IE44502B1 - A method for rearing fish - Google Patents

Description

The present invention relates to a method for rearing fish.

In the rearing of fish in land based tanks (hereinafter for convenience termed fish farming), water is pumped through a tank in xvhich the fish are held.

However, it is normally desirable to recirculate part of the water, but this poses problems in that the oxygen level in the water which can be maintained may not be as high as would be required for optimum stocking levels of the tank. Also, dirt accumulates in the water. It has therefore been proposed to inject air into tha water and to filter part of the water before recirculation. However, injection of air into the water in the tank has the effect of slowing down the velocity of the water in the region of the air injector, even when this is directed in the same sense as the water flow, and this may reduce the number of fish which can be farmed in a given volume of water.

We have now devised a method which reduces these problems.

Accordingly, the present invention provides a method for rearing fish, notably anadromous fish, which comprises rearing the fish in a vessel containing water and provided with means for circulating water within the vessel and with a separate aeration zone or chamber, said zone or chamber having means separate from the circulation means for aerating water in the - 3 4 4 5 0 3 zone or chamber and having at least one wall thereof common with the vessel, the zone or chamber being in fluid flow connection with the vessel whereby water from the vessel enters the zone or chamber, is aerated and the aerated water is returned to the vessel and supplements the circulation caused by the circulation means.

The invention also provides a fish farm (that is a series of land based tanks through which fresh and/or sea water is to be passed and in which fish are to be raised) comprising a vessel having means for causing water to circulate within the vessel, which vessel is provided with a separate aeration zone or chamber having at least one wall thereof common with the vessel and having means separate from the circulation means for aerating, water when in the zone or chamber; the separate zone or chamber being in fluid flow connection with the vessel whereby water may be removed from the vessel and aerated water may be returned to the vessel, thereby supplementing the effect of the circulation means.

A main feature of the method and fish farm of the invention is that oxygen depleted water is removed from the main flow of water through the vessel and is aerated outside the main flow before it is returned to the main flow. In this way aeration may be achieved under conditions which would not be acceptable to the fish if practised in the main flow of water, and the rate of circulation of water in the main flow is supplemented by the flow of water from the aeration zone or chamber. - 4 The vessel used in the invention may be of round, oval or rectangular plan and may be made from any suitable material, e.g. from wood, plastics or concrete,· and is erected on land rather than being a cage or similar structure floating in the sea as has been used hitherto. The water is caused to circulate around the vessel, e.g. by feeding water into the vessel tangentially and with part of the water being passed to waste. Typically, the vessels for present use have a capacity of at least 10 cubic metres, preferably at least 50 cubic metres, of water.

The aeration zone or chamber lies outside the main flow of water in the vessel. Thus, the aeration • sone or chamber may form an external construction attached to the vessel via a common wall between them. Alternatively, the zone or- chamber may lie within the vessel, as when the vessel is subdivided to form a main part and a secondary part forming the zone or chamber. It is preferred that the aeration sone or chamber lie within the vessel, notably as an internal, e.g. a generally central, construction within the vessel around which the main stream of water circulates.

The aeration means in the aeration zone or chamber may take the form of a device, such as a venturi, in which air is drawn into the water by virtue of the flow of water through the venturi.

However, we prefer to provide means which inject air under pressure into the water,-e.g, by means of an injector pipe with air holes or nozzles or by means of a porous block to provide a diffused stream of air bubbles over a length of the zone or chamber. 4 5 0 S - 5 It is preferred that the aeration moans provide air bubbles of less than 5 mm diameter, preferably less than 1 mm diameter distributed throughout the water in the aeration zone or chamber. Ideally the aeration at least saturates the water in the zone or chamber with oxygen and it should provide at least 6 ppm 02 in the water leaving the zone or chamber to enable fish to thrive in the vessel. Typically, the aeration means provides from 8 to 12 ppm 02 in the water, which typically has a temperature of from 4 to 25 0, Water is withdrawn from the main flow in the vessel, passed through the aeration zone or chamber and aerated water is returned to the vessel. This may be achieved by a suitable pump means. However, we have found that where air is injected under pressure into the zone or chamber, this may cause water to flow through the zone or chamber at sufficient rate to render a separate pump unnecessary.

Thus, where air is injected via a diffuser placed at the bottom of a chamber, the stream of bubbles from the diffuser may cause sufficient upward flow of water in the chamber. We prefer to use aeration means which cause both aeration and circulation of water through the aeration zone or chamber. Where this is done we have found that the stream of bubbles formed by the aeration means must impinge on at least two opposed lateral walls of the chamber walls.

We have found that it is desirable to use aeration means which provide impingement of bubbles on the submerged walls over as great a distance as is feasible, e.g. 40 to 80% of the whole, and it may therefore be desired to use two or more aeration - 6 means in parallel. In order to optimise the flow of water through the zone or chamber due to aeration, we prefer to have the outlet or overflow from the zone or chamber the minimum height practicable, e.g. less than 10 oms, typically less than 5 cms, above the surface of the water in the vessel.

The aeration zone or chamber is in fluid flow connection v/ith the vessel, e.g. by means of appropriate pipework or by means of underflow or overflow of a common wall between the aeration chamber and the vessel. It is preferred to remove water from a low level in the vessel, as this water will usually be that most depleted in oxygen; and to return aerated water to an upper level of the vessel, although it may be returned to a lower level of the vessel if desired.

It may be preferred to remove water for aeration from the drainage outlets in the base of the vessel, as this may aid the movement of solids deposited in the vessel, e.g excreta or surplus food, to the drainage outlets. Alternatively, movement of solids towards a drainage outlet may be aided by sloping the floor of the vessel towards the outlet. in this case, water for aeration may be removed from the top of the slope to reduce solids pick-up. It is also preferred that the Water be removed from a number of points in the vessel and returned at a number of points so as to provide a comparatively uniform flow through the aeration chamber. It is also preferred that all the aerated water should be returned directly to the vessel, i.e. it is not treated (e.g. filtered) before it is returned, although some, e.g. up to 10%, of the aerated water may be diverted and not returned directly. - 7 44S03 It is particularly preferred to form the aeration zone or chamber as a chamber having at laa3t one common wall with the vessel and having aeration means located at the base of the chamber which means circulates water through the chamber, preferably directly and without any filtration step. Thus, a fish tank may be provided with an aeration chamber at one end thereof or along part or all of one or both sides thereof. Hbwever, it is preferred that the aeration chamber take the form of a generally central hollow wall within the vessel. Preferably, such a chamber has an internal width of less than 1 metre, typically 0.2 to 0.5 metres, to assist circulation of the water by the aeration means.

It is also preferred to return aerated water through or over the opposite wall of the chamber to that through which it entered, thus aiding circulation of water around the vessel.

The invention thus also provides a vessel suitable for use in the method and fish farm of the invention which comprises a main vessel having means to cause water to circulate within the vessel and having a water inlet and a water outlet? a secondary vessel having at least one wall common with the main vessel, the two vessels being linked in fluid flow communication, the secondary vessel being provided with means separate from the circulation means for injecting air into water in the secondary vessel and thereby cause circulation of water from the main vessel, through the secondary vessel and back to the main vessel and thus to supplement the circulation achieved within the main vessel by the circulation means when in operation. Preferably, the secondary - 8 vessel is located substantially symmetrically within the main vessel to provide an annular shaped main vessel.

The invention is of especial application in the rearing of fish in land based tanks to maturity in fresh and/or salt water; notably of farmed species of fish, e.g. trout, salmon, carps, mullet, catfish and eels. However, the invention is of especial application in the farming of salmon on land using fresh and/or sea water flowing through tanks.

A preferred form of tank for use in the invention and its operation will now be described by way of illustration only and with respect to the accompanying drawings in whichsFigure 1 is a diagrammatic plan view of the IS vessel; and Figure 2 is a diagrammatic vertical section through the vessel of Figure 1.

The assembly comprises an elongated tank 1, for example built from concrete blocks, and having an internal-length of 20 metres and an internal width of 5 metres. The ends of the tank are rounded and a central partition wall 2, 15 metres long divides the interior of the tank to form an oval pond 3.

Central wall 2 contains an internal chamber 10 which is 0.3 metres wide, which acts as the aeration chamber. Clean water at 5° to 18°C is supplied to the pond via an overhead supply line 4 having outlet holes on opposed sides for each side of the pond so as to cause water to circulate around the pond as shown arrowed. The base of the tank is provided1 with drainage channels 5 and 6 on each side of the central wall 2» The channels are located along the 4 8 0« junction of the wall and the base of the tank on that section of the pond opposed to the point of introduction of the clean water. The drainage channels are thus located at the points in the pond whore the eiroulntion rate is slowest and most of the water-borne solids are being deposited. Alternatively, the base of the tank may be convex so that the slope of the base aids movement of solids to the outer rim of the tank; drainage channels 5 and 6 then being located along the junction of the side walls and the base of the tank. The channels are desirably covered with a grid. Water from the channels flows off via lines 7 and 8 to waste, carrying most of the deposited solids with it.

Wall 2 is a hollow wall defining a separate tank or aeration chamber 10 within the main tank. Chamber 10 is provided at its base with an air injector 11 along its length. This may be one or more lengths of pipe with holes in; or a porous block to which air is fed; or a more sophisticated injector with nozzles and the like. The injector may also take the form of a length of lay flat plastic tubing with fine holes along its length so as to provide a self-sealing assembly when not in use. The injector is one which delivers fine bubbles of air, typically the bubbles are less than 1 mm in diameter. Where the chamber 10 is wide it may be necessary to provide aerators in parallel along the length of chamber 10 to ensure interaction of the bubbles and the walls of chamber 10, thus achieving water circulation. The air injector is fed with air from a fan blower (not shown) which, in the case of a tank having the sizes given above, delivers 3 to 10 cubic metres of air per minute, i.e. - 10 3 0.2 to 0.7 m /metre length of chamber 10, at 0.1 to 0.5 kg/cm2 gauge.

The walls of chamber 10 are cut with inlet and outlet ports. 12 and 13 respectively. The inlet ports lead from channels 5 and 6 into the base of chamber 10. Preferably, the inlet ports lead from the upper part of channels 5 and 6 so that the majority of the solids entering the channels pass the entries of ports 12 and are drawn into lines 7 and 8 by the flow of water into lines 7 and 8. The outlet ports 13 are located near the top of chamber 10 and at approximately 0.5 to 3 ems above the expected surface of the water in the pond 3. The outlet ports are preferably located on the opposite side of chamber 10 to inlet ports 12 and are located closely downstream of the water supply line 4.

In operation, clean water is fed via line 4 to the pond 3. By virtue of the holes in line 4, approximately half the feed enters at X and the rest at S ag arrowed. The pond 3 fills with water which circulates (in this ease clockwise) under the influence of the incoming water. Some water passes via channels 5 and 6 and lines 7 and 8 to waste. The desired level of water in pond 3 is maintained by a weir or dam board, not shown. Water has also entered chamber via the inlet ports 12.and chamber 10 is full with . water to the same level as pond 3. Typically, 18002500 litres per minute of water are fed to line 4 (900-1250 litres coming out at eaeh of X and V) and the water depth is 0.95 to 1.05 metres.

Air is blow, into air injector 11 and streams of fine bubbles rise up within chamber 10 causing the water level in chamber 10 to rise, and water overflows «14 s ο :ι - 11 from outlet ports 13 to pond 3. The air streams also draw water into chamber 10 from channels 5 and 6.

This water is aerated and picks up oxygen as it pasaeti up chamber 10 before it overflows through outlet ports 13. In this way oxygen depleted water is drawn from channels 5 and 6, is aerated in chamber 10 and returned to the pond by the action of the air injector 11. By adjustment of the air pressure and air flow rate, one may vary the rate of circulation of water through chamber . Typically, with the figures given above, the total rate of circulation is 1800-5000 litres per minute through chamber 10. Thus, in effect the total rate of flow of adequately oxygenated water containing from 8 to 12 ppm dissolved in the pond is now 3600 to 7500 litres per minute; of which 1800-5000 litres are circulating through chamber 10, 1800-2500 litres are fed via line 4 and 1800-2500 litres are being withdrawn through channels 5 and 6 and lines 7 and 8 to waste. This effective increase in circulation of adequately oxygenated water through the pond means that the tank 1 may support more fish for a given capacity than a tank without the chamber 10; that less water from an outside source is required to maintain a given flow, since chamber 10 provides a proportion of the water required; the increased effective flow rate leads to enhanced cleaning action; and the water is effectively used several times without the build up of noxious materials therein which would occur with conventional re-use systems; or a combination of these.

It will be appreciated that the above form of tank may be modified in a number of ways. Thus, the tank may be of other shapes, e.g. circular or rectangular. - 12 In order to aid the flow of water caused by the air bubbles, chamber 10 may be provided with longitudinal baffles which restrict the width of chamber 10» e.g* as shown dotted in Figures 1 and 2. In place of individual 5 outlet ports 13» the wall of the chamber 10 may be formed lower over the appropriate length and aerated water may merely overflow from chamber 10 into pond 3.

It may be desired to cover chamber 10 to enable the chamber to be operated under pressure to assist pick up of oxygen by the water.

Claims (20)

CLAIMS: 1. A method for rearing fish which comprises rearing the fish in a vessel containing water and provided with means for circulating water within the vessel and with a separate aeration zone or chamber, said zone or chamber having means separate from the circulation means for aerating water in the zone or chamber and having at least one wall thereof common with the vessel, the zone or chamber being in fluid flow connection with the vessel whereby water from the vessel enters the zone or chamber, is aerated and the aerated water is returned to the vessel and supplements the circulation caused by the circulation means.

1. ·>

2. A method as claimed in claim 1 wherein the fish are reared to maturity in the vessel.

3. A method as claimed in either of claims 1 or 2 wherein the fish is an anadromous fish. 4. 4 S θ - 16 21. A fish farm or vessel as claimed in claim 20 wherein the chamber is from 0.2 to 1 metre wide. 22. A fish farm or vessel as claimed in any one of claims 17 to 21 wherein the aeration chamber has a

4. A method as claimed in claim 1 wherein the fish is a salmon. 5. Water inlet at a low level therein and a water outlet or overflow at a higher level. 23. A fish farm or vessel as claimed in claim 22 wherein the water outlet is through or over the opposite wall to the water inlet. 10 24. A fish farm or vessel as claimed in any one Of claims 17 to 23 wherein the vessel is an oval or round ended vessel having a generally centrally located aeration chamber therein. 25. A fish farm or vessel as claimed in any one 5 10. A method as claimed in any one of the preceding claims wherein the water is circulated directly without filtration from the vessel, through the aeration chamber and back to the vessel.

5. A method as claimed in any of the preceding claims wherein aeration is carried out by injecting air into the water.

6. A method as claimed in any one of the preceding claims wherein aeration provides from 6 to 12 ppm of dissolved oxygen in the aerated water.

7. A method as claimed in any one of the preceding claims wherein the level of water in the separate zone or chamber is less than 10 cms above the level of water in the vessel.

8. A method as claimed in any one of the preceding claims wherein aeration of the water causes 44S0® circulation of water through the aeration zone or chamber.

9. A method as claimed in any one of the preceding claims wherein the aeration is carried out in a chamber located generally symmetrically within the vessel.

10. Aeration is achieved by the injection of air into aeration chamber having a width of less than 1 metre and located generally symmetrically within an elongated vessel, the aerated water outlet from the chamber to the vessel being less than 10 cms above the water level

11. A method as claimed in claim 8 wherein

12. A method as claimed in claim 11 wherein the air is fed to the aeration means at a pressure of 2 from 0,1 to 0.5 kg/cm gauge.

13. A method as claimed in claim 11 wherein 20 the air is fed to the aeration chamber at a rate of from 0.2 to 0.7 cubic metres/minute/metre length of the aeration chamber.

14. A method as claimed in any one of the preceding claims wherein the aerated water is returned 25 to the vessel through or over the opposite lateral wall of the chamber to that through which it entered the chamber. 15. Of claims 17 to 24 wherein the vessel has a capacity of at least 10 cubic metres of water. 26. A vessel as claimed in claim 17 substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

15. A method as claimed in claim 1 substantially as hereinbefore described. 30 15 in the vessel.

16. A method as claimed in claim 1 substantially . as hereinbefore shown in and as described with respect to the accompanying drawings.

17. A fish farm comprising a vessel having means for causing water to circulate within the vessel, which vessel is provided with a separate aeration zone or chamber having at least one wall thereof common with the vessel and having means separate from the circulation means for aerating water when in the zone or chamber; the separate zone or chamber being in fluid flow connection with the vessel whereby water may be removed from the vessel and aerated water may be returned to the vessel, thereby supplementing the effect of the circulation means.

18. A vessel suitable for use in the method and fish farm of claims 1 and 17 which comprises a main vessel having means to cause water to circulate within the vessel and having a water inlet and a water outlet; a secondary vessel having at least one wall common with the main vessel, the two vessels being linked in fluid flow communication, the secondary vessel being provided with means separate from the circulation means for injecting air into water in the secondary vessel and thereby cause circulation of water from the main vessel, through the secondary vessel and back to the main vessel and thus supplement the circulation achieved within the main vessel by the circulation means when in operation.

19. A fish farm or vessel as claimed in either of claims 17 or 18 wherein the aeration chamber is located substantially symmetrically within the vessel. 20. A fish farm or vessel as claimed in any one of claims 17 to 19 wherein the aeration chamber comprises an elongated chamber having means for injecting air into water in the chamber.

20. 27. A fish farm comprising a vessel as claimed in any one of claims 17 to 26.