AU2019382882B2

AU2019382882B2 - System for bathing fish in marine fish farms

Info

Publication number: AU2019382882B2
Application number: AU2019382882A
Authority: AU
Inventors: Geir Terje RESSEM; Henrik TRENGEREID
Original assignee: Akvafresh As; Marine Harvest Asa
Current assignee: Akvafresh As; Marine Harvest Asa
Priority date: 2018-11-23
Filing date: 2019-11-21
Publication date: 2023-03-16
Anticipated expiration: 2039-11-21
Also published as: NO20181509A1; EP3883372A1; CA3120416A1; WO2020104604A1; AU2019382882A1

Abstract

The present invention relates to a system comprising a container (102) being partly submerged in a fish farm and filled with a softened brackish water having: a salinity in the range from 0.5 g/kg to 15 g/kg, determined as the total mass of Na

Description

SYSTEM FOR BATHING FISH IN MARINE FISH FARMS

Field of technology

The present invention relates to a system for forming a volume filled with a medication-free water having an anti-ectoparasite effect inside a marine fish farm. Background

Farmed fish is an increasingly important protein source for humans. According to Food and Agriculture Organization of the United Nations (FAO), the world production of farmed fish reached about 66 million tonnes in 2014. This accounted for about 40 % of the total human fish consumption in that year. One important species in this respect is Atlantic salmon. The world production of Atlantic salmon in 2015 was above 2 million tonnes, of which more than half was produced in floating net-cages along the Norwegian coast.

The economy in fish farming speaks for having the fish populations in the fish farms as dense as possible. However, dense fish populations increase the risk for outbursts of infectious deceases and detrimental growth of parasitic life forms in the fish population. There is a range of ectoparasites, parasites adhering to and feeding from the skin and other exterior parts of the fish tissue. For example, two parasitic organisms that cause significant losses to the fish farming industry of salmonids around the world are sea-lice and amoeba causing amoebic gill disease (AGD). Sea-lice are a family of parasitic copepods of which there are several species naturally occurring in seawater. Sea-lice spreads by releasing eggs which may float up to tens of kilometres in the surface region of the seawater and gradually develops into larvae. The larvae actively seek a host fish it may adhere to and develop into grown sea-lice. The sea-lice is an ectoparasite which eats skin, mucus and blood of the host fish and causes problems with increased susceptibility for developing infectious diseases, reduced growth, haemorrhaging of eyes and fins. The dominant sea-lice specie causing losses to Norwegian fish farmers is Lepeophtheirus salmonis.

Amoebic gill disease (AGD) is a potentially fatal disease caused by the amoeba Neoparamoeba perurans, which adheres to the gills of the host fish and causing problems with build-up of mucus on the gills and hyper-plastic lesions which gradually develops into deterioration of the gill tissue and severely compromising of the oxygen transport across the gill. Treatment costs for AGD-outbreaks are reported representing 10 - 15 % of the value of the fish stock for fish farms in Tasmania, and Australia.

Regulations set by the Norwegian Food Safety Authority has set an upper limit of 0.5 adult female sea-lice per fish in a fish farm [1] However, due to the dense population of fish in fish farms providing excellent growth and spreading conditions for the sea-louse and naturally occurring in-flow of sea-lice with the seawater, it is often necessary, at least periodically, to actively suppress the number of ecto parasites in the fish farm water and/or to treat infected fish to keep the sea-louse population at safe levels and in accordance with the regulations.

Sea-lice cause damage to salmonids by eating their mucus, skin tissue and blood paving way for other problems such as bacterial or fungal infections. According to the Institute of Marine Research [2] sea-lice are presently one of the most important causes of mortality in farmed salmonids. At a rough estimate, each year the

Norwegian aquaculture industry loses around NOK 500 million from sea-lice related damages and for costs associated with alleviating or preventing sea-lice outbreaks.

Prior art

There are known and tested several strategies for reducing/preventing sea-lice related damages causing reduced fish welfare and/or fish health in fish farms.

One strategy is to apply closed containers/tanks for raising the fish to isolate the fish environment from ambient natural (potentially ectoparasite containing) seawater, and instead supplying the tanks with seawater added one or more ectoparasite hostile compounds or removing and/or killing ectoparasites. Another strategy is to utilise that sea-lice occurs naturally in the upper layer of the seawater such that applying a sea-lice skirt or lowering the fish to deeper water will spare them from being exposed to the lice. The latter be obtained by applying submersible cages which may be submerged in periods with relatively much sea-lice in the upper layer of the sea). Both these solutions require relatively expensive infrastructure both in capital and operational costs and are thus less suited for small enterprise fish-farming.

Another strategy is medically treating the fish either orally by adding antiparasitic ally active substances (medication) the fish farm food supply or by bathing treat ments where the fish is transferred to water having one or more antiparasitically active substances and contained in this water to kill or paralyse the ectoparasites to make them release themselves from the host. This method has environmental issues such as spreading of antiparasitically active substances into the surroundings of the fish farms, fish health issues related to the fish being exposed to the antiparasitic ally active substances, and problems with reduced effect of the treatment over time due to the ectoparasites developing resistance towards the antiparasitically active substances. Medication is presently the most common treatment for sea-lice, and it is still a relative effective strategy according to the Norwegian Veterinary Institute activity report of 2015. However, this may change in near future due to the increasing resistance towards the medication. The activity report of 2015 states that sea-lice represents presently the biggest challenge for the salmon fish-farming industry in Norway.

A strategy being increasingly applied and aimed at resolving the increasing resistance towards medication is mechanically treating the fish to remove attached sea-lice. This may be obtained by brushing and/or flushing the fish with fresh- water/hot water to scour off the sea-lice. Another solution is making the fish swim though a zone where one or more lasers are applied to shoot laser pulses killing or stunning attached sea-lice and cause them to release from its host. These methods involve mechanically handling the fish which causes significant stress and reduces fish welfare to a degree which, according to the Norwegian Veterinary Institute paradoxically causes more fish deaths than the sea-lice in the present fish farming industry.

There is a need for medication free anti-lice treatments which preferably does not involve any handling or forced movement/transferring of the fish.

It is well known that salmonids swimming up in ri vers to spawn lose much of their marine ectoparasites in a matter of hours up to a week. Marine ectoparasites have in general low tolerance for freshwater. This fact has been utilised by the fish farm to treat infected salmonids for ectoparasites by transferring the fish from the net cage to a separate tank filled with freshwater and containing the fish in the freshwater for a period sufficient to kill/stun a major part of ectoparasites on the fish such that they release from their host, and then transferring the treated fish back to the seawater inside the net cage. The freshwater tank is typically on a barge which anchors next to the net cage when the fish is to be treated.

However, catching the fish inside the net cage and transferring it into the freshwater tank and back involves significant stress and handling of the fishes having a detri mental impact on fish welfare and health, and causing losses to the farming industry. It is highly advantageous if the fish could be made to swim by their own motion into the freshwater when a freshwater bath/ treatment is due and then made to swim back into the seawater of the fish farm net cage after the bath/treatment. From NO 334487 it is known a freshwater filled tank floating inside a net cage of a fish farm. A facsimile of figure 1 of NO 334487 is given in figure 1 herein. As seen on figure 1, the tank (1) is floating in the seawater inside a fish net cage. The tank (2) is closed at the top by a roof (2b) and has openings (3) at the bottom such that fish are free to swim in and out of the interior of the tank. The floor (2a) of the tank is inclined downward towards the openings (3) to better transport out excrements and other (sinking) debris from the tank and to make it easier for fish inside the tank to find the exit. The tank (1) has an air vent (6) for venting air/gases from the interior of the tank. Fresh water is fed from an external source via freshwater supply (4) and nozzles (5) to the top of the tank (1). The freshwater has lower density than seawater and will naturally accumulate at the top/upper section of the tank. The tank (1) has further one or more nozzles (7) for feeding pressurised air into the tank. It may also be inj ected oxygen if necessary or beneficial. A lamp (10) is mounted at the top inside the tank (1) to lure fish to swim into the tank by use of light, and similarly a lamp (1 1) is mounted on the exterior side of the floor (2a) to apply light to lure fish to swim out of the tank. It may also be applied smell to lure fish into the freshwater-filled tank.

The solution of forming a treatment zone of freshwater being in direct contact with ambient seawater is best suited for in-shore fish farms because the huge number of fish and corresponding dimensions of the net cages in commercial fish farming requires large volumes of freshwater to form and maintain the freshwater zone. For open-ocean fish farming the costs and practicalities required to supply the fresh water becomes prohibitive for commercial fish farming. It has also been experien ced that the fish is reluctant and difficult to be made to swim by its own motion into a freshwater zone, probably due to the osmotic shock when passing abruptly from seawater to freshwater.

From US 2012/0152721 it is known that divalent ions such as e.g. Ca²ⁱ and Mg² may be selectively extracted from seawater by use of nanofiltration.

Objective of the invention

It is an objective of the present invention to provide a system for forming a volume of a medication-free water being hostile to calcium sensitive marine ectoparasites inside a marine fish farm.

A further objective of the invention is to provide a system for forming a volume of a medication-free water being hostile to calcium sensitive marine ectoparasites preferably one or more of: Neoparamoeba perurans, Lepeophtheirus salmonis, or a Caligus specie, where the medication-free water of the water-filled volume is in fluid communication with ambient seawater such that the farmed fish may swim into the medication-free water by their own motion.

Description of the invention

The invention is based on a discovery believed to be novel and described in detail in the co-pending, yet to be published, application PCT/EP2018/063656, herein incorporated in its entirety by reference. The discovery is that water having a specific tailored salinity, hereinafter termed as“softened brackish water”, has shown to be at least as effective as freshwater in killing/ stunning Neoparamoeba perurans, Lepeophtheirus salmonis at the copepodid stage, and even more effective than freshwater in killing Lepeophtheirus salmonis at one or more of its adult life- cycle stages. This effect is observed both for non-attached free-swimming ectoparasites and ectoparasites attached to a host fish. Even though the softened brackish water applied in the present invention has been proven to be effective in killing AGD-causing amoeba and sea-lice attached to salmons, it is believed to be effective against any ectoparasite being sensitive towards water with low calcium contents.

The softened brackish water, which is suitable for use in the present invention, is characterised by having:

1) a salinity in the range of from 0.5 to 25 g/kg, determined as the total mass of Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl , HCO3 , CO3² , and SO4² ions being present in a sample of one kg of the water, and

2) a Ca²⁺ content of < 100 mg/kg, determined as the mass of Ca²⁺ ions in a sample of one kg of the water.

Thus, in a first aspect, the present invention relates to a system, comprising:

a container (102) being open in its bottom (103) towards ambient seawater (104) and open in its top (105) towards ambient air (106) and being located partly submerged in the seawater of a marine fish farm such that it partly protrudes out of the water and partly has an inner section (1 10) of the container filled with water, characterized in further comprising:

a supply (100) of softened brackish water having:

i) a salinity in the range from 0.5 g/kg to 25 g/kg, determined as the total mass of Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl , HCO3 , CO3² , and SO4² ions being present in a sample of one kg of the water, and ii) a Ca²⁺ content of < 100 mg/kg, determined as the mass of Ca²⁺ ions in a sample of one kg of the water,

an oxygen supply (101) adapted to adding oxygen to the softened brackish water,

and

a distribution manifold (107) in fluid communication (109) with the supply (100) of softened brackish water, and which has one or more injection nozzles (108) adapted to inject softened brackish water from supply (100) to the inner section (110) of the container (102).

Alternatively, the softened brackish water of the supply (100) may have a Ca²⁺- content selected from one of the following ranges; from 0.001 to 95 mg/kg, from 0.001 to 90 mg/kg, from 0.001 to 80 mg/kg, from 0.001 to 50 mg/kg, from 0.001 to 10 mg/kg, from 0.001 to 8 mg/kg, from 0.001 to 6 mg/kg, from 0.001 to 5 mg/kg, from 0.001 to 4 mg/kg, or most preferred from 0.001 to 2.5 mg/kg, determined as the mass of Ca²⁺-ions in a sample of one kg of the softened brackish water. Alternatively, the softened brackish water of the supply (100) may have a salinity selected from one of the following ranges; from 1 g/kg to 25 g/kg, from 2 g/kg to 25 g/kg, from 3 g/kg to 25 g/kg, from 4 g/kg to 25 g/kg, from 5 g/kg to 25 g/kg, from 5.1 g/kg to 25 g/kg, from 1 g/kg to 22.5 g/kg, from 2 g/kg to 20 g/kg, from 3 g/kg to 17.5 g/kg, from 4 g/kg to 15 g/kg, or from 5 g/kg to 12.5 g/kg, or most preferably from 5.1 g/kg to 10 g/kg, determined as the total mass of Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl , HCO3 , CO3² , and SO4² ions being present in a sample of one kg of the water.

Furthermore, the softened brackish water may alternatively have a salinity chosen from any of the above given alternative ranges for the salinity and calcium content chosen from any of the above given alternative ranges for the Ca²⁺-content.

An example embodiment of the system according to the invention is schematically illustrated in figures 2 a) and 2 b). In figure 2 a) the system is drawn as a cut view seen from the side in figure 2 b) the system is drawn as a cut view seen from above and taken along the dotted line A - A’ in figure 2 a). The example embodiment of the system shown in these figures has a container (102) consisting of a substantially vertically standing cylindrical tube being partly submerged such that the upper part of the cylinder protrudes a distance out of the water to avoid seawater from entering the inner space (110) by flowing/spilling over the upper edge of the container (102). The system further comprises a source of softened brackish water, here illustrated generically as a box (100) since the system according to the invention may apply any known or conceivable device and/or method for providing this water. It is necessary to add oxygen to the softened brackish water inside the container to replace oxygen consumed by the fish (respiration) etc. In this example embodiment, the oxygen is added to the softened brackish water before being injected into the container as illustrated generically by box (101). However, the invention may apply any known or conceivable way of adding oxygen to water, either before inserting it into the container such as shown in figure 2 a), or by adding the oxygen directly to the softened brackish water inside the container, or a combination of both. The system applies at least one manifold (107) having at least one nozzle (108) for injecting softened brackish water from supply (100) into the water-filled inner section (110) of the container (102). The manifold (107) of the example embodi ment shown in figures 2 a) and 2 b) is a hollow annular structure having eight injection nozzles (108) evenly distributed along its inner perimeter such that the softened brackish water is injected substantially into the plane formed by the horizontally cross-section of the manifold. The supply (100) may advantageously supply a continuous flow of softened brackish water to the one or more manifolds (107).

The system of the invention may apply any known or conceivable manifold for injecting softened brackish water into the water of inner section (1 10) of the container (102). The manifold (107) may in one example embodiment advantage- ously be located in the upper part of the water-filled inner section (110) of the container to establish a flow of the softened brackish water from the upper part of the inner section (1 10) towards the lower opening (103). The manifold (107) may further, in one alternative, be height-adjustable to enable injection of softened brackish water at several heights inside the water-filled inner section (110). The nozzles (108) may in one example embodiment advantageously be adapted to inject the softened brackish water at laminar flow conditions in a substantially horizontal direction, i.e. the softened brackish water is injected in a direction being practically normal to the earth gravity field and with sufficient low flow velocities (i.e.

Reynolds numbers) to avoid the injected water forming vortices or any other water streams characteristic of turbulence in the water mass of inner section (1 10) to keep vertical mixing of the water masses in inner section (110) at a minimum.

The flow velocities required to keep the injected water in laminar flow conditions depend on the characteristic linear dimensions of the flow, However, the concept of laminar flow is well known to a person skilled in the art such that the determination of the flow volumes of water injected through nozzles (108) to keep the injected water in a laminar flow regime may be established without undue burden on a case- to-case basis. The number of nozzles (108) on the manifold (107) may be adapted to the required flow volumes of softened brackish water to be injected into the container. The invention may further apply more than one manifold (107). The manifold (107) is set in fluid communication with the supply of softened brackish water by line (109).

The height of the part of the container (102) protruding up from the water required to avoid inflow of seawater over the upper edge depends on the weather conditions and typical roughness of the sea in the area the fish farm is located and needs to be determined for on case-to-case basis. In practice, a“free height” of at least 0.7 m has proven to be necessary but may advantageously be higher. The present invention may apply a container (102) having a vertical length providing as high free-height as necessary to keep the ambient seawater out of the interior space of the container.

The system shown in figure 2 a) is located inside a buoyancy-type net cage fish farm comprising a net (122) and a floating ring-structure (121). This is only for illustrative purpose to show that the system is to be located in the water inside a fish farm. The system according to the first aspect of the invention may be adapted to and applied in any known or conceivable marine fish farm.

The term“upper part” of inner section (110) as used herein, means unless specified otherwise, at a vertical height somewhere in the upper 1/3 of the water-filled interior space of the container (102) as indicated by the curly bracket (115) on figure 2 a). Likewise, the term“lower part” of inner section (1 10) as used herein, means unless specified otherwise, at a vertical height somewhere in the lower 1/3 of the water-filled interior space of the container (102) as indicated by the curly bracket (116) on figure 2 a). The vertical height between the lower and upper part will be termed as the middle part of inner section (110).

Without being bound by theory, it is believed that the relatively small difference in salinity between the softened brackish water and natural seawater, and thus a relatively low osmotic shock (as compared to prior art freshwater attempts) felt by the fish when swimming from ambient seawater into the softened brackish water of the inner section, is a reason why salmonids are observed to voluntary seek and swim into the water of the inner section (110) of container (102). Thus, since the softened brackish water is hostile to calcium-sensitive ectoparasites, both towards ectoparasites being present in the water and being attached to a host fish, fish swimming into the inner section of the container filled with the softened brackish water will be treated for ectoparasites without use of any medication and without any mechanical or other stress-inducing handling. Experiments disclosed in yet to be published application PCT/EP2018/063656, herein incorporated by reference, show that ADG-causing amoebas and/or sea-lice in both copepodid and adult stages attached to salmonids begin to die and fall off their host in a matter of hours when the fish dwell in the softened brackish water. The hostility of the softened brackish water towards marine ectoparasites is discussed thoroughly and verified by experiments in yet to be published application PCT/EP2018/063656 and found to be present when the calcium ion content of the water is less than 100 mg/kg.

Natural seawater has typically a calcium ion content of around 400 mg/kg such that vertical stirring of the softened brackish water, especially at the lower part of inner section (110), may result in the water of the inner section being mixed with too much seawater and thus significantly less hostile to the ectoparasites. Some in mixing of seawater is unavoidable due to the direct contact with ambient, often transversally flowing, seawater in the lower opening (103) of container (102), and further due to drag forces/water displaced by swimming fish in and out of the inner section (110) through opening (103). In one example embodiment, this in-mixing of seawater at the lower part/bottom opening (103) of the container (102) may be reduced by having a smaller cross-sectional areal of lower opening (103) than the cross-sectional area of upper opening (105). This may in one example embodiment be obtained by designing the container (102) as a frustum of a circular cone with upper diameter dtop and lower diameter dbottom, or as a frustum of a pyramid having a triangular, square, hexagonal, or a polygonal base having a maximum diameter dtop (longest possible distance between two points in the base) of the upper base and a maximum diameter dbottom of the lower base as indicated in figure 4 for the example embodiment of a container shaped as a frustum of a square pyramid having an upper opening defined by edges a, b, c, and d, and a lower opening defined by edges a’, b’, c’, and d’. The ratio dtop : dbottom may advantageously be in the range of from 1.1 to 1.5. The container (102) may be made of any material able to form a water tight wall separating ambient seawater from the softened brackish water of the inner section, and which is mechanically and chemically robust to withstand the forces induced by ambient flowing seawater and/or surface waves and the corrosiveness of marine water. Examples of suited materials include a tented tarpaulin gauze, plexiglass, stainless steel, aluminium, etc.

Depending on number of fishes being present in the inner section (110), the fish respiration may relatively rapidly consume the oxygen of the softened brackish water. The system according to the invention should therefore include an oxygen supply (101) adapted to add oxygen, preferably to saturation, to the softened brackish water either before injection the softened brackish water to the inner section (110), by adding the oxygen to softened brackish water being present in the inner section (110), or by a combination of both.

The addition of oxygen to the softened brackish water may be performed in any manner known or conceivable to the skilled person. In the case of adding the oxygen upstream of the inner section (110) the addition may e.g. be obtained by gas injection of an oxygen containing gas (bubbling) into the softened brackish water, or alternatively to natural seawater before it is desalinated to form the softened brackish water. The oxygen containing gas may advantageously be one of air, air enriched with oxygen, or pure oxygen gas.

The larger the gas bubble sizes of the injected oxygen containing gas, the less effective the mass-transfer of oxygen into the water becomes and the more vigorous stirring of the water due to rapidly rising gas bubbles becomes. Gas stirring of the softened brackish water in the inner section (110) should be kept to a minimum to avoid in-mixing of too much seawater and thus obtain too high calcium contents of the softened brackish water in the inner section (110) to maintain its intended hostility towards calcium sensitive ectoparasites. Large gas bubbles are also unfavourable by tending to rise too fast in the water to enable an effective mass transfer of oxygen from the gas phase to the water. They tend to reach the top of the water and release a substantial part of their oxygen content to the atmosphere. Thus, a particularly preferred example embodiment of the system according to the invention applies one or more nanobubble or sub-microbubble injection devices located in the middle and/or lower part of the inner section (110) to introduce an oxygen containing gas in the form of nanobubbles or sub-microbubbles, respect- ively.

Sub-microbubbles are gas-bubbles having a diameter in the range of from 1 to 10 pm, while nanobubbles are gas-bubbles having a diameter less than 1 pm. Such small gas bubbles have very low rising velocities such that they may stay entrained in the water mass for several hours to months and has very low to no stirring effect upon the water they are injected into. Furthermore, the small bubble sizes of nanobubbles and sub-microbubbles provide a very favourable surface-to-volume ratio and thus effective mass-transfer rate of oxygen from the gas phase to the water phase. These properties make injection of nanobubbles and/or sub-microbubbles into the softened brackish water very suited for use in the system of the invention since the addition of oxygen becomes highly effective without causing any significant vertical stirring of the softened brackish water, and further because the long residence times combined with the effective mass-transfer rate of oxygen into the water make it easy to add oxygen to supersaturation by simply blowing in more gas bubbles than required to satisfy the water with oxygen. When the water is saturated with oxygen, the driving force for mass-transfer of oxygen from gas to water becomes zero such that excess oxygen in injected bubbles remain in the gas phase until the oxygen concentration in the water becomes under-saturated. Thus, by injection a surplus of nanobubbles or sub-microbubbles of an oxygen containing gas, the softened brackish water may be maintained at close to saturation since the surplus of bubbles are an oxygen storage which rapidly replenishes oxygen consumed by the fish being present in the softened brackish water. The oxygen containing gas may advantageously be one of air, oxygen enriched air, or pure oxygen gas.

The system of the invention may apply any known or conceivable nanobubble or sub-microbubble injector. Examples of nanobubble and/or sub-microbubble gas injectors suited for use in the present invention include injectors based on pressurised dissolution, or static mixing. A pressurised dissolution injector utilises Henry’s law which determines that the solution degree of a gas in a liquid increase with the partial pressure of the gas. Thus, mixing oxygen containing gas with water at a pressure » atmospheric pressure dissolves relatively much of the gas into the water, such that when discharging the water supersaturated with oxygen containing gas through a venturi nozzle into a water phase, the rapid depressurisation causes formation of fine and ultrafine gas bubbles of the oxygen containing gas in the water being discharged. A static line mixer passes a pressurised water and gas mixture through a cylindrical guide having a guide vane setting the water and gas mixture in a swirl-flow and one or more current cutters along the periphery of the cylinder downstream of the guide vane. The combined action of the swirl flow and current cutters create fine gas bubbles by a combined of nucleation, cavitation, shear force and shockwaves. Static line mixers are able to treat relatively large flow volumes of water and they are relatively insensitive towards clogging. One or more static line mixers located into and thus sucking softened brackish water from the inner section (110), supersaturating it with an oxygen containing gas and then re injecting it back into the inner section (1 10), are thus suited for use in the present system.

The supply (100) of softened brackish water may in one example embodiment simply be a storage tank being fluidly connected to the manifold (107) by a pipe (109). The supply (100) may be located at a higher position (relative to the earth gravitational field) such that softened brackish water in supply (100) flows by the action of the earth gravity into the manifold (107) and further out of nozzle(s)

(108), or alternatively, the supply (100) may comprise pumping facilities for actively transferring softened brackish water from supply (100) to the manifold

(107) at controlled/regulated flow volumes. The softened brackish water may in this example embodiment be produced at a remote location and transported on a barge (not shown) etc. to the supply (100).

The softened brackish water may be obtained in any manner known or conceivable to the skilled person. In one example embodiment, the softened brackish water may be obtained by adding and dissolving one or more monovalent salts in freshwater to reach the intended salinity. In another and preferred example embodiment, the softened brackish water may be produced on site (at the fish farm) by desalination of seawater to reach the intended salinity and calcium content. The latter example embodiment has the advantage that the system becomes independent upon a source of fresh water (to be made saline by addition of monovalent salts), and thus better suited for use in open-ocean fish farms. A particularly preferred method of in-situ production of the softened brackish water is desalination of seawater by nano filtration. This enables a practically limitless supply of the softened brackish water. Nanofiltration of seawater to form softened brackish water may advantageously be obtained by use of one or more nanofiltration membrane modules. A nanofiltration membrane is a porous membrane which selectively retains molecules and solid particles in a liquid from passing through the membrane due to a sieving effect (mechanical retention) determined by the pores size, which typically is in the range of from 1 to 10 nm. Due to the pore sizes of nanofiltration membranes a relatively large fraction of divalent ions, such as e.g. Ca²⁺-ions, in the seawater is prevented from passing through the membrane, i.e. being retained in the retentate. In comparison, a relatively larger fraction of the physically smaller monovalent ions such as e.g. Na⁺, K⁺ etc. can pass through the nanofiltration membrane. The permeate flow consists therefore of seawater having strongly reduced contents of divalent ions and only moderately reduced contents of monovalent ions. Nano filtration membranes may further utilise the Gibbs-Donnan effect (electrochemical retention) to increase the selectivity in retaining charged particles and ions by incorporating charged molecules in the membrane matrix. Anionic nanofiltration membranes (having fixed positive charges in the membrane matrix) will, in addition to the sieving effect towards the relatively larger divalent ions, also retain divalent anions such as Ca²⁺ due to the electric repulsion forces between the anions and the fixed positive charges in the membrane matrix. This Gibbs-Donnan effect will be more effective towards divalent anions than monovalent anions, and thus increase the selectivity of the nanomembrane towards the dissolved salt in seawater. The working principle of a nanomembrane module is shown schematically in figure 5. The figure illustrates a cylindrical housing 220 having an inlet 221 for injection of seawater located at one side of the housing 220. The inlet 221 is supplied with seawater which passes into the lumen side 222 of a typically cylindrical hollow semipermeable membrane 223 as an inlet flow (indicated by arrow marked A on figure 5. Part of the inlet flow A passes through the lumen side 222 of the hollow semipermeable membrane 223 and exits through an outlet 224 located at the opposite side of the housing 220 as a retentate flow (indicated by arrow B on the figure). The remaining part of the injected seawater is due to an applied hydrostatic pressure on the lumen side 222 forced through membrane 223 and enters the compartment of the housing 220 on the exterior side 225 of the membrane 223 as a permeate flow (indicated by arrows C on the figure) and exits then via a second outlet 226 as a permeate flow (indicated by arrow D on the figure). The permeate flow is the softened brackish water produced by the nanomembrane module.

One advantage of employing a nano filtration membrane module to supply the softened brackish water is that the modules may be arranged in parallel to meet any demand for softened brackish water encountered in practice in fish farming.

Examples of commercially available nanofiltration membrane modules with neutral or anionic membranes suited for in situ desalination of seawater to produce the softened brackish water include Filmtec™ NF90-400 and Hydranautics ESPA 30G. The Filmtec™ NF90-400 module has a polyamide thin-film composite membrane with a total surface of 37 m². The module may operate with a trans-membrane pressure up to a maximum of 41 bar and be fed with seawater at a maximum feed flow of 15.9 m³/hour. The Hydranautics ESPA3 OG module has a composite polyamide membrane with total surface area of 37.1 m². The module may operate be with a trans -membrane pressure up to a maximum of 41 bar and a maximum feed flow of 17.0 m³/hour.

The seawater fed to the nanofiltration membrane module may advantageously be to one or more (coarser) filtration steps upstream of the nanofiltration to remove solids/microbiological species etc. e.g. having a diameter above 40 pm to avoid the relatively small pores of the nanomembrane becoming relatively rapidly clogged by particulates/microbiologic life in the ambient seawater. Any known filtration technique of water with a cut-off threshold of 40 pm may be applied upstream of the nanofiltration membrane module. An example embodiment comprises a cyclone filter removing particles above 200 pm and then a backwash-filter removing particles from about 40 pm and above.

A further advantage of in-situ production of the softened brackish water by desalination of seawater is that it becomes possible to control and regulate the temperature of the softened brackish water inside the inner section (1 10) by simply varying the water depth at which the seawater to be desalinated is collected. Tests made by the inventors indicate that having a different temperature of the softened brackish water as compared to ambient seawater may increase the salmonids’ tendency to swim into the inner section (1 10) by their own motion. These tests indicate that at locations in Norwegian waters, the collected seawater may advantageously have a temperature of from 0.25 to 1.0 °C, preferably of from 0.4 to 0.6 °C higher than the ambient surface water of the sea (as measured at a depth of 1 m) in winter time. In summertime the collected seawater may advantageously have the same temperature as the surface water. These preferences for the temperature of the collected seawater relative to the ambient surface water are dependent upon local climate conditions and may vary significantly from one geographic location to another. However, it is a matter of simple trial and error to determine which temperature difference to apply to make the salmonids’ to be lured to swim into the softened brackish water.

Thus, in one example embodiment, the system according to the invention may further comprise, as illustrated schematically in figure 6, a seawater intake (123) adapted to collect seawater at various water depths as indicated by the stapled arrows marked B and B’. The seawater intake (123) may advantageously have a grate or other screening device to prevent marine organisms/debris/solids etc. from being sucked up and/or clogging the seawater intake. The system may in one example embodiment further comprise a seawater intake filter unit (124) comprising e.g. a cyclone filter removing particles above 200 pm and a backwash-filter removing particles from about 40 pm and above.

The example embodiments of the container (102) shown in the figures have a floating structure (120) to keep the container floating partly submerged into the water. Alternatively, in another example embodiment, the container may, depending on which type marine fish farm construction it is being applied in, being made an integral part of the fish farm structure by being anchored and secured at the intended position inside the fish farm by one or more suspension rods etc.

mechanically linking the contained to the load carrying structure of the fish farm.

The container (102) may further comprise a releasable lid (127) adapted to cover the upper opening (105) of the container. The lid may be applied to protect against seawater spilling over the upper edge of the container in rough weather, to shield the fish inside the inner section (1 10) against sun-shine, etc.

In one example embodiment, the system according to the invention may comprise one or more lamps located at the upper part of the inner section of the container and/or below the lower opening enabling use of light to lure the fish in and out of the inner section of the container. The system may further comprise a fodder station located at the upper part of the inner section of the container.

List of figures

Figure 1 is a facsimile of figure 1 of NO 334487. Figure 2 a) is a cut view drawing as seen from the side of an example embodiment of the system according to the invention.

Figure 2 b) is a cut view drawing taken along the line marked A- A’ in figure 2 a) as seen from above.

Figure 3 is a cut view drawing as seen from the side of another example

embodiment of the system according to the invention.

Figure 4 is a drawing of an example embodiment of the container of the system according to the invention.

Figure 5 is a cut view drawing as seen from the side of an example embodiment of a nanofiltration membrane suited for use in the system of the invention.

Figure 6 is a cut view drawing as seen from the side of another example

embodiment of the system according to the invention.

Figures 7 a) and b) are photographs taken from below the lower opening of a test facility of the container according to the system of the present invention. Verification of the invention

The invention will be discussed in further detail by way of a test facility applied to test and verify that salmonids may be made to voluntarily swim into the softened brackish seawater in the inner section of the container.

The container of the test facility consisted of a 165 x 7 m tarpaulin gauze made into a vertically oriented cylinder of inner diameter of 52 m which is tented and made to float such that one meter of the cylinder wall protrudes up from the sea surface and forms a 6-meter-deep cylindrical wall enclosing a water-filled inner section being opening at the bottom towards ambient seawater.

A manifold was located at 1 -meter water depth inside the inner section of the tarpaulin container and injected in a horizontal direction a continuous volume flow of 600 - 650 m³/hour of softened brackish water at laminar flow conditions into the water of the inner section via a set of A manifold was located at 1 -meter water depth inside the inner section of the tarpaulin container and injected in a horizontal direction a continuous volume flow of 600 - 650 m³/hour of softened brackish water at laminar flow conditions into the water of the inner section via a set of one multi nozzle spreading the water horizontally from 200 small nozzles. The softened brackish water supplied to the manifold was made by desalination in a nano filtration membrane module section of 90 pc NF90 membranes in 5 elements pressure vessels with 18 vessels in parallel. The seawater was collected in the sea outside the fish farm at a depth of 32 meter and had a temperature of 9.6 °C, a total salinity of 15.5 g/kg, and a calcium content of 3 - 4 mg/kg after passing through the nanofiltration membrane module section. The softened brackish water was supersaturated with traditional oxygen cones that was working with a back pressure of 1.2 - 2.5 bars to reach an oxygen saturation of 200 - 250 % before being injected into the inner section.

The total salinity, oxygen level, and temperature of the water inside the inner section were measured and monitored at 1, 3 and 5 metres depth during the verification test. After stabilisation (i.e. effective replacement of the initial seawater filling the inner section before injection of the softened brackish water) the temperature was found to be 9.5, 9.4, and 9.1 °C at 1, 3, and 5 metres depth, respectively. The total salinity was found to be 15.5 g/kg, 19.0 g/kg, and 26 g/kg at 1, 3, and 5 metres depth, respectively. The oxygen content was measured to be about 150 %, 105-110 %, and 85-90 % of saturation at 1, 3, and 5 metres depth, respectively. As a comparison the seawater below the tarpaulin container was measured to have a total salinity of 32 g/kg, a calcium content of 365 mg/kg, a temperature of 8.9 °C, and an oxygen content of 80 - 85 % of saturation.

During the test, a camera located below the lower opening of the tarpaulin container took pictures upward to monitor and verify whether the salmonids are attracted and made to swim into the inner section of the container. Figures 7 a) and b) are a replicate of two typical results. The photograph of figure 7 a) shows a section of the tarpaulin wall (300) as from below. The bent rod (301) is an annularly shaped weight used to tent and keep the tarpaulin tight. The weight is attached to the lower part of the tarpaulin by ropes (302). The photograph is taken from below in upwards direction the surface of the water which is visible as the relatively bright parts (303) on both the outside and at the inside of the tarpaulin container. As seen the photo graph, the inner section (304) is filled with a huge number of salmons (305) which are free to swim in and out of the inner section. The tempered and oxygenated softened brackish water at the inner section (304) is so attractive to the salmons that they much more densely present in the inner section than outside of the tarpaulin container.

A similar result is documented by the photograph replicated in figure 7 b) which is taken from right below the lower opening and looking straight up towards the surface water (303) of the inner section (304). As seen on the photograph, numerous salmons are swimming and dwelling calmly in the tempered and oxygenated softened brackish water at the inner section (304). The water of the inner section (304) may be considered a spa bath for salmons.

These photographs and the experiments thoroughly described in yet to be published application PCT/EP2018/063656, herein incorporated in its entirety by reference, documents that at least the salmons being present in the upper and middle layers of the inner section (304) will be subject to a water having such low calcium contents that the water is hostile towards calcium sensitive ectoparasites being attached to the fish, and thus be treated for AGD, sea-louse etc. without use of any medication or any compulsory force/handling of the fish. References

1. https://www.mattilsynet.no/fisk_og_akvakultur/fiskehelse/fiske_og_skjellsyk dommer/lakselus/fakta_om_lakselus_og_lakselusbekjempelse.23766

2. https://www.hi.no/temasider/parasitter/lus/lakselus/en

3. Powell and Kristensen, 2014,“Freshwater treatment of amoebic gill disease and sea-lice in seawater salmon production; considerations of water chemistry and fish welfare.”, NIVA, Report No. 6632-2014.

4. http://www.imr.no/nyhetsarkiv/2016/juli/ferskvatn_drep_best_unge_lakselus /nb-no 5. http://www.imr.no/nyhetsarkiv/2016/juli/ferskvatn_drep_best_unge_lakselus

/nb-no

Claims

1. A system, comprising:

a supply (100) of softened brackish water having:

i) a salinity in the range from 0.5 g/kg to 15 g/kg, determined as the total mass of Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl , HCO3 , CO3² , and SO4² ions being present in a sample of one kg of the water, and ii) a Ca²⁺ content of < 100 mg/kg, determined as the mass of Ca²⁺ ions in a sample of one kg of the water,

an oxygen supply (101) adapted to adding oxygen to the softened brackish water,

and

2. A system according to claim 1 , wherein the softened brackish water of the supply (100) has:

a salinity in one of the following ranges; from 1 g/kg to 25 g/kg, from 2 g/kg to 25 g/kg, from 3 g/kg to 25 g/kg, from 4 g/kg to 25 g/kg, from 5 g/kg to 25 g/kg, from 5.1 g/kg to 25 g/kg, from 1 g/kg to 22.5 g/kg, from 2 g/kg to 20 g/kg, from 3 g/kg to 17.5 g/kg, from 4 g/kg to 15 g/kg, or from 5 g/kg to 12.5 g/kg, or most preferably from 5.1 g/kg to 10 g/kg, determined as the total mass of Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl , HCO3 , CO3² , and SO4² ions being present in a sample of one kg of the softened brackish water.

3. A system according to claim 1 or 2, wherein the softened brackish water of the supply (100) has:

a Ca²⁺ content in one of the following ranges; from 0.001 to 95 mg/kg, from 0.001 to 90 mg/kg, from 0.001 to 80 mg/kg, from 0.001 to 50 mg/kg, from 0.001 to 10 mg/kg, from 0.001 to 8 mg/kg, from 0.001 to 6 mg/kg, from 0.001 to 5 mg/kg, from

0.001 to 4 mg/kg, or most preferred from 0.001 to 2.5 mg/kg, determined as the mass of Ca²⁺ ions in a sample of one kg of the softened brackish water.

4. A system according to any preceding claim, wherein the manifold (107) is located in an upper part (115) of inner section (110) of the container (102), and optionally, is height adjustable relative to the inner section (110) of the container (102).

5. A system according to any preceding claim, wherein the manifold (107) has two or more nozzles (108), each nozzle being adapted to inject the softened brackish water into the water of the inner section (110) at laminar flow conditions, optionally in a substantially horizontal direction.

6. A system according to any preceding claim, wherein the oxygen supply (101) comprises either an injection means for gas injection of an oxygen containing gas into the softened brackish water downstream of supply (100) and upstream of manifold (107), or injection means for gas injection of an oxygen containing gas into the upper part of the inner section (110), or a combination of both.

7. A system according to any preceding claim, wherein the oxygen supply (101) comprises one or more sub-microbubble or nanobubble injection devices located at one or several locations at the lower, middle, and/or upper part of the inner section (110) of container (102).

8. A system according to claim 8, wherein the one or more sub -microbubble and/or nanobubble oxygen injection device (111) is a pressurised dissolution injection device, and/or a static mixing injection device.

9. A system according to any preceding claim, wherein the supply (100) comprises one or more nanofiltration membrane modules enabling providing the softened brackish water by desalination of natural seawater by nanofiltration, preferably either one or more Filmtec™ NF90-400 nanomembrane modules and/or one or more Hydranautics ESPA 30G nanomembrane modules.

10. A system according to claim 9, wherein the supply (100) further comprises a seawater intake (123) adapted to collect seawater at a variable water depth of from 1 to 50 metres and passing the collected seawater to the one or more nanofiltration membrane modules, and optionally, the seawater intake (123) comprises a grate.

11. A system according to claim 10, wherein the supply (100) further comprises a seawater intake filter unit (124) having a cut-off threshold of 40 pm located upstream of the one or more nano filtration membrane modules.

12. A system according to claim 10, wherein the seawater intake filter unit (124) comprises a cyclone filter removing particles above 200 pm and a backwash-filter removing particles from about 40 pm and above.

13. A system according to any preceding claim, wherein

the container (102) shaped as is either a frustum of a circular cone with upper diameter dtop and lower diameter dbottom, or a a frustum of a pyramid having a triangular, square, hexagonal, or a polygonal base having a maximum diameter dtop of the upper base and a maximum diameter dbottom of the lower base, and

a ratio dtop : dbottom is in the range of from 1 . 1 to 1 .5 .

14. A system according to any preceding claim, wherein the container (102) is made a tented tarpaulin gauze, or made of plexiglass, stainless steel, or aluminium.

15. A system according to any preceding claim, wherein the container (102) comprises a floating structure (120) adapted to keep the container floating partly submerged in the water.

16. A system according to any preceding claim, wherein the container (102) has a removable lid (127) adapted to cover the opening in its top (105).