The present invention relates to a method for formation of a vertical water column in a fish farm.

Background to the invention

A characteristic of salmon is that it naturally faces the direction of the flow of water. Other parameters that influence where the salmon chooses to be are light, oxygen and temperature. In connection with fish farming, it can appear as if

light/temperature/current are parameters that have the strongest influence on behaviour.

Description of prior art

EP0574589 A1 relates to a method and apparatus for the farming of marine organisms during the circulation of the water in the upper and lower sections of a farming tank by conducting aeration in the tank so that a vertical convection flow of the water is set up in the tank. When air is supplied to the tank via an air supply line, a vertical convection flow is set up due to the lifting force of the bubbles.

US5158037 A relates to the use of air curtains that are formed in farming appliances for fish. Oxygen/gas is supplied at the bottom sections of the appliances.

US3913525 A relates to a farming tank where water currents are formed with the help of an air supply to the bottom section of the tank.

JP56048837 A2 relates to an apparatus for the formation of a bubble blanket. It describes the use of a ring-formed pipe/hose, which, submerged in a net cage, supplies air that rises as an air curtain which spreads out towards the periphery towards the surface of the water.

US4776127 A relates to a ring-formed air supply hose placed at the bottom of a container with fish. US4488508 A relates to supply of air from diffusors in the body of water to generate currents/circulation. Furthermore, US 2010/154717 A1 shows a device for the supply of gas into water in net cages, tanks or dams with a pump and a disperser to bring the gas into contact with the water, and a water outlet. The pump is a submersed pump that sucks water directly from the net cage, the dam or the tank, and a direct contact between the pump and the disperser is made to generate a submersible unit. The gas is pressurised oxygen or oxygen enriched air, the disperser is a venturi disperser, the pump and the disperser are combined in a sealed housing and the device comprises a hook or an eyelet or another mechanical construction, permanent or adjustable, to hang up the unit.

Although it is suggested in US 2010/154717 A1 to use a ring with holes, it describes the use of a submersed pump, which together with a venturi disperser supplies oxygen rich water to the ring and where the oxygen rich water is let out into the net cage via the holes in the ring. The aim is to increase the oxygen level of the water in the net cage and, particularly, in a wide area around the ring.

Objects of the present invention

By using air bubbles of a certain size and supplied in a given pattern, it is possible to get the air bubbles to drag water along up to the surface, thereby generating a vertical water column in the body of water in the net cage.

The system can also be used to regulate the behaviour of the salmon. As the upward current that is generated has such a strong influence on the salmon, it will be collected here. By correct use, the salmon will then naturally be brought together to such an extent that it will be possible, for example, to bring it up with a suction hose without having to drive it with a sweep net.

When the salmon enters into this upward current, it will go downward in the body of water at the same time. When it gets to the end, it will go out of the current. Seeks the surface to go down again in the current. This leads to several more rounds trips from the surface and down into the deep than in normal current conditions. This will, in turn, generate several pressure changes in the course of a period of time that can influence the lice negatively. At the same time as influence from the bubbles directly, and also the reaction that takes place as they are dispersed in the sea will probably influence the lice negatively.

The idea is to be able to control which environment that is offered to the salmon, in that the water layer that is wanted inside the net cage is actively manipulated. Then, there will be a need for large volumes of water at certain periods of the year, to be brought from, for example, 25 meters depth and up to the surface.

This water has initially a greater density than the surface water so it is important that it is mixed well during its travel upwards so that is does not sink immediately.

Consequently, it is an object to provide a method and an apparatus where at least some of the mentioned advantages are obtained. Furthermore, it is an object to provide a vertical water column in a fish farm, where the water column is formed by a bubble blanket of air bubbles.

Summary of the invention

The above mentioned aims are reached with a method for the formation of a vertical water column in a fish farm, comprised of the following steps:

submersion of a circular ring equipped with a number of nozzles to a depth of more than 20 m in the body of water in the net cage,

supply of air to the submersed ring,

release of a ring-formed flow of air bubbles from the nozzles, with the openings of the nozzles being set at a diameter of between 1 -2 mm, and

control of the air supply so that the air bubbles that are released from the ring, in an area above the ring, form a bubble blanket that drags the water along to form the vertical water column. Alternative embodiments are given in the dependant claims.

The ring can be lowered down in the volume of water in the net cage to a depth of 25 m, and the bubble blanket can be formed 5 m above the ring of the ring-formed flow of rising and expanding air bubbles.

The ring can be formed with a diameter between 1 .6 - 3 m and be equipped with 20 - 40 nozzles.

In a certain embodiment, the ring can be formed with a diameter of 1 .6 m and be equipped with 20 nozzles that have an opening diameter of 2 mm. In another given embodiment, the ring can be formed with a diameter of 3 m and be equipped with 40 nozzles that have an opening diameter of 1 mm. Air is readily suppled to the nozzles of the ring at an overpressure of 5 bar. The amount of air that is supplied to the ring's nozzles can vary between 400-850 litres/hour depending on the desired water flow in the water column. Air is preferably supplied via an air hose from an air compressor.

Furthermore, it is advantageous that the ring is placed horizontally in the body of water of the net cage. Furthermore, the pattern of the air bubbles that are released from the ring can be varied in that it is possible to close nozzles.

Description of the figures

Preferred embodiments of the invention shall be described in the following in more detail with reference to the enclosed figures, in which:

Figure 1 shows a principle diagram of a fish farm and apparatus to carry out the method according to the invention.

Figure 2 shows a ring for use by the method according to the invention. Description of the preferred embodiments of the invention

Figure 1 shows schematically a fish farm 10 of a known type, be it a closed or an open net cage.

A ring 14 is placed submersed in the body of water 30 in the net cage. The ring 14 can be suspended in cables or the like in a known way, or in any other way be placed in the net cage 10. The ring 14 is equipped with several nozzles/holes 16 circumferentially placed around the ring 14. The nozzles 16 are preferentially placed evenly around the ring 14, and on the top side of the ring, i.e. the side that faces upwards to the surface 32. However, the outlet openings of the nozzles 16 can be placed arbitrarily around the circumference of the ring 14.

Alternatively, it is possible to close the nozzles 16 so that the number of nozzles 16 that shall be open can be decided on. The number of open nozzles can be decided on before the ring 14 is submersed, or the nozzles 16 can be equipped with remote controlled closing mechanisms. Thereby, the pattern of the air bubbles 40 that are released can be varied. For the supply of air to the nozzles 16 of the ring 14, an air supply hose 18 can be used that stretches down to the ring 14 from an air compressor 20 mounted on the net cage 10 or at another central place on the installation. Air is released from the nozzles 16 of the ring14 so that upwardly rising air bubbles 40 are generated and which drag water along to form a vertical water column in the body of water 30 of the net cage 10. Air bubbles which are too large will arrive at the surface without being dispersed in the body of water and they will not drag along any water. Bubbles which are too small will disperse early and not drag along any water either. The correct size, but in a wrong pattern will drag along some water but not very effectively.

It has been found out that air bubbles released from nozzles of 1 - 2 mm will have the right characteristics if they are released at a depth of 25 m. Then, it is also important that pressure and amount of air are correct. Ideally, one operates with an overpressure in the system of 5 bar at the surface (in the supply hose). The amount of air that is used ranges from 400 - 850 litres/hour according to how much water flow one wants. The pressure ought to be as given, but if one goes up or down in the amount beyond the required area the effect will reduce. With such a solution, one obtains an air expansion of the air bubbles from 25 to 10 mbs (metres below the surface) and the air bubbles are dispersed into the water before the surface.

It is also important that the air bubbles 40 are added in a correct pattern into the body of water 30. By using a ring 14 with a diameter of 1 .6 m with 20 nozzle openings of 2 mm, the air bubbles 40 that come out will rise and expand so that five meters above the ring 14 they form a dense bubble carpet 34 that drags water along.

If the ring 14 does not lie horizontally, the effect will diminish. At a 45 degrees tilt of the ring the effect is, in the main, gone. The effect also decreases with too much air.

It is important that the size of the air bubbles is such that they are, by and large, dispersed just before the surface. This ensures that the "heavy" water does not sink down again so quickly.

Accordingly, one has found that an optimal ring is 3 m in diameter and has 40 nozzle openings of 1 mm. This leads to a particularly wide and good upward flow. This will, with an installation at a depth of 25 m, lead to a good upward flow from 20 m and up to the surface, in particular, in a net cage of 160 m. With the use of the ring 14 as explained above, a water column 36 will be set up in the net cage 10, which leads to a vertical movement in the water such that the water surface 32 becomes a pressure that forces the water at the surface out to the side, and thereby against inflow of water at the surface, at the same time as the water in the net cage 10 is supplied oxygen. As a consequence of this happening, water replacement is also taking place vertically in the net cagel O, and the water becomes enriched with oxygen. The salmon in the net cage 10 will seek in towards the water column 36 and swim actively around the column, which leads to better exercise for the fish, better feeding, and also a possibly lower exposure to floating lice larvae. Such natural concentration of the fish in a concentrated area can also simplify the sorting and removal of the fish from a single net cage, as there will be more fish in one given area.

In a practical and simple embodiment example, a plastic ring can be placed at a depth of 25 m, and be made from a 32 mm water hose. The diameter is 300 cm. 40 holes/nozzles with a diameter of 1 mm are bored into the ring. The air is supplied from a compressor 20, that can supply 3600 l/min at 7 bar. The compressor 20 can be placed on the net cage 10 or more centrally on the installation, and will have sufficient capacity to distribute air to up to six net cages. In this situation, it is important that all the rings lie at the same depths in the net cages