Effect of High Nitrate Levels on Shrimps
High levels of nitrate are toxic to shrimp. This poisoning is more common in waters with less salinity. Shrimp farming produces nitrogen, in the form of ammonia, nitrite, and nitrate. In addition to animal excretion, organic solids such as uneaten feed decompose to produce ammonia.
Ammonia can be directly removed from water through absorption by heterotrophic bacteria, algae, and plants. Nitrification is another method of removing ammonia from ponds and aquaculture systems.
It is a two-step process carried out by autotrophic bacteria called nitrifiers. Ammonia is converted to nitrite and nitrite is converted to nitrate. Compared to nitrate, both ammonia, and nitrite are highly toxic to shrimp. Ammonia and nitrite levels must remain at negligible levels in mature ponds and biofilter systems, as the proposed water exchange or denitrification biological treatment technology is costly and consumes valuable resources.
Other operators feel comfortable with more than 500 mg/L in their fields. To eliminate this confusion, researchers investigated to determine what concentrations negatively affected shrimp production over a long period.
Nitrate Effects on shrimps
No statistical differences were observed in the survival or growth of shrimp exposed to 35-220 mg/L nitrate. However, at more than 220 mg of nitrate, shrimp survival and growth were significantly affected. Shrimps exposed to a maximum nitrate load of 910 mg/L performed very poorly. Several possibilities may explain the observed growth suppression and increased mortality, including reduced feeding efficiency, metabolic depression, and endocrine dysfunction.
Salinity and Nitrates
Further nitrate exposure experiments were performed over a wide range of salinities from brackish water to brackish seawater (salinity 2-18 g/L). Shrimp production in brackish waters was significantly compromised. This study found that nitrate toxicity is more of a problem for shrimps producers who farm shrimps in lower salinity waters near freshwater conditions. At lower salinities, shrimp must expend considerable energy to adjust their osmotic pressure levels to compensate for the reduced saline environment.
Health Effects
The easiest way to determine nitrate toxic effects on shrimp is to look at shrimp production numbers such as survival, growth, tentacles, gills, and hepatopancreas. Shrimp exposed to high nitrate concentrations for a long time show shorter tentacle length, gill abnormalities, and hepatopancreas lesions.
In addition to affecting animals’ health, physical and biological changes can lead to a decrease in shrimps acceptance in the market. This leads to a decrease in producer profitability.
Consequently, it is critical to consider nitrate limits when considering water and resource conservation.
