Overview:
In any aquaculture operation, whether it is a pond or recirculating system, water quality is your major concern — successful rearing of fish begins with a healthy aquatic environment.
Intensive culture, however, often taxes that environment: fish produce waste products, primarily ammonia that unless converted to non-toxic nitrate can set into motion processes that lead, in addition to direct toxicity, to high bacterial counts, oxygen depletion, fish disease, and mortality.
Management of an aquaculture system begins with monitoring for important chemical properties and water conditions — such chemical testing is the basis for remedial actions you may have to take.
The more you know about the chemistry of your system, the better decisions you may be able to make when faced with a problem. In particular, we will focus on the nitrogen cycle.
The nitrogen cycle is the movement of nitrogen, in various forms, between the atmosphere and terrestrial and aquatic environments in response to biological and physical activity.
Introduction:
Ammonia and nitrite are two of the most toxic compounds in the nitrogen cycle. This cycle begins with the direct excretion of ammonia by fish, a consequence of fish metabolizing protein and amino acids in their feed. Once excreted, ammonia can be absorbed by algae and aquatic vascular plants as their nitrogen source, or it can be oxidized by bacteria in a process called nitrification to the oxidation products nitrite and nitrate.
Because ammonia removal is dependent on natural microbial processes in most aquaculture facilities, changes in bacterial metabolism and population can produce increased ammonia levels and lead to fish stress or mortality.
Such increases can occur rapidly because of the high feeding rates necessary to support growth: high feeding rates lead to high waste excretion by fish. The protein level feed supplies may be as much as 40%; about 15% by weight of the protein supplied is ammonia. Feed not consumed by the fish or the protein and amino acids not used in growth and excreted can contribute to high ammonia concentrations.
Although fish excretion is the primary source of ammonia in aquaculture systems, other natural processes may contribute ammonia, for instance, the decline or grazing of algal (phytoplankton) blooms and DE nitrification processes in sediments.
Toxicity of Ammonia:
Just how toxic ammonia is depends on the form it takes: the un-ionized form of ammonia, NH3, is very toxic, while the charged or ionized form, NH4+, is generally non-toxic.
The form of ammonia depends on pH. As pH increases (becomes more basic), ammonia is converted to the more toxic or un-ionized form, NH3, while at low pH or acidic conditions, ammonia is mainly the ionized form, NH4+.
Ammonia toxicity, though not well understood, involves the movement of ammonia from the water into the gills of the fish.
How sensitive fish are to ammonia will vary with species, age, water quality, and acclimation to their environment. In general, ammonia levels above 0.5 parts per million at a pH above 8 may result in fish mortality.
Treatments for Ammonia:
Methods for decreasing the risk of ammonia toxicity include direct reduction of ammonia concentrations and reduction of pH.
Reducing ammonia concentrations can be achieved by diluting your system’s water with water from wells or from water storage. In recirculating systems, you can use biological filters which also remove ammonia by the action of nitrifying bacteria that sequentially oxidize ammonia to nitrite, then nitrate which is not considered toxic.
Hydroponic systems have been used to remove ammonia while in some crab shedding systems and marine aquaria, algal biological filters have been used for the same purpose. In freshwater systems, ion exchange media such as zeolite clays can be used.
The ion exchange process is the chemical “swapping” of a non-toxic charged chemical like sodium (Na+) or potassium (K+) for ammonium (NH4+). These chemical filters can be recharged by flushing them with sodium chloride solutions to displace the attached NH4+ by substituting Na+.
Maintaining a low toxic ammonia concentration by pH reduction can be accomplished in several ways. Because photosynthesis (the process by which chlorophyll-containing cells in green plants such as algae convert light to chemical energy and use inorganic nutrients to synthesize organic compounds and produce oxygen) increases pH, photosynthesis can be reduced by reducing algal populations.
Another technique is by liming or adding bicarbonate to ponds which will buffer pH at about 8 and reduce daily pH fluctuations, which can be stressful at high ammonia levels.
Testing:
Two methods are available for field determination of ammonia; both tests give a total ammonia reading but cannot distinguish between toxic and the ionized forms of ammonia. The Nesslerization method is widely used in fresh water ammonia testing.
Sample pretreatment with zinc sulfate is often required to prevent interference by calcium, iron, magnesium and sulfide. The Nesslerization procedure is not considered suitable for use in salt water without modification.
In saltwater testing the salicylate procedure is sensitive and rapid. Some testing kit manufacturers are adapting the salicylate procedure to freshwater testing because of the increased sensitivity and lack of interference.
Ammonia should be tested every day, particularly in high density aquaculture systems. Because of the dependence of ammonia toxicity on pH, you should also perform tests to verify pH is at a safe level.
Author:
Daniel E. Terlizzi
