Combined Effects of Ammonia & Nitrite Toxicity on Growth, Survivals of Vannamei Shrimps in Low Saline Culture Ponds

Shrimp farming is one of the most important aquaculture practices worldwide. In shrimp-farming operations, one of the primary wastes of concern is nitrogen. The built-up of nitrogenous waste in the form of ammonia, nitrite and nitrate from the shrimp continuously degrade the culture environment. Ammonia is excreted by animals and also arises from decomposing organic solids such as uneaten feed. The excess feed and fecal matter deposited in the bottom of the pond undergo ammonification and result in excess of ammonia formation in pond water and sediment. Generally originated high ammonia concentration from cultured ponds is the most common toxicant to the aquatic animals.

Ammonia is the main end product of protein catabolism in crustaceans and can account for 60–70% of nitrogen excretion with only small amounts of amino acids, urea and uric acid. In water, ammonia is present in both ionized (NH₄+) and un-ionized (NH₃) state, with NH₃ as the toxic form due to its ability to diffuse across cell membranes. NH₄+ is also toxic, especially at low pH levels.

This proportion of ammonia: total ammonia nitrogen is also importance in aquaculture as ammonia is toxic while ammonium is not appreciably toxic. The combination of high pH and elevated total ammonia nitrogen concentration can lead to elevated ammonia concentrations potentially harmful to culture animals.

Total ammonia nitrogen (TAN) may be used by bacteria for conversion to nitrite and nitrate, via the nitrification pathway. Nitrification is an aerobic, autotrophic process that entails two steps. Ammonium is converted to nitrite by Nitrosomonas species in the first step, and nitrite is converted to nitrate by Nitrobacter species in the second.

Nitrite is formed from ammonia and may be accumulated in aquatic systems as a result of imbalances of nitrifying bacterial activity (Nitrosomonas sp. and Nitrobacter sp.). High levels of nitrite in water are potential factors triggering stress in aquatic organisms. Nitrite is formed as an intermediate product either during bacterial nitrification of ammonia or bacterial denitrification of nitrate. The nitrite is also produced in denitrification, where biologically nitrate is reduced to di-nitrogen gas (N2) or nitrous oxide (N2O). Denitrification occurs under anaerobic conditions when heterotrophic bacteria use nitrate instead of oxygen as terminal electron acceptor in respiration. Nitrite is an intermediate in the process and may accumulate in anaerobic soils and bottom muds. Nitrite may also accumulate in water after sudden increase in ammonia concentrations following phytoplankton die-off. The decomposition of the dead plant material releases large amounts of ammonia into water. The increased availability of ammonia stimulates the activity of ammonia-oxidizing bacteria and nitrite is produced.

Factors are affecting the nitrification process include salinity, pH, temperature, concentration of dissolved oxygen, number of nitrifying bacteria, and the presence of inhibiting compounds. An increase in ammonia concentration is followed by a decrease in ammonia that is indirectly proportional to a rise in nitrite, as NH₃ is oxidized to NO₂. Plankton crashes, feed consumption drop & over feeding and organic material accumulates on pond bottoms.

The cause of toxicity of ammonia is mainly based on the irritative properties of the compound. Ammonia in the form of un-ionized ammonia (NH₃) is very toxic to aquatic animals and cause impairment of cerebral energy metabolism and damage to gill, liver, kidney, spleen, and thyroid tissue in fish, crustaceans and molluscs. The concentration of un-ionized ammonia is more toxic when dissolved oxygen concentrations are low.

The effects of chronic nitrite 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. 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.

The easiest way to determine the toxic effects of nitrite on shrimp is to look at shrimp production numbers such as survival and growth. Shrimp exposed to high concentrations of nitrite 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.

Nitrite enters the blood stream and inhibits the binding of oxygen to the iron molecule of hemocyanin. The nitrite toxicity mechanism acts on the process of oxygen transport. Hemocyanin, rather than hemoglobin, is the blood oxygen-transport protein in crustaceans and reactions of nitrite. Also the nitrite binds to hemocyanin, converting it into meta-hemocyanin, which is unable to transfer oxygen to the tissues. Nitrite has also been found to oxidize the respiratory pigment

Elevated environmental nitrite has been reported to induce methaemocyanin formation, cause hypoxia in tissue and impair the respiratory metabolism of marine shrimps. The toxicity of nitrite in pond water may deteriorate water quality, reduce growth, increase oxygen consumption and ammonia excretion, and even cause high mortality of shrimp.

The toxicity of ammonia is also influenced by salinity. The susceptibility of ammonia-N increases gradually during decreasing the water salinity. The interaction of animals closely with sediment, also affect ammonia toxicity. An increase in oxygen consumption and ammonia excretion in vannamei shrimps exposed to increasing concentrations of nitrite. The toxicity of nitrite decreased with increasing chloride ions (salinity) in fresh water but surprisingly nitrite appears to be toxic to marine shrimp.

At the most basic level, a system should be designed with the knowledge that:

In lower salinity environments several problems arise such as the decrease in the tolerance of shrimp species to nitrogen compounds which represents a great problem in low salinity inland culture.

The acute toxicity of nitrite increased with time of exposure. There is an inverse relationship between salinity and nitrite toxicity such that the toxicity increases with the reduction in salinity. The acute effects of nitrite to L. Vannamei in low salinity waters.

The increasing the salt concentration of a solution decreases the solubility of dissolved gases. As the water salinity increases, the solubility of dissolved oxygen decreases and the percentage of total ammonia present as toxic un-ionized ammonia decreases. Salinity has also close relationship with the toxicity of nitrogenous compounds such as ammonia nitrite and nitrate.

Freshwater fish and crustaceans have body fluids more concentrated in ions than the surrounding water. Saltwater species have body fluids more dilute than surrounding water. When the salinity of water is changed by more than 10% in a few minutes or hours, fish and crustaceans may unable to compensate and will be severely stressed or may die from osmoregulatory failure. Fish and crustaceans can acclimate to much lower or higher salinities within their range of tolerance if the change is made gradually. In general, the osmoregulatory capabilities of shrimp can be correlated with their salinity distributions.

Since this is a nitrogen-related problem, the most obvious preventive measures to be undertaken to reduce or minimize the amount of nitrogen compounds in aquaculture ponds.

Since ammonia and nitrite are extremely toxic to shrimp compared to nitrate. The control of ammonia and nitrite is the second most important factor impacting survival and growth of cultured organisms, followed by dissolved oxygen. Therefore, the accumulation of ammonia and nitrite may have detrimental effects on shrimp culture. The toxicity of ammonia and nitrite is heavily dependent on environmental factors and these factors play an important role in the development, growth, and survival of species exposed to ammonia and nitrite. In order to maximize growth and survival of aquatic species through the minimization of ammonia and nitrite toxic effects, more research is needed to evaluate the combined effects of factors that affect their toxicity.