upper water layers to be pressed under the lice skirt. The disclosed solution requires external mooring to separate anchoring arrangements, which renders the solution impractical in use where multiple fish farm cages are located within proximity. Furthermore, the external mooring requires a more cumbersome installation procedure and additional maintenance for the skirt sheet to stay taut. The slanted skirt of patent 335947 will also loose its effect if strong ocean currents causes movement or forces the sheet out of shape.

[0007] It is therefore an aim of the present invention to overcome the shortcomings of the disclosed prior art and to provide an improved alternative to the prior art. [0008] It is a further aim of the present invention to provide a solution where water infected with parasites in the upper water layers is not forced beneath isolating means of an aquaculture farm cage and wherein the exchange of water inside an aquaculture farm cage is improved.

Summary of the invention

[0009] The objective of the invention is obtained by the technical features of the appended patent claims.

[0010] According to the invention it is provided a foil structure adapted for a lice skirt for fish farm cages is provided for improving isolation of the fish farm cage against undesirable organisms and increase exchange of water using water current. The foil structure being positioned near the lower side of the lice skirt and extending a distance below the lice skirt and having a width less than the width of the farm cage. The material of the foil structure is comprised of a non-permeable material or a semi- permeable material thereby adapting the foil structure to direct a water flow caused by the current. The foil structure is fastened to one or more of: the lice skirt, an anchoring system of the farm cage, a separate anchoring.

[0011] In another embodiment of the invention weighted elements are arranged on or attached to a lower part of the foil structure, said weighted elements being adapted to retain the shape of the foil structure.

[0012] In yet another embodiment of the invention the material of the foil structure is comprised of a flexible material which is adapted to be shaped by the shape of the cage net by wrapping around an area of the cage net when exposed to a current and when the current vector has a direction towards the center of the farm cage .

[0013] In yet another embodiment of the invention the foil structure further comprises a lower edge and wherein at least one retaining means is fastened at said lower edge.

[0014] In yet another embodiment of the invention the foil structure comprises a upper part fastened to the lower side of the lice skirt, the lower edge of the foil structure when exposed to a current being retained at a position by the retaining means, wherein said position being further away from the fish farm cage than the upper part of the foil structure, the foil structure thereby being positioned at an angle to perpendicular with the lower end pointing away from the fish cage. [0015] In yet another embodiment of the invention the foil structure having a width and a height creating a foil area, said height being the distance extending down from the lice skirt, said foil area of the foil structure, when exposed to a current is arranged for causing a high-pressure area around and in front of the horizontally centered area and a low-pressure area at the extremities of the foil structure when exposed to an ocean current.

[0016] In yet another embodiment of the invention the foil structure is horizontally centered around the point of the farm cage where the current vector has a direction towards the center of the farm cage.

[0017] According to the invention it is also provided a system for isolating a fish farm cage comprising a lice skirt against undesirable organisms and increase exchange of water using water currents, the system comprises at least one foil structure. The at least one foil structure is horizontally centered around the point of the farm cage where the current vector has a direction towards the center of the farm cage.

[0018] In another embodiment of the invention the system may further comprises a second foil structure, the second foil structure being positioned opposite the first foil structure, wherein the second foil structure being adapted to face currents from an opposite direction than the first foil structure.

[0019] In yet another embodiment of the invention the system may comprises multiple foil structures arranged around the perimeter of the lice skirt, and wherein adjacent foil structures is separated by an open area adapted for the current to pass into the fish farm cage.

[0020] In yet another embodiment of the invention at least two adjacent foil structures that are separated by an open area has different height.

[0021] In yet another embodiment of the invention multiple foil structures are attached to a brim structure around the entire perimeter of the fish farm cage forming an continues strip at the bottom boundary of the system.

[0022] In yet another embodiment of the invention the open area between the multiple foil structures is open downwards. [0023] In yet another embodiment of the invention the multiple foil structures may have different width.

[0024] In yet another embodiment of the invention the lice skirt enclosing the farm cage and comprises a sheet which is open upwardly and downwardly and extends from an upper side connected to a cage ring of the fish farm cage and at least a distance down to an lower side, the lice skirt having a middle area between the upper side and lower side, the lice skirt being characterized by at least a part of the middle area having a smaller cross section diameter than the upper side and the lower side, the lice skirt thereby having a hyperboloid shape.

[0025] In yet another embodiment of the invention the sheet of lice skirt retains the shape of a hyperboloid of one sheet.

[0026] In yet another embodiment of the invention the lice skirt is situated on the inside of the fish farm net.

Explanation of the technical effect

[0027] When a moving body of water, such as a tidal current or ocean current, hits or encounters a fish farm cage with a non-water-permeable sheet wrapped around it, from any direction, the point of the fish farm cage which receives the current straight on, i.e. the current vector is perpendicular to a point on the front or face of the non-permeable sheet around the fish farm cage, the water molecules that meets said point will slow down, in theory, to a complete stand still close to the sheet. The water current will diverge from said point. Considering that the body of water as a moving water column, this area will be a horizontal strip receiving the current straight on, and the body of water will be divided to each side of this said strip. At this area the head on current will cause an increase in pressure, resulting in a high-pressure area ahead of the fish farm cage. This high-pressure area forces the water to move around the fish farm cage displacing said body of water and accelerating said water both around the sides of fish farm cage. In conventional lice skirts where the bottom part of the lice skirt has an even height, such as illustrated in figure 5, the current hitting the fish farm cage straight on will be pressed underneath the non-permeable sheet if the pressure of the high pressure is higher than the hydrostatic pressure from the increased draught. As the current is displaced both around and underneath fish farm cage the velocity of the current is increased, and the current will pass under the bottom edge of the lice skirt and passed the fish farm cage. Due to the low pressure of the current, the current will to less degree influence the body of water surrounded by the traditional lice skirt, but not enough to exchange the body of water within a reasonable time. If the skirt depth is increasing the time for the body of water enclosed by the lice skirt to be exchanged will only increase, and the oxygen levels inside the fish farm cage will worsen, but the number of lice transferred into the fish farm cage from surface water being pressed down will be reduced.

[0028] The effect of the invention as disclose herein is achieved partly by the added depth of the foil structure which reduces the amount of surface water being pushed below the skirt in combination with the increased in velocity of the water passing on the sides of the foil structure. The effect of the reduced amount of surface water being pushed below the skirt depth is largest where the current hits the skirt straight on, as water to the sides of the horizontal strip receiving the current straight on will to a greater degree pass to each corresponding side of the fish farm cage. A foil structure in accordance with the invention will therefor stop surface water from being push under the skirt edge. In addition, the stream of water at the depth of the foil structure will interact with the foil structure from straight on, which causes a high-pressure area in front of, or upstream, of the foil structure and a low pressure area behind, or downstream, of the foil structure. This, in combination with the effect of the free flowing current to the sides of the foil structure, will cause water to be pulled towards the low pressure areas from the high pressure, which is behind the foil structure, and new oxygen rich water is introduced by pressure up and into the body of water enclosed by the lice skirt. The low-pressure area on the downstream side of the foil structure causes turbulence and eddy currents that mixes new and old water and "pulls" the old water out. This will in turn causing a rotational movement on the body of water inside the lice skirt. The effect achieved by the invention is therefore twofold, surface water is prevented from being pushed under the edge of the lice skirt and new oxygen rich water is introduced into the fish farm cage from depths below where the sea lice lives.

Verification of the invention

[0029] The invention will be explained in further details by verification simulations to verify the effect of the invention.

[0030] A series of computational fluid dynamics (CFD) simulations where run to determine the effect a traditional lice skirt has on the water age to the body of water inside a fish farm cage surrounded by the traditional lice skirt. Further CDF simulations where run to determine the effect of foil structures in accordance with the inventions water age. The water age is the number of minutes it takes for almost the entire body of water inside the boundaries of the lice skirt to be exchanged with water supplied by a current at a constant velocity. The term "almost the entire body of water" is used herein because a boundary layer of water in close proximity to the inside of the fish farm cage will not be exchanged in accordance with the boundary layer theory within fluid dynamics, which should be known buy a person skilled in the art.

[0031] With a conventional lice skirt with a skirt depth of 10 meters comprised of a non- permeable material, the time before water was exchanged was between 200 - 2000 minutes.

[0032] With a skirt depth of 10 meters and one generally square foil structure on the side of the fish farm cage facing the current, with a width of 10 meter and a height of 5 meter, the time before water was exchanged was between 17 - 30 minutes.

[0033] With a skirt depth of 10 meters and two generally square foil structure, one positioned on the side of the fish farm cage facing the current and one on the opposing side from the first, both with a width of 10 meter and a height of 5 meter, the time before water was exchanged was between 15 - 75 minutes.

[0034] With a skirt depth of 10 meters and eighteen generally rectangular foil structures evenly positioned around the perimeter of the fish farm cage, each structure having a height of 5 meter, the time before water was exchanged was between 1 - 6 minutes.

[0035] From the CFD simulations it is clearly indicated that the effect of the foil structures in accordance with the invention has an improved effect on the time it takes for water inside a fish farm cage to be exchanged with new water from an oncoming current compared to a conventional lice skirt. These currents may be tidal currents, ocean currents, any directional movement of water driven by gravity, wind, the Coriolis Effect or water density in addition to rivers and river estuary or any other moving body of water. As fish farm cags may be located in fjords where the tidal current alternated between two opposite directions, it may be beneficial to place two opposing foil structures in accordance with an embodiment of the invention such that one foil structure is facing currents from rising tides (flood current) and one foil structure is facing currents from receding tides (ebb current). At other locations the current may be more stable from one direction, such as at open sea where the ocean current is continuous or close to a river outlet. At these locations it may be beneficial to use one foil structures in accordance with an embodiment of the invention such that the foil structure is facing the continuous current. Brief description of the figures

[0036] These and other characteristics of the invention will become clear from the following description of a preferential form of embodiment, given as non-restrictive examples, with reference to the attached schematic drawings.

Figure 1 illustrates a fish farm cage, lice skirt and a foil structure from a slanted frontal view.

Figure 2 illustrates a fish farm cage, lice skirt and a foil structure from a slanted frontal elevated view.

Figure 3 illustrates a fish farm cage, lice skirt and two foil structures as seen from the side.

Figure 4 illustrates a lice skirt with a hyperboloid shape.

Figure 5 illustrates prior art.

Figure 6 illustrates the mixing effect caused by turbulent flow.

Figure 7 illustrates a fish farm cage, lice skirt and a foil structure from a slanted frontal elevated view.

Figure 8a-8 illustrates an overview of different embodiments of the foil structure and lice skirt.

Detailed description of the invention

[0037] The following description will use terms such as "horizontal", "vertical", "lateral", "back and forth", "up and down", "upper", "lower", "inner", "outer", "forward", "rear", etc. These terms generally refer to the views and orientations as shown in the drawings and that are associated with a normal use of the invention. The terms are used for the reader's convenience only and shall not be limiting. Like numerals on different drawings describe the same feature. Numerals with apostrophe represents an additional feature represented by the same numeral, for instance the number 5 will represent one or the first of multiple or all of the multiples, and the numeral 5' represents an additional of the same feature, like a second or multiple of the same feature.

[0038] Reference in the description to "one embodiment" or "an embodiment" means that a specific feature, construction, or characterization portion described in connection with one embodiment is included in at least one embodiment of the application object. Accordingly, the use of the term "in one embodiment" or "in an embodiment" at various locations in the specification will not necessarily refer to one and the same embodiment.

[0039] Figure 1 shows an embodiment of the invention where two foil structures 5, 5 is attached to a bottom area of a lice skirt 1 for a fish farm cage 2, 6. The fish farm cage 2, 6 is comprised of a floating cage ring 2 and a net 6 supported by the cage ring 2 and mooring system 11. The mooring system 11 (shown in figure 3) connects the farm cage 2, 6 to at least another farm cage 2, 6 and/or to at least one anchor arrangement on the sea floor and/or to at least one buoy. A net 6 encloses the farmed fish. The lice skirt 1 encloses the fish farm cage 2, 6 and comprises a sheet which is open upwardly and downwardly and extends from an upper side connected to the cage ring 2 of the fish farm cage 2, 6 and at least a distance down to a lower side. The lice skirt 1 having a middle area between the upper side and lower side adapted to not let lice past the lice skirt. Preferably the lice skirt is comprised of a non-water-permeable or semi-water- permeable sheet, and even more preferably of a non-water-permeable sheet material. The foil structures 5. 5' and lice skirt 1 is adapted to improve isolation of the fish farm cage 2, 6 against undesirable organisms and increase exchange of water using water currents 3, 3' 4. The fish farm cage 2, 6 illustrated in figure 1 is round in the surface plane forming a cylinder shape which is closed in the bottom by the net 6. However, it should be understood that the invention disclosed herein may be employed for any type or shaped fish farm cages, such as a square shaped, oval, octagonal or round. In figure 1 the foil structure 5 is positioned near the lower side of the lice skirt 1 and attached to the lower side of the lice skirt 1 and extends a distance below the lice skirt 1. The foil structures 5, 5' may be attached to the lower part of the lice skirt by means of attachments means or may be an integral part of the sheet material. The higher water currents in the water column affecting the lice skirt 1 will be pushed to either side of the lice skirt. This is illustrated in figure 1 where a current 3 is directed to the right side of the fish farm cage 2, 6 and another part of the current 3' will be directed to the left side of the cage 2, 6, as view in the figure. The point, area or vertical strip that receives the current head on, or which is horizontally centered around the point of the fish farm cage where the current vector has a direction towards the center of the farm cage, is where the current 3 will be parted into at least two currents 3, 3' on either side of the lice skirt 1 and fish farm, cage 2, 6.

[0040]The foil structure 5 has a width less than the widest width of the fish farm cage 2, 6 so at least a portion of the water current 4 at equal depth of the foil structure 5 that passes directly to the sides of the foil structure 5 will at least partly pass under the skirt edge of the lice skirt 1, as illustrated in figure 1 by the deeper current 4. The foil structure 5 is horizontally centered around the point of the farm cage where the current vector has a direction towards the center of the farm cage 2, 6. In that way the orientation of the foil structure 5 is generally adapted so the point in middle of the foil structure 5 between two horizontal extremities of foil structure 5 revises the water current straight on when a fish farm cage 2, 6 comprising the invention is located in a location with at least an steady current. In the locations where the current is alternating between two opposite directions, two structures, such as a first foil structure 5 and a opposite located second foil structure 5', as illustrated in figure 1 and 3, may be employed, thereby ensuring that at least one of the two foil structure 5, 5' are facing the meeting current independent of which of the two directions the current comes from. For the foil structures 5, 5' to be able to redirect the current the foil structures are preferably comprised of a non-water-permeable or semi-water-permeable sheet material, and even more preferably of a non-water-permeable sheet material. The non-water-permeable or semi-water-permeable sheet material may be of a flexible material which is adapted to be shaped by the shape of the cage net 6 by wrapping around an area of the cage net 6 as a result from the hydrodynamical forces from when it is exposed to a current 3, 7 and when a current vector has a direction towards the center of the farm cage 2, 6.

[0041] In figure 2 the fish farm cage 2, 6, lice skirt 1 and foil structure 5 is illustrated at an oblique angle illustrating both the current 3 deflecting the lice skirt 1 and how a deeper current 7 passes the foil structure 5 thereby entering into the body of water enclosed by the fish farm net. The deeper lice free current 7 enters into said body of water where the low-pressure area behind the foil structure 5 deflects or curves the current 7 upwards and into the body of water encompassed by the lice skirt 1. The oxygen rich current 7 mixes with said body of water and pulls out oxygen depleted water 8.

[0042] In figure 3 the fish farm cage 2, 6, lice skirt 1 and foil structures 5, 5' is illustrated as seen from the side. The fish farm cage 2, 6 is moored to a mooring arrangement 11 and the foil structures 5, 5' are positional retained using retaining means 10 to hold the two opposite facing foils structures 5, 5' in a generally fixed position. The foil structures 5, 5' comprises a lower edge wherein at least one retaining means 10 is fastened at said lower edge. The retaining means 10, which may be any type of rope or line may be coupled to the mooring arrangement 11 of the fish farm cage 2, 6 as illustrated in figure 3, or it may be an independent mooring arrangement on its own with separate anchorage (not shown). To further keep the position of the foil structures 5, 5 steady, weighted elements 9 is arranged on or attached to a lower edge of the foil structures 5, 5', said weighted elements 9 being adapted to retain the shape of the foil structure 5 by weighting the foil structures 5 downwards when needed. These situations may be when currents affect the foil structure 5 from the side of from the back and when the foil structure 5 is not pushed against the net 6 of the fish farm cage. The weighted elements 9 may be point weights attached to the lower edge of the foil structures 5, 5' or it may be a longitudinal weighted element 9, 14 running the either width of the foil structure 5 or the entire circumference or perimeter of the net 6, if multiple foils structures are employed (shown in figure 8). In figure 3 the foil structure(s) 5, 5' comprises an upper part fastened to the lower side of the lice skirt 1. When the foil structure(s) 5, 5' are exposed to a current, the foil structure(s) 5, 5' are retained at a position by the retaining means 10 and the lower side of the lice skirt 1. In said position, the lower part of the foil structure(s) 5, 5' is further away from the fish farm cage 2, 6 than the upper part of the foil structure(s) 5, 5'. The foil structure(s) 5, 5' is thereby positioned at an angle to perpendicular with the lower end pointing away from the fish cage, as illustrated in figure 3.

[0043] The foil structure(s) 5, 5' has a width and a height creating a foil area, said height being the distance extending down from the lice skirt 1. Said foil area of the foil structure(s) 5, 5' causes a high-pressure area around and in front of the horizontally centered area and a low-pressure area at the extremities of the foil structure (5) when exposed to an ocean current. This causing an increase in velocity of a passing lower part of the current 7.

[0044] Figure 1-3 illustrates an traditional lice skirt 1 enclosing the farm cage 2, 6 and comprises a sheet which is open upwardly and downwardly and extends from an upper side connected to a cage ring 2 of the fish farm cage 2, 6 and at least a distance down to an lower side, the lice skirt 1 having a middle area between the upper side and lower side. In figure 4 the lice skirt 1 further comprises a part of the middle area having a smaller diameter than the upper side and the lower side, the lice skirt 1 thereby having a hyperboloid shape. This hyperboloid shape further hinder shallow water currents 3 from being pushed underneath the lice skirt 1 as the currents 3 at the same depth as the lice skirt 1 will be stopped by the lower section of the lice skirt protruding out from the center, from going underneath said lice skirt 1, but rather pass around the smaller middle section 12. The smaller middle section 12 should be understood as the middle section 12 being smaller in circumference than the upper and lower 13 part of the lice skirt 1, i.e. the circumference of the hyperboloid shaped lice skirt 1 at is thinnest. The lower edge 13 of the lice skirt may comprise a tube that may be pressurized with fluid to retain the shape of the lower edge 13 of the lice skirt 1 such that the lower edge of the lice skirt 13 is further out than the thinnest middle section 12 of the lice skirt. The deeper current 7 will pass underneath the lower boundary of the lice skirt 1 and into the net 6. In figure 4 the lice skirt is situated on the inside of the fish farm net 6 with the top part connected to the cage ring 2 and the lower part connected to the net 6, and thereby the sheet of lice skirt 1 retains the shape of a hyperboloid of one sheet. In figure 4 the fish farm cage and lice skirt are illustrated without any foil structures, but it should be understood that any number of lice skirt may be fitted to a hyperboloid shaped lice skirt 1. The hyperboloid shaped lice skirt 1 may be place inside or outside the net 6.

[0045] Figure 5 illustrates prior art where a traditional lice skirt 1 with straight skirt walls or sheets and an even edge, meaning an even height, around the entire perimeter. The current 15 when meeting the fish farm cage 2, 6 straight on will be pressed underneath the non-permeable sheet of the lice skirt 1. As the current is displaced both around and underneath fish farm cage the velocity of the current is increased, and the current will pass under the bottom edge of the lice skirt and passed fish farm cage. The current 15 will not mix properly nor adequately influence the body of water 16 inside the fish farm cage surrounded by the traditional lice skirt 1. The body of water within the boundary of the lice skirt 1 will thus rotate within said boundaries without being exchanged with new oxygen rich water. Fish within the deeper section of the net 6 (underneath the depth of the skirt 1) will receive oxygenated water caring lice from the surface, while the fish inside the upper layer of water (higher than the depth of the lice skirt 1) will experience older oxygen depleted water.

[0046] In figure 6 it is illustrated that the amount of surface water from the shallow current 15 meeting the fish farm cage 2, 6 head on is being pushed below the depth of the lice skirt 1 but is hindered from reaching underneath said lice skirt edge due to the foils structure 5. The effect of the reduced amount of surface water being pushed below the skirt depth is largest where the current hits the skirt straight on, as water to the sides of the horizontal strip receiving the current straight on will pass to each corresponding side of the fish farm cage. A foil structure 5 will therefor stop surface water from being push under the skirt edge. In addition, the stream of water from the deeper current at the depth of the foil structure will interact with the foil structure from straight on, which causes a high pressure area in front of, or upstream, of the foil structure 5 and a low pressure area behind, or downstream, of the foil structure 5. This, in combination with the effect of the free flowing current to the sides of the foil structure, will cause water to be pulled towards the low pressure areas from the high pressure behind the foil structure 5, and new water is introduced by pressure up and into the enclosed body of water of the lice skirt. The low-pressure area on the downstream side of the foil structure causes turbulence and eddy currents as is illustrated with the shaded area that mixes new and old water and "pulls" the old water out by rotating the body of water 16 from the surface and down into the turbulent area. As new water is introduced it also displaced the old water, forcing said old water out of the boundaries of the lice skirt 1. This will in turn causing a rotational movement on the body of water inside the lice skirt. The positive effect of the invention is therefore twofold, surface water is prevented from being pushed under the edge of the lice skirt and new oxygen rich water is introduced into the fish farm cage from depths below where the sea lice lives. Furthermore, figure 6 illustrates a system in accordance with an embodiment of the invention where the fish farm cage 2, 6 comprises a lice skirt 1 to protect against undesirable organisms and to increase exchange rate of water using water currents, the system comprises a first foils structure 5 and a second foil structure 5', the second foil structure 5' being positioned opposite the first foil structure 5 and wherein the second foil structure 5' is situated to face currents from an opposite direction than the first foil structure 5. If the system is employed in a location where the current alternates between two opposite directions an aligned with said alternating currents, at least one of the foil structures 5, 5' is horizontally centered around the point of the farm cage where the current vector has a direction towards the center of the farm cage 2, 6.

[0047] Figure 7 illustrates a system comprises multiple foil structures 17, 18 arranged around the entire perimeter of the lice skirt 1 and fish farm cage 2, 6 and wherein the frontal foil structure 17 comprises an uneven lower edge 20 (seen on figure 8c and 8f). The lower edge 20 of the frontal foil structure foil structure 17, is in this figure illustrated as a serrated lower portion with square downwards protruding notched at said lower edge. To each side of the frontal foil structure 17 multiple additional foil structures are arranged around the remaining perimeter of the lice skirt 1 and fish farm cage 2, 6. The adjacent foil structures 18 are separated by an open area 19 adapted for the current to pass into the fish farm cage 2, 6. The multiple foils structures may have different heights defined by the different heights of the open area. Furthermore, the width of the foil structures may also be dissimilar if multiple foil structures are utilized. In figure 7 a continues longitudinal weighted element 9 is attached to the bottom part of each foil structures and reaches around the entire perimeter of the net 6. The open areas between the foil structure can be open downwards or they can be closed by a brim structure 14 that may be a continues longitudinal weighted element 9, around the entire perimeter of the fish farm cage 2, 6 forming a continues strip at the bottom boundary of the system. When multiple foils structures 17, 18 surrounds the entire net 6 with open spacing in between each structure, the amount of turbulence created by the incoming current increases. This effect increases the rate of exchange of the water body 16 surrounded by the lice skirt.

[0048] Figure 8a-8f illustrates different embodiments of the invention with different numbers of foils structures 5, 18 are utilized. Figure 8a illustrates a single foil structure 5 protruding down from the lice skirt 1. Figure 8b illustrates an embodiment where multiple equal foil structures 18 are spaced apart by equal open areas 19 around the entire perimeter of the lice skirt. Figure 8c illustrates an embodiment where a foil structure 17 is flanked by several thinner foil structures 18 around the remaining perimeter of the lice skirt 1. In figure 8c the foil structure 17 comprises a serrated lower edge with square downwards protruding notches or structures. In a non-illustrated example, the foil structure 17 may comprise an even lower edge. Figure 8d, 8e and 8f illustrates the same configuration of foil structures as figures 8a, 8b and 8c respectively, but with a hyperboloid shaped lice skirt 1 to further reduce the amount of shallow lice infested water to be forced by an oncoming current underneath the lower edge of the lice skirt 1.

[0049] Having described preferred embodiments of the invention it will be apparent to those skilled in the art that other embodiments incorporating the invention may be used. These and other examples of the invention illustrated above are intended by way of example only and the actual scope of the invention is to be determined from the following claims.

Reference numerals

1 Lice skirt

2 Cage ring

3 Shallow surface currents

4 water current below lice skirt lower edge

5, 5' Foil structurer, first and second foil structure

6 Fish pen net

7 Water current entering into the lice skirt boundaries from below lice skirt lower edge

8 Oxygen depleted water exiting the lice skirt boundaries

9 Weighted element

10 Retaining means

11 Mooring system

12 Smaller circumference middle section of the lice skirt

13 Lower edge of the lice skirt

14 Brim structure

15 Current passing under a conventional lice skirt

16 Water body inside the boundaries of the lice skirt

17 Foil structure comprising an uneven lower edge

18 Multiple foil structures

19 Open area between multiple foil structures

20 Lower edge of the frontal foil structure