Indoor nurseries

Indoor nurseries (Fig. 7.1), also known as primary nurseries in tropical regions, vary according to country, climate, region and management strategy. Short-term indoor nurseries may serve as an extension of the hatchery phase (Cohen & Ra'anan 1989) to adapt PL to freshwater and maintain PL until sold. Aquacop (1983) found that maintaining high densities of PL (10 000 PL/m3) in hatcheries for more than a few days leads to high mortality. Typically, when the PL density in larval tanks is above 2/cm2 on the

Fig. 7.1 Indoor hatchery, Dominican Republic. Note: parallel support lines (wires) above each concrete square tank are used to hang netting for nursing the postlarvae (PL) indoors for a few days after the larval cycle is completed. (Reproduced with permission of Dallas E. Alston.)

tank bottom or 50 000/m3, newly metamorphosed PL are transferred to larger tanks to reduce density and minimise losses from cannibalism, typically for a 2- to 8-week nursery period (Aquacop 1983). The stocking density is based on the duration of the nursery period, and the target size of juveniles (Zimmermann & Sampaio 1998). In temperate regions, indoor nursery culture is carried out at the end of the winter in temperature-controlled recirculation systems provided with artificial substrate to increase the two-dimensional surface area within tanks (Valenti & Tidwell 2006). In tropical regions, indoor nursery culture may be performed throughout the year and may use recirculation, water exchange or flow-through systems (Rodrigues et al. 1991).

7.2 Outdoor nurseries

Outdoor nurseries (Fig. 7.2), also known as secondary nurseries in tropical regions, are similar to grow-out ponds. Depending on management strategies, outdoor nurseries can be stocked with newly metamorphosed PL or juvenile prawns from primary nurseries. In a 2-phase system, juveniles from primary nurseries (juvenile I) are often cultured in outdoor nursery ponds for an additional 4 to 10 weeks until they reach 0.8 to 1.5 g, at which point they are generally referred to as juvenile IIs (Valenti 1996; Zimmermann & Sampaio 1998). Other strategies may bypass the outdoor nursery and juveniles may be stocked from the primary nursery directly into grow-out ponds. The tank or pond areas of outdoor nurseries vary from 300 to 2000 m2 (Valenti 1990).

Fig. 7.2 Nursery ponds protected from predators with bird netting. Note: this system was originally used to protect these 500 m2 ponds from predators. Later, farmers started using unscreened ponds because incoming water was filtered. Predator eggs laid on the surface of the water (e.g. dragonflies) hatch out into forms too small to prey on prawn PL. (Reproduced with permission of Lucy Bunkley-Williams.)

Fig. 7.2 Nursery ponds protected from predators with bird netting. Note: this system was originally used to protect these 500 m2 ponds from predators. Later, farmers started using unscreened ponds because incoming water was filtered. Predator eggs laid on the surface of the water (e.g. dragonflies) hatch out into forms too small to prey on prawn PL. (Reproduced with permission of Lucy Bunkley-Williams.)

7.3 Nursing in cages

The nursing of PL in cages is still experimental. Paniker & Kadri (1981) reported no difference in weight gain of juveniles raised in cages compared to those raised in conventional concrete tanks. Stanley & Moore (1983) studied the growth of 3.3 g prawns stocked at low densities of 10/m2 bottom area during an 8-week period. When fedboundand unbound versions of three commercial diets designed for broiler chicken, catfish and marine shrimp, survival ranged from 96 to 100% andbiomass increase ranged from 17.6 to 21.6 g/m2.

Marques et al. (1996, 1998) stocked newly metamorphosed PL into 2m x 1m x 1m cages constructed from screen with a shade rating of 75%. The animals were fed daily at 100% of their total biomass. Stocking densities of 2,4, 6 and 8 PL/L yielded average weights and survival rates ranging from 50 to 28 mg and 100 to 79.3%, respectively. These authors also stocked cages with 0.19 g juveniles at 50 and 100 prawns/m2 Feeding rates were 10% of biomass during the first month and 5% during the second month. After 2 months, survival was 86 and 75%, and mean weights were 3.2 and 2.4 g, respectively.

Marques etal. (2000) evaluated the effects of high density cage culture of prawn PL in two phases: primary nursery (20 days) and secondary nursery (60 days). In the primary nursery phase, newly metamorphosed PL were stocked for 20 days at densities of 2, 4, 6, 8 and 10 PL/L. In the secondary nursery phase (60 days), the densities tested were

100, 200, 300, 400, 600 and 800 PL/m2, where density was based only on the cage bottom and cages did not contain additional substrate. For the primary phase, average weight gain ranged from 0.05 to 0.032 g and survival ranged from 94.7 to 62.8% at the lowest and highest densities, respectively. For the secondary phase, average weight gain ranged from 0.74 to 0.30 g and survival ranged from 87.7 to 78.3% at the lowest and highest stocking densities, respectively. Nursing in cages is inherently space limited; therefore the addition ofsubstrate should be evaluated to increase allowable stocking densities.

7.4 Multi-phase nursery systems

Multi-phase nursery systems include two phase nurseries, multi-stage stocking systems and modified batch production techniques. These systems are designed to efficiently produce larger nursed juveniles typically of more than 1 g.

Two-phase nurseries were developed in Israel during the early 1980s. In Phase I, newly metamorphosed PL were stocked at densities of 1000 to 10 000/m3 for the first 15 to 30 days (Cohen et al. 1981; Ra'anan 1983; Ra'anan & Cohen 1984). Juveniles were then stocked into Phase II ponds at lower densities, varying from 100 to 200/m2 for a 60 day period. Survival rates were reported to be 92 and 85% for Phases I and II, respectively. Commercial grow-out operations incorporating a two-phase nursery system have reported production rates of 4.5 t/ha/yr or higher (Silva & Alencar 1990; Valenti & Tidwell 2006).

Multi-stage stocking systems are designed to increase pond production efficiency by maintaining a steady-state mixed population of various sizes or age groups in Phase I (juvenile) and phase II (adult) ponds. Typically, PL are initially stocked into Phase I ponds at approximately 5 to 10 PL/m2. After 120 days, 5 to 10 g animals are transferred to Phase II ponds for grow-out and PL are restocked. Malecha (1981) described a multi-stage stocking and harvesting system to optimise growth. Initially one pond would be stocked at twice or three times the optimum stocking density for a 1 to 3 month nursery phase. Subsequent thinning and transfer of the juvenile prawns results in two or three ponds stocked at the optimum stocking density for grow-out.

In a Modified Batch System used in Puerto Rico from the 1980s, PL are stocked into nursery ponds at relatively high stocking densities of 200 to 400 PL/m2 for 60 to 90 days. Survival of PL to juveniles typically ranges from 60 to 90% in the nursery pond. Juveniles (0.3-0.5 g) are then harvested from the nursery pond and stocked at 20 to 30/m2 into Phase I ponds (juvenile ponds) without existing populations. Juvenile ponds are then harvested after 2 to 3 months and monthly thereafter to remove 9 to 15 g animals. These sub-adult prawns are then stocked into grow-out ponds with existing populations. Juvenile ponds are then either converted to grow-out ponds to allow remaining animals to grow to marketable size, or are drained and refilled. Drain-harvesting juvenile ponds into a catch basin to increase survival and reduce labour costs associated with seining, and size grading nursed juveniles into at least two size groups prior to stocking is recommended (J. Glude, pers. comm. 1988).

7.5 Facilities

Indoornursery tanks (Fig. 7.3) are usually constructed from tiles, concrete or fibreglass. The tanks, usually placed inside buildings or greenhouses, may have square, rectangular, round or octagonal shapes, with areas varying from 10 to 50 m2 (Zimmermann & Sampaio 1998) and depths of about 1 m. Supplemental aeration is required and an automatic generator or back-up aeration system is recommended (Valenti 1996).

Indoor nurseries may employ flow-through or recirculating water management systems. Flow-through systems are only used in tropical climates, as heating water is cost prohibitive in temperate climates. In flow-through systems, water continuously enters from tanks, frequently placed above the nursery tanks, and exits at the lowest point of the tank through a vertical standpipe with an outside sleeve (pipe with a larger diameter) extending higher than the water surface. This drainage system draws water from the tank bottom where food waste and detritus settle. Standpipes are covered with a 1 mm mesh screen to prevent PL from

Fig. 7.3 Typical indoor nursery tank used in the USA with substrate and recirculating biofiltration. (Reproduced with permission of Charles Weibel.)

escaping (Zimmermann & Sampaio 1998). In spite of the continuous water movement in flow-through systems, tank bottoms are siphoned periodically at some facilities to remove food wastes, faeces and decomposing organic matter (Borba et al. 1993). Some nurseries allow organic matter to accumulate to enable PL to graze on settled waste or sludge.

Nurseries in temperate climates rely on recirculating systems with bio-filtration and heaters, usually housed within well insulated buildings, where water exchange is used only to make up for losses due to evaporation and back-flushing (Coyle et al. 2003a). In temperate nursery production, it is cost prohibitive to maintain suitable water quality through water exchange due to the cost of heating and replacing water. The cost of heating water can account for more than 50% of the operational costs of prawn nursery operations in temperate climates (Coyle & Tidwell 2007). Some indoor nurseries use alternative energy sources such as geothermal water sources or sunlight to minimise their cost (Sandifer & Smith 1985; Redman et al. 1990).

In nurseries using recirculating systems, water is continuously recirculated through biological filters, which convert ammonia to less toxic nitrate. Depending on target production densities, recirculation nursery systems should recirculate water through a biological filter from 3 to 24 times/day (Sandifer etal. 1983; Zimmermann &Sampaio 1998). Addition of sodium bicarbonate is necessary to replace alkalinity lost through the nitrification process. As these systems involve limited water change, environmental conditions remain relatively stable. Also, because biomass densities are relatively low (<1 g/L; harvest density), low cost, simple filtration systems such as submerged down-flow or upwelling filter systems are suitable.

Optimum lighting conditions for prawn nurseries are not clearly defined. Photoperiod has been shown to affect food consumption, moulting frequency, the incidence of cannibalism and growth performance ofsome crustaceans. Since the nursery phase of temperate prawn production occurs indoors, the light cycle can be readily controlled if a particular light regime was determined to be significantly beneficial. Withyachumnarnkul et al. (1990) reported that juveniles grown for 110 days under total darkness (L0:D24) had higher weight gains than those grown under other light regimes (L12:D12,L16:D8 or L20:D4). Although Lin (1991) reported that growth and survival of freshwater prawn larvae increased as the period of light increased. Tidwell et al. (2001) evaluated the effect of different light regimes, using full-spectrum fluorescent lighting, on growth and survival of juvenile freshwater prawns under nursery conditions; after 60 days, survival was significantly greater in prawns reared under continuous light L24:D0 (72%) than those raised under L12:D12 (59%) or L0:D24 (58%). These authors suggested that continuous light conditions could have a positive impact on survival during the nursery phase, possibly by reducing activity of the primarily nocturnal prawns and therefore the incidence of encounter leading to cannibalism.

7.6 Water quality

The general topic of water quality in grow-out systems is fully covered in Chapter 13 but some observations that are specifically relevant to nursery systems and the rearing of juveniles are included here. Mortalities, related to water quality in indoor nurseries, usually result from low dissolved oxygen and/or high concentrations of nitrogenous compounds (Zimmermann & Sampaio 1998). Prawn PL are also extremely sensitive to contamination of tank water with toxicants such as arsenic in treated lumber, pesticides, hydrogen sulphide, and heavy metals such as copper and zinc (Coyle et al. 2003b). Further information on survival is contained in section 7.10 of this chapter.

The oxygen demand of prawns varies as a function of water temperature and size (Malecha 1983). Nelson et al. (1977) reported metabolic rates of M. rosenbergii juveniles for several combinations of water temperature and salinity, concluding that oxygen consumption rate was primarily affected by temperature and animal body weight. At a salinity of zero, the metabolic rate was higher at 20 to 27°C than at 27 to 34°C. Niu et al. (2003) evaluated the effects of temperature on oxygen consumption of freshwater prawn PL and reported weight-specific oxygen consumption rates at 23,28 and 33°C were 0.83, 1.16 and 1.49 mg O2 g/h, respectively, for 62 mg PL. Although PL will survive at dissolved oxygen concentrations of 1 mg/L for short time periods, it is recommended that dissolved oxygen levels be maintained at approximately 5 mg/L in intensive culture (Sandifer et al. 1983).

Total-ammonia concentrations above 1 mg/L have been shown to reduce growth (Straus et al. 1991); presumably levels above these will result in decreases in survival. Mortalities due to acute nitrite toxicity in commercial nursery systems have been observed to occur when nitrite levels reach more than 4 mg/L nitrite-nitrogen and/or when total ammonia-nitrogen levels reach concentrations of more than 3 mg/L. Addition of salt can help reduce the effects of toxicity to nitrite. Some prawn nurseries maintain a salinity of 3 to 5 g/L throughout the nursery phase to protect against nitrite toxicity and aid in acclimation from larval tanks.

Temperature is a major factor in animal performance (see also Chapter 9). Optimum temperatures for prawns have been reported to range from 27 to 33°C (Fujimura 1974; Smith & Sandifer 1979a; Sandifer et al. 1983). Sandifer & Smith (1985) reported that growth and survival rates decrease when temperatures are less than 22° C and greater than 32°C. However, Niu et al. (2003) reported a significant increase in weight gain at 33°C (0.82 g final average weight) compared to those reared at 23 and 28°C (0.41 and 0.62 g, respectively). Kneale & Wang (1979) found that the best growth rates occurred at temperatures of 28°C, although survival rates were 34% higher at 24°C.

Temperatures lower than 20°C have been reported to decrease growth rate and may cause mass mortality (Coelho et al. 1982; Valenti 1985). Malecha (1983) suggested that PL should not be stocked into ponds with a water temperature of less than 20°C. It has been reported that growth ceases below 19°C and mortality begins at 16°C (Ra'anan & Cohen 1982). Sarver etal. (1982) studied the low temperature tolerance of 1- to 4-day-old PL and reported a survival of 76.2% over a 24 hour period at 19°C, but only 3.8% at 17°C. Ortega-Salas & Arana-Magallon (2006) reared PL for 120 days at different temperatures and reported reduced survival in prawns reared at 20°C (67%) compared to those reared at 33°C (81%). However, Sandifer et al. (1986) reported that temperatures from 20 to 27°C decreased post-larval size variation and increased survival rates. Discrepancies in optimal temperatures maybe due to differences in experimental design, as well as strain differences in animals adapted to different geographical locations.

Sandifer et al. (1986) studied the effects of compensatory growth in PL previously held at reduced temperatures. Animals previously reared for 8 weeks at 18 to 20°C and 23 to 25°C displayed rapid growth when temperatures were elevated to 28 to 30°C. In 6 weeks, they reached the same size and weight of animals maintained at 28 to 30°C. However, this growth rate recovery was accompanied by a large variation in size distribution. Hatcheries in temperate climates operate during the winter months with a relatively narrow 'window of opportunity' for producing nursed juveniles of the appropriate size. Once nursed juveniles reach a size of more than 0.5 g, typically after 60 days of nursery culture, they become increasingly cannibalistic; this limits the duration of the nursery period. However, given the potential of compensatory gain, it may be possible to extend the production window for hatcheries (potentially allowing time for additional larval cycles) by producing PL early and maintaining them at reduced temperatures at first.

Generally, the water quality requirements of prawns raised in outdoor nursery ponds are the same as previously discussed for indoor tanks regarding water temperature, dissolved oxygen and nitrogenous compounds. The primary water quality concern unique to outdoor nurseries is photosynthetically induced high pH due to dense algae blooms, which can cause prawn mortality.

Sarver et al. (1982) reported a correlation between high mortality of stocked PL and high pH values over a 4 day period. Several authors have suggested that PL should not be exposed to pH levels greater than 9.0 for the first few weeks (Aquacop 1983; Hummel & Alston 1984; Hummel

1986; Alston 1989; Straus etal. 1991; Díaz-Barbosa 1995). Pond pH values also affect the relative toxicity ofammonia. According to Straus et al. (1991), juveniles should not be exposed to total ammonia concentrations higher than 1 or 2 mg/L for pH values of 9 and 8.5, respectively.

Hummel (1986) found that mass mortalities may occur at pH levels of 9.5 or greater in aquaria trials where pH was adjusted with sodium hydroxide. This author also tested the effects of fluctuations in pH on PL mortality in outdoor greenwater and clearwater pools. Higher mortalities were recorded for greenwater, where maximum pH values rose to 10.5 compared to 9.0 in clear water. Díaz-Barbosa (1995) exposed PL to abrupt chemical adjustments of pH. PL mortality was 100% after being subjected to pH 9.3 for 4 hours. For pH 9.1, the same mortality was reached after 32 hours. Díaz-Barbosa (1995) also determined the effects of gradually acclimatising PL to chemically adjusted pH values in two different systems. In one system, pH was decreased from 9.1 to 8.1 and then increased from 8.1 to 9.1. In the other system, pH was first increased and then decreased. The results showed that although there was a significant difference between the two systems during the initial stages of the experiments, there was no significant difference in mortality rates at the end of the experiment.

Hummel (1986) determined the effect of stocking time on PL mortality in greenwater and clearwater systems. PL were stocked into greenwater and clearwater pools at four different times of day (12:30, 18:30, 00:30 and 06:30 h). PL mortality recorded for the greenwater treatment was significantly higher than for clearwater for all stocking times. Animals stocked in greenwater at 18:30 and 00:30 hours showedlowermortalitythan animals stockedat other times. At these stocking times (18:30 and 00:30 h), pH values were relatively high (pH 9.8 and 9.6, respectively), but were decreasing. This exposure to high, then decreasing pH apparently had some acclimation value that lessened mortality in green water. Thus, the work of Hummel (1986) indicated that if it is unavoidable to stock PL with high pH, the best stocking time of the day would be during the evening or night when pH is relatively high, but decreasing.

Filling ponds with well water or clear water (lacking algae) is preferable to avoid high pH values. Some facilities rely on daily water exchange rates of 15 to 30% of the total volume to reduce pH levels. Routine application of simple carbohydrate sources such as corn, molasses or sugar at relatively low doses of 10 kg/ha/day have been used to lower pH in ponds by providing a carbon source for carbon dioxide and carbonic acid production. Clarosan (terbutryn), applied at relatively low doses of 1 to 2 mg/L, has been suggested as an algaecide to control phytoplankton blooms andthus reduce pH values in ponds (Aquacop 1979).Main-taining both total alkalinity and calcium hardness concentrations of more than 50 mg/L by addition of agricultural limestone and agricultural gypsum, respectively, is used to help stabilise pH, primarily in areas with acidic soil.

7.7 Controlling predaceous insects in nursery ponds

Predation by aquatic insects can be a particularly important problem in nursery ponds, especially when PL are stocked directly into ponds, foregoing the indoor nursery phase (Alston 1989).

Historically the standard method used for air-breathing insect eradication has been the surface application of petroleum-based products of motor oil and diesel fuel to create a thin film on the pond surface, thereby preventing respiration by air-breathing insects. D'Abramo etal. (1995) recommended a 2:1 ratio of motor oil to diesel fuel at a rate of 9 to 19 L/ha prior to stocking juvenile freshwater prawns. In an effort to avoid the environmental concerns over the use ofpetroleum based treatments, Bright et al. (2002) evaluated the effectiveness of plant and animal source oils for control of air breathing insects. Notonecta sp. was chosen as the test animal since it has been reported to be the most destructive air-breathing insect predator in nursery ponds (Gonzalez & Leal 1995). Corn oil and menhaden fish oil were found to be as effective as petroleum mixtures. Menhaden fish oil was slightly more effective than corn oil at the low application rate of 1.48 mL/m2, although both resulted in complete insect mortality at the high application rate of 4.45 mL/m2.

Dragonfly nymphs are probably the most detrimental predaceous insect and since they are not air breathers, they cannot be controlled with surface applications of oil. Ponds are frequently covered with plastic netting to avoid predators, especially dragonfly nymphs (Insecta: Odonata) (Grana-Raffucci & Alston 1987; Alston 1989; Valenti 1996; Zimmermann &Sampaio 1998). Other management strategies avoid utilising netting by filling nursery ponds 1 to 2 days before PL are stocked (Alston 1989). Cavalcanti (1997) observed that the density of dragonfly nymphs decreased during the culture period if stocking was done 3 to 7 days after filling the ponds. Ponds laden with Chara (Chlorophyta) have more dragonfly nymphs, while ponds with no weeds have more swimming predators such as backswimmers, also known as water boatmen (Insecta: Hemiptera, Notonectidae) (Grana-Raffucci & Alston 1987; Alston 1989). When pumping from surface waters to fill ponds, intakes should be filtered to prevent introduction of fish and insect larvae.

Juarez & Rouse (1983) foundthatpredaceous insects may be controlled with the organophosphate pesticide trichlor-fon (dimethyl [2,2,2-trichloro-1hydroxyethyl] phosphate). Juvenile prawns (1 g) had calculated lethal concentrations after 96 hours exposure to trichlorfon at 0.25 mg/L (LC15), 0.46 mg/L (LC50) and 0.62 mg/L (LC80). The results of this study also suggest that PL and juveniles smaller than 1 g may be more sensitive to trichlorfon. Mulla & Rouse (1985) found little growth and survival improvement after treating with 0.25 mg/L trichlorfon in plastic pools and suggested that, while trichlorfon was effective in controlling unwanted insects, unfavourable changes occurred in the natural biota.

Outdoor ponds are generally filled several days before stocking; however, the time of stocking varies. Samocha & Lawrence (1992) stocked from 10 to 14 days after filling to increase primary and secondary natural production and to stabilise the pH. However, Valenti (1995, 1996) found that predators and competitors become established quickly and recommended that ponds be stocked immediately after filling. Ponds that are stocked immediately after filling with filtered water have no predators and no photosynthetically-induced pH changes. Under these circumstances there may be a slight reduction in growth from the lack of natural food, but increased survival may outweigh this effect.

Further reading on the general topic of predation control is available in Chapter 9 (section 9.2.3).

7.8 Stocking and the use of substrates

The transfer of fragile PL from larval rearing tanks to indoor nursery tanks or outdoor nursery ponds is a delicate operation. Recently metamorphosed PL have experienced a drain on their metabolic reserves (Zimmermann & Sampaio 1998). In addition, PL moult frequently and their soft shells increase the chance of death by damage or physical trauma during handling. Sarver et al. (1982) suggested that even under the 'best conditions', 10 to 20% stocking mortality should be expected when stocking PL into ponds or nursery tanks. To determine survival rates after stocking, a sub-sample of the animals can be evaluated within a mesh bag ('hapa') suspended in the tank or pond. If survival is poor after 24 to 48 hours, more PL should be stocked. Understocking is much easier to remedy than overstocking (Sarver etal. 1982).

As prawn larvae do not metamorphose to PL simultaneously, a range in sizes from 0.006 to 0.01 g typically develops in larval tanks prior to acclimation to freshwater. The mean weights of PL are usually obtained by taking several sub-samples that are counted and weighed; these are used to estimate the total number and mean weight of the animals stocked into nursery enclosures. Several studies have evaluated automated postlarval counters with errors ranging from about 5 to 7%, with postlarval damage not significantly greater than the sub-sample process described above (West & Thompson 1983; West et al. 1983; Welch etal. 1989).

Fig. 7.4 Additional substrate constructed from plastic mesh netting placed in horizontal layers. (Reproduced with permission of Dallas E. Alston.)

PL stocking rates may be related to the maximum load-carrying capacity of the system and the amount of surface area within the tank or pond, and may be expressed both in terms of area (m2) or volume (m3). Provision of substrate increases the available surface area throughout the water column, decreases the frequency of energy requiring agonistic encounters and may provide additional food resources by the growth of periphyton on the substrate.

Stocking densities in outdoor nursery facilities vary from 75 to 1500 PL/m2 (Alston 1989; Rodrigues & Zimmermann 1997). High water exchange rates, diet and the use of aerators are factors that allow higher stocking densities during this phase. Artificial substrates (Fig. 7.4) such as plastic screens are sometimes used in the outdoor nursery phase. Results are variable, but the substrates may reduce cannibalism, increase survival, increase pond biomass and increase mean animal size (Ra'anan etal. 1984; Mulla & Rouse 1985).

Stocking densities in indoor nurseries usually vary from 500 to 6000 PL/m2, depending primarily on the duration of the culture period (Zimmermann & Sampaio 1998). Sandifer etal. (1983) suggested that stocking density should be inversely proportional to the culture period, as higher stocking densities typically result in decreases in survival and average weight. This would suggest that optimum PL stocking densities in nursery tanks will depend upon the target size of the nursed juveniles as well as the duration of the nursery period. Thus, the duration of the nursery period may be shortened when PL are stocked at lower densities. Sampaio et al. (1996) found survival rates did not change at densities from 200 to 1000 PL/m3 during 20 day culture periods. However, mortality increased with culture periods of 30 days, or with stocking rates above 2000 PL/m3. Valenti (1996) recommended stocking densities of 2000 PL/m3 with a substrate surface area of 200 PL/m2 during a 15-day period. New (2002) recommended PL densities of up to 5000 PL/m2for 1 week, or up to 1500 to 2000 PL/m2 for 1 month.

In the USA, juveniles are generally marketed as 60 day nursed juveniles but may be actually 30 to 90 day PL, depending upon many variables, including suitable water temperature for pond stocking. Tidwell et al. (2003) evaluated the production of juveniles nursed for 60 days compared to those nursed for 120 days in ponds and reported no difference in average weights at harvest. Coyle et al. (2005a) compared stocking 30 and 60 day nursed juveniles and reported significantly higher average individual weights and production with 60 day juveniles. Tidwell & Coyle (unpublished data) compared juveniles nursed for 45, 60 and 90 days and found no significant difference in final average weight or production. Longer nursery periods increase the production costs of juveniles, which may exceed US$ 100/1000 after a more than 60 day indoor nursery period (Faria & Valenti 1995).

PL and juveniles are susceptible to intra-specific attacks and cannibalism during moult and post-moult stages. Survival at the end of the nursery phase can vary substantially and is related to the territorial and cannibalistic nature of PL when cultured at high densities. Artificial substrates can be used to increase the amount of two-dimensional space available to prawns, yielding an increase in survival andpro-duction. With the addition of artificial substrate (Fig. 7.5), PL utilise the full three-dimensional volume of the tank, rather than only the walls and bottom. Artificial substrates enhance survival, production and feed conversion. Indoor nurseries utilise artificial substrates of various designs and materials to increase surface area.

A wide range of stocking densities has been used in experimental nursery systems (Table 7.1), ranging from 200

Fig. 7.5 Nursed juveniles evenly distributed on substrate. (Reproduced with permission of Charles Weibel.)

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