Background of the Invention It is known to provide a culture tank which includes a receptacle for holding a culture medium (for example salt water) and an organism to be cultured, and an aeration device for imparting oxygen to the culture medium and for maintaining the suspension of food and other particulate matter (mostly exuviae and faeces).

One such culture tank includes a filter for facilitating collection and removal of particulates such as exuviae and faeces from the tank. However, a problem with such a culture tank provided with a filter is that the configuration and location of the filter in the tank has the effect of impeding the desirable hydrodynamic flow of liquid in the tank and of increasing the risk of damage to organisms in the tank due to collisions between the organisms and the filter.

Summary of the Invention In accordance with a first aspect of the present invention, there is provided a culture tank including: a receptacle for containing a culture medium and organisms to be cultured; substantially flat filter means, the filter means being arranged to filter particulate material from the culture medium ; fluid motion imparting means arranged, in use, to introduce fluid into the receptacle such that the culture medium in the receptacle is urged to flow in a direction

substantially parallel to the filter means so as to minimise collisions between organisms and the filter means; and means for urging culture medium adjacent the filter means to flow through the filter means.

In one arrangement, the culture tank includes at least one tank compartment for containing organisms to be cultured, at least one filter compartment, and the filter means includes at least one first filter disposed between each filter compartment and an adjacent tank compartment.

Preferably, each first filter is arranged to allow particulates and culture medium to pass through the first filter and to restrict passage of organisms through the first filter.

Preferably, the at least one filter compartment and the at least one tank compartment are defined by at least one partition member extending substantially transversely across the receptacle. Preferably, the or each partition member is removable such that the longitudinal dimension of each tank compartment is adjustable. Each first filter member may be mounted on a partition member and may be removable from the partition member.

Preferably, at least one second filter is provided in the filter compartment. The or each second filter may be arranged to allow culture medium to pass through the second filter and to substantially restrict passage of particulates through the second filter.

The fluid motion imparting means may be elongate.

In one arrangement, the fluid motion imparting means is disposed adjacent an inwardly facing surface of the receptacle.

In an alternative arrangement, the fluid motion imparting means is disposed outside the receptacle.

In one arrangement, the fluid motion imparting means is an aeration member arranged to inject gas into the receptacle.

In an alternative arrangement, the fluid motion imparting means includes a culture medium injection member arranged to inject culture medium into the receptacle.

Preferably, the fluid motion imparting means is arranged to introduce fluid into the receptacle from an apical portion of the receptacle.

Preferably, the fluid motion imparting means includes a plurality of apertures for introducing fluid into the receptacle.

In one arrangement, the culture tank is provided with connection means arranged to facilitate connection of the culture tank with an adjacent culture tank so as to thereby define a receptacle of increased volume.

A known prior art culture tank includes a receptacle of generally rectangular transverse cross section. However, a disadvantage with this type of culture tank is that regions of low fluid movement occur adjacent corners of the tank which encourages settling of particulates.

In order to overcome problems with this type of culture tank, a receptacle with a base having a generally convex inwardly facing surface has been developed. With this arrangement, an aeration device is generally provided at a lowermost portion of the convex base. Due to the curved inwardly facing surface of the receptacle, regions of low fluid movement are avoided.

However, while such an alternative tank operates satisfactorily when the tank is relatively small, as the diameter of the tank is increased, regions of relatively low fluid movement develop because the efficiency of the aeration device reduces with increasing receptacle volume. While increasing the flow rate of air through the aeration device may minimise the development of static fluid regions in the receptacle, increasing the

flow rate can affect the organisms'ability to control their swimming directions and can cause damage to the organisms.

In one arrangement of the present invention, the receptacle has a substantially U-shaped transverse cross-sectional shape, and the filter means are oriented so as to extend in a substantially transverse direction relative to the receptacle.

In this way, by providing an elongate substantially U-shaped receptacle, the size of the receptacle can be increased by increasing the length of the receptacle without substantially affecting the hydrodynamic chacteristics of the receptacle.

Preferably, the transverse cross-sectional shape of the receptacle is substantially parabolic.

Preferably, the receptacle is formed of high density polyethylene material. The receptacle may be mounted on a support structure and fixed to the support structure, for example by hot air welding. The support structure may be a frame.

In accordance with a second aspect of the present invention, there is provided a method of culturing organisms, said method including the steps of : providing a receptacle containing culture medium and organisms to be cultured; providing substantially flat filter means arranged to filter particulate material from the culture medium; urging culture medium in the receptacle to flow in a direction substantially parallel to the filter means so as to minimise collisions between organisms and the filter means; and urging culture medium adjacent the filter means to flow through the filter means.

Preferably, the step of urging culture medium in the receptacle to flow in a direction substantially parallel to the filter means includes the substep of introducing fluid into the receptacle, which fluid may be a gas.

In an alternative arrangement, the step of urging culture medium in the receptacle to flow in a direction substantially parallel to the filter means includes the substep of injecting culture medium into the receptacle.

Preferably, fluid is introduced into the receptacle from an apical portion of the receptacle.

Preferably, the culture tank includes at least one tank compartment for containing the organisms to be cultured, at least one filter compartment, and the filter means includes at least one first filter disposed between each filter compartment and an adjacent tank compartment, and the method further includes the step of urging fluid to flow from the or each compartment to a corresponding filter compartment.

Preferably, the method further includes the step of arranging each first filter so as to allow particulates and culture medium to pass through the first filter and to restrict passage of organisms through the first filter.

Preferably, the method further includes the step of removably mounting each first filter member on a partition member extending substantially transversely across the receptacle.

Preferably, the method further includes the step of providing at least one second filter in the filter compartment and arranging the or each second filter to allow culture medium to pass through the second filter and to substantially restrict passage of particulates through the second filter.

In one arrangement, the method includes the steps of providing a receptacle having a substantially U-shaped transverse cross-sectional shape, and orienting the filter means so as to extend in a substantially transverse direction relative to the receptacle.

Preferably, the transverse cross-sectional shape of the receptacle is substantially parabolic.

Brief Description of the Drawings The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic perspective view of a culture tank in accordance with an embodiment of the present invention; Figure 2 is a diagrammatic cross-sectional view of the culture tank shown in Figure 1 taken along the line II-II in the direction of the arrows; Figure 3 is a diagrammatic cross-sectional view of the culture tank shown in Figure 1 taken along the line III-III in the direction of the arrows; and Figure 4 is a diagrammatic representation of a portion of a culture tank showing an arrangement for facilitating transfer of organisms between adjacent tank compartments of the culture tank; and Figures 5a and 5b illustrate alternative receptacle configurations for use with the culture tank shown in Figures 1 to 3.

Description of a Preferred Embodiment of the Present Invention Referring to the drawings, in Figure 1 there is shown a culture tank 10 for use in culturing organisms, in this example Parartemia. However, it will be understood that the invention is applicable to other organisms which are cultured in tanks, such as fish larvae, crustaceans, planktonic organisms, and gastropods.

The culture tank 10 includes a frame 12 and a receptacle 14 supported by the frame 12 and by several straps 16 extending from the frame 12 underneath the receptacle 14.

The frame 12 includes four longitudinal frame members 18, two transverse frame members 20, and four upright frame members 22, the frame members 18,20, 22 being connected together in any suitable way, for example by welding, so as to define a generally cuboid-shaped support structure. However, it will be understood that any suitable support structure is envisaged, the important aspect being that the support

structure is able to support a receptacle 14 in the orientation shown in Figure 1. In the present embodiment, the frame 12 and straps 16 are formed of steel material, although it will be understood that other suitably strong, rigid material may also be used.

The receptacle 14 includes an elongate web portion 24 of generally U-shaped transverse cross-section, in this example parabolic, and first and second end members 26 and 28 respectively. The web portion 24 and end members 26,28 together define a compartment for receiving and holding a culture medium and organisms to be cultured.

In this embodiment, the web portion 24 is formed of high density polyethylene material which has been bent and hot air welded to the frame 12, although it will be understood that other suitably strong materials may be used, including fibreglass and steel. A drain (not shown) is provided at the base portion of the receptacle 14.

It will be understood that since the receptacle has a generally U-shaped transverse cross- section, the likelihood of development of low fluid movement regions in the receptacle 14 is low. As a result, the energy required for achieving sufficient mixing of culture medium and particulates in the receptacle 14 is also relatively low. Furthermore, it will also be understood that a consequence of low mixing energy is that the likelihood of damage to the organisms is also low.

The receptacle 14 also includes a first partition member 30 and a second partition member 32, the first and second partition members 30,32 being disposed relative to the web portion 24 so as to define a filter compartment 34, and first and second tank compartments 36 and 38 respectively in which organisms are to be cultured. In this example, the first and second partition members 30,32 are disposed such that the filter compartment 34 is located generally centrally of the receptacle 14 and is narrow relative to the first and second tank compartments 36,38. However, it will be understood that a variety of configurations are envisaged.

Each of the first and second partition members 30,32 includes a first filter member 40 arranged to allow culture medium and particulate material such as food, exuviae and faeces to pass through to the filter compartment 34 whilst retaining organisms in the

first and second tank compartments 36,38. In order to retain the first filter members 40 in engagement with their respective partition members 30,32, clamps (not shown) or other suitable retaining means may be provided.

Each of the first filter members 40 is removable from its respective first or second partition member 30,32 for cleaning or for replacement with a filter member more suitable for the size of organisms presently in the tank compartments 36,38. For example, on commencement of culturing of the organisms, first filter members 40 having relatively small apertures may be used and, as the organisms grow in size, the first filter members 40 may be replaced with first filter members 40 of larger aperture size.

Provided in the filter compartment 34 between the first and second partition members 30,32 is a second filter member 42, in this example a microporous filter, arranged to allow culture medium and food to pass through the second filter member 42 whilst restricting passage of exuviae and faeces. In this example, the second filter member 42 is of generally cylindrical configuration, although it will be understood that a variety of filter types may be used.

In order to draw fluid through the first and second filter members 40,42, pump means 43 are provided, which may be an air-lift or submersible pump.

Culture medium which is drawn through the second filter member 42 may be treated prior to being returned to the first and second tank compartments 36,38. Such treatments may include micro-filtration, treatment using ultra violet radiation, biological filtration and/or treatment using ozone. The culture medium may also be treated biologically and/or using physical and/or chemical processes to remove or detoxify metabolites including ammonia, nitrites and nitrates before the culture medium is returned to the first and second tank compartments 36, 38.

The culture tank 10 also includes fluid motion imparting means 44, in this example in the form of an elongate aeration member 44 extending generally longitudinally of the

web portion 24 inside the receptacle 14 adjacent an apical portion of the web portion 24.

The aeration member 44 includes a plurality of apertures 46 arranged to introduce gas, in this example air, into the receptacle 14 during use so as to oxygenate the culture medium in the receptacle, and to urge the culture medium to flow in a direction substantially transverse of the receptacle, thereby encouraging food, exuviae and faeces to remain in suspension in the culture medium. Effective aeration also encourages even distribution of animals.

However, it will be understood that other variations are possible. For example, apertures could be provided in the web portion 24 and air introduced through the apertures from beneath the web portion 24, the important aspect being that the culture medium is urged to flow in a direction substantially transverse of the receptacle 14.

Since with this arrangement the aeration member is disposed beneath the receptacle 14, disruption of fluid flow and damage to the organisms by the aeration member 44 are minimised.

It will be understood that since the culture medium is encouraged to flow in a direction substantially transverse of the receptacle and thereby parallel to the first filter member 40, the likelihood of collisions between organisms and the filter means is minimised.

The tank compartment 34 may be used as a settling tank wherein faeces, exuviae and dead animals settle at the base of the filter compartment 34. The settling process may be accelerated, for example by temporarily turning off the fluid motion imparting means to reduce fluid movement, or by adding flocculating materials.

In order to remove settled material from the base of the culture compartment 34, a drain and controllable valve may be provided. In a preferred arrangement, the valve would be remotely actuable, for example using a solenoid, so as to facilitate automation of removal of settled material from the filter compartment 34.

The above embodiment is described in relation to a culture tank for use in culturing organisms such as Parartemia. However, for culturing larval stages of marine fish and

invertebrates such as crustaceans and gastropods, important adjustments to the culture tank should be made in order to optimise the culture tank. This is because larvae are physically more fragile than organisms such as Parartemia.

For this application, instead of providing an aeration member for introducing air into the receptacle 14, the fluid motion imparting means 44 may take the form of a culture medium injection member which may be of similar configuration to the aeration member 44. By injecting culture medium into the receptacle 14 instead of air, a more gentle laminar flow of culture medium within the tank can be generated.

The culture medium injected into the receptacle 14 may be fresh culture medium, culture medium recycled from the filter compartment 34, or a combination of fresh culture medium and culture medium recycled from the filter compartment 34.

With this embodiment, since culture medium is injected into the receptacle 14 instead of air, the culture medium in the culture tank 10 would be oxygenated using a separate oxygenation mechanism. For example, the culture medium may be oxygenated prior to being returned to the first and second tank compartments 36,38 using appropriate aeration devices, sparging devices, and so on, or by oxygen injection prior to or during pumping and delivery of the culture medium to the first and second tank compartments 36,38.

It will be understood that in order to maintain a generally constant volume of culture medium in the culture tank 10, suitable level indicators may be provided in the tank to sense the level of culture medium in the culture tank 10 and to cause an increase or decrease in the flow rate of culture medium injected into the culture tank depending on whether the level of culture medium in the culture tank is increasing or reducing. Such level indicators may include float valves, electronic water level detectors, and so on.

An example of operation of the culture tank 10 will now be described in relation to Figures 2 and 3.

As shown in Figure 2, the receptacle 14 is filled with culture medium, in this example seawater 48. The organisms to be cultured, in this example parartemia (not shown), are added to the seawater 48. A suitable feed for the organisms is also added to the seawater 48.

As indicated by bubbles 50, air is introduced into the receptacle 14 by the aeration member 44, the air serving to oxygenate the seawater 48 and to encourage mixing of the seawater and particulates in the receptacle 14. As can be seen in Figure 2, since the air from the aeration member 44 travels upwardly of the aeration member 44 in a generally vertical plane passing through the aeration member 44, the seawater 48 in the receptacle 14 is urged to circulate in a direction generally transverse of the culture tank 10, as indicated by arrows 52 in Figure 2.

As shown in Figure 3, the seawater 48, exuviae, food and faeces are drawn through the first filter members 40 from the first and second tank compartments 36,38 to the filter compartment 34, as indicated by arrows 54, for example using an air-lift or submersible pump (not shown). The organisms in the first and second tank compartments 36,38, being too large to pass through the first filter members 40, remain in the first and second tank compartments 36,38. The seawater and food in the filter compartment 34 are then drawn by the air-lift or submersible pump (not shown) through the second filter member 42, through appropriate treatment means, and back into the first and second tank compartments 36,38, as indicated by arrows 56 and 58. The exuviae and faeces suspended in the seawater in the filter compartment 34 are blocked by the second filter member 42 and remain in the filter compartment 34.

In order to change a first filter member 40, for example so as to replace the first filter member 40 with an alternative filter member 40 of larger aperture size, a user first places a barrier member, for example a plastic sheet, over the first filter member 40 by sliding the barrier member over a surface of the filter member 40 facing the relevant tank compartment 36,38 so as to prevent egress of organisms into the filter compartment 34. For this purpose, alignment means, such as grooves (not shown), may be provided on a surface of the partition members 30,32 facing a tank compartment 36,

38. The filter member 40 is then disengaged from its respective partition member 30, 32 and the desired replacement filter member engaged with the partition member 30,32.

The barrier member is then removed.

It will be appreciated that since the transverse cross-sectional shape of the receptacle 14 is relatively constant along the longitudinal direction of the receptacle, an increase in receptacle length and thereby receptacle volume has little impact on operation of the culture tank.

Although the above embodiment has been described in relation to a culture tank with one filter compartment and two tank compartments, it will be appreciated that other tank configurations are possible. For example, a culture tank having one filter compartment and one tank compartment, or a culture tank having a plurality of filter compartments and a plurality of tank compartments are possible.

It will also be appreciated that the receptacle may be formed such that the effective length of the tank compartments are adjustable depending on the requirements of the organisms to be cultured in the tank compartments. For example, for some organisms which have a relatively long larval stage, it may be necessary for logistical, husbandry and hygiene reasons to transfer the organisms to a clean tank compartment or to a bigger tank compartment, or to increase the volume of the tank compartment as the organisms grow. In order to transfer the organisms with minimum physical damage and stress, the culture tank 10 may be provided with removable partition walls.

For example, in an alternative embodiment, the partition members 30,32 defining the filter compartment 34 and the tank compartments 36,38 may be removable and replaceable with water-proof partition walls. The end members 26,28 may also be removable. In this way, a relatively small tank could be constructed by providing the partition members 30,32 to define a filter compartment 34 and by providing a partition wall at either side of the filter compartment 34 so as to define the tank compartments 36, 38. As the organisms grow, an adjacent compartment to one of the tank compartments 36,38 could be filled with culture medium and the partition wall removed so as to

create a tank compartment of increased volume. Following equilibration of animal numbers and density within the new tank volume, additional partition members 30,32 could be inserted so as to define an additional filter compartment.

It will also be understood that organisms may be transferred by providing replaceable barrier walls 60 arranged to fit with existing partition members 30,32 as shown in Figure 4 to define a passage 62 through which organisms may pass. Once a desired organism equilibration has occurred, the barrier walls would be removed and the first filter members 40 engaged with the partition members 30,32.

It will also be appreciated that the culture tank 10 may be provided at longitudinal ends with connection means which enables the culture tank 10 to connect with an adjacent culture tank so as to provide a culture tank of increased volume. As with the above alternative embodiment, removable partition members 30,32 and removable end members 26,28 may be provided so as to facilitate adjustment of the volume of the tank compartments formed by the connected culture tanks.

It will also be appreciated that although the above embodiment has been described in relation to a fluid motion imparting means 44 which is disposed at an apical portion of a generally parabolic receptacle 14, other arrangements are envisaged. For example, the fluid motion imparting means 44 may extend generally longitudinally of the receptacle 14 but be located to one side of the apical portion of the receptacle 14. Alternatively, as shown in Figures 5a and 5b, the fluid motion imparting means may be located at an apical portion of a receptacle formed so as to include a generally vertical first wall 70 and an upwardly curved second wall 72. As shown in Figure 5b, two such receptacles may be formed by disposing a generally vertical wall 74 between a generally U-shaped wall 76 and by providing each defined receptacle with a fluid motion imparting means 44.

Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.