Tilapia Topic, Aquaculture Environment, Ammonia

Ammonia is excreted by tilapia as a byproduct of protein catabolism. Although tilapia are relatively hardy they are susceptible to ammonia poisoning. If culture conditions are intensive or there is little water exchange or biofiltration available for removal of toxic levels of ammonia.

In the presence of oxygen and the appropriate bacteria ammonia undergoes nitrification and is transformed into nitrite and nitrate. However, if anaerobic conditions prevail, this process slows or can be reversed with bacterial metabolism contributing to increased ammonia loading in the water.

In water, ammonia is dissociated into ionized and un-ionized forms as depicted in the following:

The movement of the dissociation reaction is strongly dependent on pH, somewhat influenced by temperature and slightly affected by the salinity of the water. As pH is increased (becomes more basic) ammonia equilibrium shifts to favor formation of NH3. This relationship is illustrated in the graph below.

Because the ionized form (NH4+) is charged, it tends to not pass through lipid membranes and, for this reason, is much less toxic than un-ionized (NH3) ammonia.

Commercial water analysis test kits are adequate for monitoring total ammonia level in tilapia culture water. If tilapia are raised in brackish or salt water a test kit based on a salicylate method should be used for ammonia determination. Test kits based on Nessler reaction are suitable for ammonia determination in fresh but not brackish or saltwater because of interference from calcium and magnesium. Specific ammonia electrodes (probes) yield comparable results as test kits.

In intensive tilapia culture tanks or ponds bi-weekly to weekly sampling and measurement of ammonia should be sufficient. Water samples should be collected from near the bottom, the sample filtered and total ammonia measured immediately or the specimens placed on ice if there is a delay before analysis. Temperature, pH and salinity should also be determined and the time water specimen was collected recorded.

The un-ionized (NH3) ammonia level can be estimated from the total ammonia value if pH and temperature of the sample are known. Conversion tables are available in some references. HAMES preforms the calculation if total ammonia, temperature and pH measurement data are entered. If NH3 value is ó 0.1 mg/L there is probably no direct effect on tilapia health. Prolonged exposure to un-ionized ammonia level >0.1mg/L may cause growth reduction and, possibly increased susceptibility of tilapia to bacterial infection. Exposure to NH3 > 0.5 mg/L may result in mortality of tilapia.

Ammonia may interact with other water variables in complex ways that affect health of cultured tilapia.

From: http://ag.ansc.purdue.edu/aquanic/publicat/usda_rac/efs/srac/282fs.pdf "ammonia production rate is estimated to be 10 grams/100 pounds of fish/day." "The ammonia removal rate [of a biofilter] is estimated to be 0.05 grams/ft2/day"

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