SOURCE: EMANUELA D’ANTONI, DERIVED FROM VALENTI AND DANIELS (2000)
F I G U R E
17
Larval tank Sand
filter
Pump
Reservoir
UV Unit
Biofilter
NOTE: AN OPTIONAL UV UNIT IS INCLUDED IN THIS DIAGRAM
Figure 18 There can be a lot
of wasted space around circular tanks but none is wasted between these rectangular larval tanks (Thailand)
SOURCE: HASSANAI KONGKEO
they do not leak. They must have a firm, well-compacted foundation (a 5 m3 tank, for example, supports 5 mt of water, plus its own weight). Concrete pouring or facing work must be continuous so that the concrete does not dry out by sections. Failure to do this will later result in cracks and leaks at the joints. Place your larval tanks high enough so that you can drain them by gravity when the turn-down drain is operated. Construct tile, faced block or concrete drainage canals to carry the drained larval rearing water away without undermining the foundations of the tanks. Another argument against concrete tanks is that they are ‘permanent’ immovable structures. Plastic or fibreglass tanks can be pur-chased ‘off-the-shelf ’ and can be rearranged if you want to revise the layout of your hatch-ery. However, buying plastic tanks can be very expensive and many commercial freshwa-ter prawn hatcheries stick to concrete-lined or concrete tanks.
Whatever type of tanks you choose, you must ensure that they have a smooth sur-face and that all right-angled parts (where the side walls join and the bottom meets them) are ‘rounded off ’ (see Figure 18). This is essential to make efficient tank cleaning easier and to reduce the surface area available for the growth of algae, bacteria and protozoa.
Smooth surfaces also decrease the tendency of larvae to concentrate in the corners of the tank. Circular tanks avoid this problem but some hatchery operators find that food distri-bution and tank cleaning operations are more difficult in circular than in rectangular tanks, because of the difficulty of moving between them in a limited space. Some hatchery operators prefer cylindrico-conical tanks because they find them easier to clean. Figure 19 shows an interior view of this type of tank. The difficulties which hatchery operators have in working around a lot of circular tanks can be reduced by building them in groups, as illustrated in Figure 20. There are obviously many possible alternative choices of tank con-struction and layout. You must make your own choices; this manual can only point out some of the advantages and disadvantages of each type. Whatever type of tanks you choose, it is essential that you ‘age’ them when they are new by soaking them in several changes of brackishwater for several weeks. This allows soluble toxic materials to leach out.
Many hatchery managers believe that tanks with coloured (green, blue, black) inte-riors seem to give better results and there is some research evidence for this. You will note that the tanks shown in Figures 19 and 20 are painted black. Some speculate that the lar-vae can see their food more easily and are better distributed throughout each tank.
However, not all successful hatchery operators agree. Some claim that larvae find their
Figure 19 Inside of cylindrico-conical larval tank, showing the central
stand-pipe used during water exchange (Brazil)
Figure 20 Some space can be
saved by grouping tanks together but there is still some
‘dead’ space between these cylindrico-conical
hatchery tanks (Brazil)
SOURCE: EUDES CORREIA SOURCE: EUDES CORREIA
food mostly by contact, not sight, and that white tanks make it easier to clean and observe the larvae! The tanks shown in Figure 18 are painted light blue, which seems to be a com-promise. Another operator has found that painting the bottom and the lower 30 cm of the tank sides beige and leaving the rest of the tank black provides the best colour contrast to Artemia and allows the larvae to feed more efficiently in indirect light. It is therefore not possible to make a firm recommendation on tank colour in this manual (further research may make a clear recommendation feasible in the future). Individual hatchery experience, based on ease of management, observations on the larvae, and (most important of all) suc-cess in producing healthy PL in the shortest time and with the best survival rate, is what governs the choice of colour at present.
Individual tank size depends on the number of larvae you want to stock and on whether you find that operating a few larger tanks or a lot of small ones is most conven-ient. In recirculation systems, individual larval tank size generally varies from 1-8 m3and the filters can either be shared (Figure 21) or individual (Figure 22). Tanks of between 2 and 5 m3are typical in flow-through systems but some hatchery operators prefer larger tanks (e.g. 10 m3). Some hatcheries use a range of tank sizes, so that the larvae can be reared in high densities in small tanks at the beginning (which conserves water and food, and makes management easier) and moved to larger tanks later when they require more space. Other hatchery managers think that the apparent advantages of this style of
man-agement are outweighed by the larval damage and mortalities caused during tank trans-fers. For illustrating some management techniques and calculations of water require-ments, etc., a standard tank water volume of 5 m3has been used in this manual.
Good tank drainage is essential. You have to remove water (during water exchange) and, at harvesting time, PL from your tanks. The interior draining system in a cylindrico-conical tank is clearly shown in Figure 19. If rectangular tanks are used it is essential to slope them slightly toward the drain end. Use a 2 inch (5 cm) turn-down drain for a 5 m3 tank. Larger tanks will need larger bore drainage pipes (e.g. 4 inch – 10 cm – for a 10 m3 tank). Smaller tanks can use smaller pipes for drainage but it is important not to make the drainpipes too small or water exchange will take too long. These pipes must be covered with a filter sock inside the tanks made of nylon screen (Figure 23) to prevent the loss of animals during water exchange operations. They can be arranged so that they drain into
C H A P T E R 4
Figure 21 The water in
these larval tanks recirculates through a shared filter (Brazil)
Figure 22 These larval rearing tanks have individual recirculation systems (Brazil)
SOURCE: WAGNER VALENTI SOURCE: WAGNER VALENTI
a central channel, as shown in Figure 24. You will need to use a mesh size of 150-250 µm at first, because the larvae are so small. However, this mesh size drains slowly and you must increase it as the animals grow. By the time you have PL in the tanks you can use a mesh size of 1 000-1 200 µm. The filter sock is removed during harvesting operations.
You will also need other types of tanks besides larval tanks. For example, tanks for hatching live feed organisms (e.g. Artemia) are required. Mixing tanks are also needed for preparing the brackishwater to be used in the hatchery, as well as storage tanks for sea-water or brine and freshsea-water (Figure 25). Building mixing and storage tanks high enough so that the water for tanks can be distributed by gravity would seem ideal. However, the cost of constructing raised tanks is so high that pumping is normally used for this purpose, as shown in Figure 25. Your hatchery should have a total storage, holding and mixing capacity of at least twice the total volume of its larval rearing tanks (e.g. four 25 m3or two 50 m3tanks for every ten 5 m3larval rearing tanks). This capacity is necessary to allow for adequate water storage, treatment and mixing time for the production of 12 ppt brack-ishwater. You will also need to provide tanks for holding PL before sale or stocking in nurs-ery or grow-out facilities. The type, size, and shape of materials used in the construction of water storage and supply systems, as well as for postlarval holding tanks, vary according to the site and scale of operations, like the larval tanks. Some tropical hatcheries find that