W2B-039 Cultivating abalone-0.txt
Chapter 3

Cultivation methods for abalone from 8-26 mm

Juvenile abalone of size 8 mm onwards may be maintained solely on a diet of seaweed. This allows a much less specialised form of husbandry and 8 mm is the minimum size which growers are advised to purchase abalone seed. However juveniles within the 8-26 mm size range still require a more careful handling regime than larger sizes and this is why this stage of development is still classified as a nursery stage.

3.1 Cage System for 8-26mm Juveniles

A system for maintaining this stage, presently in use and developed at the S.R.L. is a cage made from modified 250 litre plastic barrels. Each barrel has three equally spaced windows measuring 85 x 20cm cut out of its side for ventilation. Holes of 2 mm are drilled around the margin of each window to enable the attachment of an appropriate sized mesh. The cage is subdivided into three compartments by disks of 1 cm thick stockboard (the plastic equivalent of chipboard). The disks fit snugly to the sides of the barrel and are attached at three points with 7 mm polypropylene rope. The flooring ensures equal distribution of seaweed and abalone and thus prevents localized overcrowding which would otherwise occur at the bottom of the cage.

Access to each compartment is via a 10 cm diameter feeding porthole which is sealed with a 10 cm (4 “) diameter sewer pipe stop end. The stop ends are held in place with rubber bands (car inner tubing) which are tied onto the cages and fastened at the opposite end with hooks fashioned from bullwire. Each stop end is also attached to the side of the barrel by a length of polypropylene twine to prevent loosing it. Additionally the inserted part of the stopend is cut to ensure that only a minimal amount protrudes into the barrel past the thickness of the walls, otherwise abalone will attach to this part and prevent its withdrawal.

The cage is surrounded by a harness consisting of three lengths of 7mm polypropylene rope which runs along its length and are held in place by two lengths of the same rope which encircles the circumference of the barrel. The three harness ropes extend beyond each end of the barrel and are tied into rope eyesplices fashioned from a short length of 10 mm rope. The short lengths of 10mm rope are eyespliced at each end. The other eyesplice end can then be connected to a suspension rope at the top end of the barrel and on to a counterweight at the bottom end. With this design the entire strain of suspension is borne by the rope harness and not the barrel. These cages have been used with considerable success by Irish Abalone/Blackshell Farm Ltd., one of the largest of the abalone companies presently operating in this country. This company suspends its cages from surface long lines which are moored in a sheltered area in Clew Bay, Co. Mayo. The company has since substituted the cage harness and the sewer pipe stop ends for a net and flower pot ends respectively. Suspension from rafts is an alternative. However, if such methods of suspension are employed it is vital that the location is sheltered enough to ensure minimal water disturbance as abalone do not seem to respond well to jerky cage movements. In less sheltered sites a more practical alternative would be a submerged longline on which the principal floats are also submerged.

Alternative designs

A very successful variation of the barrel cage system currently in use in California involves the insertion of a window in the top and bottom of the barrel. No windows are cut into the side of the barrel. These cages are suspended from a pier in a sheltered area where there is a constant gentle wave surge. This results in a continuous piston like water movement through the inside of the cages keeping the abalone free of waste and stagnant conditions at all times. Since the pier is a solid structure which is firmly fixed to the sea floor, there is no movement at the suspension point. Counterweights attached beneath the cages prevents excessive movement in the wave surge. The American abalone culturalists have an advantage when using such a system in that the seaweed they feed to their abalone are kelps with air bladders (Macrocystis and/or Nereocystis luetkeana and therefore do not sink to the bottom of the cage and block the mesh ventilation windows. Because the seaweed distributes itself uniformly throughout the cage, localized overcrowding of the abalone is also prevented as they congregate around the food source. A combination of the vertical and horizontal ventilation systems has been employed in a new much improved cage design at South West Aquaculture (see caption of Fig 2.) In this design the floors must either be of mesh (not less than 6 mm) or have mesh windows in them, otherwise vertical ventilation is cut off. Care must be taken however not to weaken the barrel by cutting out too much of the side. Obviously the size and type of barrel can vary according to suitability and preference of the grower.

Stocking densities: mesh sizes and mesh changes

Stocking density trials at the S.R.L. showed an initial density of 20 8-10 mm abalone per litre of internal cage volume e.g. a 100 litre cage would be stocked with 2000 juvenile abalone of 8-10 mm. Once a shell length of 26 mm is attained the stocking level should be reduced to 5 abalone per litre. A summary of recommended cage stocking levels and mesh sizes for juvenile abalone of 8-26 mm is presented in Table 1 below.

The ventilation factor of meshes below 6 mm is greatly reduced and consequently it is extremely important to change all meshes of below 6 mm for the next larger mesh as soon as the juveniles reach the necessary size, e.g. juveniles of mean shell length 10 mm should be contained by a 4 mm mesh and not grown on past this size in a cage with 2-3 mm mesh. Once the 6 mm mesh stage is attained the need is to transfer to larger mesh sizes becomes less urgent. At the S.R.L. when mesh changes became necessary for abalone in nursery cages it was the meshes on the cages that were changed and not the transferral of the abalone in another cage. The reason for this was that the removal of abalone from surfaces (especially if they are stuck in a corner) runs the risk of damage to the foot, particularly with abalone of less than 30 mm in size. Handling also causes stress.

The mesh change involved carefully cutting away the twine which attached the old mesh and sewing on the new and larger mesh. This exercise was undertaken in situ and care was taken to ensure that the abalone did not dry out by periodically pouring buckets of sea water over the cage. Although the sewing method was extremely laborious the advantage was that it was safe. If preferred less time consuming techniques can be used, such as mesh windows consisting of mesh sewn onto a plastic "window frame". The frames can be attached to the barrels by removable nylon nuts and bolts, or they can be permanently attached by pop riveting. It must be emphasised that the recommended cage stocking levels in this manual were derived at the S.R.L. with experimental cages (see plate 3 above).

These cages were suspended from a raft in a very sheltered area which experienced a strong tidal flow. The determining factor governing stocking levels is almost certainly ventilation, i.e. the effectiveness with which the excreted waste products of the abalone can be washed away. The presence of dissolved waste products such as ammonia above concentrations of 5 mg/litre of ammonia - N, will stress abalone and this will retard growth and make them susceptible to disease. The possibility that higher stocking rates can be obtained using other cage designs with better ventilation must be considered. Also to be taken into account is the type of water flow regime at the prospective site. A consistent but gentle flow such as the wave surge described previously is possibly more effective than an area with a strong but periodic tidal flow and both these regimes are certainly better than an area of weak tidal flow. The correct combination of cage design and water flow regime is therefore essential if stocking levels are to be optimized and the authors would suggest starting with the stocking levels given in this manual and experimenting around them.

In Japan juveniles within this size range are kept in specially designed baskets of approximate dimension 150 x 70 x 30cm. The baskets consist of a frame made from plastic piping to which is attached 2-3mm plastic mesh. The floor of each basket is covered with shelters for the abalone to retire under during the day. The shelters can be made from half section of 4 “plastic sewer pipe glued together along the sides to make a more solid and manageable structure. The baskets sit in a tank of seawater through which seawater is pumped continuously at a rate equivalent to the water capacity of the tank every 30 mins. Exchange between the tank water and basket water can be promoted in two ways: 1. By aeration through perforated piping located beneath each basket. (Plate 4). 2. By water circulated from the tank into the basket via air lift pumps. (Fig. 3).

3.2.2. U.S. A. design (Plates 5).

A Californian hatchery grows its juveniles in 2mm baskets measuring approximately 50 • 50 50cm. Plastic pipe sections are placed on their ends in the baskets to provide suitable surfaces for the abalone to adhere to. Each basket is vigorously aerated through a plastic tube. In addition to oxygenation the aeration agitates the seawater in the baskets and facilitates exchange with the surrounding tank water. Each basket is apparently stocked with 5,000 juveniles. These were the stocking levels which were volunteered by the hatchery personnel and the author has no means of checking this. It strongly recommended that those wishing to try this method experiment with different stocking levels.

Chapter 4

On-growing (26 mm - market size)

There are 3 principal methods for on-growing abalone: 1. Off bottom sea cages i.e. which are held off the sea floor and are suspended from piers, rafts or longlines (surface or submerged). 2. On bottom sea cages i.e. cages which rest on the sea floor. 3 Tank culture.

4.1 Off bottom sea cages

These are identical to the nursery cage designs in section 4.1. The only difference lies in the size of the meshes used and the stocking densities. These are summarized a in Table 3 below.

This design originally conceived in Jersey has found favour among some growers and is constructed from modified lobster pots. This technique is currently being used successfully by Irish Abalone/Blackshell Farm Ltd., a company operating in Clew Bay, Co. Mayo. The attraction of this method is that it utilizes a current technology (lobster fishing) and is therefore particularly suited for fishermen. No rafts or longlines are necessary. The pots are laid in strings on the sea floor in suitable areas and are hauled to the surface for servicing and feeding. Each string consists of 8-10 pots connected to each other by 4 meter lengths of leaded rope.

Cage design

The pots are D shaped (length 88 cm, width at base 49 cm and height 39 cm) with an internal volume of 120L. The door edge and frame of the pot are carefully covered with neoprene insulation and car tire rubber respectively to eliminate gaps through which juveniles could crawl when the pot door is closed. Car tire rubber is also wrapped around the frame at the base of the cage to prevent abrasion. Attachment surfaces and shelter are provided by plastic barrel sections held in place with lengths of rod iron and spaced with short lengths of 3/4 “plastic pipe (Hydrodare). Car inner tubing holds the assemblage together.
