In this article, I will be talking about a standarized method of selecting PLs (shrimp).

A critical stage in shrimp farming cycle is the selection of good quality post larvae (PL) and their subsequent stocking in ponds. Failure to achieve good post-stocking survival of your post larvae can reduce your harvest potential form the start of the crop, thereby reducing profitability. If post-stocking survival is low one may be forced to decide whether to restock (if additional post larvae are in fact available) or continue with a sub- optimal stocking. The decision is made more difficult by the fact that by this time in the year the growing season is usually well advanced, and, as many farmers have found from bitter experience, shorter grow-out time resulting from late season stocking typically results in smaller shrimp at harvest. The first step towards optimizing stocking success is selection of high quality post larvae.

Shrimp culture as an industry has been contributing significantly to the export earning, employment generation, poverty alleviation and to the economic development of India in recent years. India’s vast coastal belt and condu- cive environment has led to the rapid expansion of shrimp farming activities. Culture practices are gradually be- ing intensified but also vary with the investment capabilities of the entre- preneur. Since shrimp larval quality plays a key factor in influencing the successful shrimp culture, farmers and PL producers need a rapid and simple technique to evaluate postlarvae health quality with respect to future growth and survival. Characterizing good PL quality can benefit shrimp farmers by increasing crop profits and help hatch- ery managers to identify and assess the effects of factors affecting PL quality. In recent years, shrimp health man- agement has become the main focus of improving production and minimizing infectious diseases in shrimp ponds for smooth development of aquaculture industry. To accomplish this goal, one should be concerned with the quality of postlarvae especially the selection of high-health postlarvae shrimp.

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Post Larvae selection in important as it will determine the performance and production of the ponds. The objective of larvae selection is to obtain best survival and growth in the pond. As a guide, obtain post larvae from hatcheries which have produced good shrimp in the past or has good reputation. There are several characteristics of good quality post larvae, and visual observation can be used in identifying good quality or healthy post-larvae for stocking. Given below is a guide on how to indentify good healthy post larvae.

Key Instructions for Seed Selection & Stocking

• Avoid wild seed and seed form poorly managed commercial nurseries. It has high chances of importing disease. Stock same batch of seed. Avoid different batches of seed form different batches of seed from different hatcheries. Stock the shrimp seed only one time per crop. Do not continuously stock with new batches of seed.

• Place some PL (about 100 pcs) in a basin stir the water and check.
Healthy PL will swim against the current. Weak PL will gather at the center.

• Select a good registered hatchery to procure specific pathogen-free (SPF) post larvae that. Hatcheries should be designed (or modified, in the case of existing hatcheries) to ensure good bio-security. A well designed shrimp hatchery consists of separate facilities for quarantine, maturation spawning, hatching, and larval and PL rearing, indoor and outdoor algal culture, hatching of Artemia and feed preparation. Hatcheries should have bio-security system to prevent introduction of disease. Single brooder spawning must be followed. Where only one animal is kept in a spawning tank for spawning. This helps in reducing disease contamination. Hatcheries should maintain management records. Farmers should check the previous screening records of gravid shrimp for MBV and WSSV. Hatcheries should not use banned chemicals/ antibiotics. Each hatchery should have its own set of Standard Operating Procedures. Hatchery should allow access to farmer representatives at any time to observe the tanks. Hatchery should provide proper invoice/ certificate for purchase of seed. Farmer should be given a choice to reject the seed (If poor quality) till the time of packing.

• Good and healthy shrimp Larvae should be even in size. PL 10 should have a total length of 10 mm. the 3 stage form one spine so pl 10 having 3 t0 4 spine this is main character of larval age finding. Uneven size may be due to: a) Different Stage b) Under feeding c) Disease d) Poor water quality e) Over-aged Post-Larvae.

• Test the gravid for disease. Individual brooders must be transported in special brood stock transportation bags filled with oxygen, sealed and placed on ice within insulated foam boxes to maintain temperature of and less than 29 degree. Farmers must ensure the same during visit to the hatchery. Brood stock must be screened for MBV before keeping the animal for spawning. Brood stock must be screened after spawning for WSSV by collecting pleopods along with scum in disposable ampoules for PCR testing in labs that have successfully completed the ring test ( Get the at least list of successful PCR labs from NaCSA/MPEDA). Only the eggs/Nauplii tested negative for WSSV and MBV, must be used for further production of PL. Positives should be discarded after disinfection.

• Select Healthy shrimp seed. Observe the PL in a bowl by taking samples from different locations in the PL tank. Switch off the aeration briefly in the tank while taking samples. Tanks having dead pieces or showing reddish coloration should be rejected, PL tanks having good survival indicates good health of the stock. Prefer PL 15-16 stage (Total body length should be >12mm). Smaller sizes may not be ready for stocking and may not survive in the pond.

• Shrimp seed should be uniform in size and dark or light brown in colour. Seed with red, blue or green colour must be rejected. Shrimp seed should be strong and active. Collect about 500PL from the bottom of the tank pour in a round tub. Stir the water. Wait for 1-2 minutes. If many seed concentrate in the centre then do not select that seed batch. Shrimp seed should pass a salinity stress test. Collect about 100PL in a glass with tank water with ambient salinity (28 to 32ppt), and pour equal quantity of fresh water. Wait for 30min. If 100% of the seed survives, then select the seed batch. Shrimp seed should pass a formalin stress test. Collect about 100PL in a glass containing 100ppm formalin (0.25 ml of commercial formalin/L). Wait for 1hr. If more that 9% the seed survives, then select the seed batch. Select the seed batch which passes the above stress test for PCR test. Also test the PL for presence of antibiotic residues.

• Test the seed for disease and healthiness. Shrimp seed may have pathogens like WSSV. Viral related disease can cause mass mortality of shrimp in ponds. The unhealthy seed will result in poor survival and growth in ponds. Collect and pack about 500PL in a seed bag and send to a shrimp diagnostic laboratory which successfully completed ring test for disease testing. The result should be negative for MBV/HPV by wet mount and WSSV by PCR test. Take only batches which test negative for both MBV and WSSV.

• Pigmentation : (a) Shape Finger like – healthy Round Shape – unhealthy (b) Color Rust like brown / gray / dark gray – healthy Yellow – average Blue – unhealthy

• During seed packing and transportation do not mix the seed batches from different PL tanks of a hatchery or from different hatcheries. The salinity of PL tank water and pond water should be the same and should not differ by more 2 ppt. Prior to packing the seed, adjust the salinity of the PL tank water to the salinity of pond water. Start adjusting the salinity in hatchery at PL-5 stage and complete the process of adjusting at least one day prior to seed packing. Seed bags should have minimum 5L water and enough oxygen (water: oxygen =1:3). Do not pack more than 1000 PL per bag. Add Artemia in the seed bag to prevent cannibalism. Seed bags should be transported in thermo coal box or plastic tubs. Mark the seed bags/ boxes form different larval tanks. Transport time form hatchery to ponds should be less than 6 hours. If transportation time is longer, slightly reduce the temperature by placing ice bags in between two polythene layers of seed bag. Transport during cool hours (7pm- 7am).
Transport of the post larvae should be done in the early morning in order to avoid the heat. Upon reaching the grow-out pond, post larvae should be counted, checked for vigor, acclimatized and evenly distributed over the pond area. The most common method of transportation is by placing the post larvae in plastic bags filled with 1/3 of water and filling the other 2/3 with oxygen before sealing them. The packing density will depend on the age of post larvae and the expected transportation time. As a guide, the following are recommended for transportation time of less than 6 hours: PL 9 – 1500 to 2500 PL/3liter PL 10 –1000 to 2000 PL3/liter In situations where the transportation time is longer, these plastic bags can be placed into polystyrene boxes and cooled to around 20°C by placing small bags to the box. In case where transport tie exceeds 4 hours, live food should be added into the transport bags.

• Full gut indicates good health, empty gut indicates possible stress problem

• Molting problem, indicative of nutritional disorders related to sterol, phospholipids or calcium or phosphorous

• Muscle development: The ratio of width of gut and body depth below the gut at the 6th segment, healthy PL should be 1:4 or > 4

• External features: For a healthy PL, the antennal scales of head portion are Closed together whereas the tail fan (uropod) is widely opened. Closed uropods are indicative of young fry not yet suitable for stocking (less than PL 8)

• Seed should be stocked in pond during cool hours of the day i.e. after 8pm and before 8am. Make sure that plankton bloom is good and stable. Avoid stocking if pond has transparent water or dark green water. Allow to acclimatize the seed or temperature to pond. Take the disinfected 500L flat bottom, round tank and fill it with pond water up 50%. Oxygenate the water using oxygen cylinder. Open the seed bags & release the seed into the tank. Treat the PL with 100ppm formalin for 15minutes. If moulting is observed or the seed transportation period is more than 6hrs do not treat with formalin. After treatment, stir the water to create a swirl. All the dead and weak PL concentrate at the bottom-centre of the tank. Siphon off dead and weak PL using 1 inch flexi hose. Do not release seeds where water is turbid or shallow. Keep 100 PL in two small hapas and check the survival after 48 hrs. If the average survival is less than 70%, one may have to plan for restocking.

Conclusion

Stocking postlarvae of the highest quality possible, healthy and free of pathogens, is critical to the success of any shrimp farm. There are a number of well-established criteria that are used to assess PL quality, including its origin and hatchery reputation, visual evaluation, stress tests and various tests to detect the presence of pathogens. The transition from hatchery conditions to those prevailing in open grow-out systems such as tanks and ponds, where water conditions can continually or unpredictably change (day/night, dry/rainy seasons over the production cycle) can be a traumatic experience for PLs unless the transition is gradual and stress is minimized by following proper acclimation procedures. Strict use of PL quality assessment criteria in the evaluation and selection of PLs for stocking, and a careful acclimation procedure using the best quality seedstock available will be invaluable and will have a significant effect on the production and profitability of any shrimp farm, and should be standard procedures.