IL36348A - Method and apparatus for the rearing of fish in an artificial environment - Google Patents

Description

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This invention relates to a method and apparatus for th¾ rearing of fish in a controlled environment and more particularly relates to a method and apparatus for use in the raising of fish at a high density in a controlled environment in a fashion that optimizes growth and. provides use of available land, water and oxygen at hitherto unattainable high efficiencies thereby resulting in an extremely economical end product, specifically fish for huma consumption.

There are many reasons why it is desirable to raise fish in a controlled environment, namely an environment that is under the control of the fish grower as much as possible. In such a controlled environment the fish can be kept free from predators, can be protected against disease and can be fed an optimum diet so as to result in a maximum growth rate, desirable flesh con-sistency and desirable taste. Furthermore, a controlled environment makes possible the programming of the amount of fish available for market throughout the year so that the most efficient use of processing and marketing facilities can be obtained.

Raising fish in a controlled environment is desirable in -order to increase the quantities of fish for human consumption to meet the needs and expected demands for fish as a food. In the future, the natural incidence of fish to feed mankind at present and expected levels of population may be insufficient, as was the natural incidence of poultry for eggs and poultry meat. For many of the same reasons as have made the controlled raising of chickens necessary, to provide an economic product in sufficient quantity for the mass market, so, too, the controlled growing of fish \rould be comparably necessary to provide fish as food for a mass market.

Accordingly, it is a major purpose of this invention to provide a method and apparatus for the mass raising of fish in a controlled environment.

The present invention contemplates the raising of fish in one or more tanks in a high density of from about 0.5 pounds per gallon of capacity up to about 2 pounds per gallon, and, under high oxygen conditions in a density as high as 3 pounds per gallon. The usual range is about 1.25 pounds of fish per gallon of space in the tank. In addition the high density process of the present invention contemplates a water flow rate through the or each tank of about one gallon of water per minute per tank at a maximum rate of flow for each 10 to about 20 pounds of fish contained within the tank.

Because the fish environment is oxygenated water and because it is desirable and in some cases even necessary that the v/ater be within a fairly narrov range of temperature and have a certain degree of purity, it becomes necessary to use available water efficiently. Accordingly, it is another important purpose of this invention to provide an optimum use of the water available so that for any given flow rate of water a maximum amount of fish end product will be provided.

It is elementary that fish require dissolved oxygen in their water environment in order to live. Their requirement for oxygen is so critical that the level of dissolved oxygen in the water is a major factor in determining the rate of groxvth and density that the fish can tolerate while being grown and maintained in the water.

From the point of view of the economics of any large scale fish husbandry operations, it is important that none of the oxygen added to the water be wasted. In particular, this means that it is important that as much of the oxygen as possible be used by the fish. In general terras, it is known to bubble air through water in' order to provide oxygen for the fish. However, this is a relatively inefficient (in terras of use of energy) method of supplying oxygen to the fish since so much of the air involved travels through the water as bubbles which cannot be used by the fish and leaves the top surface of the water in the fish container to pass into the atmosphere. For small scale operations, such techniques may be quite satisfactory where the number of fish involved are relatively few and their packing density in the water environment is relatively low. However, the known approaches are highly impractical from a cost-efficiency point of view where applied to the growing of fish as food on a mass scale.

Accordingly, it is a major purpose of this invention to provide an improved, more efficient technique of supplying dissolved oxygen to the water environment in which fish are bein raised.

It is a further important purpose of this invention to maintain the dissolved oxygen level in the water at as high a level as is feasible. It is important not only to attain as high a level of dissolved oxygen as is feasible, but also to maintain that level throughout the water volume in which the fish are being raised so that maximum packing density and maximum growth rates may be attained.

I is a related purpose of this invention to maintain the dissolved oxygen in the water at a high level continuously so that not only is the entire volume of water at a high dissolved oxygen level but that that high level is maintained throughout the growth period of the fish so that maximum packing densities c n be attained and maintained without loss of fish or substantial decrease in rate of growth of the fish involved.

It is another important purpose of this invention to provide the oxygen in such a fashion tha all of the oxygen supplied goes into solution. This becomes particularly important where the oxygen is supplied as relatively- pure oxygen.

In order to raise fish at maximum packing densities and maximum growth rates, it is important that there be a continuous flow of water through the tanks in \fhich the fish are being raised so that undesirable waste matter will be washed away, and also so that the water may be passed through a series of fish tanks and high oxygen levels maintained in each tank.

Accordingly, an important purpose of this invention is to achieve a controlled environment within the context of a continuous flow of water throughout the tanks or containers . in which the fish ; are being raised.

Since it is important to supply water, oxygen and feed and to remove wastes, it is a broad object of this invention to provide a method and apparatus for the raising of fish in a controlled environment which will provide an iptimum use of avail-able water, of supplied oxygen and of feed while at the same time providing for an efficient removal of metabolites and other wastes so that a fish husbandry system that is practical from a cost-eff ciency point of view will be provided. As will be obvious from the following detailed description of the drawings and preferred embodiment, there are many purposes which can be stated for this invention. However, it is in the techniques which in combination provide all of these purposes that the meaningfulness of this invention lies.

To these ends the present invention provides a method of rearing fish at high density in a controlled environment, com- prising the steps of confining the fish in a vertically extending space, and continuously flowing water upwardly through said space.

The present invention further provides a method of rearing fish at high density in a controlled environment, comprising the steps of confining fish in a plurality of vertically extending spaces, and passing the water in series flow through said spaces, the water flow being from the top of each space to the bottom of the next succeeding space.

The instant invention additionally provides apparatus for use in performing either of the aforedescribed methods of rearing fish at high density in a controlled environment, said appar atus including a tank, a conduit for the supply of water to a location at the bottom of the tank, and water collecting and screening members for capturing water "flowing from the top of the tank.

Apparatus provided by the present invention may include a series of said tanks, conduits connecting said tanks for the con tinuous flow of water upwardly through successive tanks, and a device between successive tanks for aerating the water flowing from one tank to the succeeding tank.

In a preferred embodiment of the present invention, a system of a plurality of large tanks and a raceway are connected in series so that water circulates in sequence through the tanks and the raceway starting at the first tank and ending at the raceway. Each tank and the raceway contain fish.

The water introduced into the first tank is as close to saturation as possible with oxygen. This oxygen saturated water is introduced at the bottom of the first tank and rises to the screen catches the offal. The water then splashes over a funnel-like arrangement to provide turbulence. A vent tube in communication with the water flowing between tanks provides aeration.

Durin passage through the first tank, additional oxygen is introduced into the water in -order to maintain the water at as close to saturation with oxygen as possible and thus to compensate for the oxygen used by the fish grown in the first tank. This process of introducing oxygen into the water as well as aerating the water and introducing turbulence is repeated until the water has cycled successively through a plurality of tanks. On leaving the last tank, the water is again aerated and turbulence created and the water is then finally introduced into a raceway in which additional fish are also raised.

The oxygen may be injected by bubbling fine particles of oxygen, for example 0.1 to 0.5 millimeters in diameter, throughout the water environment in xvhich the fish are being raised at high densities; preferably at densities of at least six pounds per cubic foot of water. The water supply is continuously flow-ing as, for example, from the bottom to the top of a large tank. The water introduced is introduced with a dissolved oxygen content as close to saturation as is possible. In addition undissolved oxygen may be introduced in small bubble sizes at the bottom of the tank to travel with the flowing water from the bottom of the tank to the top of the tank. As the fish in the tank use the dissolved oxygen in the water, the oxygen in the large number of small size bubbles will dissolve into the water to replace the oxygen used by the fish.

Enough oxygen is supplied so that a condition as close to saturation of dissolved oxygen in water as is possible can be maintained. However, from considerations of cost, the amount of oxygen \vhich may be supplied in bubble form is kept as low as is necessary to maintain a close to saturation state throughout the tank, so that as little as is possible is lost by emis- 5 sion of oxygen bubbles at the top of the tank.

Thus water continuously flows through the system in *:hich the fish' are being raised. As it flows, metabolites, and other wastes which are added to the water by the fish, must be removed to assure maximum health and growth rate of the fish. By oxygenic) ation, aeration and turbulence these wastes may be thrown off or counteracted. A series of large tanks (four in one preferred embodiment) succeeded by a single raceway provide an efficient use of this water. Fish can be grown in four such tanks at a high packing density, while the fish in the raceway, which has the least favorable water environment, are grown at a substantially lower packing density. The combination of tanks and raceway makes for the most efficient use of the water available and oxygen supplied.

In order that the invention may be better understood and 20 more readily carried into effect, the same will now be described with reference to the accompanying drawings, in which: Figure 1 is a perspective schematic view of the system arrangement of this invention showing a preferred embodiment employing four tanks and a raceway.

Figure 2 is a schematic view in elevation of the four tanks and the raceway.

Figure 3 is a schematic showing of the water treatment arrangement between tanks.

Figure 4 is a schematic representation of one of the tanks 0 showing the technique for injecting oxygen in small particle size into the water.

With reference to the Figures, a preferred embodiment of the fish husbandry system of this invention employs four tanks 10, 11, 12 and 13 as well as a raceway 16. Water, which may be pumped up from a well, is stored in a tank 18" at a level above the top of the first tank 10 so that a head of water will be available to force water flow through the tanks 10-13.

Water flows from the storage tank reservoir 18 through conduits 19 to. be discharged into the bottom of the first tank 10. The water then flows vertically up through the first tank 10 into an overflow rim on top of the first tank 10. The water in the overflow rim flows through a treatment unit 22 and thence through a conduit 23 to the bottom of the second tank 11 where the process of upward flow of water into an encircling overflow rim and then through another treatment unit 24 is repeated.

The top level of the water in the first tank 10 is a few feet higher than the top level of the water in the second tank 11 so that there will be enough head of water to force a large volume flow of water through the second tank 11. The relationship between the second tank 11 and the third tank 12, as well as between the third tank 12 and the fourth tank 13 are essentially the same as between the first tank 10 and the second tank 11. Similarly, each of the four tanks 10, 11, 12 and 13 as well as the four treatment units 22, 24, 26 and 28 operate in essentially the same fashion. The structure of the water treatment unit 22 is shown in Figure 3· Water from the fourth tank 13, after being passed through the fourth treatment unit 2S is passed to a large raceway 16 in which fish are also raised, although not at the same density as in the four tanks 10-13.

Thus, the overall arrangement shown in Figures 1 and 2 provides a continuous flow of water through the four tanks -13 and raceway 16. It provides for a vertical, upward flow of water in each of the tanks 10-13. It further provides for water treatment in the units 22-2S in order to oxygenate the water, filter out offal and eliminate as much carbon dioxide and ammonia as possible.

In addition, at the base of each of tanks 10-13 oxygen bubbles may be injected so that the bubbles travel up with the water and maintain as close to a maximum state of dissolved oxygen as possible. Figure 4 illustrates a technique for injecting oxygen bubbles into the water in each fish growing tank.

In the embodiment illustrated, it is contemplated that each of growing tanks 10-13 will hold approximately 5,500 gallons of water. A combination of four such tanks connected for series flow of water in combination with a raceway connected to the outflow from the fourth .tank seems to provide a close to optimum arrangement from the point of view of efficient use of water. In this fashion, a maximum poundage of fish can be raised per unit flow of water per minute. The water will degrade somewhat in quality, particularly with respect. o the amount of. ammonia contained therein, as the water flows from tank to tank and finally to raceway. As a consequence, the maximum density of fish that can be grown in each tank will decrease somewhat as the tank is further along in the flow of water. Thus, the fourth tank 13 will provide a somewhat less dense maximum growth of fish than will the first tank 10. Furthermore, the raceway 16 will provide a substantially less dense growth of fish. However, oxygen will not normally be bubbled through the raceway 16 since such will be too inefficient a use of oxygen to be warranted.

However, the water and the raceway 16 by having a large- upper surface exposed to the atmosphere will tend to take on oxygen from the atmosphere and give off "ammonia to the atmosphere.

Figure 3 illustrates a preferred design for the water treatment units 22-30. Around the upper rim of the tank 10 is an encircling annular channel 40 into which the water from the tank 10 flows. An annular screen 42 prevents the fish in the tank from falling into the annular channel 40. The screen 42 is a one-eighth inch mesh screen so that it traps most of the offal waste.

The rim channel 40 has at least two openings at its outer lower corner. One opening 44 is shown. The other opening is identical to the opening 44 and is not shown. During operation, the opening 46 is plugged and water from the annular channel 40 flows through the opening 44 into the funnel-like apparatus below the opening 44 » The structure below the opening 44 includes the conduit 23 which leads to the' second tank 11. The upper end of conduit 46 flares out into a larger funnel 8 · Within the larger funnel 48 is a vent pipe 50 , which vent pipe 50 extends down into the conduit 46 by a substantial amount, such as eight inches in one embodiment. Attached to the upper portion of the vent pipe 50 is a smaller funnel 52 that is positioned directly beneath the opening 44.

Water from the opening 44 falls into the smaller funnel 52 and splashes over the smsiller funnel 52 into the larger funnel 8. This creates a degree of turbulence which serves to cause most of the carbon dioxide and some of the ammonia to be released from the water into the atmosphere. The upper end of the vent pipe 50 extends into the air and is above the level of water that exists in the larger funnel 6. The rapid large vol-ume flow of water between tanks and through the conduit (which conduit may be 4 inches in one embodiment) will cause air to be sucked in through the vent pipe 50, by a venturi-like effect, so that air bubbles. ill be injected into the flowing water and carried into the tank 11. By this means, the water initially injected into the tank 11 is assured of a close to maximum dissolved oxygen content. By means of the arrangement shown in Figure 3 , a very simple and inexpensive mechanism is provided to achieve various water treatment functions. In addition, openings in the form of holes can be made in the conduits 19, 23 in order to drav; in additional air if necessary to assure an oxygen saturated water input for each of tanks 10-13.

The offal that collects on the screen 2 will have to be periodically removed. The arrangemen shown in Figure 3 makes such removal particularly convenient and inexpensive. To remove the offal, the opening 44 is plugged and the second channel opening is unplugged. This second opening, which is substantially radially removed from the first opening 44, is not shown. An attendant then uses a brush to brush the offal which is on the radially inner surface of the screen 42 through the screen openings so that it is carried out the second channel opening by the flow of water from the tank 10. The offal is then received in a collecting tank (not shown) . After the offal has thus been removed from the screen 42, the second opening is plugged and the first opening 44 is unplugged.

The above system is designed to deal with the different types of metabolites in such a fashion as to make for an economical fish husbandry system. Certain metabolites are reduced by rel tivel sim le and inex ensive rocesses. The screen 4 eliminates most of the offal. The splash funnel 52 eliminates most of the carbon dioxide and some of the ammonia. In combination, this simple selective treatment of the water extends the useability of the water considerably. When further combined with aeration and oxygen insertion as well as the large volume continuous flow capability, the net result is an optimum efficient use of resources; in particular the resources of water and oxygen.

With reference to Figure 4, the cylindrical tank 10 is particularly adapted to be employed in connection \?ith this invention because it is relatively easy to introduce the bubbles of oxygen over the surface of the floor 2 of the tank so that the bubbles will rise throughout the entire tank and serve the function of supplying oxygen to the water 53 in all portions of the tank 10.

The floor 2 of the tank 10 can be a fine-pore porous floor through which, oxygen is injected into the tank in a bubble size preferably somewhere between 0.1 and 0.5 millimeters in diameter. The diameter of these bubbles will decrease as the oxygen bubbles move up through the tank and are dissolved into the water. ' The fine-pore porois floor can be composed of a carbu uncium stone or any other diffuser. Alternatively, the floor can be criss-crossed with fine mesh nylon txibes into which oxygen is pumped.

In the embodiment shown, the diffuser floor 59 has a chamber 54 under it into which gaseous oxygen is pumped under pressure via pump 6 from a source of oxygen $. The pressure in the chamber 54 has to be maintained sufficiently high so that the oxygen will pass through the diffuser 59 into the water environment 53 · The water is supplied to the tank 10 through a conduit 23 which has an opening 60 at or near the bottom 52 of the tank 10., The water being supplied is saturated or as closely saturated as is possible with oxygen.

It is important that the oxygen bubbles be introduced at the base of the tank 10 in approximately the same zone of the tank where the water is at its greatest pressure to assure that the oxygen will all go into solution.

It is of further importance that the tank design and the way in which water is introduced into the tank be of such a nature that the water will flow throughout substantially all of the tank and will carry oxygen throughout substantially all of the tank so that the entire tank can be fully employed in the raising of fish. In this fashion a maximum packing density can be obtained and the most efficient use of water and oxygen provided. To assure water flow throughout all of the tank 10, the water is introduced into the bottom of the tank in such fashion that it flows radially outwardly over the surface of the bottom of the tank and up throughout the tank to the top of the tank.

It should be kept in mind in connection with these arrange ments of water flow and oxygen usage, that the oxygen and water is employed to grow edible fish, at very high densities.

The density of fish in the water that this invention con-templates making possible is a density of at least six pounds of fish per cubic foot of water. To obtain this kind of density will require a continuous rapid flow of highly oxygenated water in order to clear away growth inhibiting metabolites and provide growth stimulating oxygen. Thus the actual economic considera-tion is the number of pounds per fish that can be grown per unit flow of water, per minute. It is contemplated that this invention viill permit maintaining at least fifty pounds of fish for each gallon per minute flow of water. Thus the high density that is made possible by this invention is one that involves a very intensive use of the water volume available and it requires a very substantial rate of flow of water. This means the use of a lot of oxygen and of a lot of water. Because the oxygen and the v/ater cost money, efficient use is essential to keep the costs of the end product, fish for use as food, to a minimum. It is for this ultimate reason that it is of considerable importance that the oxygen be introduced where the water is introduced and that the oxygen and water flow and travel throughout the entire volume of the tank involved.

An advantage of this optimum use of v/ater in fish husbandry is that it makes possible the use of cascaded series of silo tanks for raising fish. This type of tank arrangement is highly efficient, but cannot work well unless one can provide v/ater extremely rich in oxygen.

A further advantage of raising fish in water in which oxygen is abdundant is that the copious oxygen can compensate partially for levels of toxicity and waste in the v/ater. Fish do quite well in water saturated in oxygen, notwithstanding that the toxicity in the v/ater would, at normal oxygen levels, be harmful.

The v/ater introduced is introduced in a condition where it is as nearly fully saturated as is possible with oxygen. For water at a temperature of approximately 50°F., a close to saturation dissolved oxygen level may be between 9.5 and 10.0 milligrams per liter of v/ater. The oxygen bubbles that pass through the diffuser 59 then maintain dissolved oxygen saturation as much as possible throughout the tank. This is of major importance in order to assure that the fish grow as fast as possible. Essentially, the rate at which fish grow will be a function of the amount of dissolved oxygen available.

Although applicant believes it preferable to employ oxy- en in as pure a form as possible as the gas which is injected into the tank, it may be possible and sometimes even economically desirable to employ air instead of oxygen. It will undoubtedly be a function of the cost of providing relatively pure oxygen and of the size of the fish growing installation that will de-termine the point at which it is economically preferable to employ oxygen rather than air.

However, for the smaller fish growing installations and in those situations where the cost of supplying oxygen may be too high, air might well be employed, though -with somewhat less effectiveness, than oxygen. The same basic considerations that apply to the more preferred form of this invention in which oxygen is employed will also employ to the embodiment in which air is employed. The air' particle sizes should be small, somewhere in the range of 0.1 and 0.5 millimeters in diameter where they are injected into the water. The water supplied should be as near saturated as possible with oxygen. The bubbles in the water should flow together from one introductory zone of the tank to an exit area of the tank.

When air is employed, however, there will be bubbles com-ing off of the outlet or surface of the tank involved because so much of the air will contain nitrogen. Where the tank involved is deep so that relatively substantial pressure is developed at the bottom of the tank, the air introduced in small bubble form will have to be introduced at sufficient pressure so that it will be injected into the tan. This can cause embolism and damage to the fish. However, the use of a continuously bubbling stream of air throughout the tank should have the effect of picking up much if not most of this excess nitrogen and thereby reducing the risk of embolism and fish damage. However, the risk is there and this is one of the reasons why a relatively pure oxygen bubble insertion is preferred.

When air is involved, the continuous flow of the water becomes of additional importance in order to make sure that carbon dioxide from the air does not build up. However, since a continuous flow of water will be necessary in any case where fish are being raised at high density levels, the carbon dioxide build up problem should not be significant.

Each of the tanks may be enclosed at the top to prevent additional contaminants from the air being introduced into the , tanks. In addition the enclosed top and the use of opaque material for the body of the tanks permits the control of lighting in the tanks which in some respects and with certain species controls or influences behavior and activity of the fish.

The water may be pumped through the tanks in intermittent varying velocities to conserve pumping costs while still providing sufficient oxygen and waste control.

The preferred location for a system according to the present invention is geographically where suitable ambient temperature is present on a year round basis and a su ficient volume of naturally existing water is available for the species being grown. The present invention, as described above, provides a method and apparatus for the dense rearing of fish at hitherto unattainable ef iciencies. It permits the grower to isolate the fish from predators and to restrict the consumption of food by the fish only to those determined to be most suitable for the ^ fish to provide the desired flesh quality and amount.

This invention provides efficient and accurate control of the temperature, waste, oxygen and food. It also provides instantaneous and continuous surveillance of the fish so that any disease or parasitic attacks can be caught at the inception and appropriate prophylactic measures taken before any serious losses occur.

Claims (27)

36348/2 / C L A I M S: 1. A method of rearing fish at high density in a controlled environment, characterized by the steps of confining the fish in at least one vertically extending space which contains a body of water at or near oxygen saturation level, continuously supplying water at or near oxygen saturation level to the bottom of said space, and continuously overflowing used vater at the top of said space, the flow of water through the space being a quiescent Substantially unidirectional laminar flow substantially exclusively in an upward direction. 36348/2 CLAIMS

1. A method of rearing fish at high density in a controlled environment, characterized by the steps of confining the fish in at least one v-ertically extending space which contains a body of water at or near oxygen saturation level,, continuously supplying water at or near oxygen saturation level to the bottom of said space, and continuously overflowing used water at the top of said space the flow of water through the space being a quiescent .substantially unidirectional laminar flow substantially exclusively in an upward direction.

2. The method according to claim 1, in which the space is completely enclosed.

3. The method according to claim 1 or claim 2, characterized by the step of continuously oxygenating said water prior to its upward passage through said space.

4. The method according to claim i characterized by the stop of continuously bubbling oxygen or an oxygen containing gas into said water at the bottom of said space.

5. Tho method nocording to claim 4, chnrnctori'/.od 'in tohwjb' flh kl I mMil nn <1« »iii h i h t..ly ΜΓΠ of A tl:t ttiii«t.«>' liq 0.1 and 0.5 mllliinatora,

6. The method according to claim 4 or claim 5i characterized in that the bubbles are bubbles of substantially pure oxygen.

7. The method according to claim 4 or claim 5> characterized in that the bubbles are bubbles of atraospheri air. 36548/2

8. The method according to any one of claims 1 to 7, characterized in that the density of the fish in the space is in the range of 0.5 to 3 pounds per gallon (U.S. measure), and preferably in the order of 1.25 pounds per gallon.

9. The method according to claim 8, characterized in that the rate qf water flow through the. space is in the range of 0.1 to 0.05 gallons (U.S. measure) for each pound of fish confined within the' space.

10. The method according to any one of claims 1 to 9, characterized by the .step of intermittently varying the rate of flow of the water through said space.

11. The method according to any one of claims 1 to 10, characterized by adding nutrients to the water prior to its entering the space.

12. The method according to any one of claims 1 to 11, characterized by adding compatible antibiotics, fungicides pesticides or antiparasitical agents to the water prior to its entering the space.

13. The method according to any one of the preceding claims, characterized by confining fish in a plurality of said vertical spaces, and passing the water in series flow through said spaces, the water flow being from the top of each space to the bottom of■· the next succeeding space.

14. · The method according to claim 13 i characterized by aerating said water during its passage between said spaces.

15. The method according to claim 13 or claim 1 i characterized by introducing turbulence into the wa.ter leaving the top of said spaces prior to the water being intro 36348/2

16. The method according to claim 15 > characterized in that said turbulence is effected in the presence of atmospheric air.

17. The method according to claim 16 , characterized by introducing atmospheric air into the water subsequent to introducing turbulence into the water flow.

18. 10. The method according to any one of claims 13 to 17 , characterized by passing the water leaving the terminal space of said plurality into a raceway, and confining fish within said raceway.

19. The method according to any one of claims 13 to 18 , characterized by removing debris from the water as it leaves the respective spaces.

20. The method according to any one of claims 13 to 19 i characterized by at least partially reducing the concentration of dissolved carbon dioxide and ammonia in the water leaving the respective spaces. · '

21. % Apparatus for uao in performing tho method of any one of the preceding claims, characterized by a tank, a conduit for the supply of water to the tank at a location adjacent the bottom of the tank and for flowing water through the tank in a quiescent substantially unidirectional laminar flow substantially exclusively in an upward direction, a perforate screen extendin peripherally of the upper end of said tank for screening the water overflowing therefrom, and a channel extending peripherally of tho upper end of said tank for collecting wator overflowing thorofrom. 36348/2

22. Apparatus according to claim 21, characterized by a top closure member for the tank.

23. Apparatus according to claim 21 or claim 22, characterized in that the tank is of a height greatly in excess of its width.

24. ·. Apparatus accord ng to any ono of cloima ?.l to 23, characterized by a device for injectin a stream of small diametor gas bubbles containing oxygen into said tank in a sufficient quantity to maintain the dissolved oxygen content of the water at near saturation, . said injecting device ,' being in communication with the, tank' at a position such that the gas bubbles will be carried substantially throughout the tank, the diameter of said gas bubbles being sufficiently , small so that substantially all of. the oxygen contained there-, in will dissolve into- the circulating water' before the bubbles . escape from the water. ,

25. Apparatus according to claim 24, in which the injection device is such as to form gas bubbles having a dia* meter of between 0.1 and 0*5 millimeters.

26. Apparatus according to cla^im or claim 25, ■ . characterized in that the gas injection device is comprised · by a plate of foraminous material constituting one wall of a, closed chamber. ' 1 ' · · "J..

27. Apparatus according to olaim 24 or olaim 25, characterized in that the gas injection device is comprised by perforate tubes of plastics material* 28 Apparatus according to any one of claims 21 to characterized by a selectively controllable variable displacement pumping system for supplying water to said conduit at determined volumes for selected periods of time. 29 Apparatus according tc any one of claims 21 28, through 9, including a series of said tanks, conduits connecting said tanks for the continuous flow of water upwardly through successive tanks, and a device between successive tanks for aerating the water flowing from one tank to the succeeding tank. 29 30 ^i. Apparatus according to claim ^€f, characterized in that said aerating devices include a vent tube having an input end and an output end, said vent tube being inserted into the conduit between successive tanks, the output end of. said vent tube having its axis substantially aligned with the direction of flow of water through said conduit, the input end of said vent tube being exposed to atmosphere, whereby rapid flow of water through said conduit v/ill draw a substantial quantity of air into the water flow. 2 30 31 z. Apparatus according to claim o or claim ^&, including a water treatment device between successive tanks for at least partially purifying water flowing from one tank to the next succeeding tank. 31 32 Apparatus according to claim characterized in that the treatment device incorporates members for breaking up the water flow and for recombining the water flow with a splashing action. 5 33 yli. Apparatus according to any one of claims to y , characterized in that the terminal tank of said series discharges into a raceway, and aerating and water treatment devices are incorporated into a discharge conduit from said terminal tank. 3 A method of rearing fish at high density in a controlled environment, substantially as hereinbefore described with reference to the accorapanying drawings. 35 Apparatus for use in rearing fish at high density and which constitutes a controlled environment, substantially as hereinbefore described with reference to the accompanying drawings. - 23 -