03.01.16- Shrimp nursery technology: System design and management for cost-effective results Part 1. Design considerations
By Craig Browdy, Peter Van Wyk, Chris Stock, Thomas R. Zeigler and Ramir Lee
Properly designed shrimp nursery systems are high-biosecurity facilities to grow post larvae at high and hyper intensive densities, from 2 mg to as large as 3 g. The aim is to produce healthy, strong and uniform juveniles with significant potential for compensatory growth after their transfer for final grow out. In this two part article, we will first discuss basic design considerations. Part 2 will discuss water quality, feeds and feed management for these nursery systems.
Hyper intensive nursery systems for juvenile shrimp production have been around for decades, with varying popularity. In recent years they have gained recognition as a valuable tool for the shrimp farming industry to increase efficiency and profits, and in some cases to help exclude (particularly with early mortality syndrome/acute hepatopancreatic necrosis disease (EMS/ AHPND) and EHP caused by the microsporidian Enterocytozoonhepatopenaei as well as manage other diseases.
System design and management help to facilitate a consistent, low-risk “factory model” defined by foreseeable production inputs and stable, predictable, operating results. Currently, there is limited information available on standard design and management of shrimp nursery systems, and there is much variability among systems in use.
Many shrimp farmers in Latin America are using nurseries as part of their production strategy, particularly larger, integrated companies. The systems are somewhat less widely used in Asia, but are fast gaining popularity and becoming a trend. In general, nurseries typically involve lined tanks/ponds covered by plastic greenhouses or roofs suspended by cables, with an area of 300- 7,500 m2. They can be rectangular (with continuous, rotating water current around a central baffle), square or round (typically with circular flow around a central drain). Stocking densities range from 500 to 10,000 or more post larvae (PL)/m3, with harvest sizes of 0.3-3 g and harvest biomass of 1-3 kg/m3.
“System design and management has also improved to facilitate a consistent, low-risk “factory model” defined by foreseeable production inputs and stable, predictable, operating results.”
Advantages of nursery systems
In general, nursery systems permit better and more precise management and manipulation of the young shrimp within practical and advantageous economic boundaries, which may not be economically practical in larger pond systems. Specifically these systems have the following benefits:
Control and biosecurity
By reducing the culture area and volume into a series of intensively-stocked tanks, a higher degree of control is possible over environmental conditions, water quality and feeding, resulting in greater economic efficiency. Managing smaller unit areas/volumes improve the exclusion of pathogens and predators. Similarly, greater accuracy is possible in estimating the juvenile populations and providing important advantages in managing feed during the grow out phase.
A two or three stage production system using a one or two phase nursery provides the opportunity to increase biosecurity in the quarantined nursery areas. Consequently, this usually produces higher overall survival rates and production per unit area than in single-phase grow out systems.
Well managed nursery systems provide for faster growth (with some density dependency) and production of bigger and stronger juveniles with better survival and a high potential for compensatory growth. Stocking juveniles from nursery systems rather than direct stocking of post larvae, increases the number of grow out production cycles (turnovers) by reducing culture time to market size in the grow out ponds. This in turn, can increase the number of crops per year allowing the final grow out ponds to be used more efficiently and significantly improving the farm’s bottom line.
Improved health and disease management
Larger, older shrimp will have a more developed immune system. As a management strategy for early mortality syndrome/acute hepatopancreatic necrosis syndrome (EMS/AHPND), a nurseryphase permits the stocking of large juveniles, with betterresistance to the disease. The small confinement provides forsignificant control of water quality, disease exclusion and properfeeding. In addition, as a management strategy for white spotsyndrome virus (WSSV), animals in nurseries can be maintainedin water temperatures above 30°C during seasonal periods whentemperatures in outdoor ponds are lower, making the animalsmore susceptible to this disease. In a properly managed nursery, disease control and diagnostic measures can be implemented allowing the farmer to detectinfected animals in the nursery before they are transferred tothe final outdoor grow out ponds. This enables the option ofterminating the crop at an earlier stage and restarting, thusreducing the financial impact. This is particularly important withthe most recent and devastating shrimp diseases, EMS/AHPND,and EHP which can be caught early through available and improving histopathology and molecular diagnostic techniques.
“Nursery systems offer the shrimp industry an important opportunity to increase profits.”
Enclosed intensive nursery systems can further broaden the effective stocking windows for seasonal hatchery outputs. This allows greater efficiency for both the hatchery and the farm. For farms not integrated with hatcheries, intensive nursery headstart strategies may allow the purchase of seedstock in advance of the peak demand periods, at lower cost and with greater probability and certainty of seedstock delivery. Additionally, for shrimp farms in areas of lower salinities, the nursery can be used as an acclimation system.
Nursery systems have a few disadvantages. These include greater infrastructure investments (higher construction costs than conventional pond systems) higher operational costs, and increased labour requirements as such systems require trained biologists (often to the level of hatchery operators). Prevention of component/equipment failures is critical. With increasing stocking density and lower water exchange, one important risk is higher organic loading leading to poorer water quality and animal health. Increased stress to animals due to more handling and transferring, can increase susceptibility to diseases. For these reasons, proper management by properly trained personnel of the nursery system is a critical prerequisite for success.
The ideal location for a nursery system is in close proximity to or integrated within the grow out farm. Access to high quality water and appropriate infrastructures are critical. The water source should preferably come from the farm intake canal or before thepump station, to allow draining and dry-out of the main reservoirchannel without affecting nursery operations. Water should draininto the farm discharge channel and away from the intake. To lower costs it is advisable to locate nurseries close to an existingmain and emergency back-up power source. If possible, it shouldbe near the farm main administration area close to offices,supplies and personnel. Ideally, construction location should behigh enough for good drainage and no more than 5-10 minutestravel time to the farthest pond in the farm.The size and shape of the nursery system and its tanks are veryvariable, and it can be a single-phase or a multiple-phase design.
Single phase nursery tanks
Single phase tanks are typically designed to produce 100, 200 or 300 mg juveniles. Tanks are typically from 40 to 200 m3 and under greenhouse or shade cloth coverings. Four different tank shapes are typically used. Most of these tank systems are built out of concrete, wire mesh, wood, plastic, fiberglass or small reservoirs with formed (compacted) soil with a plastic liner (HDPE or EPDM of non-toxic liner material) or epoxy coating. Stocking densities range between 8 to 50 PL/L to produce harvest-size juveniles of 0.1-0.3 g and a final harvest biomass of 1-5 kg/m3.
Round and rectangular tanks
Round tanks with centre drain have good water circulation for feed distribution and sludge removal (i.e self-cleaning). Many nurseries use round tanks made of fiberglass, concrete, or the most locally cost-efficient materials. However, there are physical size limits to circular systems, as round tanks larger than about 40 m in diameter lose some of their most desirable characteristics. As the diameter increases, more energy is spent maintaining flow velocities to scour and entrain solid waste, and the tanks tend to be less effective in delivering solid waste to their collection points.
Rectangular tanks are easy to build, are space-efficient under standard greenhouses, and hatchery technicians readily relate to their management. But they are less efficient in the removal of suspended solids and sludge, and in feed distribution. They can also be more expensive to build, operate and manage.
Oval raceway tanks have good water circulation for keeping solids in suspension; they fit well under greenhouses, and can be designed to readily remove wastes. However, they can be somewhat more difficult to build and manage than other designs. Regarding production, consistently high amounts of shrimp biomass have been achieved with these systems.
Oval or plug flow “raceways” are typically 33-66 m long by 5-9 m wide, and depths of 0.7- 1.2 m. While they fit nicely into commercial greenhouses and into the conventional freespan width of a closed building, they require more energy ingenerating the water flow necessary to deliver solid wastes to the designated removal points.
“In design, consider the factory model: well-defined inputs resulting in steady state operations with steady production.”
To maintain an organised flow, the oval raceway is usually divided down the middle by a centre wall. In oval (and rectangular) raceways there can be two drains and two points to pick up solid waste.
Stacked, shallow (10-20 cm of water) rectangular raceway systems have been designed for super-high densities and space efficiency. These can maximise the biomass per m2 footprint (up to 10X that of other systems), and are unmatched at efficiency to control temperature, feeding and personnel inputs. They are ideal for extreme environments where space and temperature are limitations. These systems are relatively new innovations with initial production results from some promising prototypes suggesting potential for very high production rates. As these systems are commercialised they will have the potential to become much more common, supporting hyper-intensive growout operations and inland shrimp farming in cold environments.
Second phase nursery tanks
These are very similar to the single phase systems but built at a larger scale. Stocking is usually of 0.1-0.3 g juveniles transferred from the first phase system. The reason for a second phase nursery at the farm level is related to stocking a greater number of larger juvenile shrimp into production ponds than is possible from just a single-phase system. In markets where seasonal production favours higher prices for early harvests, a two phase system is most advantageous. These systems are typically built in three, differently shaped, covered tanks or ponds (rounded centre drain, oval and rectangle), with an average volume ranging from 300 to 7,500 m3.
Optimisation of the design and management of nursery systems has been the subject of a great deal of research. Large scale commercial implementation is growing. Nurseries have the potential to enable increasing biosecurity and better disease control which can help to increase stocking flexibility and reduce production costs.
The future holds great promise for expanding application of nursery systems globally. The most successful systems are those designed to take advantage of the lessons learnt from the growing numbers of commercial systems in operation today.
In part 2 we will address water quality, feeds and feeding arguably the most important aspect of the management of nursery systems.
AQUA Culture Asia Pacific Magazine - March/April 2016 Voume 12 #2