WO2003037077A1

WO2003037077A1 - Method and apparatus for aquatic animal husbandry

Info

Publication number: WO2003037077A1
Application number: PCT/AU2002/001481
Authority: WO
Inventors: David Mann; Tamao Asakawa; Maurizio Pizzutto
Original assignee: The State Of Queensland, Through Its Departement Of Primary Industries
Priority date: 2001-11-02
Filing date: 2002-11-01
Publication date: 2003-05-08
Also published as: AUPR865101A0

Abstract

A method and device for raising marine animals, especially crustaceans such as crabs (27). The method involves isolation of berried females (27) and rapid removal of the female (27) after hatching. Larvae (32) are transferred to a larval hatch tank (30) where water conditions and diet are controlled. After metamorphosis, juveniles are transferred to a pond (34) and raised for a preferred period. The pond (34) may include a settlement substrate formed from soaked straw. The device for performing this metod also includes a tank (28, 33) with a conically inwardly sloping bottom region and cylindrical top region. The larval hatch tank (33) is part of a two tank system with a sump tank (37) connected in a water recirculation system and including filters and heaters.

Description

METHOD AND APPARATUS FOR AQUATIC ANIMAL HUSBANDRY

FIELD OF THE INVENTION

This invention relates generally to a method and apparatus for aquatic animal husbandry and in particular for marine animals. The invention is particularly directed toward incubation, hatching and rearing of crustaceans, especially crabs, but is not necessarily restricted to this process.

BACKGROUND OF THE INVENTION

Aquatic animals have long formed part of the staple diet of many cultures. With the gradual world-wide depletion of the numbers of edible marine, estuarine and fresh water species, attention has inevitably turned to aquaculture as a substitute for wild-sourced food supplies for both people and animals.

After early initial difficulties in establishing a viable industry, significant advances have been made in relation to the artificial rearing of numerous fish and crustacean species. Land-based trout and salmon farms, as well as oceanic pens for rearing the same species, are now widely-known forms of farming practice. Prawn farming has also made significant advances to the point where artificially reared prawns form the basis of significant commercial exploitation. Crabs have also to a limited degree been the subject of some forms of aquaculture, which may include the collection of reproductive material, such as fertilised eggs. This step is usually followed by attempts to hatch and raise zoeal stages and juvenile crablet stages with subsequent further development by seeding these juveniles into a wild environment or, alternatively, attempting to raise them in a crab farm situation.

Although the numbers of fertilised eggs in these species may be very high, the developmental process is also accompanied by very high mortality rates which on occasion may approach 100%. The early stages of aquatic life are particularly predisposed to mortality as a result of environmental stress from sudden changes in water characteristics such as temperature, concentration of salt, other dissolved components, toxins and pathogens. Failure to successfully negotiate the various life stages in early development of aquatic animals has led to significant difficulty in obtaining adequate numbers of some species to warrant the associated economic outlay and input of labour and resources of artificial rearing.

It would be of assistance to provide a system of aquatic animal husbandry which resulted in higher production rates in the early stages of animal development. It would also be of advantage if such a system provided good growth rates in those early stages.

SUMMARY OF THE INVENTION

In one aspect, although it need not be the only or indeed the broadest aspect, the invention resides in a method of aquatic animal husbandry comprising the steps of collecting fertilised eggs, incubating the eggs until hatched as larvae, culturing the larvae until they metamorphise into juvenile animals and, at least partially, raising the juvenile animals.

Preferably the aquatic animals are marine animals. The marine animals may be crustaceans. The crustaceans may be crabs. In this specification, "marine" may be understood to include a reference to an estuarine habitat.

Collecting fertilised eggs may include the step of locating at least one egg carrying female in a quantity of liquid. The liquid may be in a container. The liquid may be water. The container may be a tank. The tank may be an incubating tank. The method may include the step of at least partially filling the tank with seawater. The seawater may be pre-treated. Pre-treating the sea-water may include the step of filtering to remove particulate matter. The particulate matter removed may be of a size greater than 1μm. Pre-treating the seawater may include the step of allowing the seawater to settle for a period of time after filtering. The period of time may be around a minimum of 7 days.

The method may include the step of maintaining the water in the incubating tank with certain desired characteristics. Those characteristics may be one or more of a temperature of 27-28°C, salinity of 32-35 grams per litre, pH of 8 - 8.5, dissolved oxygen at saturation level, low ammonia level which may be as low as less than 0.1 part per million.

The method may include the step of cleaning the incubating tank on a daily basis. Cleaning the incubating tank may include cleaning the bottom of the incubating tank on a daily basis by scrubbing.

If the aquatic animal is a crab, berried or egg-bearing females may be stocked in the incubation tank at a density of less than 1 per m² of tank bottom surface area.

The method may include the step of microscopically examining a small sample of eggs from one or more of the females. The females may be culled from the incubation tank if their eggs fail to meet certain pre-selected criteria which may be one or more of, less than 70% fertility in the eggs, greater than 5% egg mortality, the presence of fungal hyphae in and around the eggs, the presence of bacteria on the surface of the eggs particularly dense filamentous bacteria, high rate of egg loss from the sponge of eggs and a high range of developmental variation throughout the sponge of eggs. If the aquatic animals are crabs, the method may include the step of not feeding the crabs while in the incubation tank.

The method may further comprise the step of transferring the egg-bearing females prior to hatching. Suitably at least one female may be transferred to a second tank. The second tank may be a hatch tank. Preferably the female is transferred approximately one hour before hatching is due. The hatch tank may suitably contain seawater with the same properties as that of the incubation tank. The seawater may be filtered to remove particulate matter of greater than 1μm and may have been allowed to settle for a period such as 7 days. The hatch tank may be aerated at a relatively low rate.

Preferably the egg-bearing female is located individually in a cage within the hatch tank. The cage may be a floating cage. The egg-bearing female may be removed from the hatch tank soon after hatching is complete.

The method may include the step of collecting hatched larvae from the hatch tank. The hatched larvae may be collected from the tank after a preferred or preselected time interval. The preferred time interval may be between about 15 to about 30minutes. The larvae may be collected using collection means. The collection means may be a scoop and/or include syphoning of fluid from the hatch tank. Preferably the larvae are concentrated in a small volume of water during collection.

Preferably, the hatch tank is cylindro-conical in shape and has a capacity of approximately 1 ,000 litres.

The method may further comprise the step of transferring the larvae to a third container. The third container may be a larval culture tank assembly. The larval culture tank assembly may comprise a single tank which is preferably cylindro- conical in shape. Its volume may suitably be in the range of around 1 ,000 to around 5,000 litres. Preferably, the conical shaped portion of the tank is lowermost and has a slight angle of slope of approximately 10 degrees. The larval culture tank assembly may suitably comprise a main tank and a sump tank with fluid communication means for transferring water between the main and sump tanks. The fluid communication means may comprise recirculation means. The recirculation means may provide fluid flow from the main tank to the sump tank through an airlift system and from the sump tank to the main tank by an overflow system. The overflow system may provide flow from the sump tank to the main tank from an outlet position towards an upper edge of the sump tank, said outlet position higher than an inlet or open top of the main tank so that overflow water can return to the main tank.

Water in the larval culture tank assembly is preferably circulated through the system constantly. The rate of circulation may suitably be 1 ,000 litres per hour.

The method may further include the step of filtering outlet water from the main tank to prevent larval migration into the sump tank. Appropriate filter sizes may be 300 micrometres for larval stages zoea 1 to zoea 3; 500 micrometres for zoea 3 to zoea 5 and 800 micrometres for megalopae.

The method may further include the steps of altering water characteristics in the main tank by effecting changes to contents of the sump tank. The changes may include heating the contents of the sump tank. Heating may suitably be accomplished with immersion heaters. Other alterations of water characteristics may include alteration of salinity, nutrient levels or other components of the water mixture.

The method may further include the use of at least one filtration medium. The at least one filtration medium is suitably located in the sump tank. The at least one filtration medium may comprise biofiltration media.

The method may include the step of filtering water from the sump tank to control the concentration of particulate matter. This step may include applying filtering means over the outlet from the sump tank. Suitably the filtering means may be a filter bag. The filter bag may have a pore size of around 60 micrometres. The method may further include the step of aerating the larval culture tank by passing air through airstones immersed in the water of either or both the main tank and sump tank.

If the aquatic animal is a crab, an effective stocking rate of the larval culture tank may be approximately 100 larvae per litre.

Preferably, the method includes the step of feeding the larvae a controlled diet. The step of feeding a controlled diet may include the step of feeding rotifers of L- strain to zoea 1 and zoea 2 stages. The method may include the step of feeding Anemia nauplii to zoea 2 and subsequent zoeal stages through to the megalop stage. Artemia juveniles may be fed to the final zoea stage, through the megalop stage and to an early crablet stage. Artemia adults may be fed at the megalop stage and also to crablets.

A particulate diet may be beneficially fed to all stages preferably commencing at approximately half way through the first zoeal stage. The particulate diet may suitably comprise microparticulate commercial, prawn feed mixed with particularised freeze-dried krill. The particulate diet may be suitably increased in size with progressive stages of larvae being around 100 micrometres at zoea 1 , 250 micrometres from zoea 3 to the final zoea stage, 500 micrometres at the megalop stage and 800 to 1 ,000 micrometres at the first crablet stage.

The method may further include the step of preparing a habitat for juvenile animals. The habitat may be a nursery pond. The nursery pond may suitably be located outdoors. It may be lined with a suitable lining material such as high density polyethylene ("HDPE"). Preferably, the nursery pond is drainable to a collection pit. The nursery pond may be stocked with sea water. The sea water may be fertilised with suitable materials at a preferred time prior to introduction of crablets. The suitable material may be urea and/or superphosphate. The preferred period may be approximately 3 weeks. The method may include the step of forming a settlement substrate. The settlement substrate may be formed by distributing pre-soaked straw throughout the pond.

Prior to introduction of crablets to the nursery pond, the method may include the steps of altering the larval tank assembly water to more closely correspond with that of the nursery pond by infusing nursery pond water into the larval tank assembly. Preferably the infusion is gradual. Preferably, the method involves collecting the larvae after metamorphosis to megalopae. Most preferably, collection occurs one day after metamorphosis. Collection may be effected using a plankton mesh filter bag to capture the megalopae.

When the aquatic animals are crabs, the method may include feeding megalopae and crablets in the nursery pond.

The crablets may be fed commercial prawn food. Preferably, the commercial prawn food is between 0.8 to 1.5 millimetre diameter during the first week of occupation of the nursery pond. Feeding may be accomplished by distributing feed at the rate of 0.25 gram/m² of tank surface area twice daily. After the first week feeding may be effected by offering commercially available prawn feed with a diameter of 1.5 to 2.5 millimetres. Preferably, the feeding practice may be monitored. The method may include harvesting of the nursery pond. Monitoring of feeding practice may comprise sampling the pond bottom to determine the presence of uneaten food and break down products.

Harvesting of the nursery pond may suitably occur at crab 8 to crab 9 stage. Harvesting may be affected by slowly draining to the collection pit and using a collection cage. After harvesting the juvenile crabs may be placed in holding bins with constant water flow.

In a second aspect the invention lies in a method of incubating eggs of an aquatic animal including the step of collecting fertilised eggs. Collecting fertilised eggs may include the step of locating at least one egg carrying female in a quantity of liquid. The liquid may be in a container. The liquid may be water. The container may be a tank. The tank may be an incubating tank. The method may include the step of at least partially filling the tank with seawater. The seawater may be pre- treated. Pre-treating the sea-water may include the step of filtering to remove particulate matter. The particulate matter removed may be of a size greater than 1 m. Pre-treating the seawater may include, the step of allowing the seawater to settle for a period of time after filtering. The period of time may be around a minimum of 7 days.

The method may include the step of maintaining the water in the incubating tank with certain desired characteristics. Those characteristics may be one or more of a temperature of 27-28°C, salinity of 32-35ppt, pH of 8 - 8.5, dissolved oxygen at saturation level, low ammonia level which may be as low as less than 0.1 part per million.

If the aquatic animal is a crab, berried or egg-bearing female may be stocked in the incubation tank at a density of less than 1 per m² of tank bottom surface area.

The method may include the step of microscopically examining a small example of eggs from one or more of the females. The females may be culled from the incubation tank if their eggs fail to meet certain pre-selected criteria which may be one or more of, less than 70% fertility in the eggs, greater than 5% egg mortality, the presence of fungal hyphae in and around the eggs, the presence of bacteria on the surface of the eggs particularly dense filamentous bacteria, high rate of egg loss from the sponge of eggs and a high range of developmental variation throughout the sponge of eggs. If the aquatic animals are crabs, the method may include the step of not feeding the crabs while in the incubation tank.

In a third aspect, the invention resides in a method of hatching the eggs of an aquatic animal, including the step of transferring at least one egg-bearing female prior to hatching. At least one female may be suitably transferred to a second tank. The second tank may be a hatch tank. Preferably the female is transferred approximately one hour before hatching is due. The hatch tank may suitably contain seawater with the same properties as that of the incubation tank. The seawater may be filtered to remove particulate matter of greater than 1//m and may have been allowed to settle for a period around a minimum of 7 days. The hatch tank may be aerated at a relatively low rate.

The method may include the step of collecting hatched larvae from the hatch tank. The hatched larvae may be collected from the tank after a preferred time interval. The preferred time interval may be between about 15 to about 30minut.es. The larvae may be collected using collection means. The collection means may be a scoop and/or include syphoning of fluid from the hatch tank. Preferably the larvae are concentrated in a small volume of water during collection.

The method may further comprise the step of transferring the larvae to a third container. In a fourth aspect, the invention resides in a method of the culture of larvae of an aquatic animal comprising the steps of introducing larvae to a container. The container may be a larval culture tank assembly. The larval culture tank assembly may comprise a single tank which is preferably cylindro-conical in shape. Its volume may suitably be in the range of around 1 ,000 to around 5,000 litres. Preferably, the conical shaped portion of the tank is lowermost and has a slight angle of slope of approximately 10 degrees . The larval culture tank assembly may suitably comprise a main tank and a sump tank with fluid communication means for transferring water between the main and sump tanks. The fluid communication means may comprise recirculation means. The recirculation means may provide fluid flow from the main tank to the sump tank through an airlift system and from the sump tank to the main tank may be by an overflow system. The overflow system may provide flow from the sump tank to the main tank from an outlet position towards an upper edge of the sump tank, said outlet position higher than an inlet or open top of the main tank so that overflow water can return to the main tank.

The method may further include the steps of altering water characteristics in the main tank by effecting changes to contents of the sump tank. The changes may include heating the contents of the sump tank. Heating may suitably be accomplished with immersion heaters. Other alterations of water characteristics may include alteration of salinity, nutrient levels or other components of the water mixture. The method may further include the use of at least one filtration medium. The at least one filtration medium is suitably located in the sump tank. The at least one filtration medium may comprise biofiltration media.

The method may include the step of filtering water from the sump tank to control the concentration of particulate matter. This step may include applying filtering means over the outlet from the sump tank. Suitably the filtering means may be a filter bag. The filter bag may have a pore size of around 60 micrometres.

The method may further include the step of aerating the larval culture tank by passing air through airstones immersed in the water of either or both the main tank and sump tank.

Preferably, the method includes the step of feeding the larvae a controlled diet. The step of feeding a controlled diet may include the step of feeding rotifers of L- strain to zoea 1 and zoea 2 stages. The method may include the step of feeding Artemia nauplii to zoea 2 and subsequent zoeal stages through to the megalop stage. Artemia juveniles may be fed to the final zoea stage, through the megalop stage and to an early crablet stage. Artemia adults may be fed at the megalop stage and also to crablets.

A particulate diet may be beneficially fed to all stages preferably commencing at approximately half way through the first zoeal stage. The particulate diet may suitably comprise microparticulate commercial prawn feed mixed with particularised freeze-dried krill. The particulate diet may be suitably increased in size with progressive stages of larvae being around 100 micrometres at zoea 1 , 250 micrometres from zoea 3 to the final zoea stage, 500 micrometres at the megalop stage and 800 to 1 ,000 micrometres at the first crablet stage.

In a fifth aspect, the invention resides in a method of rearing juvenile aquatic animals. The method may further include the step of preparing a habitat for juvenile animals. The habitat may be a nursery pond. The nursery pond may suitably be located outdoors. It may be lined with a suitable lining material such as high density polyethylene ("HDPE"). Preferably, the nursery pond is drainable to a collection pit. The nursery pond may be stocked with sea water. The sea water may be fertilised with suitable materials at a preferred time prior to introduction of crablets. The suitable material may be urea and/or superphosphate. The preferred period may be approximately 3 weeks. The method may include the step of forming a settlement substrate. The settlement substrate may be formed by distributing pre-soaked straw throughout the pond.

Prior to introduction of crablets to the nursery pond, the method may include the steps of altering the larval tank assembly water to more closely correspond with that of the nursery pond by infusing nursery pond water into the larval tank assembly. Preferably the infusion is gradual. Preferably, the method involves collecting the larvae after metamorphosis to megalopae. Most preferably, collection occurs one day after metamorphisis. Collection may be effected using a plankton mesh filter bag to capture the megalopae.

In a sixth aspect, the invention resides in a tank assembly for aquaculture purposes wherein the tank assembly comprises a first tank means, a second tank means and a fluid connection means for providing fluid connection between the first and second tank means. The first tank means may be a main tank and the second tank means may suitably be a sump tank. The sump tank may be 20 to 25% capacity of the main tank. Either or both the main tank and the sump tank may be cylindroconical in shape. The cylindrical portion of the tank may be suitably located above the conical portion. The slope of the sides of the conical portion may be shallow and in the order of 10 degrees. The fluid connection means may comprise a first conduit for fluid flow from the main tank to the sump tank and a second conduit for fluid flow from the sump tank to the main tank.

The fluid connection means may further include an air inlet situated in a lower region of the first conduit and adapted to provide an airlift mechanism in operation so that water is urged from the main tank to the sump tank. Preferably, the air lift mechanism is adapted to provide a flowrate of 100% of the volume of the main tank every 1 to 5 hours.

The first conduit may comprise a main tank outlet, an ascending section and a sump tank inlet.

Suitably a plankton filter may be located on the main tank outlet of the main tank and sized to restrict or prevent larval migration from the main tank to the sump tank. The filter size of the plankton filter may be varied depending on the larval stage in the main tank being in the case of crabs, suitably 300 micrometres from zoea 1 to zoea 3; 500 micrometres for zoea 3 to zoea 5 and 800 micrometres for megalop. The sump tank inlet may be located in or adjacent an upper most region of the ascending section. The sump tank may suitably include a filtering medium. The second conduit may suitably comprise an overflow arrangement. The overflow arrangement may comprise an outlet from the sump tank, a discharge pipe connected to the outlet from the sump tank and an inlet to the main tank. In a preferred arrangement, the outlet from the sump tank and the discharge pipe may be situated higher than the inlet to the main tank which may be simply an open upper region of the main tank. The second conduit may further include a filtering means. The filtering means may be a filter bag. The sump tank may include a filtering mechanism. Preferably the filtering medium comprises biofiltration media.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a perspective view of a larval culture tank assembly of the present invention.

Figure 2 is a chart of a feeding schedule for juvenile crabs. Figure 3 is a flow chart of a crab rearing system.

DETAILED DESCRIPTION OF THE DRAWINGS

The invention is directed to a tank assembly for use in rearing juvenile and larval forms of aquatic animals and is also directed to a method of rearing those juvenile or larval forms. The device and method may be applied to a number of Phyla. The method and device are both well suited for use with Subphylum Crustacea, Order Decapoda and Infraorders Anomura and Brachyura. When used with crabs the method and device may both be suitable for members of Scylla species (S. serrata, S. paramamosain, S. tranquebarcia, S. olivacea); Portunus spp. (P. pelagicus, P. trituberculatas, P. sanguinolentus); Charybdis spp.; Callinectes spp.; Carcinus spp.; Olivipes spp.; Ranina ranina; southern and northern king crab - Paralithodes sp; Eriocheir sinensis (Chinese mitten crab); Birgus latro (coconut crab). The method is particularly suited for Scylla serrata and Portunus pelagius.

Referring to Figure 1 there is shown a tank assembly generally indicated at (10) and comprising a main culture tank (11) and sump tank (12). The tanks shown are both cylindro-conical with a shallow angle to the conical bottom located below a substantially cylindrical upper portion. The angle of the conical portion is suitably a slight or shallow angle of less than 10 degrees . Both tanks have conical bottoms (13, 14 respectively). The present shape is described by way of preferred example only. It is clear that a variety of tank shapes and sizes can be used with various liquid capacities. The inventors however, have found that a convenient volume of tank is approximately 1000-5000 litres in the main tank. The smaller sump tank is preferably sized to hold 20-25% of the volume of the main tank.

The tanks are in fluid connection via a main tank outlet (15) which is engaged with an elbow (16) which in turn is connected to a substantially vertical uplift pipe (17) having a gas outlet (18) at its upper region which communicates with the environmental atmosphere. The uplift pipe (17) also includes a T-piece (19) which connects to a delivery pipe or sump tank inlet (20) for discharging water into the sump tank. In operation air is pumped into the uplift pipe (17) to provide a bubbling air lift effect for transporting liquid from the main tank (11), up the uplift pipe (17) and subsequently along the delivery pipe (20) to discharge into the sump tank (12). Air pumped into the uplift pipe (17) is discharged via the gas outlet (18). Coarse bubbles are introduced at the bottom of the vertical section of the pipe connecting the 2 tanks through an air diffuser. Coarse bubbles are preferred as they cause less damage to the live food animals. The rate of air bubbling is such that it is maximal to the point where foam will start to form at the top of the pipe. This ensures that the live feed animals are not damaged in the movement of the water. The rising air in the vertical pipe acts to draw the water from the culture tank to flow into the sump tank at its outlet and the sump tank is positioned such that the height for the water to be lifted from the culture tank to flow into the sump tank is minimised. This system therefore, provides circulation of water from the main tank (11 ) to the sump tank (12).

The sump tank (12) is supported by a base (21) which is formed to locate an upper edge (22) of the sump tank (12) on a plane higher than the main tank upper edge (23). A sump tank outlet and discharge pipe (24) is adapted to provide a simple overflow mechanism for returning water from the sump tank (12) to the main tank (11 ).

The sump tank (12) is provided with a drain (25) for discharge of liquid from the sump tank thus allowing for removal of a predetermined volume from the system. A particular advantage of the relative sizes of the sump tank being approximately VΛ of the size of the main tank is that the maximum change of water usually required in the system (ie. 25%) can be accomplished with a single emptying of the sump tank. The assembly may also include a heater (26) which may be of the immersion type.

A particular advantage of this system is that it allows alteration of characteristics of the water or culture medium in the main tank (11 ) indirectly through addition or removal of ingredients from the sump tank (12). Introduction of the changes may therefore be gradual and relatively even, being controlled by the flow rate of water through sump tank overflow (24). It is well known that sudden alterations in conditions inside the main tank can lead to stress on the life forms contained within it. As an example of this, the use of immersion heaters in the main tank is known to increase the mortality rate. Using the heater as an example it is clear that the localised area of heating which occurs around such a device is quarantined from the main tank in the present invention and transfer of heat will be gradual through the mixing in the main tank of warmed water from the sump tank. This assembly also facilitates cleaning as the main tank may be maintained as a self contained entity while the sump tank is drained and scrubbed, resulting in the removal of particulate matter trapped therein.

The airlift mechanism may maintain a continuous flow of culture medium recirculating through the system preferably at a rate of between 100% turnover every 1-5 hours for 1000 litre and 5000 litre culture tanks respectively. That is the rate of 1000 litres per hour or thereabouts is a reasonably effective rate of circulation.

A plankton filter may be applied to cover the main tank outlet (15) channel to prevent larvae and other small life forms from entering the airlift and being discharged to the sump tank. The filter should be sized to allow circulation of food. Preferred filter pore sizes when raising crab larvae are 300//m for zoeal to zoea3; 500μm for zoea3 to zoeaδ and 800μm for megalop.

The sump tank may be used to remove particulate matter that would foul the system and is also used to remove uneaten feed; renowned for contributing to deterioration of water quality. If the level of food is too high or simply as an ongoing husbandry habit a filter bag of approximately 60 /m pore size may be secured over the discharge into the sump tank (20) and left as long as appropriate to filter food particles from the water. This may be a set time or alternatively may be assessed by determination of the level of non-desired particles of food in the liquid.

Light aeration may be supplied to the main culture tank (11 ) through airstones of sufficient number and spacing to ensure there is slow and continuous movement of water in all parts of the tank. A light aeration rate may be around 250mL of air per minute introduced from a small air diffuser (25 mm long, 10 mm diameter) at the bottom of the tank. Multiple diffusers may be used. In a preferred embodiment biofiltration media are included within the sump tank to act as a particulate matter trap and control nitrogen metabolite levels. A suitable medium is a small plastic spheroid product that is the same as that used for aquariums (ie aquarium beads, biofilter beads). They are designed to have a very high surface area to volume ratio. Preferably, the pore spaces are wide enough to allow passage of the live feeds (rotifers and artemia) and to allow quick and effective cleaning while still giving adequate nitrogenous waste conversion efficiency. The particulate material is easily removed which also reduces the risks of proliferation of undesirable bacteria. As described, in-flow of water into the main culture tank (11 ) promotes a lateral flow of water. This allows larvae more stability within the water column and also allows them to remain in the optimum position, particularly in relation to live food which is able to aggregate near the surface of the liquid. This effectively increases the food density in the area where the larvae tend to place themselves and reduces damage to the larvae. The flow rate into the main culture tank (11 ) may be altered and adjusted to suit the requirements of different larval stages.

As the use of the airlift pipe provides a high level of aeration and gas exchange, there is a minimal requirement for extra aeration means in the main culture tank (11).

The optimum physico-chemical parameters of the water or culture medium for different stages and species vary in relation to features such as salinity and temperature. It is therefore important that the culture system allow for control of these parameters to maintain a healthy stable environment. Where culture conditions need to be changed during, for example, the larval culture cycle, it is preferred to have a system where manipulation can be performed easily and without shock and stress to the larvae. The present assembly provides a volume of culture medium outside the culture tank where changes can be made and then slowly introduced, for example, the larval environment after partial dilution in the sump tank.

While emphasis in the description has been placed on crab larva rearing, this is by way of example only. The tank assembly may be used in many additional applications.

In a further aspect, the invention resides in a method of husbandry of juvenile or larval forms of aquatic animals comprising the incubation of eggs, hatching of eggs, culturing of larvae and maintenance and rearing of juveniles. The following description is directed towards crabs but it should be understood that the method may be applied to species other than crab and indeed to species that are not crustaceans. To incubate eggs, berried females (egg carrying crabs) are held in a large tank at low density which is preferably less than 1 crab per square metre. The conditions in the tank may be controlled to provide a low stress hygienic environment, so that the eggs may proceed through embryonic development to the pre-hatching stage. It is preferred that feed is not supplied to the spawner crabs during incubation. This husbandry technique has the advantage of avoiding an increased risk of bacterial and fungal growth within the sponge (or egg mass).

Water used in the tank is preferably seawater and may be pre-treated by filtering to 1μm and subsequently allowed to settle in a storage tank for at least seven days, after which the water may be used to fill the egg incubation tank. Use of pre-treated seawater is advantageous over other prior art methods as failure to treat the seawater increases the risk of bacterial and fungal infection of eggs. There are methods known which include the use of chemical pre-treatment of the seawater, for example, with chlorine or antibiotics. Settled seawater gives reliable results and is also chemical free thereby establishing a more stable vigorous and healthy microbial flora. The maintenance of hygiene during the holding period reduces the threat of potential disease organisms. Methods which require the use of antibiotics or antimicrobials to prevent contamination of the eggs are undesirable for sustainable long term culture techniques. In addition the use of prophylactic antibiotics is well known for the tendency to select for resistant bacterial strains and subsequent long term tank contamination.

It is important for close attention to be paid to the conditions inside the tank and water. The temperature may be held steady and preferably in the range of about 27 to about 28°C. This temperature may be controlled by heaters if necessary in a more temperate climate. Salinity is preferably maintained stable between around 32 to around 35ppt and pH is preferably maintained in the range of 8.0 - 8.5. Oxygen may be maintained at dissolved saturation levels and ammonia is preferably kept to negligible levels of less than 0.1 part per million. A preferred husbandry practice is to clean the tank bottom of debris daily and also scrub or brush the bottom with an appropriate device.

The development and condition of the eggs are monitored on a regular basis by microscopic examination of small numbers of excised eggs. Culling thresholds may be instituted based on pre-selected criteria such as fertilisation rate of the eggs being less than 70%; egg mortality rate being greater than 5%; proliferation of fungal hyphae; dense growth of filamentous bacteria on the surface of the eggs; high rate of egg loss from the sponge and a high range of developmental rates throughout the sponge. Removing crabs that fail one or more of these threshold tests results in the maintenance of high viability, healthy eggs, with relatively consistent rates of development throughout the sponge.

The day of hatching of the eggs may be predicted by the stage of embryo development as seen under a microscope. On the day of hatching the spawner may be transferred from an incubation tank to a hatch tank about an hour before hatching is due to commence. Hatching time of the larvae is predictable and typically will occur within a short time of sunrise. Preferably the hatch tank may be a cylindro-conical shape and have a capacity of around 1000 litres. A light aeration rate may be supplied from the bottom of the tank to ensure slow water movement. Preferably settled seawater is used to fill the hatch tank. The physico-chemical parameters of the hatch tank may be the same or similar to the seawater used in the incubation tank.

In some existing methods the spawners are added directly to a larval culture tank omitting the hatch tank step. This type of approach increases the potential for disease transmission from the spawner crab to the newly hatched larvae.

Preferably the spawner crabs are held in a floating cage with one spawner per hatch tank. The advantage of placing a spawner in an individual cage allows for the hatched spawner to be removed from the hatch tank as soon as hatching is complete, reducing the risk of disease transmission to the newly hatched larvae and subsequent contamination of the larval culture tank with egg casings and spawner faeces.

Preferably the spawner is monitored on a continuous basis around the time of hatching and as soon as hatching is complete the crab is removed from the tank. Completion of hatching may be detected by the absence of eggs under the abdomen and cessation of spawner hatching behaviour. Spawner hatching behaviour in a crab is shown by the female becoming more agitated with increased movements, walking around the cage and rising up high on her legs. When hatching is imminent, a "flicking" or "flexing" of the abdomen occurs. This effects a dislodgment of eggs and larvae from the sponge.

The larvae may be collected from the hatch tank by one of two preferred methods depending on the normal larval behaviour of the species being reared. The first method involves scooping with a small ladle from a dense aggregation at the surface of the water. Alternatively, if the larvae aggregate near the bottom of the tank, syphoning may be used with discharge from the syphoning hose passed through a collection sieve. The larvae are preferably collected from the hatch tank shortly after hatching and most preferably within 15-30minut.es. This collection time allows sufficient time for the larvae to moult and harden their shell before collection and to subsequently stock them to a lower density prior to their health being compromised through remaining in the hatch tank for extended periods of greater than 1 hour. In transferring the larvae from the hatch tank to the larval culture tank it is preferred to concentrate the larvae into a small volume of water to avoid or minimise the chance of cross contamination from the tanks.

The water quality of the hatch tank is preferably kept at the same as the larval culture tank to limit or minimise any shock to the larvae upon transfer. As a guideline, if there is greater than a 1°C or 3ppt salinity difference then the collected larvae may first be added to a sieve and larval culture tank seawater may be slowly passed through the sieve to allow for a 20-30 minute acclimation period before the larvae are added to the culture tank. The tank assembly previously described may be used during any of the stages of development of aquatic animals. However, it is particularly suited to act as a larval culture system. The optimum physico-chemical parameters of the culture medium for larvae such as salinity and temperature may vary among species. It is therefore important that the system allow for control of the parameters to maintain a healthy and stable environment.

Newly hatched larvae are stocked into the culture tank at a preselected rate. The preferred stocking rate is approximately 100 larvae per litre.

During the larval stage, feeding of the developing life forms becomes important for the first time. The feeding of the various life stages may be of great importance in terms of survival rate, general health and growth rate. Feed-type transitions may be instituted at zoea 2 to zoea 3 and at zoea to megalop. Preferably these transitions involve overlap of the feed types so that larvae are acclimated to new feed over approximately 2 days. In addition, an important feature of the feed relates to particulate diet size, which may be increased with progressive stages of the larvae, and may be 100 /m diameter at zoeal , 250μm starting at zoea 3 to the final zoeal stage 500μm at the megalop stage and 800-1 OOOμm at the first crablet stage. These size specifications are approximate only and may vary quite significantly. However, the trend of increasing particle size with increasing maturity is preferred.

The particulate diet preferably consists of a combination of artificial microparticulate feed such as that produced for prawn larvae and particularised freeze-dried krill. The inventors have shown that using krill particles greatly reduces the occurrence of moult death syndrome in the larvae thereby improving production rates. However, the benefits of krill may depend on the strain of Artemia used. The inventors have also shown that strains of Artemia derived from geographical regions in Asia and in particular from China and Vietnam, may be nutritionally sufficient to support normal growth and survival of crab larvae without the addition of krill or other additives. These strains of Artemia are currently more expensive than the most widely used Artemia available commercially which are predominantly from the Great Lakes in the USA. As this difference in cost may sometimes be by a factor of 3, the commercial incentive to minimise the expense of Artemia is high. As the USA and similar grade Artemia are readily available for fin fish and prawn hatcheries, their combination with krill is commercially attractive. Some suitable prawn feeds and their specifications are shown in the following table.

Figure 2 is a schedule for a suggested feeding regime for crabs whose life cycle involves between 4-6 zoeal stages.

Preferably rotifers, L-strain are fed to zoeal , zoea2 and just into the zoea3 stage as a transition. Particulate diet may be commenced approximately half way through the zoeal stage and continued throughout the subsequent stages into the crablet stage with preferably increasing particulate diet size as discussed above. Artemia nauplii may be commenced approximately half way through the second zoeal stage and continued through all subsequent zoeal stages until terminating at the commencement of the megalop stage. Artemia juveniles may then preferably be fed from around the middle of the final zoeal stage through the megalop stage and into the commencement of the crablet stage as a transition while Artemia adults may be fed from towards the end of the megalop stage and subsequently. Traditionally larval nutrition has been an impediment to the achievement of good megalop production rates using standard feeding regimes. Most prior art regimes nutritionally enrich the live foods with artificial formulations designed for fin fish larvae or with selected micro-algae species. These methods are time consuming to conduct, prone to bacterial contamination and do not necessarily deliver the nutritional profile appropriate to crab larvae. Krill is attractive to the larvae and is not likely to be the source of pathogens due to it being a freeze dried product from high latitude cold water. It is also envisaged that a particulate supplement may be produced to replace krill and prawn larvae feeds.

After the crablet stage is reached, the animals may be moved to a nursery pond which is preferably outdoors. The pond may be lined with high density polyethylene (HDPE) and sited so that it can be completely drained into a collection pit. Approximately 3 weeks before stocking, the pond may be filled with seawater and fertilised with urea and superphosphate to promote phyto-plankton and zoo-plankton bloom. The proliferation of live food organisms constitute an important initial part of the crablet diet, promoting high growth rate and survival rate.

Pre-soaked straw is scattered throughout the pond forming complex 3D structures to act as a settlement substrate for colonisation and shelter by the megalopae and crablets. The use of straw has distinct advantages as it does not compromise pond management, is desirable for utilisation by crabs, is not expensive, is readily available and is not excessively labour demanding. The straw requires soaking so that it will sink. Usually soaking occurs over approximately 10 days in a tank of seawater, preferably with some water exchange to prevent build up of tannins which can lead to unhygienic conditions. One of the advantages of using an organic substrate like straw as opposed to an inert one like plastic is that it promotes the development of benthic organism assemblages that form part of the diet for the small crablet stages. Using straw to provide refuge substrate for juvenile crustaceans as well as to promote development of feed organisms in marine/brackish water systems is a novel feature of the invention.

A short period, preferably one day, after all of the zoea have metamorphosed to the megalop stage in the larval culture tank, they may be harvested by collection in a plankton mesh filter bag and stocked into the nursery pond. To reduce shock on the megalopae, water quality of the larval culture tanks maybe modified slowly to be similar to that of the nursery pond water, by gradual infusion of new water from the pond to the larval culture tank.

For the first week in the nursery pond and up to crab2 stage, selected prawn feeds of 0.8 to 1.5mm diameter size, are distributed over the pond at approximately 0.25g per m² twice per day. Following this period, feeds in the size range of 1.5 to 2.5mm diameter may be fed at an appropriate rate. The rate may be accurately determined by pond bottom sampling to detect the presence of excess feed and presence of nitrogen thereby allowing appropriate modification of feeding amounts.

Pond management may follow standard procedures developed for pond aquaculture of prawns, using application of fertiliser, water exchange and aeration to maintain a phytoplankton bloom and desirable water quality conditions. These methods are well known to a person skilled in the art.

Crablets are harvested at the crabδ to crab9 stage (2.5 to 6g) by slowly draining the pond into a collection cage covering the outlet. As the water drops the crablets migrate down with the water towards the outlet. As the crablets accumulate in the cage they are netted out and placed into holding bins that preferably receive a constant through-flow of seawater. Figure 3 is a schematic representation of the husbandry scheme for tending larval and juvenile crab stages. A crab (27) is seen in an incubating tank (28). The crab (27) is laden with a sponge (29) of eggs.

Upon it becoming apparent that the crab is due to hatch she is moved to a hatching tank (30) and placed in an individual cage (31 ), from where her eggs hatch to produce larvae (32). The larvae are collected and transferred to a larval culture tank (33) for subsequent rearing through to the megalop stage, after which they are transferred outdoors to a nursery pond (34) for growing through to the crabδ to crab9 stage, after which they are harvested by flow through of the water (35) in the pond (34) through a collecting sieve (36) located to accept water draining from the pond. The larval culture tank (33) is connected to sump tank (37) to provide a circulatory loop between the two tanks.

Throughout this specification, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element or integer or group of elements or integers but not the exclusion of any other element or integer or group of elements or integers.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that that prior art forms part of the common general knowledge in Australia.

Throughout the specification the aim has been to describe the preferred embodiments of the invention without limiting the invention to any one embodiment or specific collection of features. Those of skill in the art will therefore appreciate that, in light of the instant disclosure, various modifications and changes can be made in the particular embodiments exemplified without departing from the scope of the present invention. All such modifications and changes are intended to be included within the scope of the appendant claims.

Claims

CLAIMS:

1. A method of animal husbandry for use with aquatic animals, especially crustaceans, said method including the steps of: pretreating a suitable liquid by one or more of filtering and allowing the suitable liquid to stand for settling; locating the suitable liquid in a tank; and locating at least one egg bearing female in the suitable liquid.

2. The method of claim 1 wherein the suitable liquid is seawater.

3. The method of claim 2 wherein the seawater is filtered to remove particulate matter of around 1 μm diameter or larger and the seawater is allowed to stand for settling for 7 days or longer.

4. The method of any one of the preceeding claims wherein the aquatic animal is a crab.

5. The method of any one of the preceeding claims including the step of maintaining the suitable liquid with one or more of a temperature of 27-28°C, salinity of 32-35 ppt, pH of 8-8.5, disolved oxygen at saturation level and a low ammonia level.

6. The method of claim 4 further including the step of: cleaning the tank daily including scrubbing an inner surface of the tank.

7. The method of any one of claims 4 to 6 further including the step of: stocking female crabs at less than 1 per square metre of tank bottom surface area.

8. The method of claim 7 further including the step of: examining microscopically a small sample of eggs from one or more of the female crabs to assess egg viability.

9. The method of claim 8 wherein the step of assessing egg viability includes the step of: culling female crabs if their eggs have one or more of less than 70% fertility in the eggs; greater than 5% egg mortality; presence of fungal hyphae in and around the eggs; presence of bacteria on the surface of the eggs; high rate of egg loss from a sponge of eggs; and a high range of developmental variations throughout the sponge of eggs.

10. The method of claim 7 further including the step of: starving the crabs while in the tank.

11. The method of any one of the preceding claims further comprising the step of: transferring egg bearing females to a hatch tank prior to hatching of the eggs.

12. The method of claim 12 wherein the female is transferred approximately 1 hour before hatching is due.

13. The method of claim 11 wherein the hatch tank contains liquid with the same properties as liquid in the incubation tank.

14. The method of claim 13 further comprising the step of: aerating the hatch tank at a relatively low rate.

15. The method of claim 11 further comprising the step of: locating the egg bearing female individually in a cage located within the hatch tank.

16. The method of claim 15 wherein the cage floats in the hatch tank.

17. The method of claim 11 further including the step of: collecting hatched larvae from the hatch tank.

18. The method of claim 17 wherein the hatched larvae are collected between 15-30 minutes after hatching.

19. The method of claim 18 wherein the larvae are concentrated in a small volume of water during collection.

20. The method of any one of claims 11-19 wherein the hatch tank is cylindroconical in shape and has a capacity of approximately 100 litres.

21. The method of any one of the preceeding claims further comprising the step of: transferring the larvae to a larval culture tank assembly.

22. The method of claim 21 wherein the larval culture tank assembly comprises a single tank which is cylindroconical in shape and has a volume in the range of 1,000-5,000 litres.

23. The method of claim 22 wherein the larval culture tank has a lowermost conical shaped portion with a slight angle of slope of around 10 degrees.

24. The method of claim 23 wherein the larval culture tank assembly further comprises a sump tank and fluid commumcation means for circulating water between the main tank and the sump tank.

25. The method of claim 24 wherein the fluid communication means comprises an air lift system for providing fluid flow from the main tank to the sump tank and an overflow system for providing fluid flow from the sump tank to the main tank.

26. The method of claim 25 further comprising the step of: circulating water through the tanks at a rate of approximately 1000 litres per hour.

27. The method of claim 26 further comprising the step of: filtering outlet water from the main tank to prevent larval migration into the sump tank.

28. The method of claim 27 further comprising the step of altering water characteristics in the main tank by effecting changes to the contents of the sump tank, said water characteristics including one or more of temperature, salinity and nutrient levels.

29. The method of claim 28 further including the step of: filtering water in the sump tank with a filtration medium.

30. The method of claim 29 wherein the filtration medium is a biofiltration medium.

31. The method of claim 29 further including the step of: filtering outlet water from the sump tank to remove particulate matter.

32. The method of claim 31 wherein the method includes using a filter bag with a pore size around 60 micrometres.

33. The method of claim 32 further including the step of: aerating the larval culture tank by passing air through air stones immersed in the water of at least one of the main tank and sump tank.

34. The method of claim 33 including the step of: stocking the larval culture tank at a rate of approximately 100 larvae per litre.

35. The method of claim 34 further including the step of: feeding a controlled diet to the larvae.

36. The method of claim 35 wherein the step of feeding a controlled diet includes the steps of: feeding rotifers of L-strain to zoea 1 and zoea 2 stages, feeding Artemia nauplii to zoea 2 and subsequent zoeal stages through to megalop stage, feeding artemia juveniles to the final zoeal stage and through the megalop stage and to an early crablet stage and feeding artemia adults to the megalop stage and to crablets.

37. The method of claim 36 further comprising the step of: feeding a particulate diet to all stages commencing at approximately half way through the first zoeal stage.

38. The method of claim 37 wherein the particulate diet comprises microparticulate commercial prawn feed fixed with particularised freeze-dried krill.

39. The method of claim 38 wherein the particulate diet is increased in size with the progressive stages of larvae being around 100 micrometers on average at zoea 1, 200 micrometers on average from zoea 3 to the final zoeal stage, 500 on average micrometers at the megalop stage and 800-1,000 micrometres on average at the first crablet stage.

40. The method of claim 39 further comprising the step of preparing a habitat for juvenile animals.

41. The method of claim 40 wherein the habitat is a nursery pond.

42. The method of claim 41 wherein the nursery pond is an outdoor construction lined with high density polyethylene.

43. The method of either one of claim 40 or claim 42 further comprising the step of stocking the nursery pond with seawater.

44. The method of claim 43 further comprising the step of: fertilising the seawater with urea and/or superphosphate at a preferred time prior to introduction of crablets.

45. The method of claim 44 wherein the preferred time is approximately 3 weeks.

46. The method of claim 44 further comprising the step of:

forming a settlement substrate.

47. The method of claim 46 wherein forming the settlement substrate includes the step of: distributing presoaked straw throughout the nursey pond.

48. The method of claim 47 further comprising the step of: altering the larval tank assembly water to more closely correspond with that of the nursery pond by infusing nursery pond water into the larval tank assembly prior to introducing crablets from the larval tank assembly to the nursery pond.

49. The method of claim 48 including the step of collecting the larvae after metamorphosis to the megalop stage.

50. The method of claim 49 wherein collection occurs approximately 1 day after metamorphosis.

51. The method of claim 50 including the step of: feeding crablets in the nursery pond with commercial prawn food.

52. The method of claim 51 comprising the step of: feeding the crablets by distributing feed at a rate of 0.25 g per square metre of nursey pond surface area twice daily.

53. The method of claim 52 further comprising the step of: sampling the pond bottom to determine the presence of uneaten food and break down products and altering the feeding rate accordingly.

54. The method of claim 53 further comprising the step of: feeding prawn food at between 0.8-1.5 mm diameter during the first week in the nursery pond and subsequently feeding commercially available prawn feed with a diameter of 1.5-2.5 mm.

55. The method of claim 54 further including the step of: harvesting the nursery pond when crabs are at crab 8- crab9 stage.

56. The method of claim 55 wherein harvesting of the nursey pond includes the step of: slowly draining the pond to a collection pit and through a collection cage; and locating collected juvenile crabs in holding bins with constant water through flow.

57. A tank assembly for aquaculture comprising: a main tank; a sump tank of approximately 20-25 % capacity of the main tank; fluid connection means for circulation fluid between the main tank and sump tank; wherein: either or both the main tank and the sump tank are cylindroconical in shape with the cylindrical portion of the tank located above the conical portion which slopes inwardly in a downwards direction.

58. The tank assembly of claim 57 wherein the slope of the sides of the conical portion is approximately 10° or less.

59. The tank assembly of either one of claims 57 and 58 wherein the fluid connection means comprises a first conduit for fluid flow from the main tank to the sump tank and a second conduit for fluid flow from the sump tank to the main tank.

60. The tank assembly of claim 59 wherein fluid is urged along the first conduit by an air lift mechanism.

61. The tank assembly of claim 60 wherein the air lift mechanism is adapted to provide a flow rate of 100% of the volume of the main tank every 1-5 hours.

62. The tank assembly of claim 61 wherein the first conduit comprises a main tank outlet, an ascending section and a sump tank inlet.

63. The tank assembly of claim 62 wherein a plankton filter is located on the main tank outlet and sized to restrict or prevent larval migration from the main tank to the sump tank.

64. The tank assembly of claim 63 wherein the plankton filter is varied in pore size and provides a 300 micrometre mesh up to zoea 3, 500 micrometre mesh for zoea 3 to zoea 5, and 800 micrometre mesh for the megalop stage.

65. The tank assembly of claim 59 wherein the second conduit comprises an overflow arrangement.

66. The tank assembly of claim 65 wherein the overflow arrangement comprises an outlet from the sump tank, a discharge pipe connected to the outlet and an inlet to the main tank.

67. The tank assembly of claim 66 wherein the overflow arrangement includes a filter bag for removing particulate matter from the fluid.

68. The tank assembly of claim 67 further comprising a filtering mechanism.

69. The tank assembly of claim 68 wherein the filtering mechanism includes a biofiltration medium.

70. A crustacean when produced according to the method of any one of claims 1 to 56.

71. A crab when produced according to the method of any one of claims 4 to 55.

72. A method of aquatic animal husbandry substantially as described herein with reference to the figures.

73. A tank assembly for aquaculture substantially as described herein with reference to the figures.