Darryl E. Jory-1997
Aquaculture Magazine, 23 (2): 95-103
Shrimp seedstock from hatcheries or collected in the wild can be directly stocked into production ponds, or the animals can go through a transitional growout phase called the nursery. A nursery phase can be an important management strategy in the growout process for penaeid shrimp under semi-intensive and intensive conditions. This phase involves holding shrimp postlarvae for around 25-30 days, at very high densities and with very high inputs in the form of feed, monitoring of water parameters and high rates of water exchange, and labor. There are several advantages to using a two-stage growout system incorporating an intermediate or transitional nursery phase (Samocha and Lawrence, 1992). Production systems that use the nursery phase usually have higher overall survival rates and production per area than one-phase growout systems, where postlarvae are stocked directly into growout ponds. Nursery systems provide a relatively high degree of control over environmental conditions and feeding, and exclusion of predators and competitors is facilitated. In addition, these systems increase the number of production cycles per pond (turnover) by reducing culture time in growout ponds, thus enhancing profitability. In temperate areas a nursery phase provides additional advantages to a shrimp farm. The growing season can be extended through a "headstarting" program (by stocking postlarvae in early spring) that permits the production of two crops in a year. It can be also be used to stockpile seedstock and extend the productive season of hatcheries and increase their profitability. Nurseries can also be used to overwinter shrimp broodstock. The drawbacks to using a two-phase growout strategy include additional cost of required infrastructure, skilled labor and supplies, and varying degrees of mortality due to handling stress (depending on skill and experience of workers and transfer system used) during the transfer to growout ponds. The nursery phase is generally implemented in two types of systems: earthen ponds, and above-ground tanks and raceways.
Readers interested in detailed information on earthen nursery pond preparation and management are directed to the references by Villalon (1991, 1993) and Samocha and Lawrence (1992), and for information on intensive shrimp nursery systems to the reviews by Sturmer et al. (1992) and Samocha et al. (1993). The Shrimp Mariculture Project at Texas A&M University has been a leader in the development of intensive raceways systems for penaeid shrimp, and there are numerous publications (some listed in the references section) with results of these studies.
Earthen Nursery Ponds
The typical nursery pond is between 0.5 and 1.5 hectares in area, and located very close to or adjacent (even connected) to growout ponds, to minimize time and transport distance required which stress animals during transfer. Transfer is facilitated when the nursery ponds are connected to the growout ponds, as final stocking can be generally accomplished by opening up the drainage gate between the two ponds and allowing the nursery pond water to freely flow into the growout pond along with the shrimp juveniles, after adjusting water levels in the ponds. Stocking densities vary but are generally high (over 100 postlarvae/ m^2). Their management is very similar to that for a larger growout pond, including pond preparation (drying, liming, fertilization, etc.) and monitoring and management during a production cycle. Management normally involves very high inputs of feed, aeration and water exchange. As with growout ponds, the objective is to provide the stocked postlarvae with the best possible environment in terms of water quality and bottom conditions, an ample supply of natural food organisms, and eradication of potential predator and competitor species. For additional information see Villalon (1991, 1993) and Samocha and Lawrence (1992). Variants of this system includes the stocking and maintenance of seedstock in a small area enclosed with netting, usually a corner, within a growout pond; after the nursery period is over the netting is removed and the animals are allowed to distribute throughout the pond. Greenhouse enclosures built inside the growout ponds have been used in Texas, using PVC liner walls, paddlewheel aerators and air-diffusers. Stocking densities between 2,000 and 2,300 postlarvae/m2 were used, producing juveniles of an average weight of 1 gram after eight weeks, and with survival rates of 80% and higher being possible (Samocha and Lawrence, 1992).
Above-Ground Nursery Tanks and Raceways
Intensive nursery tanks and raceways provide more control over several parameters than nursery ponds, and can yield higher survival rates. Typical nursery tanks are round, with a central drain and are made of concrete or other material and plastic liners. Raceways are typically 10 to 30 meters long by 59 meters wide, and depths of 0.7 to 1.0 m. Tanks and raceways may be enclosed within a greenhouse structure. Aeration is very important and is diffusers. Optimum water quality must be ensured by using elaborate water filtration and conditioning, when necessary.
Above-ground nursery systems are typically stocked at 500-5,000 postlarvae/m^2. Shrimp growth is highly dependent on stocking density, and provided by paddlewheel aerators or air densities must be reduced if larger animals are desired. In studies conducted by Sturmer and Lawrence (1987b), raceways stocked at 4,400 postlarvae/m^2 yielded animals averaging 1.3 g each (2 kg/m^2) after 42 days of nursery culture. At stocking densities of 2,200 postlarvae/m^2 the average weight increased to 1.7 g (1.5 kg/m^2). Careful monitoring of water parameters, particularly dissolved oxygen, pH and metabolite levels, is critical. High density nursery systems are ecologically unstable and must be closely watched to prevent massive mortalities. The main problem that must be dealt with in the operation of intensive nursery tanks and raceways is the rapid buildup of metabolic wastes and the need to remove these through high water exchange rates. Disease outbreaks can also be a problem due to the very high shrimp densities involved, and adequate food levels must be maintained around the clock.
Nursery Systems Around the World
It is difficult to generalize on the use of nursery systems on a global scale, except to say that there probably is nursery system infrastructure in at least some farms in all shrimp growing countries. The nursery phase is implemented or not according to prevailing environmental, logistic and economic conditions. Stern and Letellier (1992) presented a review of penaeid nursery systems and their management in Latin America, based on a survey conducted among over 50 shrimp farming companies in the region. Their findings were based on limited responses (27% of companies responded), but were representative of the industry at the time. The survey showed that most farmers used earthen nursery ponds from 0.5 to 1.5 hectares (1.24 to 3.71 acres) in area, that no more than 5% of the total production area was devoted to nursery activities, stocking densities commonly used are 100-150 postlarvae/m^2, feeding was generally done only once a day and using nonspecialized, crumbled feeds having 30-45% protein, and there was overall agreement that the nursery phase should not last more than 25 days. About one-third of the respondents indicated that they were experimenting with and/or using commercial scale nursery raceways, where live Artemia were used in addition to standard feeds. There was a generalized consensus among respondents that the use of nursery systems resulted in several benefits, including improved accuracy when estimating growout pond stocking rates, improved size uniformity at harvest of growout pond, better use of farm infrastructure and increased pond turnover. The main drawback of nursery systems was additional stress and mortality due to increased handling levels when animals were reared in these systems for more than 25 days.
Nursery systems are also widely used in Asian countries. According to Qimgyin et al. (1995) in China growout ponds were directly stocked with 5-7 day old postlarvae until the mid-1980's. In the past few years the use of both separate nursery ponds and nursing areas within growout ponds (typically a corner of the pond separated with a net) have been used, with the nursery period lasting two to three weeks. About one-third of farms in operation use a nursery phase. Before the introduction of nursery systems, postlarvae mortalities were extremely high due to poor handling practices, and farmers were forced to stock three to four times the seedstock number the farmer would normally had to stock, in hopes of offsetting high mortalities (Forbes, 1988).
In a recent personal communication, Mr. Alexandre Alter Wainberg (Secretary, Brazilian Association of Shrimp Farmers) reported on Brazilian shrimp farming and the use of nursery systems in this country. Preliminary results of an ongoing survey (about 30% completed at this time) of the industry suggest an expansion of pond area to nearly 5,000 hectares by the end of 1997, almost a two fold increase over 1995 figures, and most of it on semi-intensive culture systems. I am reproducing here excerpts of Mr. Wainberg's communication which I feel clearly illustrate some very important, contrasting points and opinions about the nursery systems. According to Mr. Wainberg: "Nursery ponds are not in use anymore (in Brazil). The majority of the farms, including me, stock PL10 directly in the ponds. I know only 2 farms (names omitted) still using outdoors nursery tanks, with densities of 5-20 postlarvae per liter, with aeration provided by blowers for about 5-15 days and survival rates of 60- 80%. I know also of 2 farms that nurse the postlarvae at their own laboratory. In my opinion all the postlarvae must be nursed before transfer to grow-out ponds. The reasons have been exhaustively described in the literature. To me, the only reason to not have a nursery stage is when a farm is under construction and it is better for the owner to invest in more grow-out ponds than in nursery systems."
Mr. Wainberg further added: "The comparison between nursery tanks and nursery pond is more difficult to do, and the personal opinion of the pond manager is very important for system choice. I offer my particular opinions:
1. In terms of the product, pond juveniles are bigger and stronger than tank juveniles, and the result is shorter culture time and higher survival in growout ponds. The survival is usually the same (60-90%) but the weight is much smaller in tanks (0.05-0.1 grams) than in ponds (0.5-2.0 grams).
2. The main disadvantage of ponds (when compared to) tanks is the stressful and labor intensive conditions of the transfer process with larger shrimp. When the nursery pond is located near the grow-out pond, transfers can be done at night or early in the morning with less than 2 % mortality. Remember that there is also some mortality on the transfer process of tank juveniles, and the assessment of shrimp numbers is very difficult without injury to the small shrimp. A very good Brasilia pond manager (name omitted) developed a transference system that "pumps" 0.5-4.0 grams juveniles (small is better!) up to 650 meters with little mortalities.
3. An important advantage of the tank system is the accuracy of the control of density, survival, feed, and health of the culture stock, but sometimes the system overloads with some harmful effects on juvenile quality and the need of intermediary transference between two tanks. In Brazil there are no suppliers of high quality, low pollution feeds for postlarvae and very small shrimp...... Farms must use frozen adult Artemia, crab meat and commercial feed, and these are not good for water quality.
4. When a farm does not have energy, or the energy supply is not reliable (very common in Brazil), the need for alternative diesel generators increases the construction costs of nursery tanks. Nursery ponds use the same water infrastructure of the farm.
5. The choice between systems is not a case of land availability in Brazil. For big farms it is better to have nursery tanks because their high frequency in stocking grow-out ponds and the fact that night labor is expensive in Brazil (75% more). Big farms also have energy lines and generators. Small farms do not have nursery ponds or tanks because of the lack of technology and money;
6. Nursery systems are labor intensive and people in the tropics do not like to do much work. I'm a shrimp farmer and know the difficulties to have good employees. The more complex the nursery system is, the more the need is for skilled personnel, not always available, and the fragility of the system and the potential for human error. Some farms used to stock grow-out ponds directly with postlarvae because of the "excess work " of nursery systems.
7. Nursery systems usually provide expensive stock for grow-out ponds because of the production costs and mortality inherent of the process. The farmer must do his own cost/ benefit study for the particular situation, and decide if a nursery system will be more profitable to the overall production of the farm. I think that there is a strong direct relationship between the intensity of the grow-out culture method and the profitability of using nursery systems. This relationship also stands over the intensification of the nursery system applied: more intensive growout, more intensive nursery."
In some Venezuelan shrimp farms the use of above ground, round, concrete tanks known as "Hawaii" tanks is widely preferred over earthen nursery ponds, which are considered "troublesome". These round tanks are used to grow hatchery postlarvae (Penaeus vannamei and P. stylirostris) from PL4-5 to hardy, strong PL25-30 which survive and grow extremely well after transfer to growout ponds. Tanks are set up with central drainage pipes, and both natural and commercial feeds are used; natural plankton blooms are stimulated and maintained through selective fertilization regimes, and close monitoring of several key water parameters is emphasized (anonymous, personal communication).
In Ecuador, the top shrimp producing country of the Western Hemisphere and one of the top world shrimp producers for many years, the two-phase production system using earthen nursery ponds was very popular, widely used and significantly contributed to the country's prominence as a farmed shrimp producer. However, the use of earthen nursery ponds has been declining steadily in recent years, although these systems are still in use at some farms. Reasons for the decline include reduced availability of wild postlarvae, questions about the quality of laboratory-reared seedstock, and overall cost. Currently, most farms that still use nursery ponds use them only during the winter season, and generally stock them with wild postlarvae only, considered more resistant to handling than hatchery-produced seedstock. Both semi-intensive and intensive nursery systems are used, and nursery pond size is typically 1 hectare (2.47 acres). In the semi-intensive nursery system, the stocking density is 10 to 25 postlarvae/ m^2, daily water exchange is 3-10%, animals are fed a 38-45% protein commercial diet; survival after the nursery phase (which lasts 25-30 days) is 50-70%. Overall survival after final growout to market size is over 40%, and animals weigh 0.3-0.8 grams. In the intensive nursery ponds the stocking density is 150 to 200 postlarvae/m^2, daily water exchange is 3-5%, animals are fed a 40% protein commercial diet; survival after the nursery phase (which lasts 21-28 days) is 50-60%, overall survival after growout to market size is around 60%, and animals weigh 0.3-0.9 grams. (Econ. Nancy Cely Icaza, Executive Director, National Aquaculture Chamber, Ecuador; personal communication). The author has visited farms in Ecuador where the use of above ground tanks with elaborate water filtration and recircu1ating infrastructure has been tested as a management tool to prevent the onset of Taura Syndrome outbreaks and improve shrimp survival during the early growout phase, when shrimp appear to be much more susceptible to Taura Syndrome.
Perspectives
According to Dr. Tzachi Samocha (Texas A&M University, personal communication), intensive nursery systems can be very cost effective in terms of efficient facility usage, potential for high shrimp survival (over 95%), and a high carrying capacity (over 3 kg/m^2). These systems also have much potential for the production of live bait shrimp for the recreational fishing industry, and can be an essential tool for shrimp farmers to produce two crops per year in temperate climate areas. Dr. Samocha further noted that with the recent Taura Syndrome outbreaks in Western Hemisphere shrimp farms, farmers keeping young shrimp postlarvae at high stocking densities (as required by nursery systems) have suffered high losses, and for this reason most farmers now prefer bypassing the nursery phase and implement direct stocking of growout ponds. But it is anticipated that the development of new Taura-resistant shrimp strains will bring back the use of nursery systems.
The nursery phase is probably the most difficult one to manage, but the benefits derived from a two-phase growout strategy, using first a nursery system followed by final growout to market size, will likely result in an increased incorporation of this strategy into a growing number of farms around the world. This will apply particularly to those operations located in semitropical and temperate regions, where two crops per year are not possible without a nursery phase. Further refinements in shrimp nursery technology and the need to maintain profitability of operations will likely result in increasing use of this strategy in farms located in tropical regions.
References
Forbes, A. 1988. Marine shrimp culture in the Republic of China: one man's experience. World Aquaculture 19(4): 7-12.
Qingyin, W., Y. Conghai and Y. Jia. 1995. The shrimp farming industry in China: past development, present status and perspectives on the future. pp. 1-12.
In: C.L Browdy and J.S. Hopkins, editors.-Swimming Through Troubled Waters. Proceedings of the Special Session on Shrimp Farming, Aquaculture '95. World Aquaculture Annual Meeting, San Diego, California. 2-6 February 1995.
Rhodes, R.J., S. Sureshwaran, C. Greene, C.L Browdy, J.D. Hollowat, Jr., and T.M Samocha. 1995. Production effects of a greenhouse enclosed nursery system on the projected financial performance of a South Carolina marine shrimp farm. Technical Report No. 85. Marine Resources Division, South Carolina Department of Natural Resources. 19 p.
Samocha, T.M and A.L Lawrence. 1992. Shrimp nursery systems and management. pp. 87-105.
In: Wyban, J., (editor). 1992. Proceedings of the Special Session on Shrimp Farming. World Aquaculture Society, Baton Rouge, LA. U.S.A.
Samocha, T.M., A.L Lawrence and W.A. Bray. 1993. Design and operation of an intensive nursery raceway system for penaeid shrimp. pp. 173-210.
In: J.P. McVey (editor). CRC Handbook of Mariculture. 2nd Edition. Volume 1. Crustacean Aquaculture. CRC Press. Boca Raton, FL
Samocha, T.M., A.L Lawrence and J.M. Biedenbach. 1993. The effect of vertical netting and water circulation pattern on growth and survival of Penaeus vannamei postlarvae in an intensive raceway system. Journal of Applied Aquaculture 2(1): 55-64.
Samocha, T.M., F.L Castille, A.L Lawrence and S.E. Talley. 1993. Early spring growth trial of Penaeus vannamei postlarvae at high stocking densities in raceways. np. J.K Wang (editor). Proceedings of an Aquacultural Engineering Conference. 21-23 June 1993,WA. American Society of Agricultural Engineers.
Stern, S. and E. Letellier. 1992. Nursery systems and management in shrimp farming in Latin America. pp. 106- 109.
In: Wyban, J., (editor).1992. Proceedings of the Special Session on Shrimp Farming. World Aquaculture Society, Baton Rouge, LA. U.S.A.
Sturmer, LN., T.M. Samocha, and A.L Lawrence. 1992. Intensification of penaeid nursery systems. pp. 32321-344.
In: A.W. Fast and L.J. Lester (editors). Marine Shrimp Culture: Principles and Practices. Elsevier Scientific Publishing Company. Amsterdam.
Sturmer, LN. and A.L Lawrence. 1987a. Intensive pond management strategies for nursery production of Penaeus vannamei juveniles. Journal, World Mariculture Society 18: 28A (abstract).
Sturmer, LN. and A.L Lawrence. 1987b. Effects of stocking density on growth and survival of Penaeus vannamei and P. stylirostris postlarvae in intensive nursery raceways. Journal, World Mariculture Society 18: 6A (abstract).
Sturmer, LN. and A.L Lawrence. 1988. Feeding regimes for enhanced Penaeus vannamei production in intensive nursery raceways. Journal, World Mariculture Society 19:68A (abstract).
Villalon, J.R.1991. Practical Manual for Semi-lntensive Commercial Production of Marine Shrimp. Texas A & M University Sea Grant College Program, College Station, Texas. TAMU-SG-91-501.
Villalon, J.R 1993. Commercial semi-intensive penaeid growout techniques in Ecuador. pp. 237-273.
In: J.P. McVey (editor). CRC Handbook of Mariculture. 2nd Edition. Volume 1. Crustacean Aquaculture. CRC Press. Boca Raton, FL.
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