Are you aware of what the silent killer in high-density aquaculture is?
It’s not necessarily the disease pathogens or low oxygen levels, although these also play a part.
It is ammonia buildup from uneaten fish feed and fish waste that allows productive systems to become toxic death traps. MDPI research has shown that the intensification of shrimp farming and the increased level of feed input can often lead to high ambient ammonia levels that are a significant factor in the high fish mortality rate resulting from stress. In agreement, studies conducted by ScienceDirect have established that high ammonia levels in aquaculture systems are viewed as a very serious threat and cause of shrimp mortality during the culture of Pacific white shrimp. The 48-hour LC50 (Lethal Concentration of ammonia that will kill 50 pct of shrimp) in regards to Pacific white shrimp is 71.68 mg/L based upon results in the International Journal of Molecular Sciences. To others, it sounds like a time bomb. However, this is the reality of ammonia toxicity and the loss of profitably in fish farms around the world.
The Nitrogen Waste Crisis:
It’s hard to ignore the evidence showing how nitrogen from aquaculture farms severely affects the environment. Research published through PMC found that only 20%-30% of nitrogen used in incorporating aquatic species into farming systems was converted from the original nutrient source (in this case animal feed) into biomass (wet weight of the fish), with the rest of the nitrogen in the aquatic environment created through the conversion process causing water pollution containing ammonia and nitrite.
A study on recirculating aquaculture systems conducted through research on ResearchGate cited an increase in total ammonia nitrogen (24.20 mg/L) of 40% mortality in shrimp; however, this resulted from an 81.2% increase in feed input.
The effects are devastating. Research published through PMC found that increased ammonia concentrations in the water column reduced growth and increased damage to the hepatopancreas (the fish equivalent of the liver) and gills, increased oxygen consumption rates, decreased osmoregulatory ability, reduced hemolymph’s ability to deliver oxygen to the tissues, reduced frequency of molting, and caused increased mortality. Studies from ScienceDirect showed that ammonia concentrations above 5 mg/L created additional stress by increasing oxygen consumption rate, thus affecting the physiological processes of the animals (growth, ability to produce and release digestive enzymes) and decreasing overall survival of the aquatic animals.
Research published in MDPI demonstrated the effects of ammonium toxicity on white shrimp. White shrimp exposed to lethal ammonia concentrations for 48 hours resulted in increased mortality from lipid accumulation in the hepatopancreas, decreased gut microbial diversity, reduced diversity of beneficial bacteria, and increased proliferation of pathogenic Vibrio sp.
Is there an economic impact? Obviously, there are significant production losses associated with aquaculture because of unavoidable water quality issues according to studies conducted through ResearchGate.
The Immunosuppression Cascade
Now for what makes ammonia particularly nefarious. An analysis, as referenced from ScienceDirect using transcriptome, found that toxicity evoked by ammonia suppresses immunity defense and heightens the virulence of infection by pathogens, and that shrimp infected from WSSV and stressed from contact with ammonia have significantly increased mortalities than WSSV-infected shrimp in normal water after 120 to 144 hours.
Research through MDPI established that as a result of exposure to ammonia, the integrity of intestinal health was compromised in terms of mucosal integrity, antioxidant activity, immunity, energy metabolism, and the microbes present in the intestines. Research established some of the major causes of deaths involving shrimp as a result of exposure to ammonia in the water, including the occurrence of oxidative stress and the interference with cellular metabolism and mitochondrial functions.
As cited by a research published by PMC, high amounts of ammonia inhibit the immune system by reducing blood cell lymphocyte proliferation, making them much more susceptible to regular infections and diseases that could otherwise be managed.
Why This Matters for Every Pond Operation?
The reason for concern is due to the fast accumulation of waste loads as a result of intensive feeding of fish beyond what can be processed naturally by available biological processes in a pond. Research conducted by numerous academic institutions has indicated the harmful impacts of nitrogen compounds on aquatic organisms, through direct toxicity, immunosuppression and increased pathogen growth, resulting in significant production loss from aquaculture operations. We’ve focused on feeding more to grow faster but 70-80 percent of that nitrogen becomes toxic waste that kills through multiple pathways.
