FIELD OF THE INVENTION

This invention relates to a method of and apparatus for cultivation of crustaceans.. The invention has been devised particularly, although not solely, for cultivation of freshwater crayfish such as marron.

BACKGROUND QF THE INVENTION

Freshwater crayfish are in high demand owing to their good eating qualities. Marron is an example of a freshwater crayfish which is in particular demand as it is the largest of a number of different species of native freshwater crayfish in Western Australia. There are several disadvantages in supplying this demand with freshwater crayfish taken from their natural environment. In particular, freshwater crayfish in their natural environment are difficult to catch, and over-exploitation of areas containing such crayfish result in depletion of numbers for breeding purposes. In response to this, there have been several attempts to breed freshwater crayfish on a commercial basis by aquaculture. This is facilitated by certain characteristics of crayfish in that they grow fairly rapidly, do riot have a very complicated life cycle, and are - exploitable commercially without harming stocks remaining in the natural environment.

Traditionally, aquaculture techniques for commercial cultivation of freshwater crayfish including marron have utilised large tanks or farm dams. This is not- altogether satisfactory, however, as marron are of a cannibalistic nature and so there is a very high rate of mortality of marron fingerlings. Furthermore, farm dams do not provide optimum conditions for cultivation of

485 _- ,2 _- PCT/AU89/00375

marron as factors such as high rainfall can produce high mortality rates of marron. While fresh water tanks do overcome the problem relating to heavy rainfall or other variations in climatic conditions, they still suffer from the problem of the inherent cannibalistic tendencies of marron.

It is therefore evident that the marron need to be maintained in an environment which is conducive to their development and in which they are isolated from one another.

There have been proposals to cultivate marron and other freshwater crayfish in compartments within a water environment but such proposals have not proved altogether satisfactory as the water environment has not been particularly conducive to growth.

SUMMARY OF THE INVENTION

The present invention seeks to provide a method of, and apparatus for, cultivation of marron or other crustaceans in an environment which is conducive to their development.

In one form the invention resides in a method of cultivating crustaceans comprising the steps of: providing a plurality o.f growing compartments in series, each compartment being separated from the next succeeding compartment by wall means permeable to water; establishing a water flow through the compartments at a level below the top of the compartments; regulating the rate of water flow to promote the growth of algae on said wall means between successive compartments; introducing one crustacean into each compartment where the crustacean can proceed to mature.

It is believed that the step involving the promotion of growth of algae on said wall means is particularly important as the presence of algae provides a food source for the crustaceans and reflects a natural environment where algae would be present.

Preferably, said method further comprises the step of introducing bark into each compartment. It is believed that the bark provides a further source of food for the crustaceans and also provides a body on which algae can grow.

Preferably, said bark is treated to remove, or at least reduce the amount, of tannin within the bark. One way of treating the bark is to soak the bark in water for an extended period of time such as approximately three months.

Preferably, said method further comprises the step of aerating the water. The water may be aerated by allowing it to cascade into a tank in which said compartments are located.

The rate at which water flows through the compartments is important inasmuch as a flow rate which is too slow can lead to stagnation of the water and a flow rate which is too fast can cause mortality in the crustaceans, the reason for which is unknown. A suitable flow rate has been found to be in the order of 10 litres per minute.

In another form the invention resides in apparatus for cultivation of crustaceans comprising: a growing tank; means for establishing a water flow through the tank while maintaining a substantially constant water level below the

top of the tank; and partition means dividing the tank into a plurality of growing compartments, said partition means being permeable to water whereby water can flow through each compartment, said partition means being adapted to support the growth of algae.

Preferably, said partition means is black or dark in colour to promote algae growth.

Preferably, said tank is elongated and said water flow is from one end of the tank to the other end of the tank.

Preferably, each compartment is elongated and disposed transverse to the longitudinal axis of the tank whereby water flow through each compartment is across the compartment.

Preferably, said tank is provided with a removable closure on top of the tank to provide selective access to each compartment.

Preferably, said removable closure is perforated for ventilation of the region above the water level within each compartment.

Preferably, water flows through the tank substantially continuously.

Preferably, means are provided for selectively discharging water from the tank from time to time to remove debris and other unwanted matter in the tank.

Preferably, recirculating means are provided for recirculating water flowing through the tank. A filtering means is also preferably provided to filter the recirculating water.

Preferably ^' , aeration means are provided for aerating the water flowing through the tank.

Preferably, a plurality of said tanks are disposed in series with respect to' water flow such that the water flows successively through the tanks.

Preferably, said tanks are disposed one above another such that water flowing from one tank enters the tank immediately below. For preference the tanks are longitudinally offset.

Preferably, water from one tank cascades into the tank immediately below to facilitate aeration of the water.

Preferably, said apparatus further comprises a quantity of bark within each compartment. For preference said bark is treated to remove or at least reduce tannin within the bark before being introduced to the compartments.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be better understood by reference to the following description of one specific embodiment thereof as shown in the accompanying drawings in which:-

Figure 1 is a schematic view of a cultivation apparatus comprising an assembly of growing tanks;

Figure 2 is a schematic side elevational view of the assembly of growing tanks;

Figure 3 is a detailed view of part of one of the growing tanks with a lid section raised to show

growing compartments within the interior of the growing tank;

Figure 4 is a detailed view showing a partition means which divides each tank into growing compartments; and

Figure 5 is a schematic view showing the arrangement of growing tanks in series for water flow successively through the tanks.

DESCRIPTION OF PREFERRED EMBODIMENT

The embodiment shown in the drawings is directed to apparatus for cultivating marron.

The apparatus comprises a frame structure 11 supporting a plurality of growing tanks 13 in a series of vertical arrays. The vertical arrays of growing tanks are arranged in pairs with a walkway 15 between each pair of vertical arrays, the walkway being supported on the frame structure 11. The walkway provides for convenient access to the tanks in the vertical arrays to the sides of the walkway.

The growing tanks 13 in each array are positioned in vertically spaced relationship and are in a staggered formation (as best seen in Figure 2) such that each tank is longitudinally offset with respect to the tank immediately below it, the purpose of which will be explained later.

Each growing tank 13 comprises a bottom wall 17, a pair of side walls 18 and a pair of end walls 19. The tank has an open top. A closure in the form of a hinged lid 21 is

provided for the open top of the tank. The hinged lid is formed in sections 23 for convenient access to the interior of the tank. The lid is provided with perforations 25 for ventilating the interior of the tank while inhibiting the entry of debris into the tank.

Water is supplied to the uppermost tank in each vertical array and flows sequentially through the tanks in the array while maintaining a substantially constant water level within each tank. For this purpose the uppermost tank of each vertical array is provided with an inlet means 27 in the form of an inlet pipe connected to a supply line 29 by way of a flow control valve 31. The supply line 29 delivers water from a source such as a reservoir (not shown) . The inlet means 27 is disposed above and inwardly of one end of the tank such that water discharging from the inlet means cascades into the tank. Each tank also has an outlet means 33 in the form of a pipe extending a short distance beyond the end wall 19 of the tank. The outlet end of the outlet pipe 33 is positioned above and inwardly of one end of the growing tank immediately below such that water discharging from the outlet pipe cascades into the tank immediately below, as shown in Figure 5. Water cascading into each tank enters the tank at one end through the open top of the tank, there being an opening provided in the lid 21 for this purpose. The water leaves the tank through the outlet pipe 33 at the other end of the tank, also as shown in Figure 5 of the drawings.

The outlet pipe 33 of each.tank is spaced both from the top and the bottom wall of the tank. With this arrangement, the outlet pipe does not let the water level 36 within the tank fall below the level of the outlet. The flow rate of water entering the uppermost tank is

regulated by valve 31 such that the water level within the tank is at the level of the outlet means. In this way, an air space 35 exists in the region of the tank above the water level 36 and below the top of the tank. The purpose of the air space is to provide an area in the compartment into which the marron can surface, as it would do in its natural environment.

The outlet means 33 on the lowermost tank is connected to a return line (not shown) which returns the water to the reservoir for ' subsequent re-circulation after being filtered and aerated. Aeration may be achieved in any suitable way such as by agitating the water in the reservoir or pumping air into the water. Aeration is also facilitated by the water cascading into each tank.

A drainage system is provided for selectively draining each series of tanks from time to time to remove debris and other unwanted matter. The draining system includes a drain pipe 37 connecting each tank to a downpipe 38. The drain pipe 37 is connected to the lower region of each tank at the end of the tank in which the outlet 33 is fitted. Therefore, with this arrangement, there are two downpipes 38 for each vertical array of tanks, one at each end of the tanks. A valve 39 is provided in each discharge pipe 35 and water drains from the tank upon opening of the valve.

A partition means 41 divides the tank into a plurality of growing compartments 43. The partition means may be of any suitable material although it is preferable for such material to be resistant to the corrosive effects of water within the tank. The partition means comprises a plurality of transverse partitions 45 which extend across the tank between the side walls and a plurality of central

partitions 47 extending longitudinally of the tank between adjacent transverse partitions 45. The transverse partitions 45 extend transversely of water flow through the tank and are provided with perforations 49 to allow water to flow through the screens. The perforations in the screens are of a size which permits water flow while isolating crustaceans on opposed sides of the screen from each other. Each transverse screen is connected adjacent its corners to longitudinal rails 51 which are positioned within the tank. The central partitions 47 are received in tracks 53 provided in the transverse partitions (as best seen in Figure 4) and are provided with clip formations 55 which engage the upper edge of the adjacent transverse partitions.

Each lid section 23 for the tank is pivotally connected to one of the longitudinal rails 51, as best shown in Figure 4.

When the apparatus is used for cultivating marron, a layer of bark is deposited on the floor of each compartment so as to some extent reflect a natural environment for marron. The bark is treated before use to remove, or at least reduce the amount of, tannin in it. The treatment process involves soaking the bark for an extended period of time (typically about three months) in water. This treatment of the bark also opens up pores within the bark for easier feeding by the marron. The marron are then put into each of the compartments. As marron are of a cannibalistic nature, only one animal is put into each compartment. The lid 21 prevents each marron from escaping from its respective compartments and the partition means prevents the marron from attacking marron in neighbouring compartments. Water is delivered to the

uppermost tank via delivery line 29 and the flow rate is controlled by valve 31. The flow rate is adjusted so that the water within each tank is maintained at the level of the outlet 33. Water flows through the various compartments in each tank to the outlet 33 from where it cascades into the tank immediately below it through the open top thereof. This cascading flow facilitates aeration of the water. After the water has passed sequentially through the tanks in the vertical array, it is returned to the _. reservoir. The recirculating water is filtered and aerated. Additionally, nutrient material may be added to the water if found necessary.

The flow rate of water through the tanks requires careful control inasmuch as a flow rate which is too slow can lead to stagnation of the water and a flow rate which is too fast can cause mortality of the marron within the tanks . It has been found that a suitable flow rate is in the order of 10 litres per minute.

The conditions within the compartment are conducive to algae growth on the transverse partitions 45. This is particularly important as the presence of algae provides a food source for the marron and to some extent reflects a natural environment for marron. This results in an almost self-sustaining environment where very little additional food is required to be added to the water. Where food is added, it is in a form which floats and which is of a size that cannot pass through the perforations 49 in the partitions at least when first introduced into the compartment. Food in the form of floating pellets is particularly suitable. Because the food floats on the water surface, any food not consumed by the marron over a given period can be seen and removed before it decays. As the food is in a form which cannot

pass through the perforations in the compartment, it cannot be carried away by the flowing water before the marron has had an opportunity to consume at least some of it.

The circulating water to some extent removes debris and unwanted material within the compartments but it is necessary from time to time to operate the drainage system to flush the tanks.

Marron are introduced into the growing compartments while in the fingerling stage and remain there until mature and ready for harvesting. When the marron are mature and ready for harvesting, the drainage system can be operated to remove water within the tanks to facilitate capturing of the marron.

From the foregoing it is evident that the present invention provides a growing environment for marron and other crustaceans which to some extent reflects their natural environment inasmuch as it is conducive to algae growth and also protects the crustaceans against cannibalism as they mature. As the marron are maintained in separate compartments, they can be inspected and supplied with additional food on an individual basis.

It should be appreciated that the scope of the invention is not limited to the scope of the embodiment described.