Running Mortality Syndrome in Shrimp: Symptoms, Causes & Management

Running Mortality Syndrome in Shrimp: Symptoms, Causes & Management

Running Mortality Syndrome, commonly called RMS, is a term used in shrimp farming to describe a pattern of ongoing, usually low-level mortality rather than one sudden mass mortality event.

The subject is important because the term RMS is not always used in exactly the same way in different regions or publications. In India, ICAR-CIBA has historically described RMS as a farmer-named syndrome involving continuous mortality, while more recent CIBA work continues to investigate the causes of running mortalities and emphasizes that several factors may be involved. (ciba.res.in)

This distinction matters.

RMS should not automatically be treated as one specific pathogen or one specific disease.

Recent ICAR-CIFRI field surveillance in West Bengal, for example, found RMS and EHP among possible contributing factors in a mortality event, but explicitly stated that the observations were preliminary and not a confirmed diagnosis. Laboratory testing and histopathology were required for confirmation.

So the best approach for farmers is:

Observe → Sample → Test → Identify the likely cause → Correct farm conditions → Monitor

What Is Running Mortality Syndrome (RMS)?

RMS describes a situation where shrimp mortality continues day after day over an extended period, rather than ending quickly after a single event.

Older ICAR-CIBA field guidance described Indian RMS as continuous low-level mortality that could reduce survival and production. That guidance reported that the condition had been observed in P. vannamei farming areas of Andhra Pradesh and Tamil Nadu and that mortality often began after approximately one month or around 40 days of culture.

More recent CIBA work in 2025 reported that running mortalities remained an important production problem and that investigations were needed to determine the actual causes in affected farms. CIBA noted associations with factors including Vibrio, EHP, white-feces syndrome, white-muscle syndrome and poor water quality in affected crop failures.

Important: RMS Is Not Always the Same as CMNV

This is an important update for anyone writing about RMS.

A 2026 ICAR-CIBA training manual uses the term "running mortality syndrome" in the context of viral covert mortality disease (VCMD) caused by covert mortality nodavirus (CMNV), explaining that VCMD's continuous daily mortality pattern was initially called RMS.

For farmers, the safest rule is:

RMS is a mortality pattern that requires diagnosis—not a diagnosis by itself.

What Does RMS Look Like?

Symptoms can vary depending on the underlying cause.

Historical Indian RMS descriptions included:

Continuous daily mortality

Usually low mortality on any single day

Reduced overall survival

Production loss

Whitish patches in the abdominal muscle of affected shrimp

Mortality often becoming noticeable after approximately 30–40 days of culture

Common Signs Farmers Should Watch For

A farmer should pay attention when several of these signs occur together:

Mortality

Shrimp die every day or repeatedly over an extended period.

White or opaque muscle

Affected shrimp may show whitish areas in the abdominal muscle. Historical CIBA RMS descriptions identified this as an important field sign.

Reduced feeding

Shrimp may show reduced or inconsistent feeding activity.

Slow growth

The surviving shrimp may show poor or uneven growth.

White feces

Some current Indian running-mortality investigations have involved persistent white feces together with mortality.

Survival decline

Even when daily mortality appears small, cumulative mortality can become economically significant.

Why Is RMS Difficult to Manage?

The main challenge is that the same mortality pattern can have different underlying causes.

A pond with continuous mortality may have:

Poor water quality

High organic loading

Vibrio-associated problems

EHP

White-feces syndrome

White-muscle syndrome

Viral disease

Multiple simultaneous problems

Environmental stress

Other causes that require laboratory investigation

Possible Factors Associated With Running Mortality

1. Poor Water Quality

Water-quality fluctuations can place shrimp under physiological stress.

Recent ICAR field surveillance associated mortality investigations with fluctuations in:

Dissolved oxygen

pH

Alkalinity

Organic loading

Pond-bottom waste

Poor Pond-Bottom Condition

Accumulated organic matter can create a stressful pond environment.

Check for:

Black sediment

Bad smell

Excess sludge

Uneaten feed

Dead organisms

Poor circulation zones

Pond-bottom management should therefore be part of any investigation into prolonged mortality.

Vibrio and Other Bacterial Problems

Earlier CIBA investigations of Indian RMS found Vibrio species, including Vibrio parahaemolyticus and Vibrio azureus, predominantly in haemolymph samples from affected shrimp. However, the investigation did not establish Vibrio as a single definitive cause of the syndrome.

This is an important distinction:

Detection of a bacterium does not automatically prove that it is the sole cause of mortality.

A proper diagnosis requires the clinical picture, laboratory testing and other evidence.

EHP

Enterocytozoon hepatopenaei (EHP) is another possible factor that may occur in farms experiencing running mortality or poor production.

However:

RMS ≠ EHP

A farm experiencing mortality should not be labelled as EHP-positive based on mortality alone.

EHP requires appropriate diagnostic testing.

White Feces Syndrome

White feces can occur alongside running mortality in some Indian farm investigations.

CIBA's 2025 farmer-researcher initiative specifically investigated running mortalities associated with white muscle and white feces

White Muscle Syndrome

White muscle abnormalities can also occur in affected shrimp.

CIBA training material describes white-muscle syndrome separately and notes that sudden changes in environmental conditions such as salinity, temperature and dissolved oxygen can be associated with the condition.

Environmental Stress

Rapid environmental changes can weaken shrimp and make existing health problems more difficult to control.

Important factors include:

Temperature → Salinity → DO → pH → Alkalinity → Organic load

High Biomass and Management Pressure

As shrimp grow, the pond carries a much larger biological load.

Higher biomass generally means greater demands on:

Oxygen

Feeding

Water quality

Aeration

Waste management

Monitoring

When Does RMS Usually Appear?

There is no single universal day of culture at which RMS begins.

Historical CIBA guidance reported mortality beginning after approximately 30–40 days, while CIBA's 2025 investigation highlighted some recent Indian crop failures particularly around 50–70 days of culture.

Step-by-Step RMS Management Guide

Step 1: Do Not Panic

Finding a few dead shrimp does not automatically mean the pond has RMS.

First determine:

How many shrimp are dying?

How many days has this continued?

Has mortality increased?

Are there other symptoms?

Step 2: Start a Mortality Record

Record daily mortality.

Day Estimated Mortality Feed DO pH Remarks

45 150 75 kg 5.0 7.8 Normal feeding

46 180 74 kg 4.7 7.7 Slightly reduced feed

47 220 70 kg 4.4 7.6 Reduced feeding

48 250 68 kg 4.2 7.5 Monitor closely

Numbers are illustrative only.

The value of this record is the trend, not the example numbers.

Step 3: Check Dissolved Oxygen

Check DO as soon as continued mortality is noticed.

Pay particular attention to the early morning.

MPEDA recommends early-morning DO monitoring and provides 4–6 ppm as a reference range for shrimp farming.

If DO is low:

Improve appropriate aeration → improve circulation → recheck DO → investigate the cause.

Step 4: Check pH

Measure:

Morning pH

and

Afternoon pH

This helps determine whether there are significant daily fluctuations.

Step 5: Check Alkalinity

Test total alkalinity.

Low or unstable alkalinity can make pH more difficult to manage.

Also check whether:

Heavy rain occurred

Water exchange recently occurred

The farm is at low salinity

Nitrification activity is high

Step 6: Check Ammonia and Nitrite

Measure:

Total ammonia

Unionised ammonia where available

Nitrite

CAA's L. vannamei guidance includes total ammonia, unionised ammonia and nitrite among water-quality parameters requiring routine monitoring.

Step 7: Check Temperature and Salinity

Record:

Temperature = ____ °C

Salinity = ____ ppt

Look for sudden changes rather than focusing only on the current value.

Step 8: Inspect the Pond Bottom

Check:

Black soil

Bad smell

Excess sludge

Uneaten feed

Dead shrimp

Poor circulation

Organic deposits

Step 9: Review the Feeding Program

Ask:

Has feed recently increased?

Are feed trays showing uneaten feed?

Has feeding declined?

Did mortality begin after a major feed increase?

Feed management should be adjusted according to actual shrimp response and pond conditions.

Do not continue increasing feed simply because the shrimp are expected to be larger.

Step 10: Observe the Shrimp

Collect representative shrimp and examine:

Body color

Muscle condition

Hepatopancreas appearance

Gut fullness

Shell condition

Swimming behaviour

White feces

Growth variation

Remember:

Visual signs are useful for screening, not definitive diagnosis.

Step 11: Collect Proper Samples

When mortality continues, send appropriate shrimp and environmental samples to a competent diagnostic laboratory.

CIBA maintains specific guidance on collection, preservation and dispatch of shrimp samples for disease diagnosis, emphasizing the importance of proper sampling for meaningful diagnostic results.

Step 12: Test for Relevant Diseases

Depending on the symptoms and local disease situation, a diagnostic laboratory may investigate conditions such as:

WSSV

EHP

IMNV

Other viral pathogens

Bacterial problems

Other relevant diseases

Step 13: Do Not Assume RMS From White Muscle Alone

White muscle can be observed in different shrimp-health problems.

Historical RMS descriptions included white abdominal muscle patches, but newer investigations also distinguish white-muscle syndrome and other disease conditions.

Therefore:

White muscle → investigate

not:

White muscle → automatically diagnose RMS

Step 14: Review Farm History

Check:

Seed hatchery

Seed testing

Stocking density

Stocking date

Feed records

Water-quality records

Previous disease

Previous crop performance

Water source

Water exchange

Pond preparation

Biosecurity

Rainfall

Aeration

The history can help the diagnostic team understand what changed before mortality began.

Step 15: Strengthen Biosecurity

When disease is suspected:

Do not move nets, buckets, equipment or workers unnecessarily between ponds.

Pay particular attention to:

Visitors

Equipment

Water movement

Birds

Crabs

Dead shrimp

Pond-to-pond movement

Step 16: Manage the Pond, Not Just the Symptom

Once mortality is identified, focus on correcting the underlying stressors.

Priorities generally include:

Water quality

Maintain stable DO, pH, alkalinity and other relevant parameters.

Feed

Avoid unnecessary feed accumulation.

Pond bottom

Manage excess organic matter.

Biomass

Understand how much shrimp the pond is carrying.

Biosecurity

Reduce opportunities for disease spread.

Diagnosis

Use laboratory evidence before claiming a specific disease.

Step 17: Continue Monitoring Daily

After making management changes, don't stop testing.

Continue recording:

Mortality

DO

pH

Temperature

Salinity

Alkalinity

Ammonia

Nitrite

Feed

Shrimp behavior

Improvement should be visible in the trend.

Step 18: Consider Early Harvest Based on Evidence

Persistent mortality can make continuing a crop economically risky.

If mortality is substantial, growth is poor, FCR is deteriorating and diagnosis/management indicates that recovery is unlikely, the farmer and technical adviser may need to consider partial or early harvest.

This should be an economic and biological decision, not a panic response to one bad day.

Can RMS Be Prevented?

There is no single RMS-prevention product or one-step treatment that can guarantee prevention.

The best strategy is to reduce the factors that make mortality more likely.

1. Start With Quality Seed

Use properly tested and authorized seed from approved sources.

1. Prepare the Pond Properly

Start the crop with good soil, water and biosecurity conditions.

1. Avoid Unnecessary High Stocking Pressure

Choose stocking density based on actual farm carrying capacity.

1. Maintain Stable Water Quality

Avoid large fluctuations.

1. Manage Feed Carefully

Minimize feed wastage.

1. Maintain Good Aeration

Especially as biomass increases.

1. Monitor the Pond Bottom

Prevent excessive organic accumulation.

1. Maintain Biosecurity

Reduce the movement of possible disease agents between ponds.

1. Monitor Shrimp Health

Don't wait for mass mortality.

1. Keep Complete Records

Good records make abnormal patterns much easier to recognize.

RMS and Water Quality

One of the clearest lessons from recent Indian investigations is that disease and environmental management cannot be separated.

This supports a practical farm principle:

A healthy shrimp pond needs both disease prevention and environmental stability.

RMS and Feed Management

Persistent mortality can quickly affect FCR.

For example:

Mortality increases → surviving biomass becomes lower than estimated → feed remains high → feed efficiency deteriorates

Therefore, when mortality begins:

Recalculate biomass → review feed → check tray response → monitor FCR

Don't continue feeding based solely on the original stocking estimate.

RMS and FCR

FCR can become an important economic warning indicator.

Formula

FCR = Total feed used ÷ shrimp biomass produced

RMS and Dissolved Oxygen

DO deserves special attention because poor oxygen conditions can stress shrimp and worsen other pond problems.

Recent ICAR field surveillance specifically identified DO fluctuations among important environmental observations in a mortality investigation.

During a running-mortality event:

Check DO particularly during:

Early morning → after cloudy weather → after heavy rain → during high biomass

RMS and pH

Rapid pH changes can be another warning sign.

Check:

Morning pH

Afternoon pH and calculate:

Daily fluctuation = Afternoon pH − Morning pH

A changing pH pattern should trigger investigation of:

Plankton

Alkalinity

DO

Weather

Organic loading

RMS and Ammonia

When mortality increases, don't forget ammonia.

Check:

Total ammonia + unionised ammonia + pH + temperature

Because pH and temperature influence the proportion of ammonia present in the more toxic unionised form, an ammonia reading should not be interpreted independently.

What Not to Do During an RMS Event

Don't assume every mortality is RMS

First investigate the cause.

Don't assume white muscle proves RMS

Several conditions can produce similar signs.

Don't immediately add multiple products

Identify the underlying problem first.

Don't continue feeding blindly

Reassess biomass and feed consumption.

Don't ignore early-morning DO

Low oxygen can rapidly worsen pond stress.

Don't move equipment between ponds without proper sanitation

This can increase disease-spread risk.

Don't dispose of dead shrimp beside the pond

Manage mortalities according to appropriate farm biosecurity procedures.

Don't claim a disease is confirmed without diagnostic evidence

Recent ICAR field investigations explicitly distinguish preliminary observations from confirmed diagnosis.

Complete RMS Monitoring Checklist

Daily Mortality

☐ Count dead shrimp

☐ Record mortality by pond

☐ Compare with previous days

☐ Check whether mortality is increasing

Shrimp Observation

☐ Check activity

☐ Check feeding response

☐ Check swimming behaviour

☐ Check body colour

☐ Look for white muscle

☐ Look for white feces

☐ Check growth variation

Water Quality

☐ DO

☐ pH

☐ Temperature

☐ Salinity

☐ Alkalinity

☐ Ammonia

☐ Nitrite

☐ Transparency

☐ Water colour

Pond Bottom

☐ Check black soil

☐ Check bad smell

☐ Check sludge

☐ Check uneaten feed

☐ Check dead organisms

☐ Check circulation

Feed

☐ Record daily feed

☐ Check feed trays

☐ Recalculate biomass

☐ Review FCR

☐ Adjust feeding when required

Biosecurity

☐ Restrict unnecessary visitors

☐ Disinfect equipment

☐ Avoid sharing equipment between ponds

☐ Control crabs and birds

☐ Manage dead shrimp properly

☐ Check incoming-water protection

RMS Investigation Checklist

When mortality continues for several days:

☐ Start mortality log

☐ Measure early-morning DO

☐ Measure morning and afternoon pH

☐ Check temperature

☐ Check salinity

☐ Check alkalinity

☐ Check ammonia

☐ Check nitrite

☐ Inspect pond bottom

☐ Review feeding records

☐ Review biomass estimate

☐ Collect representative shrimp samples

☐ Contact a competent diagnostic laboratory

☐ Test for relevant pathogens

☐ Review previous crop history

☐ Review seed source

☐ Review weather and rainfall

☐ Review biosecurity

☐ Monitor daily response after intervention

RMS Emergency Checklist

When mortality suddenly increases:

First

Check DO immediately.

Second

Check shrimp behaviour and collect samples.

Third

Review feed and biomass.

Fourth

Check pH, ammonia, nitrite, temperature and salinity.

Fifth

Inspect the pond bottom.

Sixth

Strengthen biosecurity.

Seventh

Send appropriate samples for diagnosis.

Eighth

Continue monitoring until the trend stabilizes.

How Aqua Culture Guide Can Make This Article More Useful

This article can be connected to several practical tools:

RMS Mortality Calculator

Farmer enters:

Pond area → estimated mortality/day → number of days → estimated cumulative mortality

FCR Calculator

Feed used ÷ shrimp biomass produced

Water Quality Checker

Inputs:

DO + pH + ammonia + nitrite + temperature + salinity + alkalinity

Shrimp Health Checklist

A farmer can select observed signs such as:

White muscle / white feces / poor feeding / surface swimming / mortality

and receive a recommendation to seek appropriate diagnostic testing, rather than receiving an unsupported disease diagnosis.

RMS and Other Shrimp Diseases: Don't Confuse Them

Condition / Term What It Means

RMS A term used for a pattern of continuing mortality; underlying cause needs investigation

EHP A specific microsporidian infection requiring diagnosis

WSSV A specific viral disease requiring diagnosis

WFS A syndrome/clinical condition involving white feces; cause may require investigation

WMS White-muscle syndrome with characteristic muscle lesions

VCMD Viral covert mortality disease associated with CMNV

Conclusion

Running Mortality Syndrome is one of the more difficult shrimp-farm problems because continuous mortality can have multiple possible explanations.

The biggest mistake is to see daily mortality and immediately assume:

"This is RMS."

A better approach is:

Observe → Record → Test → Diagnose → Correct → Monitor

Start with the basics:

Good seed + strong biosecurity + stable water quality + proper feed management + good pond-bottom condition + regular shrimp health checks

When mortality continues, investigate:

DO → pH → alkalinity → ammonia → nitrite → temperature → salinity → feed → biomass → pond bottom → disease testing