Fig. 5.3 Brick larval rearing tanks.

and size of tanks. Tanks may be oval, cylindrical or rectangular (with smooth corners), and may have flat, conical or U-shaped bottoms. They may be constructed of fibre-glass, polypropylene, epoxy-painted wood, cement-covered brick (Fig. 5.3) or asbestos (Fig. 5.4), or some other nontoxic material. It is essential that the materials used are inert. In particular, the interior of cement or cement-lined tanks must be painted with epoxy resin to avoid toxic chemicals leaching into the larval rearing water. Law and Yeo (1997) stated that cracked fibreglass tanks should not be used because phenolic compounds are leached out from the fibreglass into the water.

Larval rearing tank capacity typically ranges from 1 to 10m3. Small hatcheries use 1 to 3 m3 tanks to minimise water consumption and facilitate maintenance. The 3 to 10 m3 tanks that tend to be used in medium- to large-scale hatcheries are more difficult to maintain (feeding, cleaning, aeration, etc.) properly. The depth should be approximately

Fig. 5.4 Asbestos larval rearing tanks.

1 m and the water column not more than 0.9 m. However, quite shallow water depth is reported to be used in China (Miao Weimin, pers. comm. 1999) and a 2m water column is used in a hatchery in Brazil (H.P. de Barros, pers. comm. 2008). Some hatcheries use small pre-stocking tanks to optimise the use of water and food and to enhance management (Hsieh et al. 1989, Valenti & Tidwell 2006, Valenti 2007). This system may increase hatchery productivity and optimise fixed costs. However, it is not normally used in Thai prawn hatcheries which, though recognising that it is easier to manage small high-density tanks, consider that the damage and loss of larvae during transfer outweighs the apparent advantages. Valenti et al. (1998) described the main characteristics of tanks used for recirculation systems. According to these authors, tank size generally varies from 1to5m3 and the water column should be 0.8 to 1.0 m deep with a freeboard of 0.15 m.

One of the most important aspects in choosing the tank design and configuration is proper water circulation to allow adequate removal of waste products (nitrogen and solids) and uneaten feed. Tanks with corners or 'dead spots' (i.e. poor or no water circulation) will trap solids and prevent complete waste removal and proper suspension of food. Therefore, cylindrical (or cylindro-conical) or U-shaped tanks are better. However, most hatcheries use flat-bottomed tanks. According to New (2002), circular tanks are acceptable but once the capacity of the hatchery is increased a lot of small tanks or a few very large ones would be needed. Large circular tanks are rather cumbersome to use. Round tanks allow better larval distribution, but food distribution and tank cleaning operations become difficult, especially in the centre of the tank when tanks are too large. Such problems are not experienced in rectangular tanks; however, there is a tendency for larvae to concentrate in the corners and angles of these tanks, which should therefore be rendered curved. Cylindrical-conical tanks present easy handling. Smooth-sided tanks are advisable since rough-textured sides will trap solids and potentially be abrasive to the exoskeleton of larvae, thus causing injury.

Improper flow rates orpoor circulation within atank will prevent adequate removal of wastes. As the stocking rate is raised, the turnover rate must be accordingly increased. The highest stocking rates may require a maximum turnover rate of 70 to 100% per hour. A 50 000 L larval culture system requiring a 70% turnover rate would require a flow rate of 35 000 L/h. All pump, filter or disinfecting systems need to match these performance requirements. Water circulation within the culture tank is usually performed using aeration and is such that water circulation is from the centre of the tank to the sides and from the top to the bottom. Failure to provide adequate circulation will lead to 'dead spots' where larvae and feed drop out and are trapped near the tank bottom. In addition, solid wastes will not be removed from the culture tank to be processed by the filters. Such conditions can lead to excessive build-up of heterotrophic bacteria within the culture tank.

The bottom of larval tanks is best if sloped slightly towards the drain. Drainage is provided by means of a turn-down PVC pipe having a diameter varying from 38 to 76 mm, which is protected by a 150 to 1000 ^m nylon screen. The choice of screen mesh size depends on the larval development stage and needs to be just small enough to retain larvae during water exchange and siphoning processes. Some hatcheries use a 150 ^m nylon screen throughout all phases of culture to avoid lost uneaten Artemia nauplii. Larval tanks should be positioned so that they will drain by gravity when the turn-down drain is operated. Each tank requires an independent supply of air. A brackish and freshwater supply should be available close to the tanks to fill up the tanks and to reduce salinity when necessary for emergency use and for tank cleaning operations between larval rearing cycles, respectively.

Tank colour, like many other facets of prawn hatchery technology, remains controversial. To some, the colour of the wall sides and bottom is very important. Some operators observe that a dark (black, blue, green) interior gives the best results, because animals can detect their food more easily against a dark background and larval distribution throughout the tank is more uniform, thus ensuring proximity to food. Lin & Omori (1993) demonstrated that feeding rate decreases with lighter colours. The best survival and food intake occurs when black tanks are used (Lin &Omori 1993; Rodrigues etal. 1998). Figure 5.5 shows an overview of rearing tanks in a freshwater prawn hatchery. Yasharian et al. (2005) observed that tank colours had no significant impact on final postlarval size or days required to reach postlarval stage, but they detected a significant difference on survival and obtained 71, 78 and 84%, respectively, for black, green and red tanks. However, other hatchery operators believe

Fig. 5.5 Overview of rearing tanks.
Fig. 5.6 Rearing tanks at Mississippi State University. (Reproduced with permission of William Daniels.)

larvae find their food not by sight but by tactile response. These operators often prefer white tanks to observe the larvae and cleanliness of the tank more easily. Rodrigues et al. (1998) reported that they found no evidence that either sunlight or tank colour affected the capture or ingestion of food by freshwater prawn larvae. However, these authors found that greater numbers of PL were obtained with the use of black, rather than white, tanks irrespective of the incidence of sunlight. Furthermore, they reported that larger and heavier PL were produced when black tanks were used and sunlight was present. However, at Mississippi State University (Fig. 5.6) the larval rearing tanks used by Daniels et al. (1992) were 8t in capacity; the bottom and the lower 0.3 m of the tank sides were painted beige while the rest of the tank was left black. With the indirect natural light source, this gave a good colour contrast to Artemia and allowed the larvae to feed more efficiently. This matter is discussed further in section 5.2.6 in the context of lighting.

Although a UV system is an expensive item, its purchase is well worth the expense, especially for the novice. When water is properly filtered and has good clarity, UV light will kill organisms in the water column. Daniels et al. (1992) reported that UV light typically reduced the number of bacteria from a very high concentration of 106 to 109 bacteria/ml at the UV device inlet water to a low concentration of 1 to 10/ml at outlet. The size of the UV light system depends upon the flow rate and the type of organism to be killed. Other types of disinfectant systems, such as ozone or chlorine, may also be applicable.

5.2.4 Biological filters

Biological filters (biofilters) are used in recirculation systems to remove the inorganic nitrogenous wastes, which come primarily from the ammonia produced by larvae and live feeds (e.g. Artemia sp.) during excretion and from the decomposition of organic matter. The nitrogen is converted from ammonia to nitrite and subsequently to nitrate by bacteria through the nitrification process. Spotte (1979), Kaiser &Wheaton (1983) and Brock etal (1994) described the nitrification of ammonia, which results from a sequential action of the nitrifying bacteria. The first group oxidises ammonia to nitrite and is represented by the genera Nitro-somonas and Nitrosococcus; the second oxidises nitrite to nitrate and includes the genera Nitrobacter, Nitrospira and Nitrococcus (Brock et al. 1994). The nitrification process involves both oxidation and synthesis and consequently results in oxygen consumption and alkalinity destruction (Kaiser &Wheaton 1983).

The type and size of biofilters vary. Many different types of biofilter designs are acceptable, provided they effectively remove all ammonia and nitrite produced during the cycle. Valenti et al. (1998) described the main types of biofilters which are used in freshwater prawn hatcheries (Fig. 5.7). These authors stated that submerged biofilters are preferred because they are efficient, simpler and cheaper than others. Chowdhury etal. (1993) reported that the submerged biofilters which are horizontally divided into chambers are more

Fig. 5.7 Main types of biofilters used in freshwater prawn hatcheries. (Modified from Valenti et al., 1998, with permission of Sao Paulo State University.)

Key: A = submerged downflow; B = submerged up-flow; C = submerged horizontal divided into chambers; D = emerged trickling filter; E = biodrum (rotating cylinder containing plastic substrate); F = rotating biological discs.

efficient. Adequate aeration is necessary to maintain proper dissolved oxygen (DO2) levels for nitrifying bacteria in submerged biofilters. Griessinger etal. (1989) and Daniels et al. (1992) utilised biofilters equivalent to approximately 6% of the volume of the larval culture tanks. Biofilter size has varied from 4 to 20% of the rearing tanks.

Typically, some calcareous material is used as the filter medium; smaller particles, such as crushed oyster shell or coral, are recommended. Non-calcareous substances (e.g. plastic beads) can be used but lack the buffering capacity to assist in regulating pH in the range 7.0 to 8.5. If a non-calcareous medium is used, pH may have to be maintained through the addition of chemicals such as bicarbonates. Substrate material can be enclosed in bags fashioned from plastic or nylon netting of the appropriate mesh to facilitate handling. The total amount of biofilter media required depends on the daily maximum ammonia-nitrogen load expected in the larval culture system (based on the desired level of postlarval production) and the bacterial carrying capacity of the filter media being used. The filter media carrying capacity is dependent upon several factors, including the surface area to volume ratio. Media with a higher surface area to volume ratio will provide more area for bacteria to grow. This area depends mainly on the porosity and diameter of the substrate. Valenti et al. (1998) recommended using 5 mm particles of crushed shells. Daniels etal. (1992) provided an example for calculating the amount of crushed coral needed to adequately convert the amount of ammonia (60 g of NH3-N) produced by 2 million prawn larvae in a 24 hour period. This amount was based on empirically derived data of 30 |ig/larvae/day. This is equivalent to 226.8 g of ammonium chloride (i.e. 1.0 g of NH3-N per 3.78 g of ammonium chloride) being completely oxidised by the biofilter media. A bag of well-maintained crushed coral weighing 2.26kg usually contains a good population of nitrifying bacteria which will nitrify 1 g of ammonium chloride in 24 hours (Daniels et al. 1992). The required number of bags of crushed coral would, therefore, be 227.

Biofilters provide media for colonisation by nitrifying bacteria. Biofilters are 'living' systems that must be 'activated' (or colonised), fed and maintained. Griessinger etal. (1989) emphasised the importance of biofilter activation in promoting the growth of beneficial bacteria and in the efficient removal ofnitrogenous waste products. Nitrifying bacteria are ubiquitous, but inoculation and activation will enhance the efficiency and reduce the start-up time of the filter. Many hatcheries do not add 'activated' substrate to their biofilters and yet obtain good production. However, the use of 'activated' substrate reduces the likelihood of spikes in the level of ammonia or nitrite during the cycle. Some hatcheries maintain a tank with activated substrates only for emergency situations, when activated substrates are transferred to biofilters. When feasible, the biofilter media should be activated in a separate pre-conditioning tank and added to the biofilter as needed to compensate for increases in nitrogenous wastes as feeding and larval biomass increase. This allows the media to be maintained at maximum bacterial loading capacity. Temperature and salinity in the pre-conditioning tank should be stable and equal to conditions in the larval culture tank. The amount of 'activated' substrate in the biofilter at any time should be able to nitrify 100% ofthe maximum expected ammonia produced by the larvae in 24 hours. Griessinger et al. (1989) stated that the maximum substrate (coral) volume, representing less than 4% of the total rearing volume, is reached by day 17 of rearing and a larval stage index (see section 5.4.3) of8.5.

A procedure for the 'activation' or pre-conditioning of filter media has been described in Daniels et al. (1992). It starts with an inoculation, using water or media from an existing system, or can start from scratch. Initially, 10% of the required total ammonia is added to the water containing the substrate material in the form of ammonium chloride (NH4Cl) or another inorganic source. Once this amount is consumed, the same amount is added until that amount can be completely converted to nitrates within a 24 hour period. The amount of ammonia is then doubled until it is completely converted in 1 day. As each level of ammonia is consumed within the desired 24 hour period, the amount is doubled until the maximum required load is consumed daily. The addition of 10 g of NH4Cl and 6.25 g of sodium nitrite (NaNO2) foreach 1000 Lofwateractivatesammoni-fication and nitrification at the same time. This practice has been used successfully in the Aquaculture Centre at Sao Paulo State University, Brazil for many years. The 'breaking in' of biofilters is essential in temperate regions and may require 4 to 6 weeks (Valenti & Tidwell 2006).

Once the maximum bacterial load is achieved, the production cycle can begin. However, the bacterial population on the media must be maintained at the maximum level of ammonia and nitrite consumption. Addition of media to the biofilter should coincide with the increase of NH3-N produced by an increase in the larval biomass (Griessinger et al. 1989). Typically, beginning 3 days post-stocking, increasing amounts of 'activated' media must be added daily to the biofilter tank. The bacterial population provided through daily addition of media should always be sufficient to remove all ammonia and nitrite. Upon completion of the larval cycle, the biofilter media can be removed, thoroughly rinsed, and either stored dry or returned to the pre-conditioning tank to re-establish and maintain the bacterial colony. Alternatively, the substrate can be chlorinated to kill all bacteria, de-chlorinated, and then re-seeded with stock bacteria from another pre-conditioning tank.

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