WO2004017725A1

WO2004017725A1 - Net cage installation for fish farming

Info

Publication number: WO2004017725A1
Application number: PCT/NO2003/000293
Authority: WO
Inventors: Kurt André STRØMMEN
Original assignee: Stroemmen Kurt Andre
Priority date: 2002-08-26
Filing date: 2003-08-25
Publication date: 2004-03-04
Also published as: CA2497065A1; AU2003256179A1; GB2407748A; NO316102B1; NO20024062D0; GB2407748B; NO20024062A; GB0503691D0

Abstract

A net cage installation arranged for replacement of the net while fish remain in the net cage (10) is described. The net cage installation comprises a net (6a), a floating construction with a walkway (15) circumferentially arranged around the net (6a) and a number of net suspensions (20) placed internally of and circumferentially on the walkway (15). A rail system (5) is arranged internally of the walkway (15) and is used during net replacement.

Description

Net cage mstallation for fish farming

The present invention relates to a net cage installation arranged for replacement of the net while fish can remain in the net cage, said net cage installation comprises a net, a floating frame construction circumferentially placed around the net and a number of net suspensions placed internally of and circumferentially on the frame construction.

The invention also relates to a method for carrying out the replacement of a net.

The net cage installation is particularly developed for use with an installation where a floating frame construction having a walkway is assembled of several floating elements that are articulately connected to each other and that individually follow the movements of the sea. However, it must be understood that the net cage installation also can be used with installations having a rigid frame construction.

The traditional replacement of a net on fish farming installations is a time consuming process requiring much manpower. The net replacement takes place in that a fish farming net is prepared for deployment in a net cage, i.e. that the net is properly prepared and arranged, and is usually delivered on a pallet to ensure simple transportation and storage. The net is delivered with a top net, i.e. a jumping net that prevents the fish from jumping over the edge of the net, lying on top. When the net is to be moved from a boat to a net cage, the fish farming net is simultaneously turned so that the bottom of the net is lying uppermost on another pallet.

The pallet with the fish farming net is placed on the opposite side to where the net that is to be replaced is drawn up from the sea. Before the net replacement can begin, the net must be released from weights or other appliances that are secured to the net to keep it down. The net, i.e. the jumping net, must first be released from the edge of the net cage/railing.

After the net is in position at the middle of one side of the net cage, the fish farming net is put out with the bottom first. Almost the entire net is put into the sea between the walkway and the net that is to be replaced. When about four metres of the net remains on the walkway, the net is temporarily secured so that one can find the net corners. When all four corners have been found, they are drawn to the side, and the fish f-uτning net is now ready to be pulled out.

One side of the new net is drawn out by hand and hooked onto the railing. This side is to be sewn together with a corresponding side of the net that is to be replaced. The opposite side of the net is drawn in the same way and secured to the railings. The two other sides of the net are drawn out to each side of the net cage and sewn together on the walkway. When both nets are sewn together, those sewn parts are ready to be lowered into the sea.

The net is now ready to be drawn towards the outer edge of the net cage installation. On the two sides where the new net is sewn together, it can be released, and one can start to pull both the new net and the net that is to be replaced forward. This is very time consuming and heavy work since one must carry the weight of both nets as they are pulled forward. During the entire replacement process, it is very important that the net does not go below the surface of the water so that fish can escape. Just at this stage there is a great risk for fish to escape. When the entire net that is to be replaced has been drawn forward a few meters, there will be an "excess" of net at the outer side where the fish farming boat will be laying. It is now ready for the crane on the boat to pull the surplus net onboard.

The net is pulled onboard by the crane with the help of straps that are temporarily fastened to the net. In this way, by continuously fastening straps to the net and then pulling, the net will gradually be drawn onboard. The fence on both nets that are in the net cage is moved forwards to the outer edges of the net cage, where the boat is located. This must be done by hand. After the net that is to be replaced has come all the way to the outer side, the new net will finally have taken over the place in the net cage. The seam is released and the remaining portions of the net that is to be replaced are taken onboard the boat. The net is then threaded with a rope through the top of the net, i.e. the jump net, thereafter to be tied in a bundle at the top. The tying together is secured or knotted, and will thus ensure that the net does not get tangled up during washing/transportation. The net is now ready to be taken away and washed.

Replacing a dirty fish farming net as described above will require 4 to 5 persons to obtain efficiency. The time consumption will be about 6 hours including the time to remove weights/attachments from the dirty net and securing the same to a clean net after the net is in place. The transportation of the nets to and from the fish farm installation comes in addition.

More recently, a new installation that uses drums to coil up/uncoil the nets has entered the market. Such an installation is described in Norwegian Patent 308 761. However, this installation is intended for rigid frame constructions and cannot be used in a net cage installation having an articulated frame construction. .

In accordance with the present invention a net cage installation of the introductorily described type is provided, which is characterised in that a rail system is internally arranged to the frame construction, said rail system is utilised during replacement of the net and comprises an inner rail to which the net that is to be replaced can be connected via a number of suspension means that are displaceable along the rail, and a dual rail nearest to the frame construction to which the new net can be connected via a number of suspension means that are displaceable along the dual rail, said new net, prior to the net replacement itsejf, is connectable to a respective rail of the dual rail so that the net is hanging twofold down into the water, and external of the net that is to be replaced, and that the dual rail comprises either respective turning means that turn the suspension means around and thus turns the inside out of portions of the new net to be the outside when the net passes the turning means, or alternatively that the suspension means are turned manually.

Preferably, the net is supplied in the form of a closely packed magazine supplied on a shorter dual rail from which the net hangs twofold down via said suspension means. In one embodiment each suspension means can be a hanger having a roller designed to run along a rail, and also fixing means for the net.

In a preferred embodiment intended for an articulated frame construction, the rail system comprises a number of flexible points (joints) that are arranged near articulated locations of a frame construction/walkway of the articulated type.

In one embodiment each hirning means constitutes part of the rail system, where the outer rail of the dual rail is continuous, the inner rail in the dual rail is discontinuous and makes a jump down at the turning means and continues in a same first direction, in the opposite direction the inner rail forms an arch that terminates at one end which points more than 90° back in the first direction, said end makes a jump to an underlying rail which constitutes the extension of the inner rail of the rail system in said opposite direction.

Additional and -further objects, features and advantages will appear from the following description of one presently preferred embodiment of the invention, which is given for the purposes of description, without thereby being limiting, and given in connection with the appended drawings, in which:

Fig. 1 A shows a net cage installation according to the invention seen from above.

Fig. IB shows in more detail a section of the lower left corner of fig. 1A.

Fig. 2 shows in perspective view a turning means for a net cage installation according to the invention, at a first stage of a net replacement. Fig. 3 shows in perspective view the turning means according to fig. 2 at a second stage of a net replacement.

Fig. 4 shows in perspective view the turning means according to fig. 2 at a third stage of a net replacement.

Fig. 5 shows an outline of a suspension in the form of a davit that supports the rail system.

Fig. 6 shows an outline corresponding to that shown in fig. 5 as the nets hang before a net replacement begins. Fig. 7 shows a magazine for storing, transportation, washing and placing of a net. Fig. 8 shows a simpler embodiment of the invention shown in fig. 1 A, but which requires somewhat more manual labour.

Fig. 9 shows a variant of the simpler embodiment of the invention shown in fig. 8, and Fig. 10 shows how the rail system according to the invention is arranged for use on a fish farming net cage of substantially circular type.

Reference is first made to fig. 1A that generally illustrates a fish farming net cage 10. A fish fanning installation can have more net cages, normally arranged in pairs in a longer row. The net cage 10 comprises an encircling frame construction with walkway 15 for personnel. Larger installations can also have a centrally located track intended for vehicle access. In a common installation the frame construction is often assembled of floating sections 16 which are articulately connected to each other to cope with and adapt to the wave motions of the sea. A number of such articulated connections are shown in figure 1 with reference number 17.

A rail system 5 is arranged along the inner circumference of the frame construction around the net cage 10. The rail system 5 comes into use during a net replacement and is otherwise not used. The rail system 5 stands at a predetermined height above the sea level, for example, about 1.5 - 2 metres. The rail system 5 is connected to the frame construction by means of a number of suspension means 20 in the form of davit-like constructions. In each corner of the net cage 10 the individual rails 5a, 5b, 5c of the rail system 5 form an arch which turns each rail 90°. At, or adjacent to each articulated connection 17 of the frame construction/walkway 15, the individual rails 5a, 5b, 5c are connected to each other in an articulated way so that they flex when the floating sections 16 follow the motions of the waves. This is shown in further detail in fig IB, where the joints are marked with the reference number 12. For a fish farming installation of the rigid type (without articulated connections), the rails do not need to have flexible points, i.e. do not need to be linked.

In more detail, the rail system 5 comprises an inner rail 5 a that runs along a first, second and third side 1, 2, 3 of the net cage 10 and with an opening 7 for pulling up of a net (not shown in fig. 1 A) on the fourth side 4. The fourth side 4 with the opening 7 may face the open sea for possible access for crane or winch vessels. The rail system 5 further comprises a dual rail 5b, 5c that lies between the inner rail 5a and the walkway 15. The dual rail 5b, 5c extends from one corner at the one end of the fourth side 4, along the entire first side 1, the entire second side 2 and halfway down the third side 3. Which corner at the end of the fourth side 4 one starts from is optional.

Approximately in the middle of the first and third sides 1,3 the rail system 5 has turning mechanisms 8a, 8b respectively, which are only shown schematically in fig. 1. Their construction and shape will now be explained ftirther with reference to fig. 2. Fig. 2 also shows a net 6a that is suspended from the inner rail 5a with the help of a number of suspension means 9 which are displaceable along the rail 5a. The net 6a is the old net that has been in the sea for some time and is to be replaced. How the net replacing occurs will be described later in connection with figs. 3 and 4.

Each turning mechanism 8a, 8b constitute apart of the rail system 5. The outer rail 5c of the dual rail is continuous. The inner rail 5b of the dual rail is discontinuous and at the turning mechanism makes a jump di down and continues in the same first direction Rl. In the opposite direction R2, the inner rail 5b forms an arch 5bι that terminates at an end 5b₂ which points more than 90° back in the first direction Rl . This end 5b makes a jump d₂ down to an underlying rail 5aι which constitutes the extension of the inner rail 5a of the rail system 5 in the mentioned opposite direction R2. Furthermore, the inner rail 5a is discontinuous and makes a jump d₃ down in the mentioned opposite direction R2 at the turning mechanism. The toning mechanisms 8a, 8b, on the respective sides of the net cage 10, are basically alike, but inverted, and with the difference that on the one side the outer rail 5 c of the dual rail terminates here.

Fig. 5 shows a typical suspension means in the form of a davit 20. The davit 20 is rigidly fastened to the frame construction/walkway 15. A number of fastening means 21 are arranged on the davits 20 to secure the horizontal beams that together with the davits 20 form a railing on the walkway 15. From a cantilever 22 of the davit is a structure 23 that supports the rail system 5 suspended. The supporting structure 23 is suspended in chains 24 with associated shackles 25. As illustrated, the supporting structure 23 fixes the inner rail 5a and the two rails 5b and 5c of the dual rail.

In the shown embodiment the rails 5a, 5b, 5c are in the form of circular pipe profiles. Each suspension means 9 constitutes a hanger 9a, a lower eye 9b and an upper roller 9c. The lower eye 9b is intended for suspension of a net. In longitudinal section, the roller 9c is twin concave and with a radius of curvature that is adapted to the diameter of the pipe profiles of the rails. This results in the desired property that when the roller 9c rolls along a rail, it is guided sideways at the same time as the hanger 9a can pivot or oscillate about the longitudinal axis of the rail.

As mentioned previously, the rail system 5 is used at net replacement. Normally the net hangs in hooks 28 arranged on each support structure 23 at the same time as the net is drawn out to the side and hooked onto lower hooks 27 on the davit 20. Then the suspension means 9 hangs freely down along the net in its eye 9b. That portion of the net, which is above the water level and hangs from the hooks 28 and via the hooks 27, forms a jumping net for the net cage so that the fish cannot jump out. Further hooks 26 are arranged on the cantilever 22 and are intended for securing a bird net thereto, which will extend as a "roof over the entire net cage.

How a net replacement is carried out will now be described. At first, the net 6a that is to be replaced is hanged on the inner rail 5a by means of the suspension means 9 such that each roller 9c on the hanger 9a is hooked onto the pipe-formed rail 5a. The net 6a will then hang down into the water as shown in fig 5. It is to be understood that the entire net 6a is still hanging down into the water as a net bag that surrounds the live fish that might be there. Then the new net 6b is hanged via the suspension means 9 onto the dual rail 5b, 5c for one half of the net cage 10. With reference to fig 1A, the new net 6b will then hang down into the water as a "twofold curtain" from the dual rail 5b, 5c, but in a limited extent from each turning mechanism 8a, 8b, and along the other side 2 which is opposite to the fourth side 4 with the opening 7. In this half of the net cage 10, the new net 6b will hang externally to the net 6a that is to be replaced, as shown in fig. 6. Note that the space, which after the net replacement is to constitute the net bag, is in fig. 6 defined between the downwardly hanging net sections of the net 6b. This means that when the net bag shall be developed to become the new net bag, the inner net section that hangs from the rail 5b must "en route" be turned inside out. This means that the side of the mentioned inner net section that faces the net 6a will be on the outside of the net bag after the net replacement.

The further net replacement continues, in that, along the entire side 2 and somewhat around the arch in each corner, the top of the net 6a is sewn together with the top of the nearest placed net section of the net 6b. Thereafter, the suspension means 9 are lifted, over the same length extension as the nets 6a, 6b are sewn together, off from respective rails 5a, 5b, and lowered into the water together with the sewn together section of the nets. Then, the sewn together section of the nets 6a, 6b hangs in an arch down into the water, perhaps 3-4 metres at the deepest. This form an escape opening between the nets 6a, 6b that the fish gradually will use as the net replacement proceeds.

The next step will be to get hold of the net 6a at the opening 7 and start to pull the net 6a at this place. The suspension means 9 will then be displaced, i.e. in the case illustrated, roll along the inner rail 5a. The suspension means 9 will roll, one by one, on the inner rail 5a and move towards the opening 7 where they jump off and follow the net 6a up from the net cage 10. Reference is now made to figs. 2-4 that illustrate how the new net 6b is turned or turned "inside out". As mentioned, it is only one half of the net 6b that is turned, in particular the part that hangs from the rail 5b nearest the net 6a that is to be replaced. In the first stage of the pulling in, the suspension means 9 roll on the inner rail 5a as shown in fig. 2. Because the nets 6a, 6b are sewn together, the part of the net 6b that hangs down from the rail 5b is drawn along the rail 5b via the rollers 9c. At the same time, the part of the sewn together section of the nets 6a, 6b that hang down into the water will be drawn along. Some fish will now be able to swim over into the new net 6b.

When the sewn together section of the nets 6a, 6b approaches the toning mechanisms 8a, 8b, a situation arise as illustrated in fig. 3. What initially was the outer part of the net 6b, will all the time hang approximately stationary down into the water from the rail 5 c, via respective suspension means 9, here illustrated with the one suspension means 9'. The inner part of the net 6b will, because of the sewing together, be drawn along "from behind" and will collect accordion-like near the toning mechanism.

By further pulling of the net 6a, the foremost roller on the rail 5b will after a while make a jump d down onto the extension 5aι of the inner rail 5a and roll towards the end of the rail. The suspension means 9 is now turned approximately 180°, see fig 4. Thereafter, the other suspension means 9 follow in turn and make the jump d₂ down onto the extension 5aι. When all suspension means 9 have passed the jump d₂ down to the extension 5a., their order is reversed and all have turned approximately 180°. Thus the new net 6b is also turned or turned "inside out" when it passes. At the same time, the jump dι-d₃ is made as small as possible, i.e. so that a roller can pass between them.

When the last suspension means 9 has passed the jump d₂, this will be the first that is drawn along the rail 5a when pulling-in continues. The others follow in turn, at the same time as more fish swim over into the new net bag that is being formed. When the entire net 6a that is to be replaced has reached the opening 7, the sewing together of the net bags is released and the rest of the net 6a is removed. The new net 6b is now in place and hooked onto the suspension hooks 28 on the support structures 23 and the hooks 27 at the same time as the suspension means 9 are unhooked from the rail 5a.

In addition, in order to simplify the work with the net replacement, a magazine that stores the new net 6b in a compact way has been developed. The magazine stores the net 6b via the same suspension means 9 on a short dual rail with the same distance between the rails as the dual rail of the rail system 5. This magazine is indicated schematically in fig. 1A with the reference number 11, and an exemplified embodiment of the magazine 11 is shown in fig. 7. Two parallel rails 13a, 13b are secured to the magazine housing 14 by brackets. The distance between the rails 13a, 13b, corresponds to the distance between the individual rails of the dual rail 5b, 5c. In addition, two parallel rails 15a, 15b are arranged over the rails 13a, 13b and at such a distance between them that a roller 9c can freely pass between them, but still not jump off. The magazine housing 14 can be closed in one end. The other end can advantageously be closed with a cover (not shown). The cover can have openings through which the rails 13a, 13b project. The magazine 11 can be connected directly to the dual rail 5b, 5c, the cover is removed and the suspension means 9 can be pulled continuously and in parallel onto the dual rail 5b, 5 c, and further all the way round to one end reaches the second turning mechanism 8b and the other end reaches the first toning mechanism 8a.

Fig. 8 shows a simpler embodiment of a net cage 10a than the net cage 10 shown in fig. 1 A, in that the turning mechanisms are omitted. This means that the individual rails 5 a, 5b, 5c are continuous and lie in a straight line also where the turning mechanisms are located. However, this solution will require somewhat more manual labour. With such a solution, the new net is pulled out from the magazine 11 in the same way as before. After the nets are sewn together, the sections of the new net that go forward on each side 1, 3 can be drawn to their own corner at the ends of the side 2. When one starts to pull in the net that is to be replaced, the suspension means 9 that hang on the middle rail 5b must be manually lifted from the rail, toned 180° and be hooked onto the inner rail 5a. One starts with the suspension means 9 on the middle rail 5b that is nearest the corner and continues with the next suspension means 9 etc until the middle rail 5b is emptied. When the middle rail 5b is emptied, one continues to pull the net that is to be replaced and the suspension means 9 on the outer rail 5c that is ftirfhest from the corner moves first and the remaining suspension means 9 follow in turn. Note that one does not need to turn or lift these suspension means 9 over onto the inner rail 5a, but can let them roll along the outer rail 5c. When the suspension means 9 on each side that initially are located furthest from the corner, reach the middle of the sides 1 and 3, the new net is substantially pulled out and in place. The net can now be hung onto the respective hooks 27 and 28.

Alternatively, a procedure more like the one described first can be used, i.e. let the new net hang all the way round from approximately the middle of each side 1 and 3. This means again that the suspension means 9 on the middle rail 5b must manually be lifted from the rail, turned around 180° and hooked onto the inner rail 5 a. This can occur either when the sewn together part passes the suspension means 9 on the middle rail 5b, or near the middle of the sides 1, 3 after the new net, still on the middle rail 5b, has aggregated at the mentioned place. Still with the suspension means 9 that is nearest the corner first. When the rail 5b is emptied, the new net will substantially have been pulled out and in place. The net can now be hung onto the respective hooks 27 and 28.

Fig. 9 shows yet another embodiment of a net cage 10b that still is somewhat simpler that the embodiment 10a shown in fig. 8. However, this requires more manual labour. As before, one has the inner rail 5a nearly all the way round. The dual rail 5b, 5c is now only at the side 2 which is opposite to the side 4 with the opening 7. The magazine 11 can normally be placed in one of the corners at the ends of the side 2. The new net is pulled out on the dual rail and hangs twofold into the water. One finds the middle of the net on the middle rail 5b and pulls the suspension means 9 out towards each corner and thereby tightens the net on the middle rail 5b so that this section of the net can be sewn together with the corresponding section of the net that is to be replaced. Thereafter, the sewn together section is let into the sea so that it hangs in an arch down into the water. As the net that is to be replaced is drawn in, the suspension means 9 on the middle rail 5b must be lifted from the rail manually, turned round 90° and hooked onto the inner rail 5a. Firstly, the rail 5b is emptied and thereafter one continues to lift a number of suspension means 9 onto the outer rail 5c until the new net is drawn out along the side 2 simultaneously with it being stretched out along the sides 1 and 3. The net can now be hung onto the respective hooks 27 and 28.

Fig. 10 shows yet another net cage 30 that illustrates how the rail system according to the invention is adapted for use on a fish farming net cage of a substantially circular type. As before, the inner rail 5a runs nearly all the way round and ends at an opening 7. The dual rail 5b, 5c only run about halfway round the net cage and at the part that is opposite to the opening 7. The magazine 11 can optionally be placed at one of the ends of the dual rail 5b, 5c. As before, the new net is pulled out onto the dual rail 5b, 5c and hangs twofold down into the water. The new and the old net are sewn together over a sufficient length, possibly along the entire length in the longitudinal direction of the dual rail 5b, 5c and thereafter lowered into the water. The suspension means 9 on the middle rail 5b is either lifted manually over onto the inner rail 5 a, or one can use turning mechanisms as described previously for a more automated net replacement.

Furthermore, it is to be understood that a rail system can be used without being suspended from davits. The rails can be fastened to brackets, which in turn are secured directly to existing railings, or the rails are fastened to brackets directly secured to davits. To make this possible, the rails must have flexible joints to take up motions in the fish farming installation. This can be attained by use of rubber joints, hinged joints, rails of plastic or the like. It is also possible to use rails that are not made of circular section pipes. Alternatives are rails of square sectioned pipes, angled profiles, U-shapes, etc and partly closed profiles where pulleys or rollers run inside the closed profile, in which a bottom slit is provided in the longitudinal direction of the rail.

Furthermore, the rails can be placed down along the edge of the net cage. By doing it in this way, the rails will be less exposed to the movements of the fish farming installation. Then plastic pipes or the like can be used along the railings to keep the jumping net of the net standing.

Claims

P a t e n t c l a i m s

1.

Net cage installation arranged for replacement of a net while fish can remain in the net cage (10), said net cage installation comprises a net (6a), a floating frame construction circumferentially placed around the net (6a) and a number of net suspensions (20) placed internally of and circumferentially on the frame construction, characterised in that a rail system (5) is internally arranged to the frame construction, said rail system (5) is utilised during replacement of the net and comprises an inner rail (5a) to which the net (6a) that is to be replaced can be connected via a number of suspension means (9) that is displaceable along the rail (5a), and a dual rail (5b, 5c) nearest the frame construction to which the new net (6b) can be connected via a number of suspension means (9) which is displaceable along the dual rail (5b, 5c), said new net (6b), prior to the net replacement itself, is connectable to a respective rail of the dual rail (5b, 5c) so that the net is hanging twofold down into the water and external of the net (6a) that is to be replaced, and that the dual rail (5b, 5c) either comprises respective turning means (8 a, 8b) that turn the suspension means (9) around and thus turns the inside out of portions of the new net (6b) to be the outside when the net passes the turning means (8 a, 8b), or alternatively that the suspension means (9) are toned manually.

2.

Net cage installation according to claim 1, characterised in that the net (6b) is supplied in the form of a closely packed magazine (11) supplied on a shorter dual rail from which the net (6b) hangs twofold down via said suspension means (9).

3.

Net cage installation according to claims 1 or 2, characterised in that each suspension means (9) comprises a hanger (9a) having a roller (9c) designed to run along a rail, and fixing means (9b) for the net (6a, 6b).

4.

Net cage installation according to claims 1, 2 or 3, characterised in that the rail system (5) comprises a number of flexible points (joints) (12) which are arranged near articulated locations (17) of a frame construction of the articulated type.

5.

Net cage installation according to any of the claims 1-4, characterised in that each turning means (8 a, 8b) constitutes part of the rail system (5), in which the outer rail (5 c) of the dual rail is continuous, the inner rail (5b) of the dual rail is discontinuous and makes a jump (di) down at the turning means and continues in a same first direction (Rl), in the opposite direction (R2) the inner rail (5b) forms an arch (5bι) that terminates at one end (5b ) which points more than 90° back in the first direction (Rl), said end (5b₂) makes a jump (d₂) to an underlying rail (5aι) which constitutes the extension of the inner rail (5a) of the rail system (5) in said opposite direction (R2).

6.

Net cage installation according to claim 5, characterised in that the inner rail (5a) makes a jump (d₃) down in said opposite direction (R2) at the toning means (8a).

7.

Method for replacement of a net in a net cage installation while fish can remain therein, said net cage installation comprises a net, a floating frame construction circumferentially placed around the net and a number of net suspensions placed internally of and circumferentially on the frame construction, and a rail system arranged internally of the frame construction, characterised in that the method comprises the following steps: a) a new net is provided on the rail system partially around a net cage, hanging twofold down into the sea and externally of the net that is to be replaced, b) the top of the inner part of the twofold net is sewn a predetermined distance to the upper edge of the net that is to be replaced, c) the sewn together net sections are lowered into the sea so that a passage is formed between the nets, d) the net that is to be replaced is pulled out of the sea at the opposite end of the net cage at the same time as the net pulls the new net along the rail system, e) the new net is turned during the pulling according to point d) so that the inside of the initially twofold net is now the outside, and f) the pulling continues until the entire net that is to be replaced is out of the sea and the sewn together net sections are released and the new net is suspended from the edge of the net cage.

8. Method according to claim 7, characterised in that the new net is supplied in the form of a closely packed magazine for fitting directly to the rail system for transfer of the new net to the dual rail of the installation.

9. Magazine for use with a net cage installation according to any of the claims 1-6, characterised in that the magazine (11) comprises a magazine housing (14) which opens downwards and carries a net (6b) in a closely packed form, in which the net (6b) hangs twofold down from a shorter dual rail (13a, 13b) via suspension means (9).