20 Jun 2019
Ammonia Stress in Pond-Based Aquaculture
Written by Zach Stein

Ammonia is a natural by-product of any fed aquaculture and if unmeasured and not taken care of can lead to significant long term stress on shrimp and fish in ponds.

In this post, we’ll explore the mechanics of how ammonia builds up in a pond, when to be most aware of potential ammonia danger, and what the risks are of long term exposure to high-ammonia situations.

How Ammonia Builds Up in a Pond

Ammonia is part of the nitrogen cycle in a pond. Ionized ammonia occurs when the NH3 gas molecule bonds with hydrogen, creating NH4. While NH4 is not toxic to aquatic life, NH3 is a toxin and can lead to significant stress or even mortalities in extreme cases.

Ammonia is excreted by aquatic animals as a part of their waste. In a well circulated pond that does not have a build up of sludge on the bottom, excess ammonia is usually offgased through the surface into the atmosphere around.

But if stocking densities and feeding rates are high with low aeration and the right weather the pond can produce more ammonia than the surface can off-gas or can be handled by microorganisms in the nitrogen cycle.

Times of Greatest Danger for Ammonia Stress

While NH4 (ammonium) is not toxic to aquatic animals, NH3 (ammonia) is and can lead to a multitude of undesirable outcomes.

The level of NH3 in a pond or system is related to three variables, the temperature of the water, the pH, and the levels of NH4 present.

The higher the temperature and the higher the pH, the greater risk there is for ammonia toxicity. For pond-based systems like most shrimp farms, these can mean towards the end of a harvest cycle when the animals are being fed the most, meaning the most nitrogen is being added into the system, it is the most critical to closely monitor ammonia levels to ensure the animals remain in a stress-free environment. This can be compacted by low-water exchange during these times if there has been heavy rains as animals can be more resistant to ammonia toxicity if the salinity is higher.

Risks of Increased Ammonia in a Pond

While warm water species like shrimp can be quite tolerant to high levels of ammonia in terms of mortality, even low levels can lead to animal stress which can have a string of undesirable results:

  • Poor food conversion
  • Slow growth
  • Greater risk of disease

While many farmers today are not testing ammonia daily, we believe this will need to change as farmers push for higher input efficiencies at higher densities. Farmers should not feed in even medium ammonia scenarios as not only will the animals not be able to process those calories into growth, but it will lead to higher ammonia levels in the pond, causing the animals to consume more oxygen and stressing them even further.

Conclusion

Ammonia (NH3) levels of even 0.05 mg/L can lead to animal stress. Especially as you get to the end of a cycle or if there is rain in the forecast and your pH has been climbing into the high 8’s, monitoring your ammonia. Should you detect high levels, stop feeding (don’t waste money), exchange water, aerate (if you can), and retest later. Your animals will be less stressed and will be that much better able set to eat (and grow!) when water conditions return back to optimal levels.