High Nitrate Levels Toxic To Shrimp
Studies by the authors determined that under culture with water at one-third of ocean salinity, shrimp survival and growth were significantly impacted at levels of nitrate above 220 mg/L. The nitrate may have reduced feeding efficiency, depressed metabolism and impaired endocrine function.
Nitrate toxicity is more of an issue for shrimp raised in lower-salinity waters. Shrimp exposed to high concentrations of nitrate exhibit shorter antennae, gill abnormalities and hepatopancreas lesions.
In shrimp-farming operations, one of the primary wastes of concern is nitrogen, which appears as ammonia, nitrite and nitrate. Ammonia is excreted by animals and also arises from decomposing organic solids such as uneaten feed.
David D. Kuhn, Ph.D., Department of Food Science and Technology, Virginia Tech, Blacksburg, Virginia 24061 USA
Stephen A. Smith, DVM, Ph.D., Department of Biomedical Sciences and Pathology, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech, Blacksburg, Virginia, USA
George J. Flick, Jr., Ph.D., Department of Food Science and Technology, Virginia Tech
This ammonia can be removed from the water directly via assimilation by heterotrophic bacteria, algae and plants. Ammonia can also be removed from ponds and aquaculture systems via nitrification. This is a two-step process performed by autotrophic bacteria called nitrifiers. Ammonia is converted to nitrite, and nitrite is then converted to nitrate.
Compared to nitrate, both ammonia and nitrite are extremely toxic to shrimp. Ammonia and nitrite levels should remain at negligible levels in mature ponds and biofilter systems as presented in Figure 1. Nitrate, however, will continue to accumulate, and high concentrations of nitrate are also toxic to shrimp.
Standard options for reducing nitrate levels (water exchanges or denitrifying biological treatment technology) are costly and use up valuable resources.
The effects of chronic nitrate toxicity include dark cuticular lesions on the carapace and anterior segments of the tail, an indicator of stress in shrimp. The upper shrimp is missing an antennae, and one anterior appendage is abnormally shortened.
Pacific white shrimp, Litopenaeus vannamei, were exposed to varying levels of nitrate in small-scale 150-L systems over six weeks at approximately one-third of ocean salinity (11 g/L salinity). Table 1 shows a summary of the nitrate concentration impacts on shrimp production.
Table 1. Long-term effects of elevated concentrations of nitrate on shrimp production.
| Average Nitrate|
|Final Survival|| Average Final|
Weight of Shrimp
|35 mg/L||87%||9.3 g|
|220 mg/L||87%||8.7 g|
|435 mg/L||64%||7.5 g|
|910 mg/L||15%||5.0 g|
No statistical differences were observed for survival or growth of shrimp exposed to 35-220 mg/L nitrate. However, above 220 mg of nitrate, shrimp survival and growth were significantly impacted. Shrimp exposed to the maximum nitrate load of 910 mg/L did extremely poorly. Several possibilities may explain the suppression of growth and increased mortalities observed, including reduced feeding efficiencies, metabolic depression and impaired endocrine function.
Salinity And Nitrate
Additional nitrate exposure experiments were conducted over a wide range of salinities from brackish water to half-strength seawater (2-18 g/L salinity). Shrimp production was significantly compromised in the lowest salinity group.
The easiest way to determine the toxic effects of nitrate on shrimp is to look at shrimp production numbers such as survival and growth. Aquaculturists interested in preserving the health of their stocks can also evaluate other physiological attributes, such as antennae, gills and the hepatopancreas.
Shrimp exposed to high concentrations of nitrate over a long period of time exhibited shorter antennae length, gill abnormalities and lesions in the hepatopancreas. Short antennae and gill abnormalities are often considered early clinical signs of decreasing shrimp health.
The hepatopancreas organs in shrimp produce digestive enzymes and are responsible for promoting the normal absorption of digested food. One type of lesion observed in shrimp exposed to elevated levels of nitrate is shown in Figure 2. These cells were dilated and devoid of epithelial membranes, probably a consequence of not eating well or not metabolizing feed normally.
In addition to affecting the health of the animals, physical and biological changes can result in the shrimp having a reduced market acceptance, which results in reduced profitability for a producer.
It is important to consider pushing the limits of nitrates when considering water and resource conservation. However, it is also important to not compromise shrimp health or production.
This trial suggested caution should be used when considering raising shrimp in waters that contain nitrate levels exceeding 220 mg/L. In addition, marine shrimp raised in very low salinities exhibit a reduced tolerance to nitrate. It is hoped this information will provide aquaculturists information to help them make managerial decisions regarding the balance between resource conservation and nitrate control.
Source: The Global Aquaculture Advocate