By: Jaime Ulises Paz Lopez
Vannamei shrimp (Litopenaeus vannamei) aquaculture is a vital global economic activity, heavily reliant on optimal water quality for the shrimp’s survival, growth, and productivity. Poor water quality can lead to stress, increased disease susceptibility, higher mortality rates, and reduced profits for farmers. Key water quality indicators include nitrogen compounds such as ammonium, nitrite, and nitrate, which can be toxic at elevated levels and are by-products of shrimp metabolism and organic matter decomposition. While Vannamei shrimp can tolerate a wide range of salinities, the salinity of the water significantly affects their physiological processes and the toxicity of nitrogen compounds. This article aims to review safe concentration levels for total ammonium, nitrite, and nitrate in Vannamei shrimp aquaculture across various salinity levels (1 ppt to 30 ppt), providing essential guidance for shrimp farmers to maintain water quality and optimize shrimp health and productivity.
1. Introduction
Vannamei shrimp (Litopenaeus vannamei) aquaculture is an economic activity of great importance throughout the world. The success of your shrimp farm depends largely on maintaining optimum water quality, which is essential for the survival, growth, health and overall productivity of the shrimp. Poor water quality can stress shrimp, increase their susceptibility to various diseases, lead to higher mortality rates and ultimately reduce farmers’ profits. Therefore, understanding and effectively managing water quality parameters is essential for sustainable and profitable aquaculture production.
Among the various water quality parameters, nitrogen compounds such as ammonium, nitrite and nitrate are key indicators of the health of the aquatic ecosystem and can be toxic to cultured organisms if their levels are high. These compounds are by-products of shrimp metabolism and the decomposition of organic matter present in the culture tanks. The accumulation of these nitrogen compounds requires particular attention, as they can adversely affect the physiology and well-being of the shrimp.
Vannamei shrimps are known for their euryhaline nature, which means they can tolerate a wide range of salinities, from 0.5 to 40 parts per thousand (ppt). However, although this adaptability to different levels of salinity is an advantage for cultivation in different geographical regions, the salinity of the water has a considerable influence on its physiological processes, particularly osmoregulation. Furthermore, salinity can also modulate the toxicity of various compounds present in the water, notably ammonium and nitrite. Consequently, the safe concentrations of total ammonium, nitrite and nitrate for Vannamei shrimp can vary depending on the salinity of the water in which they are reared.
The aim of this article is to provide a comprehensive review of safe concentration levels for total ammonium, nitrite and nitrate in Vannamei shrimp aquaculture at different specific salinity levels: 1 ppt, 5 ppt, 10 ppt, 15 ppt, 20 ppt, 25 ppt and 30 ppt. This information is crucial for shrimp farmers to maintain adequate water quality and optimise the health and productivity of their crops under various salinity conditions.
2. Nitrogen dynamics in shrimp farming ponds
The nitrogen cycle in aquaculture systems is a complex process that directly influences water quality. Shrimp excrete ammonia (NH₃) as their main metabolic waste product. In the culture tank, nitrifying bacteria transform this ammonia, in a process called nitrification, first into nitrite (NO₂-) and then into nitrate (NO₃-). Understanding this sequential transformation is essential for anticipating and managing the levels of each of these nitrogen compounds.
Total ammonia in water (TAN) exists in two forms in equilibrium: un-ionised ammonia (NH₃) and the ammonium ion (NH₄⁺). Of these two forms, un-ionised ammonia (NH₃) is significantly more toxic to aquatic organisms than ammonium ion (NH₄⁺). The relative proportion of NH₃ and NH₄⁺ in water is determined mainly by pH and temperature. Higher pH and temperature shift the equilibrium towards the more toxic form, NH₃. Therefore, simply monitoring TAN may not be sufficient to assess the real risk to shrimp; It is essential to consider the factors that influence the proportion of un-ionised ammonia.
Nitrite (NO₂-) is an intermediate compound in the nitrification process and is also toxic to shrimp. Nitrite can interfere with oxygen transport in shrimp haemolymph, leading to a condition known as ‘brown blood disease’ and potentially death. Although it is a transition product in the conversion of ammonia to nitrate, nitrite accumulation poses a direct threat to shrimp health and highlights the importance of a balanced and efficient nitrification process.
Nitrate (NO₃-) is the end product of the nitrification process and is generally considered less toxic than ammonia and nitrite. However, at high concentrations, particularly in low salinity conditions, nitrate can still cause stress in shrimp, impair growth and even lead to mortality. Although its acute toxicity is lower than that of other forms of nitrogen, its accumulation in limited water exchange systems requires appropriate management to avoid chronic effects on shrimp health.
3. Safe levels of total ammonia and influence of salinity
In general, safe levels of total ammonium (TAN) in Vannamei shrimp aquaculture are considered to be kept below 1 part per million (ppm) or milligram per litre (mg/L). Some sources suggest a maximum limit of 2 ppm. However, the critical factor is the concentration of un-ionised ammonia (NH₃-N), which must be kept below 0.1 ppm, and even lower levels, such as no more than 0.01 ppm, are recommended by some sources. Exposure to a concentration of 0.45 mg NH₃-N per litre has been shown to reduce shrimp growth by 50%, highlighting the high toxicity of this form of ammonia.
Salinity plays an important role in ammonia toxicity in Vannamei shrimp. In general, a decrease in water salinity leads to an increase in ammonia toxicity. Studies have shown higher LC50 (median lethal concentration) values for ammonia-N at higher salinities (35 ppt) compared to lower salinities (15 ppt). This indicates that shrimp can tolerate higher ammonia concentrations in saltier water. The estimated ‘safe level’ for ammonia-N for juveniles was less than 15 ppt (2.44 mg/L) compared with 25 ppt (3.55 mg/L) and 35 ppt (3.95 mg/L) at pH 8.05 and 23°C. The corresponding safety levels for NH₃-N were 0.12, 0.16 and 0.16 mg/L respectively. Another study found that survival rates at 5 ppt and 10 ppt were significantly lower than those at 20 ppt when exposed to high concentrations of ammonia. The lethal concentration of ammonia at 72 hours was significantly higher at 30 ppt (32 mg/L) than at 5 ppt (18 mg/L). Furthermore, shrimp at a salinity of 3 ‰ were found to be the most sensitive to ambient ammonia-N, with a 96-hour LC50 of 9.33 mg/L. These results clearly demonstrate an inverse relationship between salinity and ammonia toxicity, implying that stricter control of ammonia levels is required in low salinity environments.
Based on the information gathered, the following recommended ranges for total ammonia at specified salinity levels can be proposed:
- 1ppt: Due to the increased toxicity at very low salinity, it is recommended to keep the TAN as close as possible to 0 ppm, ideally below 0.5 ppm. NH₃-N should be < 0.01 ppm.
- 5ppt: A TAN below 0.75 ppm is recommended, with NH₃-N < 0.015 ppm.
- 10ppt: The target should be a TAN of less than 1 ppm, with an NH₃-N < 0.02 ppm.
- 15ppt : It is recommended to keep the TAN below 1.5 ppm, with an NH₃-N < 0.03 ppm.
- 20ppt: The target should be a TAN below 1.75 ppm, with an NH₃-N < 0.035 ppm.
- 25ppt: A TAN of less than 2 ppm is recommended, with an NH₃-N < 0.04 ppm.
- 30ppt: The aim should be to keep the TAN below 2 ppm, with an NH₃-N < 0.04 ppm.
It is important to note that these are general recommendations and must be adjusted according to other water quality parameters, such as pH, temperature and dissolved oxygen, as well as the specific life stage of the shrimp.
