The concept of Nurseries in Shrimp aquaculture first arose when farms were badly hit by WSSV in the early 90’s. Farmers suddenly became aware of the need to prevent this disease at any cost before entire farmers were wiped out. It was evident that healthy seed were the key to a good healthy harvest. Managing and controlling infection in the early stages of development and maintaining a good healthy stock was of prime importance. What better way to tackle this issue than to minimize the area of operation to a manageable sale, where critical parameters could be monitored constantly, and simultaneously risks were also enormously reduced. Thus started the introduction of the three-phase farming system in the Shrimp aquaculture sector, with intermediary nursery rearing at the farm sites, before stocking the animals into grow-out ponds.
The idea of the nursery system was already prevalent in fish farms and other aquaculture systems. Nurseries range in complexity from simple use of net cages inside the grow-out pond to Circular collapsible tanks on the pond dyke, to brick/RCC rectangular tanks in green houses close to the grow-out area to very complex high-tech raceway systems completely monitored by computers round the clock. An evolution in design of nursery systems took place for holding the seed for longer duration, in higher biomass to produce larger post larvae.
Nursery systems for P. vannamei culture across the world are reaching new heights with high density stocking and very intensive farming systems with zero water exchange in order to keep all parameters constant. Prerequisites are highly efficient filtration systems similar to those used in hatcheries. Apart from use of probiotics to control water quality, regular siphoning is done. The target is to produce healthier and stronger animals before transfer to the pond.
There has been an evolution in tank shape for nursery systems in Mexico. Circular tanks from sixes of 50 tons in 2008 have evolved to raceways of 100 tons in 2009 and then to larger raceways of 500 tons in 2011. These are now bigger, with rectangular tanks of 800 to 1000 tons in curved or flat green houses. Since 2012, modifications in transfer technology led to the use of pumps for more efficient transfers. This reduces stress and mortality during transfers to only 3-5%. Transfer process has been refined such that the animal does not stay out of water longer than 30-60 seconds. In 2013, the maximum distance for transfers with pumps were to ponds located up to 3km form the race way units. The transfer capacity with pumps is 15kg per minute. A recent development in Mexico is to reduce stocking density to achieve higher weight of PL as well as higher survival rates in the raceways. Biomass of 6.9 kg/m3 has been achieved.
These experiences in Mexico were then transferred to Asia. In a farm in Malaysia, PLs were stocked in raceways for 25-30 days to produce 1-1.5 g PL. These were then stocked into grow-out ponds at a density of 70-80 PL/m. FCR in the raceway was 1.0-1.5 depending on the duration of culture. In the grow-out ponds, shrimp grew to 17-18 g and FCR was 1.3. Survival in the grow-out pond was 90-95%.
In Thailand there were mixed results. There was success in the nursery stage but due to the health condition of the animal during transfer, results were poor in the grow-out stage. In recent times nursery rearing and transfers to grow-out ponds have improved a lot and are being viewed as the best way to combat EMS and EHP diseases, which is widely prevalent in Thailand.
In Vietnam, the implementation of the nursery system was hampered by low investment and technology as well as poor bio-security.
The Indian Experience
In India the nursery system has been in use for several years’ right from the time Penaeus monodon was cultured predominantly. It helped to hold the stock in a protected area while the grow-out pond was being prepared. While the animals were growing in nurseries, farmers could carry out he grow- out area preparation process thoroughly, without compromising on the procedures due to lack of time. The stocking tie into grow-out ponds could be delayed without missing out the prime season, by stocking at the right time, to harvest at the right time, and to reach the market at the right time.
This system also helped immensely to tide over the vagaries of nature. When water availability was low, the farmer could start the culture operation in a small nursery area while the rest of the farm was being prepared for the grow-out phase. This way he could maximize the utility of the farm until favorable conditions were reached without compromising on the economic front.
In the nursery system, the animals could be monitored better as it was a smaller area of operation. It could check the occurrence of disease right to start when the infection sets in. If he nursery animals are found to be infected by diseases, the stock could be discarded and a fresh batch could be quickly started, without jeopardizing the entire farm. This way the entire farm will not have to be cleared and disinfected. It saved a lot of time and energy and is economically a better option.
In areas of low salinity, the nursery area could be used to acclimatize the animals slowly over a period of time to the correct salinity without undue stress on the animal.
While shifting seed to the grow-out ponds, the actual numbers could be accessed and therefore the feeding regime in the grow-out ponds could be more accurate. This is major saving on feed input This resulted in less pollution caused by accumulation of uneaten feed and also improved the FCR. In all, managing one nursery area for about a month is a lot more economical than managing the same stock spread out over a huge area. In the long run, the three phase farming system looks to be the most practical sustainable and sensible way to farm shrimp with maximum financial benefits and optimum utilization of available resources.
India has also experimented on modern hatchery systems in raceways under controlled conditions with efficient filtration systems. In the year 2008, our own Certitude Farms along with Mr.Anil Ghanekar had reared Penaeus monodon PL to one gram size in 40 days and successfully transported the same in 1000 liter tanks by road for 7 hours to our farm with nil loss recorded in the GAA magazine May 2009 issue. Mr. Ghanekar has since then continued t set up commercial nurseries with Zero discharge. With a stocking density 5-15 per liter and using RAS and Biofloc technology, he got a survival rate of 80% with stunted growth rate for ease of transport in 21 days. Maximum transportation distance was 400 km with 95% survival.
The following report is based on our own experience in Certitude Farms, Nagaptinam.
In the year 2015 when EHP became rampant, we decided to construct a nursery in our farm to rear the animals to juveniles. We thought if we reared the animals to juveniles in controlled conditions with better nutrition, we could strengthen the health of the animals at the early stage. Equally importantly we wanted to avoid culture during the peak summer months and the best way to do that is to stock in late January or early February. But historically we have seen that stocking culture ponds during January poses a big risk of WSSV infection due to cross contamination. Hence we thought a closed nursery system will be the best way forward.
Construction of the nursery:
We constructed and square earthen pond of 2000 (45mX45m) and fully lined it with HDFE sheet. It is best to construct the nursery in a higher level as compared to rest of the farm. This enables better drainage, better bio security and quick cleaning. We installed a central drainage system for flushing out he accumulated waste. The pond bottom is sloped gradually towards the centre to ensure efficient drainage in the pond. Nursery has a water depth of 1.5m. This is sufficient to supply juveniles to 8 hectare water spread area of grow out ponds. Any one of the grow-out ponds could also be converted to function as the nursery. This way, the farmer is not stressed about managing it, as it is just like any other grow-out pond, only it is more controlled area. The total cost of construction of our 2000m2 nursery was approximately Rs. 7.5 lakh.
It is critical to maintain higher than 6ppm Dissolved Oxygen levels at all times in the nursery pond. Eight paddle wheel aerators are placed in concentric rows inside the pond, one row is place 7m away from the dyke and the second row runs 15 m away from the dyke. Each row creates a centrifugal action ensuring that all the dirt moves to the central drainage area, which is the lowest point in the pond. This system helps in efficient water exchange and maintaining stable water quality parameters by draining out he centrally collected sludge through gravitational flow.
Bio-Security is critical in the nursery system, as all animals are in one pond, in high density and any mistake could collapse the entire system. Crab fencing and bird fencing are mandatory to protect the stock. The top is completely covered by shade nets to keep the temperatures down in summer. During winter, plastic sheets are used to cover the top to maintain higher temperatures during cold conditions. Covering the top also prevents any undue bird droppings which is usually the cause for cross contamination on the farm. Hand and foot dips are placed at all entry points just as it is in shrimp hatcheries. Hygiene and cleanliness are followed strictly in nursery ponds. In large commercial farms, it is advisable to house the nursery complex far away from the grow-out ponds. This will help in minimizing chances of contamination from the grow-out area to the nursery area.
Normally PL10 size seed are stocked in the nursery. Stocking density varies form 0.5-5 per liter. At our farm in Nagapatinam we stock 1 animal/liter. They are reared in the nursery until they reach PL 40 size in one month, and about 1 gram ABW. Stocking densities will vary according to the type of pond and the additional facilities available.
Initially we use the same PL feed as used in the hatchery. After the third day, the animals are gradually weaned to the farm PL feed. Feeds with 45% crude protein and high energy feeds are fed five times a day. The feed is broadcast manually around the pond. It is critical to use the best quality feed during the nursery phase. Lower quality feed will affect water quality and animal growth. The Nursery phase ensures less wastage of food as the area is localized and limited.
It is important to regularly ad probiotics and minerals to the nursery system, since the stocking density is high. This helps to mitigate disease attacks and also boosts the animal’s immune system. Minerals are very important for molting and the dosages are adjusted according to water condition.
Maintenance of water quality is the key to healthy juveniles and eventually a good crop. After 15 days of culture in the nursery, water exchange is carried out 5cm-10cm daily, depending on feed management. Vibrio levels, dissolved oxygen levels and ammonia levels are checked once a day. DO and ammonia are critical components in a nursery system. Salinity is maintenances between 15-25 ppt, pH around 8 and temperatures between 28-30c.Usually, in the nursery phytoplankton bloom will be very high because we will be using a lot of feed in a very small area. This has to be controlled with good water exchange.
Transfer of Juveniles to Grow-out pond:
This is one of the main bottle-necks in having a nursery phase in shrimp farming. The effectiveness of a three- phase culture depends largely on the method used for transferring juveniles to the grow-out ponds. Managing the nursery systems to produce healthy animals is of prime importance. While there are plenty of experiments for the transfer of juveniles, one has to consider the risks and possible losses when implementing them. The transferring process should have minimal stress on the animal. In our farm, harvest is done using a drag net. About 2 kg of small animals of 1 gram each are placed in 20 kg buckets filled with water saturated with oxygen. Maximum transportation time form nursery to grow-out pond was 5 minutes. Mortality was around 5-10%. Mortality is directly related to condition of animal and stress during transfer. Transfers are carried out in the early morning hours.
A volumetric assessment is done to ascertain numbers of shrimp transferred to each pond, just as the seed quantity is measured in the hatchery. Harvesting by drag net odes churn up the bottom but 90% of the stock is retrieved. The balance 10%, which is not harvestable, is allowed to remain in the nursery pond and grow there to marketable size. In this method, we have achieved 80-90% survival.
We have followed this system of nursery rearing for the past 3 crops on our farm and have succeeded very well. We are happy with the outcome and feel it is the best way to move forward.
Advantages of going through nursery phase:
After several years of significant loses with white spot syndrome virus (WSSV) out breaks, a nursery phase is now a common practice even in P.vannamei shrimp farming industry. Since the occurrence of white faeces disease through the EHP pathogen in 2015, it has gained even more significance. It is very difficult to control this disease and there is no known treatment to overcome it. So, it has become more important to prevent the attack by strengthening the animals in nursery phase. A nursery phase allows farmers to focus on controlling the vibrio bacteria and delaying exposure to the pathogenic agent. Good nutrition and good health develops immunity and promotes resistance to the infection.
Farmers have noticed a significant chance to prevent EHP by introducing the nursery phase.
Significance of nursery systems in the current scenario:
Nursery systems are more relevant in today’s scenario to promote sustainable farming practices. The two main considerations are diseases prevention and profitability optimization. Stocking post larvae (PL) after the nursery phase (usually up to PL45) instead of PF 10-12 direct from hatcheries can reduce the duration in grow-out ponds by 20-25 days and improve feed conversion ration by 10-20%. This is now one of ht strategies to mitigate EHP (and EMS in affected Countries), by stocking larger size PL into grow-out ponds.
The three-phase system has various benefits for the entire supply chain. In a nursery system there is better control of water quality and shrimp health. There is optimization of space and better feeding efficiency. The target is to produce stronger post larvae which have been acclimatized to the pond environment. These results in better survival during the first days of culture and minimize exposure to disease vectors. Nursery systems allow more crop rotation in the ponds. Farmers require more PL and hatcheries sell more PL when producers have more crops.
The highest cost in shrimp farming is during the grow-out stage. The shorter cycle means savings in costs of feeds and energy. However, all of this can be achieved only with the use of proper technology and with full control of the system. In poorly constructed or managed nursery systems, low survivals may result, and weak animals are likely to be transferred to ponds. Two biggest costs in shrimp farming are feed and duration of culture so introduction of the nursery phase helps t reduce risk and improve profitability.
The need for commercial nurseries:
Moving forward our sector is going to see a lot of farmers investing in nurseries for ht farms, as this is found to be very effective in controlling diseases, costs and saving item. There is a need for the services of commercial nurseries which can cater to the needs of small and marginal famers. Hatchery operators are the most suitable to set up these commercial nurseries for maintaining quality and traceability. Ideally these nurseries should be situated in isolation form farming areas for avoiding obvious bio security hazards. So farmers setting up nurseries for should keep this in mind and find an isolated spot within or outside their farm to prevent contamination.
We are going through challenging times, and the presence f new diseases and adverse environmental conditions are pushing the industry to change and use new tools. It is best for each farmer to use a system that is best suited to his farm. Although many experiments are conducted and many results are published, it best to adapt these learning to reach a method that is conducive to the conditions prevalent in his area. The system should be practical and yet allow for improvements and up gradations from time to time to improve the health, size and survival rate of the juveniles.
There is still room improvement such as adjusting feeding regimes, probiotic protocols, and temperature conditions in order to achieve larger PL in the nursery phase and better FCR in the grow-out pond. We should concentrate n adapting proven technology appropriate for our own unique situations. In this way we can move to more controlled sustainable shrimp farming.