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.
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.
- 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.
- 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 <290C. 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.
Laboratory Standards for PL selection
|WSSV||Absent||2 step Nested PCR|
|Stress Test||100%||50% Salinity drop for 30min|
|>90%||100ppm foramlin for 1hr|
|Muscle to Gut Ratio||04:01||Microscopy|
|Hepatopancreas||Full with oil globules||Microscopy|
|Gut||Full& without Gregarines||Microscopy|
|Dorsal Rostral Spines||>5||Microscopic|
- 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).
- 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.
Source : NaCSA