The invention relates to methods and apparatus for aquaculture feeding, with particular applicability to salmonid farming.

Sea lice are a major problem in the fish farming industry. Sea lice belong to the copepod family, and are found naturally throughout the northern hemisphere. Sea lice have affected salmon fishing for a long time. For example, the salmon louse, Lepeophtheirus salmonis was described by the zoologist Henrik Nikolai Krayer in 1837. Sea lice are host-specific, and depend on their host (e.g. salmonids) to complete their life cycle.

In general, the saltier the water, the more the sea lice thrive. They tend to fall off salmon when the fish head up river. When lice are found on a salmon caught in a river, it is commonly seen as a sign that the fish recently entered the river, but laboratory experiments have shown that sea lice can remain on salmon for up to 14 days after entering fresh water. Sea lice cause damage to salmonids by eating their mucus, skin tissue and blood. This paves the way for other problems such as bacterial or fungal infections and also affects the osmotic balance of the fish.

Sea lice are today one of the most important causes of mortality in farmed salmonids. At a rough estimate, each year the Norwegian aquaculture industry loses at least NOK 500 million as a result of direct losses, the cost of chemicals, extra work associated with delousing, weight loss due to stress, loss of fish, etc. These days there are limits on the number of sea lice permitted on salmon in fish farms, and for example the Norwegian Food Safety Authority has provided clear guidelines on how to combat the parasite.

The problem is not limited to farmed fish. Increases in sea lice numbers pose a threat to wild salmon as the lice can transfer from fish pens to wild fish. Although the main impact of sea lice is reduced growth, in the worst case scenario, reduced growth in vulnerable wild populations can harm their reproductive potential. A recent trial used a funnel that forced the fish to stay lower in the cage and resulted in a reduction of infestation by 66 to 84%. This trial suggests that keeping the fish lower in the water may reduce the infestation by reducing the time that the fish stay in the area where infestation typically takes place.

The mismatch of swimming depth of the fish and the copepodids resulted in reduced lice infestations in various environmental conditions. The behaviour of the fish was observed to be normal. However, a growth reduction of 33% was observed, possibly caused by under-feeding due to biofouling in the net roof.

The cage environment is the key to good fish production. Several factors affect the environment within the cage and affect the depths at which the fish choose to swim. These factors include the availability of food, appetite, light levels, currents, oxygen levels, temperature, salinity, the presence of chemicals and perceived fear (e.g. of predators). These factors can result in the fish spending an increased amount of time at a particular preferred depth and can result in greatly increased fish densities when compared with an even distribution within the cage. The density may be increased at these depths by a factor of 1.5 to 5, or in some cases up to 20. Thus the whole cage is not being used and the increased density may be detrimental to the fish, for example it may facilitate the transfer of sea lice between hosts, increasing infection levels.

According to a first aspect, the invention provides an aquaculture feeding apparatus comprising: a feed supply attached to a float of variable buoyancy.

Viewed from another aspect, the invention provides an aquaculture feeding apparatus comprising: a feed supply attached to a float of variable buoyancy; and a flexible element connected between the variable buoyancy float and a mount structure, the flexible element being arranged to hang in an arc between the variable buoyancy float and the mount structure.

A feeding apparatus with adjustable buoyancy allows feed to be distributed to farmed aquatic animals (including fish, shellfish, molluscs, etc. but in most preferred cases farmed fish such as salmon) at a variable depth within the water. The system may be used in either freshwater or salt water environments, but will gain particular benefit in sea farms where sea lice are more problematic. The buoyancy of the variable buoyancy float can be adjusted so as to control the floating depth of the float, i.e. the depth below sea level. As the feed supply is attached to the float, varying the depth of the float also varies the depth of the feed supply. The availability of food is one of the major factors that affects the preferred depth at which fish swim. In traditional fish farms feed is supplied close to the sea surface which is also where the conditions are ideal for sea lice to breed. The fish are thus drawn closer to the surface in order to feed and are thus more susceptible to sea lice infestation. By lowering the depth of the feed supply, the fish can be

encouraged to spend more time at greater depths where they are less susceptible to sea lice. More specifically, by providing a feed supply with an adjustable depth the feed supply depth can be combined with other environmental factors to influence the preferred swim depths and optimize the conditions for maximum health and growth of the animals.

By way of example, a deep feed supply is not necessarily an ideal solution. While this may draw the animals away from the surface regions (where lice are present) in order to feed, if the feed level is uncomfortable, e.g. due to non-optimal temperature and light levels, the animals will simply return to the surface after feeding.

However, by adjusting the feed supply level to take account of other environmental factors, the animals may be encouraged to remain in the vicinity of the feed and away from the surface. As the other environmental factors that influence swim depth vary with time, the adjustable depth feed supply can be adjusted regularly (or continually) so as to maintain optimal conditions for the animals.

Another example is that in certain conditions the surface temperature may be colder than the temperature at a depth of, e.g. 10 metres. Where the surface is uncomfortably cold, the animals may then prefer to swim at this greater depth and avoid rising to the surface. With a traditional surface feed supply the animals will simply wait for the feed to descend down to their preferred swimming depth.

However, if strong currents are present, feed will be swept out of the cage without being consumed. As well as being inefficient, this can make it difficult to provide enough feed for optimal growth. With a feed supply attached to a float of variable buoyancy the feed supply can be lowered to supply feed at, or closer to, the preferred swim depth, increasing efficiency of feed supply and facilitating increased growth and health by increasing feed uptake.

The use of an adjustable buoyancy float to control depth is particularly

advantageous because it defines the feed supply depth in relation to the current water level rather than in relation to a fixed point on the land or seabed. The water level (with respect to the land) can vary e.g. with tides and this will in turn affect the preferred swim height (with respect to the land) for aquatic animals. The buoyancy of the float may be varied in different ways. For example it can be varied by adjusting the mass while keeping the volume constant or it can be adjusted by adjusting the volume while keeping the mass constant, or a

combination of both. In preferred arrangements, a fluid with a different density to that of the ambient water is used to adjust the buoyancy. The fluid may be of a greater or lesser density or fluid of one density could be replaced with fluid of a different density so as to alter the buoyancy of the float. The float may comprise a fluid chamber or reservoir to contain the fluid of different density to the ambient water. The chamber may be an expandable chamber, e.g. a chamber that can be inflated and deflated by adding and removing fluid. Alternatively, the chamber may be a fixed volume and the amount of a fluid within the chamber can be adjusted by adding or removing that fluid to change the mass within the chamber. In preferred embodiments, the fluid supplied to the chamber is gas, more preferably air. The quantity of air held within the float (e.g. within the chamber) determines its buoyancy either by water replacement (forcing water out or drawing water into the chamber) so as to alter the mass or by inflating and deflating the chamber to change its volume. In an adjustable volume chamber, the volume of the chamber may be adjusted by compressing and decompressing gas, e.g. using a compressor or a pneumatic cylinder without requiring any external supply. However such systems are expensive and require maintenance. Therefore the aquaculture feeding apparatus may further comprise a fluid supply line arranged to supply fluid of different density to the ambient water so as to vary the buoyancy of the float. Such systems are inexpensive and easy to maintain. Similarly, the float may further comprise a fluid outlet through which fluid may be expelled so as to change the buoyancy. This may be to reduce the volume of an expandable chamber.

Alternatively, air can be vented into the surrounding water without harm. In some preferred arrangements, a fluid less dense than water (preferably air) is supplied to and drawn from the chamber through an upper opening. Supply of the less dense fluid through the upper opening forces water out of a lower opening thus decreasing the mass within the chamber. Drawing the less dense fluid from the upper opening draws water in through the lower opening thus increasing the mass within the chamber. This arrangement provides a simple and inexpensive system which is easy to maintain. Only a single connection is required to the chamber and all pumping apparatus can be located above the water level, e.g. on land or on a nearby vessel.

The feed supply could be from a feed reservoir held on the float and which requires periodic restocking. However, it is preferred that the feeding apparatus further comprises a feed supply line attached to the float. The feed supply line can deliver feed from a much larger reservoir or store, e.g. on nearby land or on a nearby floating vessel such as a barge which is easier and less expensive to restock.

Additionally, the weight of feed does not then need to be supported by the float. A plurality of supply lines could be used or a single supply line (or multiple supply lines) may branch to provide a plurality of outlets so as to provide a more even distribution of feed.

The aquaculture feeding apparatus may further comprise a fixed buoyancy float in addition to the variable buoyancy float. The fixed buoyancy float may be designed to support the majority of the weight of the underwater part of the system so that the variable buoyancy float only needs to overcome a small portion of the total weight in order to adjust the height.

The aquaculture feeding apparatus preferably further comprises a flexible element connecting between the variable buoyancy float and a separate mount structure. The flexible connecting element is preferably made from a material of a different density to the ambient water. The flexible connecting element is preferably more dense than the ambient water so that it sinks and provides a downwards force on the variable buoyancy float and on the mount structure. The downward force helps to stabilize the feeding apparatus in the water. The use of a flexible connecting element means that as the end points are moved to different relative vertical positions, the weight distribution between the different support structures (the variable buoyancy float and the mount structure) is changed so that the different supports have to support different weights. This has the effect of reducing the sensitivity of the system to the buoyancy of the float and thus facilitating control over the float depth (and thus the feed supply depth). For example, when the buoyancy of the variable buoyancy float is increased, the float will start to rise. As it rises, the fraction of the weight of the connecting element that is supported by the float rather than by the mount structure increases until it balances the increased buoyancy force. The size (and thus weight per unit length) of the connecting element can be selected so as to provide the required level of control. This must be balanced against the available buoyancy force and the depth range required for the float. The flexible element is preferably arranged to hang in an arc between the variable buoyancy float and the mount structure at certain depths of the variable buoyancy float. The mount structure could be fixed relative to the land, e.g. it could be mounted to a jetty or pier or to the land itself, or to the seabed. However, such systems would be complicated and expensive and it is thus preferred that the separate structure is a floating structure. This floating structure floats on the surface of the body of water and thus moves up and down with the tides and with waves. Although the oscillatory vertical movement of the floating structure caused by waves causes movement and a change in the relative weight distribution of the connecting element between the floating mount structure and the submerged float, the frequency of oscillation is sufficiently high in relation to the movement rate of the submerged float that it does not cause any substantial movement of the submerged float which thus remains at a relatively stable depth even during periods of high waves.

An alternative arrangement is to have the mount structure mounted (e.g. anchored) to the seabed. The flexible connecting elements can partially rest on the seabed when the feeding device is in a lowered position, thus adjusting the weight distribution of the flexible element between the variable buoyancy float and the seabed.

The flexible connecting element preferably comprises at least one rope, cable, chain or similar. The material of the connecting element is preferably a dense and/or heavy material compared to the ambient water, such as a metal. The connecting element may comprise different sections of different construction, for example a length of heavier chain to provide weight, connected to the separate structure via a length of lighter weight rope. A single connecting element may suffice in some circumstances. However preferably two or more connecting elements attached to different positions on the variable buoyancy float are used to provide balance and stability. These plurality of connecting elements may be attached at the other ends to a common attachment point on the mount structure or to different points.

The flexible connecting element may pass through an aperture in the variable buoyancy float. This has the advantage that the variable buoyancy float and the separate structure on or near the water surface can be kept substantially in line with one another and thus provides an easy way of controlling and/or monitoring the lateral position of the variable buoyancy float. It will be appreciated that the aperture may also be formed in the fixed buoyancy float if used and, depending on structure, through a feed supply chamber.

By having a plurality of connecting elements attached to the variable buoyancy float and connected to different positions on the mount structure (or structures) can provide stability by holding the float in a balanced position between the attachment points and thus holding the float stable against currents. At the same time, as the flexible elements hang in an arc, there is slack in the system such that waves do not cause high tensions which could snap the flexible elements if they were already taut. This is important as the float and feeding device are located within the cage. If the float and feeding tube, or severed cables/ropes were able to drift to the edge of the cage, there would be a risk that they could catch and/or tear the cage netting allowing the farmed animals to escape. The aquaculture feeding apparatus may further comprise an air retaining structure on the variable buoyancy float, the air retaining structure having a roof and side wall and being open at the bottom so that air can be retained within the air retaining structure while underwater to create an underwater air/water interface. The provision of an air water interface underwater that can be depth adjusted (by adjusting the buoyancy of the float) is advantageous because it provides a local source of air for aquatic animals, e.g. for fish to gulp air so as to fill the swim bladder. Normally, animals would have to return to the surface to do this, but as described above, the surface may not be optimal in other respects, particularly due to the presence of sea lice larvae or lower temperatures. Thus the provision of an air/water interface underwater and at variable depth can improve the environment by reducing the necessity to return to non-optimal conditions.

Preferably air is supplied to the air retaining structure via an air supply line that also supplies the variable buoyancy float. A single supply can then be used for both purposes and can be controlled simply by different valves.

According to another aspect, the invention provides a method of supplying feed in aquaculture, comprising: adjusting the buoyancy of a variable buoyancy float so as to adjust the floating depth of said float; and supplying feed through a feed supply attached to said variable buoyancy float.

Viewed from another aspect, the invention provides a method of supplying feed in aquaculture, comprising: adjusting the buoyancy of a variable buoyancy float so as to adjust the floating depth of said float and thus changing the distribution of weight of a flexible element connected in an arc between the variable buoyancy float and a mount structure; and supplying feed through a feed supply attached to said variable buoyancy float.

The preferred features described above in relation to the apparatus apply equally to this method. In particular, said adjusting step may comprise: supplying fluid of different density to the ambient water through a fluid supply line so as to vary the buoyancy of the float. Said adjusting step may comprise: expelling fluid through a fluid outlet of said float. Said supplying step may comprise: supplying feed through a feed supply line attached to the float.

A flexible connecting element may be provided between the variable buoyancy float and a separate structure and said adjusting step may comprise: changing the distribution of weight of the flexible connecting element between the variable buoyancy float and the separate structure. The separate structure may be a floating structure. The flexible connecting element may comprise at least one rope, cable, chain or similar.

The method may further comprise supplying air to an air retaining structure on the float, the air retaining structure having a roof and side wall and being open at the bottom, so that air is retained within the air retaining structure creating an underwater air/water interface. The air may be supplied to the air retaining structure via an air supply line that also supplies air to the variable buoyancy float. The aquaculture feeding apparatus may further comprise a winch arranged to be able to raise or lower the variable buoyancy float. The winch may act on the flexible connecting element to reel in or pay out the flexible connecting element, thus adjusting the weight distribution between the variable buoyancy float and the mount structure. Alternatively, the winch may be connected to the variable buoyancy float by a separate line and be capable of directly raising and/or lowering the variable buoyancy float and hence the feeding device. The winch is preferably provided on a mount structure floating on the sea surface.

The adjustment of the weight distribution of the flexible elements by reeling them in and paying them out (or otherwise adjusting their length) from the mount structure can also be used with a fixed buoyancy attached to the feed supply. The fixed buoyancy can be arranged to support the weight of the feed supply and a certain length of flexible element(s). Adjusting the length of the flexible element connected between the mount structure(s) and the fixed buoyancy will therefore adjust the depth of the fixed buoyancy and thus the depth of the feed supply. Using a fixed buoyancy simplifies the system a little and is particularly suited to situations where adjustment is required relatively infrequently e.g. once a week or less rather than more dynamic systems which will likely benefit more from a variable buoyancy implementation.

Therefore according to another aspect, the invention provides an aquaculture feeding apparatus comprising: a feed supply attached to a float of fixed buoyancy; and a flexible element connected between the fixed buoyancy float and a mount structure; wherein the length of the flexible element connected between the fixed buoyancy float and the mount structure is adjustable. The preferred features described above in relation to the variable buoyancy arrangement also apply to this aspect of the invention. Thus preferably the flexible element is arranged to hang in an arc between the fixed buoyancy float and the mount structure. The aquaculture feeding apparatus preferably further comprises a winch on said mount structure arranged to reel in and pay out said flexible element.

According to another aspect there is provided a method of supplying feed in aquaculture, comprising: adjusting the floating depth of a fixed buoyancy float by adjusting the length of at least one flexible element attached between said fixed buoyancy float and a mount structure; and supplying feed through a feed supply attached to said fixed buoyancy float.

The preferred method features described above also apply to this method. In particular, the adjusting step preferably comprises: changing the distribution of weight of the flexible element between the fixed buoyancy float and the mount structure. The adjusting step preferably comprises winching said flexible element.

As above, two or more flexible elements and mount structures may be provided. In such case, a winch may be provided for each flexible element, e.g. at each mount structure.

Viewed from an alternative aspect, the invention provides an apparatus for feeding aquaculture fish at various depths comprising: a feeding device that distributes feed in the cage, said device containing a possibility to displace water with gas to allow raising or lowering the device (feed spreader) in the water; a floating element; and a connection between the feeding device and the floating element, the connection having a specific weight not equal to water, the floating element holding the weight of the feed spreader and the connection, and wherein the feeding device can be inflated with gas to an amount that relates to a specific water depth .

Preferred embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Fig. 1 shows a first embodiment of a variable buoyancy feeding system; Fig. 2 shows a detail of the feeding system of Fig. 1 ;

Fig. 3 shows a second embodiment of a variable buoyancy feeding system;

Fig. 4 shows a third embodiment of a variable buoyancy feeding system;

Fig. 5 shows a detail of the embodiment of Fig. 4; and

Figs. 6 and 7 show a fourth embodiment of a variable buoyancy feeding system.

Fig. 1 shows an aquatic feeding system that is suitable for salmon farming amongst other applications. The feeding system comprises a submerged feeding device 10 that includes a fixed buoyancy float 11 , an air container 12 that provides an adjustable buoyancy and a feed container 13 from which feed is supplied to aquatic animals such as farmed salmon. Feed is supplied to the feed container 13 via feed supply line 14 that is connected to a feed source or feed store (not shown) on land or on a nearby vessel such as a barge. Feed is distributed to the aquatic animals through a plurality of feed outlets 15 from the feed container 13. Air or other gas is supplied to the air container 12 via supply line 16. In some embodiments air container 12 may have a variable volume that depends on the quantity of gas held within it. However, as shown in Fig. 1 , the air container 12 has a fixed volume and the buoyancy is altered by changing the ratio of air to water within the container 12. As air is supplied to container 12, e.g. pumped in through supply line 16 connected to upper inlet/outlet (upper valve) 18, water in the container 12 is forced out through lower inlet/outlet (lower valve) 17 thus increasing the buoyancy of submerged feeding device 10 so that the feeding device 10 rises in the water. Thus air container 12 acts as a variable buoyancy float that can be used to adjust the overall buoyancy of the submerged feeding device 10. Air (or other gas) can also be vented from the air container 12 back up through line 16. When air is vented up line 16, water is drawn in through lower inlet/outlet (lower valve) 17 into the air container 12 so as to decrease the buoyancy of air container 12.

A surface floating device 20 floats on top of the surface of the water. This floating device 20 moves along with the waves. In addition two flexible connecting elements 21 are connected at one end 22 to the submerged feeding device 10 and at the other end 23 to the surface floating device 20 which serves as a mount structure. The use of more than one connecting element 21 helps to keep the feeding device 10 horizontal.

In this embodiment, the flexible connecting elements 21 include a relatively heavy chain 24 that is connected to the submerged feeding device 10. The chains 24 are then both connected to a common rope 25 of lighter weight material that connects the chains 24 to the surface floating device 20. As an alternative to chains, lead rope may be used. In some examples, lead rope with a weight of approximately 2 kg / m may be used.

The rope 25 extends from the surface floating device 20 through an aperture 26 in the submerged feeding device 10 (in fact through apertures in each of the fixed buoyancy 1 1 , variable buoyancy 12 and feed container 13). This keeps the submerged feeding device 10 in line with the surface floating device 20.

To describe the operation of the system, if air is let out of the air container 12 by venting air through vent pipe 17, the submerged feeding device 10 will start to sink. As the submerged feeding device 10 sinks, more of the weight of the chains 24 is transferred over to the surface floating device 20. The overall buoyancy of the submerged feeding device 10 should be adjusted so that it will keep the feeding device 10 at the maximum recommended feeding depth for the aquatic animals. The maximum depth at which the animals (e.g. fish) are happy to feed is preferred so as to minimize the chances of sea lice infection which tends to occur closer to the surface. Cages can be constructed in a large variety of sizes with varying depths. For example some cages may have a depth of 50 metres. The feeding device 10 may be adjusted so as to provide feed at any depth within the cage or it may be restricted to a certain range of depths within the cage. For example, it may not be necessary to deploy the feeding device 10 to the very bottom, particularly in very deep cages. Therefore the feeding device may be restricted e.g. to 70% of the cage depth. A winch or similar may be provided on the floating device 20 that can draw in or let out the rope 23 and thus can be used to raise or lower the feeding device. This provides an alternative mechanism for depth adjustment that can be utilized in addition to or instead of using air supply to the air container 12 through the line 16.

On the other hand, if one pumps air into the air container 12, the submerged feeding device 10 will become lighter (more buoyant) and will start to pick up weight from the chains 24. The submerged feeding device 10 will at some stage come to an equilibrium at a water depth that relates to the amount of air held within the air container 12.

It can be seen that the surface floating device 20 can move up and down with the waves. This motion will move the weight of the chains 24 back and forth between the surface floating device 20 and the submerged feeding device 10. This will not cause the submerged feeding device 10 to move up and down significantly since the movements of the floating device 20 caused by the waves are rapid in comparison to the sluggishness of the feeding device 10. Therefore the feeding device 10 remains quite steady in the water with regard to vertical movements. The response of the feeding device 10 to the driving force of the waves will depend on many factors, including the size of the cage, the weight of the chains 24 and the size and frequency of the waves. However, purely as an example, waves of a few metres in amplitude may result in movement of the feeding device 10 with an amplitude of only a few tens of centimetres. In this embodiment, if the air container 12 is completely filled with air, the submerged feeding device 10 will float all the way to the surface and stay under the surface floating device 20. This provides an upper level for the feed supply that is close to the surface (which will sometimes be the optimal feeding level) and also allows for easy access to and maintenance of the submerged feeding device 10.

By varying the feeding depth in this manner, the farmer can regulate the feeding depth continuously according to the varying ambient environmental conditions and accordingly a degree of influence can be had on the preferred swim depths of the farmed aquatic animals. For example, salmon can be influenced to swim deeper when a high concentration of sea lice is present near the surface. This will not only benefit the farmed fish by reducing infestation, but will also reduce the breeding of sea lice and their potential to infest wild fish that pass near to the farm.

Although it is an extremely important consideration, the benefits of this system are not limited solely to the reduction in sea lice infestation, but also extend to more sophisticated control of feeding behavior and fish density within the farm cages. By monitoring the farmed animals and the various environmental factors including any of temperature, salinity, oxygen levels, chemical levels, light levels and currents, the farmer can detect abnormal behavior and adjust the feeding height to alleviate stresses for the animals so as to promote health and growth, thus increasing production.

Fig. 2 shows an alternative view of the submerged feeding device 10, in particular showing the rope 25 passing through an aperture 19 in the centre of the feeding device 10, passing through all of the fixed buoyancy unit 11 , the air container (variable buoyancy) 12 and feed container 13.

Fig. 3 shows a second embodiment of a variable depth aquatic feeding system that is similar in operation to that shown in Fig. 1 , but also showing the boundary of cage 150. However, instead of a single surface floating device, this embodiment provides two (or more) spatially separated structures 120 (for the purpose of illustration, these may be fixed relative to land or they may be floating structures). A chain 124 and rope 125 is connected between each structure 120 and the submerged feeding device 110. The symmetry of the system keeps the submerged feeding device generally centrally positioned with respect to the structures 120.

The air supply 116 in addition to being used to adjust the buoyancy of submerged feeding device 110 is also used to supply air to a conical structure 130 which is open at the bottom and closed at the top. This provides a localized air/water interface 131 lower than the air/water interface at the sea surface 140. This localized air/water interface 131 allows fish to gulp air to fill their swim bladders without having to visit the upper region of the cage 150 which may be sub-optimal, e.g. due to an abundance of seal lice larvae. Fig. 4 shows a third embodiment of the system. Fig. 4 is similar to Fig. 3, showing the cage walls 250 and sea surface 240. A feeding device 210 is connected to each of two (or more, not shown in the Figure) spatially separated surface structures 220 (which may be fixed relative to the land or seabed or may be floating on the sea surface) by chains 224 and ropes 225 (together forming flexible connecting elements 221). A feed supply unit 260 (this may be a buoy or similar) is also shown, floating on the sea surface 240 with a first feed supply line 261 that supplies feed (generally in the form of pellets) to the feed supply unit 260. A second feed supply line 214 then supplies feed to the feeding device 210 from the barge 260. A water supply line 262 provides water to the feed supply unit 260 for hydrating and softening the feed pellets.

Fig. 5 illustrates the operation of the third embodiment in more detail. Feed pellets are supplied through supply line 261 by blowing air through the line 261. As the pellets reach the feed supply unit 260, they fall to the bottom of the unit 260 while the air from supply line 261 is vented through an opening in the top of supply unit 260. Meanwhile, compressed air is supplied through pipe 263 to the bottom of water supply line 262. As the compressed air rises up water supply line 262, water is carried with it. Upon reaching the feed supply unit, the compressed air is released through the opening in the top of supply unit 260, while the water is delivered to the base of the unit 260. As the feed pellets are delivered into water within the feed unit 260 they absorb water, becoming softer and more easily digestible by the animals in the cage 250. Being mixed with the local sea water also means the feed pellets have the right salt balance. The softened pellets are then delivered through second feed supply line 214 down to the submerged feeding device 210 with variable buoyancy and thus variable (and controllable) height/depth so that they can be delivered to the animals at the optimal depth for feeding and health. Submerged feeding device 210 has a fixed buoyancy part 211 that supports some (but not all) of the weight of the feeding device 210. The rest of the support comes from the variable buoyancy air container 212. The ratio of air 212a to water 212b can be varied by supplying air or drawing air back through an air supply pipe (not shown in this figure). The air container 212 in Figs. 4 and 5 is open at the bottom so that water may be readily drawn in or pushed out under the changing volume of air 212a. As the container 212 is fully open at the bottom, the animals may also swim up into the container 212 where they can access the air 212a. This allows fish to gulp air to fill their swim bladders without needing to swim up to the sea surface 240 where they would be at more risk of lice infestation.

The submerged feeding device may of course be provided with sensors so as to sense the depth and/or the amount of air 212a and/or water 212b present within the air chamber 212. The sensed data may be fed back to a controller (along with other useful information such as temperature, salinity, current direction and velocity data and other indicators of animal wellbeing or stress that may be useful for controlling and adjusting the depth of feeding device 210. Figures 6 and 7 show a fourth embodiment of a variable depth aquatic feeding system that is similar in operation to that shown in Figs. 1 and 3. The boundary of cage 350 is shown. A surface floating device 320 is shown in the form of a floating ring (best seen in Fig. 7 which shows a top view). This floating surface device may be fixed relative to land, e.g. anchored to the seabed or it may be a purely floating structure, e.g. anchored to a barge. Two flexible connecting elements 321 are provided, each comprising a weighted part such as a chain 324 and a lighter part such as rope 325. Each is connected to the surface ring 320 and the submerged feeding device 310. The symmetry of the system keeps the submerged feeding device generally centrally positioned with respect to the structure 320. In particular, the two flexible elements 321 are connected to points on the ring about 60 degrees apart so as to form a triangle. Opposite these two connections on the structure 320 is the feeding hose 314. This is typically a strong plastic hose, e.g. made from polyethylene and extends in the opposite direction to the two flexible elements 321. Together these three connections hold the feeding device 310 stably in

approximately the centre of the cage, i.e. in the centre of the surface ring 320. As the two flexible elements 321 hang in arcs between the feeding device 310 and the surface ring 320 they can accommodate movements of the cage that may occur due to wind and waves and currents. Thus there will be no sudden jerks or high forces on the feeding device 310 which remains relatively stable both in height and in lateral position. There is thus a low risk of breakage of the chains/ropes 324, 325 and therefore low risk of the associated damage to the cage walls 350 that may occur if any part of this mounting system were to break free.

Figure 7 shows the system from above and illustrates several feeding outlets 315.

Figure 6 shows the system from the side and also shows the air hose 316 and a camera 370 which hangs below the cage to monitor feeding activity. The camera 370 is adjustable in height by adjusting the length of camera line 371 which runs round a pulley 372 on feeding device 310 to keep it vertically underneath the feeding device 310 for best monitoring. Line 373 is also connected to camera 370 to allow the camera 370 to be pulled up easily for cleaning or maintenance. A data connection cable may also be integrated with either one of lines 371 or 373 or may be provided separately. Lines 371 and 373 connect back to a camera operation point 374 which may be located on the floating ring 320 or on a barge.