How to choose good whiteleg shrimp postlarvae: PL size, stress tests, PCR testing and quarantine certificates

Postlarvae are the first stage of the whole culture cycle and the hardest to correct: stock a weak or pathogen-carrying batch and, however carefully you farm, it is difficult to recover. This article explains how to choose good whiteleg shrimp postlarvae step by step: visual inspection at the tank, microscope examination, stress testing, reading the PCR test report, checking quarantine certificates and choosing a reputable hatchery.

Why can't postlarvae be chosen by eye alone?

According to Jory (Global Seafood Alliance, 2017), postlarvae (PL) quality cannot be measured by a single indicator; it requires a combination of observation, microscope examination, stress testing and the hatchery's history. Many dangerous diseases do not show at the PL stage. Guidance from the Department of Fisheries and Fisheries Surveillance (2026) notes:

• White spot syndrome virus (WSSV) and IHHNV can be transmitted vertically from broodstock to eggs and larvae even when the broodstock show no signs.
• Acute hepatopancreatic necrosis disease (AHPND/EMS) usually occurs in the first 10–45 days after stocking, and mortality can reach 70–100% within the first 20–30 days.

Healthy-looking shrimp are only a necessary condition; stress tests and PCR results are also required.

Step 1: Visual inspection at the tank

According to the National Agricultural Extension Centre (NAEC, 2016) and QCVN 02-19:2014/BNNPTNT, acceptable whiteleg shrimp postlarvae should meet the following:

Tips for checking at the hatchery:

• Scoop shrimp into a glass and hold it up to the light to look at the gut. An empty or discontinuous gut is a warning sign; the Department of Fisheries and Fisheries Surveillance (2026) describes shrimp with AHPND as having an empty, discontinuous gut and a shrunken, pale hepatopancreas.
• Watch for deformities: a bent rostrum, curled antennae and a crooked tail are signs of IHHNV infection in whiteleg shrimp, together with 10–30% reduced growth and size variation (Department of Fisheries and Fisheries Surveillance, 2026).

Lee (Global Seafood Alliance, 2005) recommends stocking whiteleg shrimp 9 mm or longer, because larger PL almost always tolerate stress better.

Step 2: Microscope examination

NAEC (2016) advises placing the shrimp on a glass slide with a drop of seawater, examining it at 100–150× magnification, and looking at the appendages (rostrum, antennae, legs, tail segments) for attached protozoa and at the shell for any lesions. Jory (2017) adds: gut fullness, fouling and debris on the setae, opacity of the swimming legs and tail muscle, and deformities. Wouters et al. (2015) include hepatopancreas size and colour among the criteria for microscope examination.

Step 3: Salinity or formalin stress test

A stress test lets you compare tolerance between batches; choose one method and apply it the same way to every batch.

NAEC method (2016): put about 20 shrimp into a 300 ml glass beaker, abruptly lower the salinity to 15 ppt with fresh water (or use formalin), and observe for 2 hours. Survival of 95% or more is a pass.

Methods compiled by Jory (2017):

Note: stress test results largely reflect size (Lee, 2005), so only compare batches of the same age. Stress tests also cannot detect viruses, so PCR is still required.

Step 4: PCR testing for dangerous diseases

PCR detects the pathogen's genes, even when the shrimp show no signs yet. Guidance from the Department of Fisheries and Fisheries Surveillance (2026) requires hatcheries to routinely PCR test 100% of PL batches before sale, and farmers to keep the negative test report for each batch of postlarvae.

Diseases to request testing for in whiteleg shrimp postlarvae, and the corresponding standards, according to the Department of Fisheries and Fisheries Surveillance (2026):

At a minimum, PL should test negative for WSSV, AHPND, EHP and IHHNV; the guidance above also lists infectious myonecrosis (IMNV), Taura syndrome and yellow head disease. See also: white spot disease (WSSV), acute hepatopancreatic necrosis disease (EMS), EHP microsporidian.

How to take PCR samples correctly

According to Jory (2017):

• To detect a 2% infection level with 95% confidence, each tank or batch of more than 100,000 shrimp needs at least 150 shrimp (pooled into one sample, or split into 5 samples of 30 shrimp with all 5 tested).

Samples are fixed in 90–95% alcohol (1 part sample to 10 parts alcohol) and sent to a competent testing laboratory (Department of Fisheries and Fisheries Surveillance, 2026). Ideally, farmers should take the samples themselves from the tank to be shipped.

Interpreting PCR results correctly

Jory (2017) notes that PCR detects gene fragments, which does not necessarily mean the pathogen is alive; there can be false positives due to sample contamination, or false negatives due to damaged samples or insufficient sampling. IHHNV in particular can insert its genes into the shrimp genome, which readily gives false positives (Department of Fisheries and Fisheries Surveillance, 2026).

Step 5: Documents, quarantine and origin

QCVN 02-19:2014/BNNPTNT requires stocked postlarvae to have invoices and documents clearly stating their origin and a quarantine certificate from the competent authority. Guidance from the Department of Fisheries and Fisheries Surveillance (2026) requires the use of PL from specific pathogen-free (SPF) broodstock that have been quarantined, and keeping records including the invoice, the quarantine certificate and the negative test report for each batch.

Step 6: Choose a reputable hatchery

Lee (2005) considers genetics and freedom from viruses to be the most important factors in PL quality but hard to measure, and therefore recommends buying SPF postlarvae from biosecure hatcheries with a long history free of disease outbreaks.

Farmers should ask the hatchery: are the broodstock SPF, are there a PCR report and quarantine certificate for this very batch, have the PL been acclimated to the salinity of my pond, and may I take my own samples for testing? Keep a record of each postlarvae batch and the results of each crop to choose a hatchery for the next crop.

Preparing the pond before receiving postlarvae

The pond must be ready before the delivery date, following the pond preparation procedure before stocking; for ponds that have had EHP, see PCR testing for EHP during pond preparation. When the postlarvae arrive, follow the articles on how to stock postlarvae with salinity and temperature acclimation and biosecurity in the first 30 days. See also the solution page for low oxygen, shock and stress and the care schedule by days of culture.

Supporting Noben products

Support for pond preparation and stocking, used according to the label:

• SUPER VIDAL: supports stable algal density and pH while building water colour before stocking; for algae stabilisation use 50 g/3,000 m³ of water, applied in strong sun at 9–10 a.m., not when it is raining.
• IODINE 99: disinfects the water source, reducing harmful bacteria, fungi and protozoa; for pond preparation use 1 litre/3,000 m³ of water, applied in the cool late afternoon; wait 2–3 days before building water colour and adding probiotics.
• VITA C (pond application): vitamin C for pond application to help prevent shock and reduce stress; before stocking use 5 kg/2,000 m³ of water, applied 30 minutes before stocking.
• BETA 1.3: strengthens resistance and boosts the immunity of shrimp; mix 5–10 g/kg of feed.

Use the dose calculator to convert label doses to your pond volume.

References

• How good are your shrimp postlarvae? – Global Seafood Alliance (D. E. Jory), 2017
• P.L. size, source strong early measures of shrimp grow-out performance – Global Seafood Alliance (D. O'C. Lee), 2005
• Sound hatchery protocols enhance shrimp postlarvae quality – Global Seafood Alliance (R. Wouters et al.), 2015
• Stocking shrimp postlarvae and pond care and environmental management (in Vietnamese) – National Agricultural Extension Centre, 2016
• Guidance on prevention and control of several dangerous diseases in brackish-water shrimp (in Vietnamese) – Department of Fisheries and Fisheries Surveillance, 2026
• QCVN 02-19:2014/BNNPTNT on brackish-water shrimp farming establishments (in Vietnamese) – Ministry of Agriculture and Rural Development, 2014