05.01.16- Shrimp nursery technology: System design and management for cost-effective results Part 2. Water quality, biofloc technology, feeds and feed management
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. Part 1 of this article in issue March/April (pp 39-43) covered design considerations.
Hyper-intensive shrimp nurseries maximize production efficiencies. At very high stocking densities, high feed inputs demand careful attention to water quality. From initial filling through the production cycle, system management based on a sound fundamental understanding of microbial and water quality dynamics coupled with the use of high quality specialised feeds and precision feeding strategies are essential.
Water should be pumped into reservoir tanks from a high quality source and properly treated. Most nursery systems should implement at least some degree of filtration and disinfection of incoming water, including sand filtration, cartridge filtration (5-50 microns; layers of 1-micron, filter bags are highly recommended) and chlorination (20-30 ppm). De-chlorination can be accomplished by aeration or addition of thiosulfate, but the latter approach is discouraged because of possible detrimental effect of the chemical to the animals. Vitamin C addition and natural de-chlorination with aeration are two of the best approaches.
Without exception, all nurseries should strive for the highest levels of biosecurity and implement additional post-filtration of incoming water, including the use of ozone or UV sterilisation and final cartridge filtration.
Support equipment for nurseries must include suitably-sized incoming water pump(s), distribution pipes and blowers with sufficient capacity and emergency back-up power. Maintaining oxygen levels and mixing is absolutely critical. Round tanks usually use a single 0.5-1 hp blower for each 100-m3 of tank volume. Rectangular tanks usually use 2-3 hp blowers per 100-m3 of tank volume. Newer designs normally use venturis orair injectors that both aerate and circulate the water without theneed for any blowers. The most efficient designs require lowerinitial capital investments and have lower operating costs whileincreasing the system’s carrying capacity.
Shrimp nurseries operate at very high stocking density and biomass, imposing the need for continuous monitoring of critical water quality and animal health parameters. Aside from the essential equipment for water quality monitoring in a pond and hatchery, ORP meters, power backup systems and backup oxygen supplies are required. Monitoring of ionic balance including calcium, magnesium and potassium levels and their ratios is required when using lower salinity water. Larger and more complex nurseries can also have automated monitoring systems with a variety of sensors and alarms to promptly warn operators of system failures and problems.
Proper aeration is absolutely critical in nursery systems, not only to provide oxygen to shrimp for effective feed utilization and growth, but just as importantly to oxidise the liquid, solid and gaseous waste of the system. Water aeration, circulation and circular flow are taken care of by various equipment, including spray bars from high pressure pumps, nozzles and/or airlift pumps. Depending on the system, paddle wheels can be used for older juveniles but are discouraged at initial stocking due to potential physical damage to the young shrimp. Systems using both spray bars and airlift pumps have the advantages of redundancy if either the water pumps or air blowers have mechanical failures.
Along with proper water temperature (normally not an issue in indoor systems), DO is the most important water quality parameter. It must be maintained at or above 4ppm, however, 6ppm is recommended to support optimum growth. With high inputs of feed, there is high demand for oxygen by the target crop and by the microbial community in the water. Any change in aeration efficiencies or power failures can quickly become catastrophic. Thus, monitoring of equipment and oxygen levels should be carried out continually or at least 12 times per day. DO levels of less than 4 ppm require prompt measures like water exchange and sludge removal, suspension of feeding, probiotic treatments, and the addition of pure oxygen.
One of the keys to successful operation of intensive nurseries is management of nitrogenous wastes. Ammonia can be quite toxic at high pH and nitrite toxicity can easily cause mortality in these systems if not monitored and properly managed as discussed below. Any settling of organic material in the system allows for anaerobic decomposition producing harmful compounds like H2S which can quickly compromise the health of the crop.
Biofloc technologies are at the core of the design and operation of modern hyper-intensive nursery systems. When managed correctly, a diverse healthy microbial community contributes directly and indirectly to shrimp nutrition and growth while processing excess nitrogen in the system. Once established, the community becomes stable, competitively excluding harmful opportunistic Vibrios, improving shrimp health and immune competence before transfer to growout ponds. The key to maximizing these benefits is in understanding and managing the microbial community in the system.
Fundamentally, the microbes can be best understood through three key functional groups: heterotrophic bacteria, nitrifiers, and microalgae. Heterotrophic bacteria take up nitrogen and carbon from the system building more bacterial cells. Feeds which drive the system are high in nitrogen, particularly high protein feeds for young animals. Addition of carbon drives production of heterotrophic bacterial biomass pushing them to higher and higher concentrations as evidenced by increasing floc volume. Nitrifiers cycle ammonia to nitrite and nitrate. They are slow growing, requiring time to become established unless head started through water reuse or direct addition. Once functional, the chemoautotrophic nitrifiers can cycle all excess nitrogen through to nitrate if alkalinity and oxygen levels are maintained. Photoautotrophic algae can grow as long as there is light penetration. Growth of beneficial algae like diatoms have been correlated with higher shrimp growth. The challenge for the grower is to understand and manage these components, balancing functional roles and driving the community to maintain water quality, growth and health.
When ammonia and nitrite levels are problematic, addition of molasses, sugar or other carbon sources provides a short term solution, building more microbial biomass and tying up organic material until the cells die and decompose. Continual addition of carbon results in very high floc volume, and excess organic material which can build up as sludge. Thus, the system should be managed to maximize nitrification. Understanding when and how much carbon addition to use is the first key to successful water quality management. High floc levels cause gill fouling, increase propensity for sludge accumulation and shade light reducing algal productivity. All of these reduce growth and lead to health problems. Thus, the second key to system management is maintaining optimum floc levels. This is achieved through use of settling chambers and foam fractionation in closed systems and/or through strategic collection and flushing of sludge with water exchange or recirculation. On the other hand, too much water exchange can lead to system instability, algal blooms and crashes and pH swings. Thus, managers must actively measure and regulate floc composition and density.
To establish a biofloc, the following procedure is recommended by well-known shrimp farming and biofloc expert Dr Tzachi Samocha. Firsly, add a probiotic such as EZ Bio daily, beginning before the start of the production cycle. Proper use of an effective probiotic can be an important management tool, particularly at startup. Ongoing R&D is providing more empirical data to advance strain selection, dosages and functionality. Secondly, at the start of the production cycle, add sugar in proportion to the amount of feed added (30%) and thirdly, measure all forms of nitrogen (NH4-N, NO2-N, NO3-N) daily. When nitrate appears, gradually decrease the amount of sugar added eventually eliminating carbon addition after 5-7 days. With proper startup and ongoing management, biofloc stability can be achieved and maintained.
Feeds and feed management
In hyper-intensive nursery systems as in all shrimp farming, it is the feed inputs that drive success or failure of the crop. Feeds directly determine shrimp growth and health while indirectly driving key water quality parameters. Flocs can contribute some micronutrients for shrimp and can enhance growth but floc is not a reliable, constant predictable source of food for shrimp. Although the importance of proper feed selection and management is fundamental to successful operation of hyper-intensive nursery systems, feed selection and management strategies vary greatly from producer to producer. Practices typically originate from traditional procedures modified through personal experiences and preferences. A scientific approach to feed selection and management should focus on the evaluation of three key variables. Feed formulation, feed physical characteristics and feeding protocols. The key to success is to strive towards providing each shrimp exactly what they need, exactly when and where they need it. Use of regular pond feeds in hyper-intensive nursery systems significantly compromises performance. Waste feed and undigestible nutrients go directly or indirectly into the water, driving water and bottom quality deterioration, reducing growth and compromising health.
During the nursery phase shrimp are growing at an exponential rate. Genetic strains utilized today have significantly higher growth potential requiring adequate nutrients for maximum performance. Specialized nutrient dense nursery feeds are available from Zeigler today to provide 100% of the nutritional requirements with the correct balance of essential nutrients. They have to be highly palatable, and very digestible. The feed should be precisely formulated to support shrimp health, enhance immune system balance and reduce stress during the transfer to the grow-out pond.
The particle size and uniformity of the feed used depends on shrimp size and its uniformity. Particle sizes and shapes should be optimized for the size range of the shrimp population in the system. Table 1 above includes pertinent information based on our proven Precision Feeding Program (PFPTM), for various shrimp stages, weights and production biomass.
If there are animals of different sizes within a tank, then feed particles of various sizes need to be used. Optimally, every shrimp in the tank should have access to a properly sized particle of feed at each feeding. It is the producer’s responsibility to check labels for manufacture dates to ensure feed freshness. Good packaging is essential to retain feed quality and nitrogen should be used in the packaging to extend its shelf life and palatability. The feed must have a balance between good water stability to retain nutrients but not be so hard that it loses its attractability and texture.
The goal of a precision feeding program is to provide just enough feed to achieve the desired growth objective. Precise calculation of feeding rates requires accurate prediction of shrimp survival, growth and feed conversion efficiencies for the selected shrimp strain and diet. Optimizing feeding rates can be greatly facilitated using feeding programs that adjust feeding recommendations according to population status and system temperature while allowing operators to make proactive adjustments during the cycle based on sampling data, potential for compensatory growth, and observations of molting, water quality, excess feed etc. For example, our PFPTM program incorporates sophisticated algorithms to adjust for predicted temperature changes and enables daily user input to adjust feeding according to conditions in each nursery unit.
Continuous feeding using automatic feeders is recommended because shrimp are constantly feeding or grazing. The minimum recommended feeding frequency is once every two hours (12 times per day) with equal quantities of feed for each feeding. This is because the nutritional value of aqua feeds begins to decline immediately when placed in water and can lose much of their value after only one hour in water. Constant daily feeding rates enhance conditions for desirable and beneficial steady-state water quality. At each feeding, inputs of organic material are reflected in increased oxygen demand. More frequent application of smaller inputs reduces fluctuations in oxygen demand. This in turn reduces peak aeration requirements and changes in dissolved oxygen levels.
The feed should reach 70% to 80% of the area of the system within several minutes of each feeding. This can be achieved by physically broadcasting the feed across the surface or by allowing the water circulation to distribute it. Excessive buildup of feed in localised areas such as tank corners, can promote low oxygen conditions and produce harmful hydrogen sulfide. Overfeeding must be avoided. The goal is to feed the proper quantity of feed to each shrimp where they can easily access and promptly consume the feed.
It is typical that in well managed hyper-intensive nurseries shrimp grow at a rate below their apparent genetic potential at lower densities. Thus, size of shrimp at harvest should be expected to be somewhat smaller than that of animals stocked directly into a pond at the same temperature. However, shrimp have been shown to recover this growth difference in a very short time if the producer adapts initial pond feeding rates to support compensatory gain after the successful transfer of healthy nursed juveniles to grow-out ponds. This is one of the most important tools in pond management, to shorten time in grow-out ponds to market size, reduce disease risk and increase profitability.
Properly designed and operated shrimp nurseries provide proven benefits and allow for much more efficient use of the carrying capacity of ponds with direct stocking of post larvae, while significantly manage risk during the first 20-40 days of the cycle. Shorter pond cycles, daily fixed costs are reduced for each kg of shrimp produced.
The best economic gains from the proper use of nurseries are realised. There is the opportunity to stock larger post larvae in ponds, reduce the total duration of a cycle to market size, hold post larvae when temperatures are too cold for open pond stocking or to have a large quantity of juveniles ready to stock when temperature increase or regulations permit. Producers can also reduce time to harvest by having juveniles ready to re-stock a pond following a harvest, increasing the number of cycles per year or improving the size of shrimp at harvest. An additional benefit is to have juveniles large enough to go directly on to pelleted feeds when stocking where primary productivity is low (oceanic water).
Shrimp farmers continually look for ways to lower risk while increasing operational efficiencies and profitability. Effective implementation of hyper–intensive nursery systems is a fast and cost-effective way to accomplish this. Proper management of water quality and precise specialized feed inputs in shrimp nursery systems are critical prerequisites to success. Adoption (and in some cases re-adoption) of improved nursery systems technologies, is increasingly common in Asia and their growing role for future industry success is evident.
AQUA Culture Asia Pacific Magazine - May/June 2016 Voume 12 #3