The March/April 2016 issue of AQUA CULTURE Asia Pacific, edited by Zuridah Merican (firstname.lastname@example.org), contains an important article titled H2S–The Silent Killer, authored by Soraphat Panakorn (email@example.com), a shrimp farming specialist at Novozymes Biologicals in Thailand.
From Wikipedia: Hydrogen sulfide is the chemical compound with the formula H2S. It is a toxic, colorless gas with the characteristic foul odor of rotten eggs. It is heavier than air, very poisonous, corrosive, flammable and explosive. Hydrogen sulfide often results from the prokaryotic breakdown of organic matter in the absence of oxygen in swamps and sewers, a process commonly known as anaerobic digestion. Dissolved in water, hydrogen sulfide is known as hydrosulfuric acid or sulfhydric acid, a weak acid.
Soraphat Panakorn begins his article with the following sentence: “Losses from hydrogen sulfide toxicity in shrimp farms override mortality from other causes and it is time farmers realized this.”
H2S is a silent killer. It increases production costs by reducing average daily growth and survival rates and increasing feed conversion ratios and susceptibility to diseases. Often, farmers encounter H2S problems but don’t know how to manage them.
H2S is always present in the pond, possibly killing shrimp slowly every night. Farmers may have experienced losses of about 10% per crop and generally have accepted this as the norm when they do not know the real cause of the mortality or how to handle it. As of now, an estimated 4 million metric tons of shrimp could have been lost to H2S toxicity.
What is Hydrogen Sulfide? H2S is generated when sulfate consuming bacteria digest organic matter under anaerobic conditions (no oxygen) in water or under wet conditions. In a shrimp pond, the bottom layer of mud, sludge and bioflocs produce H2S.
How Does H2S Harm Shrimp? The first action of H2S is to block shrimp from taking up oxygen. When H2S levels are low, this weakens shrimp, makes them sluggish and increases their vulnerability, even when exposed for a short period. When H2S levels are high, mass mortalities occur, even when exposed for a short period. H2S can also cause tissue corrosiveness by irritating soft tissues in the gills, gut, stomach walls and hepatopancreas. H2S also stresses shrimp, lowering their resistance to infection.
A safe level for H2S in giant tiger shrimp (Penaeus monodon) ponds is 0.033 parts per million (ppm). White shrimp (P. vannamei) postlarvae can tolerate up to 0.0087 and juveniles up to 0.0185 ppm.
Detection Methodology: The detection of H2S is quite complicated and more difficult than detecting ammonia or nitrite. Most shrimp farmers don’t test for H2S despite the fact that the safe level of H2S is much lower than ammonia and nitrite. At the same concentration, H2S is 100 times more dangerous than ammonia and 1,000 times more dangerous than nitrite!
H2S Toxicity: Toxicity of H2S is dependent on three key parameters: pH, temperature and dissolved oxygen. H2S interferes with the shrimp’s oxygen transfer processes. Dissolved oxygen levels above 3 ppm help to block H2S production. A combination of low pH, oxygen and temperature makes H2S more dangerous. Therefore, monitoring these three parameters is key to mitigating H2S toxicity.
How to Check for H2S: Check bacterial count by TCBS agar plate in water samples collected 2-5 cm from a sludge area. Normal Vibrio will show up as green or yellow colonies; sulfate reducing bacteria will show as a black colony. If you see a black colony, it means H2S is being generated. Immediate action should be taken.
What Causes H2S?
- In ponds with clear water before stocking, algae grows on the bottom of the ponds.
Then, when phytoplankton blooms in the pond water, it blocks the light for the
algae on the bottom, causing it to crash, which, in turn, leads to the generation of H2S.
- Ponds with sandy or loose soils.
- Very deep ponds with insufficient oxygen create anaerobic conditions that lead to
the production of H2S.
- Ponds containing high concentrations of suspended organic matter. When the organic
matter settles to the pond bottom, it creates conditions that favor H2S production.
- Ponds with leaky, high-density polyethylene liners. When organic matter seeps under
these liners, into an area that is devoid of oxygen, H2S is generated.
- Ponds with high levels of feed wastes that experience a plankton crash.
- Acid sulfate ponds with low pH, and high loads of organic matter support the
release of H2S.
How to Prevent H2S
- At 3:00 a.m., always make sure the dissolved oxygen levels in your shrimp ponds are
above 3 ppm three meters from the edge of any sludge and 30 centimeters above
the pond bottom.
- Avoid farming in loose or sandy soil or in acid sulfate areas.
- Keep pH between 7.8 to 8.3 during the entire crop. The daily pH
range must be less than 0.4.
What to Do During Heavy Rain
During heavy rains, water parameters change that encourage H2S production. Rains cause low temperatures, dissolved oxygen and pH, as well as lower alkalinity and mineral levels. Sounds and waves created by wind also stress shrimp and cause them to crowd together on pond bottoms and in sludge areas, places where H2S is generated. When this happens, farmers should do the following:
- Stop feeding during rainy conditions.
- Check your pH and apply lime if necessary to maintain optimal pH levels.
- Keep aerators running all the time.
- Remove fresh water from the pond, if possible.
- Don’t allow floodwaters to enter ponds.
- Following the rains, have mineral and salt solubles ready to mix into your shrimp feed.
- Apply H2S consuming bacteria.
What to Do During a Plankton Crash
Once there is a plankton crash, pH will immediately drop. Organic matter concentrations will increase suddenly, resulting in a sudden uptake of oxygen. Toxic gases will be released and bacteria will bloom. The farmer must take the following steps:
- Cut feed amounts by 50-60%.
- Apply fine lime to maintain pH and to flocculate the dead plankton.
- Run aerators to force organic matter into the center of your pond.
- Exchange water by siphoning out center-of-the-pond sludge.
- Apply H2S consuming bacteria.
What to Do When H2S Is Detected
- Immediately cut feed amounts by 30-40% for at least 3 days until conditions
return to normal.
- Increase aeration immediately (but be aware of sludge disturbance during
installation of a new aerator).
- Exchange water to make sure it remains clear and apply probiotics.
- Apply lime immediately to increase pH to over 7.8.
- Apply microorganisms that can consume H2S, Paracoccus pantothrophus, for example.
Source : https://www.shrimpnews.com/FreeReportsFolder/SpecialReports/HydrogenSulfideTheSilentKiller.html