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Tidwell etal. (2005)

a Data are arranged in ascending order by prawn/m2 (total surface). b With fertiliser, no feed. c With fertiliser and trichlorfon, no feed.

d Tanks were covered with Eichornia crassipes but only the surface area of plants was used in total surface area calculation. e Temperature maintained at 24° C. f Temperature maintained at 28° C. g Reared under continual light (L24:D0).

h Reared under 12 hours light and 12 hours darkness (L12:D12).

1 Reared under continual darkness (L0:D24).

' Plastic mesh substrate oriented horizontally. k Plastic mesh substrate oriented vertically, n.a. - not available to 6000 PL/m2 of surface area provided by the tank bottom and addition of substrate. Coyle at al. (2003a) evaluated PL stocking densities of 215, 430 and 860/m2 relative to the amount of artificial substrate (bird netting) provided and reported reduced survival at 860/m2 (62%) comparedtothe other treatments; 215/m2 (94%) and 430/m2 (78%) after a 60 day nursery period. These authors reported that even with reduced survival, the higher stocking density (860/m2) produced greater numbers of nursed juveniles on a volume (5.5/L) and area (527/m2) basis than lower densities with no significant decrease in average weights. Average variable cost computations indicate that the 860/m2 stocking density was the most economical choice of the 3 densities evaluated. However, Coyle et al. (2003a) concluded that in situations where PL are expensive or difficult to obtain and high survival is essential to the nursery operator, relatively low stocking densities (200-400/m2) and/or culture periods (40-50 days) may be advantageous.

Many different types of natural and artificial substrates have been used in prawn nurseries to allow increases in stocking densities and limit the effects of cannibalism on survival. Initially, aquatic plants, palm leaves, branches, pebbles or shells were used in nursery tanks to provide shelter for animals to increase survival rates (Ling 1969a; Fujimura & Okamoto 1972; Sandifer & Smith 1977). To avoid inefficient and impractical natural habitats, Smith & Sandifer (1975) utilised artificial substrates in the form of plastic mesh in high-density tanks. Coyle et al. (2003a) used artificial substrates in the form of horizontally-layered sheets of 0.625 cm black plastic mesh with a 10 mm opening, supported by PVC frame and with a 5 cm separation between layers, and reported good growth and survival.

Artificial substrate designs are often based on a behavioural characteristic of prawns, termed the 'edge effect' (Sandifer et al. 1983). When substrates are more or less solid layers, prawns exhibit a pronounced preference for the edges. Mesh layers increase the amount of surface edges available to the prawns in both vertical and horizontal planes. Artificial substrates may be constructed with plastic netting (screen) in several layers over wood, aluminium or PVC pipes (Sandifer et al. 1983; Smith et al. 1983; Faria & Valenti 1995; Sampaio 1995). Substrates should be suspended above the tank bottom by 0.1 m or more to facilitate siphoning wastes. Sandifer et al. (1983) suggested that 50 mm-wide strips placed horizontally could serve as feeding stations.

Substrate can be installed in either horizontal or vertical orientations. Vertical orientation may be preferred because it could allow increases in inclusion rates and potentially improve water flow and solids collection in nursery tanks. Tidwell et al. (2005) evaluated substrate orientation where substrates were either positioned horizontally or vertically in the water column. After 30 days, average weight of prawns and survival were not significantly different between the vertical and horizontal substrate treatments, which overall averaged 0.27 g and 92%, respectively. After 60 days, there was no significant difference in average weight or survival between populations in the horizontal (0.57 g; 81%) and vertical treatments (0.56g; 84%); this indicated that substrate orientation has no significant impact on prawn nursery survival or average weight. New (2002) suggested that hanging the mesh vertically allows the prawns easy access to the tank bottom to search for feed and allows detritus to fall to the tank bottom, where it can be siphoned out.

7.9 Feeding strategies

Information on the nutrition and feeding of prawns is reviewed thoroughly in Chapter 12, while the nutrition and feeding of larvae is covered in Chapters 5 and 6. This current section refers specifically to factors especially relevant to nursery systems.

PL are able to consume larger food items than larvae. Reported feeds include particles of shrimp or fish, corn, soybean or wheat (Sampaio 1995). They will also consume larval and adult insects, snails, oligochaetes, fish, fish faeces and other PL.

Fresh foods are often used in pond based nursery systems and can be used in small nursery tanks employing intensive management techniques, although deterioration of water quality may occur. Valenti (1996) recommended feeding two separate daily rations consisting of 3 g of beef liver/1000 PL. Borba etal. (1993) used one to four separate daily rations consisting of eggs, fishmeal, cod liver oil, powdered milk, molluscs, gelatin and Spirulina; this mixture was cooked in a water bath and sieved. Coyle et al. (1996) evaluated the relative acceptability and suitability of naturally occurring pond organisms identified as likely natural food items. Growth and survival were highest for juveniles fed live zooplankton compared to those fed oligochaetes, gastropods or a nutritionally complete pelleted diet. The authors suggested that pondmanagement strategies to maximise zooplankton production and/or supplemental provision of zooplankton in nursery tanks should be evaluated.

In ponds, fresh trash fish can be fed directly or as a supplement to prepared diets. Culled female tilapia fingerlings from mixed-sex tilapia populations can serve as a food source after tilapia males have been manually separated for monosex grow-out. The female tilapia may be ground together with equivalent portions of a 32% protein commercial tilapia feed and frozen until fed. Since decaying fish flesh may cause water quality problems, this management technique needs to be used cautiously.

There has been a tendency for nursery operators to supplement prawn nursery rations with diets designed for other

Table 7.2 Weekly feed projection for freshwater prawn nursery. (Modified from D'Abramo etal. 1995, copyright 1995 with permission of Mississippi State University.)

Week 1 2 3 4 56789 10

Average weight (g) 0.01 0.015 0.02 0.04 0.07 0.1 0.2 0.3 0.4 0.5

species (e.g. catfish, marine shrimp) (M. New, pers. comm. 2008). This practice has also been common in research work. For example, Sandifer et al. (1983) and Smith et al. (1983) suggested supplementing a 25% crude protein ration with a mixture of cooked eggs, fish gonads, squid, fresh fish and spinach. Molina-Vozzo etal. (1995) supplemented a commercial fish feed with 20% beef liver, creating a diet with a combined protein level of at least 40%, to obtain better growth and survival. Cohen etal. (1981) and Ra'anan & Cohen (1982) utilised a mixed ration which consisted of 43% (by weight) minced fresh fish (tilapia) fed twice a day, 46% of live Daphnia and 11% carp ration (25% crude protein) once a day. Although costly, they considered this diet contained essential nutrients. The diet was used with a 100% daily water exchange to minimise water quality and disease problems.

In the USA, prawn nurseries rely solely on prepared crustacean diets or trout feed during the nursery phase. This may be in part due to the universal use of recirculation systems; in these, fresh feeds can result in deterioration of water quality requiring daily water exchange. Crustacean diets are different from those for finfish in that they contain additional binding agents or differences in the manufacturing process that increases pellet stability. This is important as prawns are generally considered grazers that feed continuously throughout the day and night. In addition, some manufactured crustacean diets may contain feed attractants that aid in chemoreception.

Although there is ample information available regarding the nutritional requirements of juveniles, most nursery producers in the USA feed relatively high protein marine shrimp diets containing >40% protein due primarily to increased availability. Typically, PL are fed a 0.5 mm crumble for the first 30 days and a 1.0 mm crumble sinking diet for the final 30 days. Hari & Kurup (2003) evaluated different dietary protein levels and protein sources for juvenile prawns and reported the highest growth rate and maximum utilisation of protein in prawns fed 300g/kg dietary protein; further increases in dietary protein did not have any added advantage. These authors also found that equal proportions (1:1) of plant and animal protein resulted in better growth performance than ratios of 1:2 or 2:1.

In most nurseries, the amount of food presented varies from 10 to 20% of initial prawn biomass (Sandifer & Smith

1975;Mancebo 1978;Corbin etal. 1983; Sandifer etal. 1983; Karplus et al. 1990; Heinen &Mensi 1991) and diminishes with time. However, Sampaio et al. (1997) improved survival rates by increasing feeding rates from 30 to 60% of initial prawn biomass. Increased survival was attributed to increased food availability. In some nurseries, food is supplied in excess and quantities are adjusted according to the quantity of uneaten food from the previous feeding (Sampaio 1995). D'Abramo etal. (1995) provided a detailed feeding chart for nurseries based on assumed weekly weight gain over a 60-day nursery period typical of temperate climate nursery production. This chart has been adapted and presented as Table 7.2.

Lovell (1978) suggested that multiple daily rations in prawn hatcheries increase consumption and nutrient intake, reduce losses of soluble nutrients into the rearing water, and minimises food disintegration and wastage. However, Sampaio et al. (1997) considered that feeding more than once a day did not change postlarval development in an indoor nursery. Heinen & Mensi (1991) reported that PL with initial weights ranging from 9 to 16 mg showed higher weight gain and survival rates when fed only a single daily ration. They concluded that the prawns had an opportunity to graze during a 24 hour period on the submersed feed colonised by micro-organisms, thus providing a supplementary food source. In practice, some commercial nurseries feed twice daily (once in the morning and once in the afternoon) while others feed only once, usually in the afternoon.

7.10 Survival

The factors controlling the growth and survival of freshwater prawns are reviewed in a number of other chapters, particularly Chapters 9 and 12, while the special conditions influenced by heterogenous individual growth (HIG) are mentioned briefly in section 7.12 of the current chapter and covered in Chapter 16. This section refers specifically to the survival of freshwater prawns during the nursery period, which seems primarily affected by two related factors, water volume/surface area and the incidence of encounter (social activity). Mortalities may result from disease, poor water quality (see section 7.6), system failure and cannibalism;

however, cannibalism appears to be the primary cause of mortality during the indoor nursery stage. In spite of M. rosenbergii being a relatively disease-resistant species, diseases can cause mortality during the nursery phase. The relative incidence of focal spontaneous muscle necrosis lesions in nursed juveniles may be an indicator of postlarval fitness (Sarver etal. 1982).

Kneale & Wang (1979) reported the effects of age, temperature, stocking density and substrate on the survival of prawns nursed for 60 to 80 days. These authors reported that survival was generally inversely proportional to the initial stocking density, showing a high of 84% at 150 PL/m2 and a low of 39% at 1500 PL/m2. Average individual weight was greater at lower stocking densities of less than 600/m2 (0.8-1.0 g) compared to higher stocking densities (0.4 g). Survival was higher in prawns cultured at 24°C (78%) compared to 28°C (43%). Temperature also significantly increased overall growth, yielding an average of 0.41 g at 28°C compared to 0.23 g at 24°C. Temperature can also influence size variability. Sandifer et al. (1986) reported less size variability at culture temperatures of 18 to 25°C than at higher temperatures.

Kneale & Wang (1979) also noted a rapid shift in mortality rate, referred to as the 'breakpoint', that occurs between the 8th and 9th weeks of age for juvenile prawns. These authors indicated that the occurrence of the breakpoint was not influenced by temperature, number of habitats per tank, density, mean weight or biomass for tanks stocked at densities of more than 900 PL/m2. However, no breakpoint was observed for tanks with stocking densities below 600/m2, which suggests that density may effect the incidence of the breakpoint. Sandifer & Smith (1975) stocked cylindrical tanks at 10 to 200 PL/m2 and also reported a rapid decline in survival between days 42 and 57. Willis et al. (1976) observed a similar occurrence during the second week. In the USA, juveniles are typically sold as '60 day nursed juveniles'. Sixty day juveniles have become an industry standard over time as pond producers, realising that pond production of large prawns is clearly a function of the size of the animals at stocking, demanded the largest juveniles possible. Subsequently, commercial nursery operations determined that, at practical stocking densities of 200-600/m2, losses from cannibalism after 8 weeks become significant and cost prohibitive (Craig Upstrom, pers. comm. 2008). These observations support the 'breakpoint' phenomena suggested by Kneale & Wang (1979).

D'Abramo et al. (2000) evaluated individual as well as synergistic effects of water volume, surface area and rate of water replacement on weight gain of individually cultured prawn PL. After 60 days, weight gain of juvenile prawns was 232% greater under conditions of both greater water volume and surface area (100% increases). Higher water replacement did not compensate for lower volume. A sec ond trial was conducted to determine at what point growth reduction identified in the first trial became operative. Significant reductions in weight gain began to occur when a critical biomass density of approximately 500 mg/L was attained. This response reflects the relationship between maximum density and body size relative to culture volume. This is especially important in research, where growth is used as the primary or only response variable to compare different treatments.

To maximise weight gain over time, production based systems for the commercial culture of crustaceans must incorporate designs to minimise reductions in growth resulting from high biomass density or high incidence of energy demanding aggressive encounters. The observations of Kneale & Wang (1979) and D'Abramo et al. (2000) are related to the operative maximum carrying capacities ofex-perimental nursery culture systems. Although there are obviously differences between producers and even individual units, in indoor commercial nursery operations the typical maximum biomass densities achieved are normally < 1 g/L. Knowledge of the maximum carrying capacity of the animal and production system allows the implementation of efficient management strategies such as selective harvest or stock manipulation/movement to maximise overall production per unit of time or space (D'Abramo et al. 2000).

7.11 Harvesting

The harvesting of prawn larvae is covered in Chapter 5. Juveniles are harvested from indoor nurseries in tropical areas whenever grow-out ponds are available. However, in subtropical and temperate regions, prawns are usually maintained at optimum temperatures in indoor nurseries until the grow-out ponds reach temperatures of more than 20°C.

Nets or dip nets (3 mm mesh) are used to transfer juveniles from indoor tanks to outdoor nursery ponds or to grow-out ponds (Borba et al. 1993). Estimates of the numbers of juveniles present are based on counting and weighing several sub-samples. More counts are necessary for batches where the size distribution is broad. Average weights need to be determined during the beginning, middle and end of each tank harvest, as smaller animals are typically captured first (Coyle et al. 2006). Manually harvested individuals are then transferred to outdoor nurseries or grow-out ponds. However, to reduce stress, animals can be transferred by gravity-induced flow into plastic tubes or through small channels connecting tanks.

In outdoor nursery ponds, juveniles from 0.2 to 2.0 g are harvested by seining ponds 2 or 3 times with a 5 to 6 mm mesh seine, or by draining ponds completely (Alston 1989; Zimmermann & Sampaio 1998). If ponds are drain harvested, they should contain a sufficiently large catch basin or box at the end of the drain or outlet (Alston 1989). After juveniles are transferred to grow-out ponds, the bottoms of nursery ponds should be disinfected by applying 1 kg of lime/1 m2 to kill pathogens.

7.12 Size grading

Cohen et al. (1981) first described a heterogeneous individual growth (HIG; see also Chapter 16) effect among juveniles after 50 days of nursery culture, which increases over time in indoor nurseries. These authors noted less growth with less HIG in high-density nurseries (1000 and 5000 PL/m3). This effect probably begins in the hatchery, since male larvae metamorphose into PL before females, shifting population growth curves to the right (Karplus et al. 1990). HIG of juveniles is accentuated by morpho-typic changes that occur during the extended culture time associated with multiple phase nursery operations employing outdoor nursery ponds. During a typical nursery period of approximately 60 days, juveniles attain average weights of 0.1 to 1.0 g with innate high variability due to HIG.

Coyle & Tidwell (unpublished data) cultured PL under commercial conditions for 45, 60 and 90 days at two densities, 215 or 430 PL/m2, to determine if size variability may be influenced by the length of the nursery period and the culture density. At harvest, 3 samples ofmore than 100 individuals were randomly taken at the beginning, middle and end of the harvest to simulate commercial conditions for determining average weights for each tank. Results indicate that size variability increases as the length of the culture period increases and as the culture density increases; coefficients of variation ranged from 112 to 320%. In commercial production, pond allocations of juveniles are determined gravimetrically using average weights. These authors suggested that large ranges in individual sizes may cause difficulty in obtaining accurate average weights, which in turn may result in errors in pond allocations.

Based on these data, a series of trials were conducted to quantify the potential error in pond allocation for both un-graded and graded (Plate 7b, facing page 254) populations of nursed juveniles. Coyle et al. (2005c) reported a trial in which average weights were determined for juveniles of different ages that were then stocked into holding tanks as they would be allocated for pond stocking; subsequently these groups were individually counted. Actual numbers were 15, 19 and 26% below the predicted numbers for 30, 45 and 60 day nursed juveniles, respectively. Grading significantly improved these predicted values to 7, 9 and 10% below actual values for 30, 45 and 60 day nursed juveniles; however, it did not entirely eliminate the problem. This indicates that there is a systematic problem with the current methodologies that are used to make pond allocations due to the large variation in individual weights inherent in this species; size grading significantly reduces this problem. These discrepancies could account to a large extent for the apparently poor survival (50-60%) reported for commercial production ponds; in reality the ponds may have been stocked with fewer animals than intended, based on these erroneous average weight estimates. It is recommended that nursed juveniles are size graded prior to making pond allocations and that an additional 10% by weight is added to compensate for the potential error associated.

In addition to the improvement associated with providing accurate counts, size grading of nursed juveniles prior to pond stocking (to reduce the effects of HIG) has been found to increase both mean harvest size and total pond production in temperate climates, thereby enhancing economical viability. Size grading is essentially a husbandry technique designed to separate the largest, fastest-growing prawns from others in the nursery population, as faster growing individuals are able to retard the growth of other individuals in the population (Karplus etal. 1986). Removal or separation ofthese fast-growing individuals improves the prospects that other individuals will achieve their growth potential. The smaller individuals in the prawn population, generally males, respond by increasing growth rates to compensate for their small size. Essentially, size grading succeeds in disrupting the continuation of the socially induced, differential growth rates that result in a wide variation in size within the population over time.

Size grading can be accomplished by using finfish bar graders (Fig. 7.6) or specially designed rectangular tanks

Fig. 7.6 Conventional 'fish grader' used for size grading nursed juveniles. (Reproduced with permission of Charles Weibel.)
Fig. 7.7 Grading tank designed to passively grade juvenile prawns as they swim through increasing smaller bar sizes to escape a concentration gradient. (Reproduced with permission of Charles Weibel.)

with fixed grader panels (Fig. 7.7) that allow juveniles to passively grade themselves as they swim through a series of grader panels decreasing in grader bar width. The desired separation is achieved through knowledge of the size distribution of the population, combined with selection of the appropriate bar width. Ra'anan & Cohen (1983) suggested grading juveniles into two or three groups. This increases productivity by decreasing competition in grow-out ponds through reduction in HIG. Daniels & D'Abramo (1994) separated 50 day juveniles into 5 groups, obtaining a total biomass 43% greater than in non-separated populations. The final value of the harvested prawns was 73% higher. Malecha (1988) suggested that the fraction containing the smallest prawns should be discarded. However, most producers use a 50%:50% (upper-lower) or40%:60% (upper-lower) size grade and use both populations for stocking. A higher percentage of the upper class reduces the overall effect of size grading on production characteristics in ponds. Some commercial nurseries in the USA grade into 4 different sizes to reduce size variation and increase accuracy in pond allocation.

As the duration of the nursery phase increases, size variation increases due to differential growth rates that may contribute to poor survival due to cannibalism by larger individuals. Therefore, size grading juveniles at some time during the nursery period might decrease size variation and increase survival. Tidwell et al. (2005) described an experiment in which juveniles were size graded at the midpoint of the culture period (30 days) to evaluate the effect of separating the 2 size classes for the remaining 30 days on growth and survival during a 60 day nursery period. After 30 days, 8

tanks were harvested, pooled, size graded using a #8 grader bar (6.35 mm or 8/32 inches) into 2 size classes, and subsequently restocked at the same density (500 PL/m2) but into 3 tanks per treatment (either high or low grade). After 60 days, average weight of the high grade prawns and ungraded prawns was significantly higher than that of the low-grade fraction. High-grade and ungraded treatments were not significantly different. Survival of prawns for the low grade fraction (89%) was significantly higher than that for prawns from either ungraded (81%) or the high-grade fraction (76%). These data suggest that larger individuals within nursery populations negatively impact survival. To maximise survival in the nursery phase, it may be beneficial to remove the larger fraction to other tanks or to stock the larger fraction first into grow-out ponds to maximise survival in the nursery. In addition, these authors found that survival at 30 days (92%) was significantly higher than at 60 days (82%), indicating that nursery duration does have a significant effect on survival at a stocking density of 500 PL/m2.

Further information on the value and effect of size grading is contained in Chapters 10 and 16.

7.13 Transporting juveniles to grow-out ponds

According to Zimmermann &Sampaio (1998), indoor and outdoor nurseries should be constructed near grow-out ponds to reduce mortality from excess handling and transportation. After transporting and stocking juveniles, tanks should be dried, disinfected with chlorine or lime and left to dry for at least 48 hours to minimise problems with pathogens. Regional nurseries, which provide seedstock to nearby pond producers, have developed in the fragmented and widespread industry in the USA to reduce stress, mortality and increased transport costs associated with long distance transport.

Of the published studies related to transport of juveniles, most have used sealed, oxygenated polyethylene bags (Smith &Wannamaker 1983; Alias &Siraj 1988; Coyle etal. 2001). The efficacy of such procedures is constrained by the inevitable reduction of DO2 levels and increases in dissolved ammonia concentrations in the shipping water, due to the metabolism of the animals (Schmitt & Uglow 1996). Smith & Wannamaker (1983) successfully shipped 6g juveniles in plastic bags at 18 g/L for 24 hours and reported that survival in transport was more closely related to DO2 concentrations than any other water quality variable. Another study found that densities of approximately 15 g/L resulted in survivals of more than 90% after 6 and up to 12 hours (Alias & Siraj 1988). Coyle et al. (2001) reported acceptable transport densities for juvenile prawns of 10 to 20 g/L when transported in sealed oxygenated plastic bags, which resulted in survival percentages of more than 90% after 24 hours.

Transport of juveniles in plastic bags is not economically feasible for transport of large quantities of juveniles. Therefore transport of juveniles in trucks equipped with larger (300-1000 L) transport tanks supplied with a continual supply of oxygen is more commonly practised. Coyle et al. (2005b) evaluated the effect of biomass density, substrate and salinity on water quality and transport survival of juvenile freshwater prawns in open vented containers continuously supplied with pure oxygen to simulated truck transport conditions. These authors suggested that allowable transport densities in vented containers are similar (20 g/L; 96% survival) to sealed bags (10-20 g/L) and noted that in vented containers supplied with a continual source of oxygen, un-ionized ammonia will probably become the first limiting factor. In this trial, water temperatures increased from 20°C at the beginning to 24°C after 24 hours. Better maintenance of transport temperature could have increased allowable transport densities.

The benefit of providing substrates in transport containers is questionable. Smith & Wannamaker (1983) reported that increased substrate did not benefit survival when 6 g juveniles were shipped in aerated plastic bags at 18 g/L for 24 hours. Coyle et al. (2005b) reported that the presence or absence of substrate had no significant effect on any measured water quality variable or prawn survival after 24 hours of simulated transport. However, Alias & Siraj (1988) reported increased survival of PL by providing polypropylene netting as habitat during transport. Vadhyar et al. (1992) evaluated different substrate materials (plastic straws, plastic ribbon and plastic netting) at different densities (100, 200, 250, 300, 400 and 800 PL/L) in model transport containers to determine the effect on the safe duration (time). Only plastic straws at the lowest densities (100 and 200 PL/L) resulted in a significant increase in safe transport duration compared to the controls. Additional research is required to evaluate the effect of different types of substrate on transport survival of juvenile prawns.

The addition of salt in the transport water of aquatic animals is a common practice to provide a more isotonic environment, which reduces the metabolic demand for os-moregulation and generally reduces oxygen consumption and nitrogenous waste production and stress. However, increasing salinity lowers the solubility of oxygen in water (Boyd 1979). New & Singholka (1985) reported that some hatcheries added seawater to transport water, claiming that survival rates are better in brackishwater than in freshwater. However, Smith & Wannamaker (1983) reported that increased salinity did not benefit survival of juveniles transported in sealed polyethylene bags. On the other hand,

Coyle etal. (2005b) reported that a salinity of 6 p.p.t. during the transport of juveniles decreased un-ionized ammonia concentrations and therefore should be considered beneficial. The isosmotic point for M. rosenbergii is reported to be 17p.p.t. (Sandifer etal. 1975), which could potentially further reduce ammonia efflux rates and oxygen consumption (Singh 1980).

Increases in transport temperatures significantly increases oxygen consumption and nitrogenous excretion (Chen & Kou 1996); therefore reducing acceptable biomass densities or transport times. Harrison & Lutz (1980) reported that survival of juvenile prawns increased when transported at reduced water temperature (20° C). A direct relationship between temperature increase and nitrogen efflux rates was reported for freshwater prawns by Schmitt & Uglow (1996). Temperature changes during the shipment of freshwater prawns should be avoided or made gradually, thus indicating the importance of insulated transport containers. Transport temperatures of less than 20°C (but not <16°C) may allow higher biomass densities and reduce the accumulation of nitrogenous by-products. This assumption has not been tested experimentally but commercial live haulers of juveniles routinely transport at water temperatures of 17 to 19°C, using well insulated transport containers and through the addition of ice. This practice implies that juveniles have been previously acclimatised to near 20°C to minimise the stress associated with dramatic temperature change.

The condition and moult stage of juveniles prior to transport is important because juveniles have a tendency to 'stress moult' shortly after handling and/or water exchange. Commercial nurseries in the USA routinely perform more than 50% water exchange 3 to 4 days prior to harvest of nursery tanks to acclimate juveniles to cooler water temperatures of 20 to 22°C and to stimulate a moult in individuals in the pre-moult stages. This practice is thought to result in fewer moults during harvest and transport and to increase transport survival, although this has not been tested experimentally.

Information on the transport of larvae is contained in Chapters 5 and 9, while the transfer ofbroodstock is covered in Chapter 4.

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