Healthy water quality is the first concern you will have in the rearing of aquatic species — it is up to you to ensure that your fish or shellfish or plants have a healthy environment.
Fish produce wastes that can be toxic, in particular, ammonia and nitrite; unless they are converted to non-toxic nitrate, you can get high bacterial counts, oxygen depletion, disease, off flavors and mortality.
Monitoring for important chemical properties like ammonia and nitrites and for water conditions such as temperature and pH is essential — it provides the early warning signals you will need in order to take actions to protect your aquatic crop.
Introduction:
Nitrite (NO2) is an intermediate compound in an oxidation sequence called nitrification and occurs naturally as part of the nitrogen cycle, the movement of nitrogen through an ecosystem.
First, ammonia from fish excretion or other sources is oxidized to nitrite by the bacterial genus Nitrosomonas. Nitrification is completed by Nitrobacter species that convert nitrite to nitrate (NO3-).
The rate of nitrification is influenced by water temperature, pH, oxygen and other environmental factors that affect growth and metabolism of the nitrifying bacteria; it is also influenced by the abundance of nitrifiers, which are themselves influenced by the availability of attachment surfaces, or substrate.
When the nitrification activity of natural bacterial populations or in biological filters is inhibited, you may see increases in nitrite.
Toxicity of Nitrite:
Once in the blood, nitrite readily oxidizes hemoglobin to methemoglobin, which cannot transport oxygen. This condition, known as brown blood disease, causes suffocation of fish, even with adequate levels of oxygen.
In freshwater fish, nitrite levels as low as 0.20 parts per million (ppm) can cause deaths and in sensitive fish like rainbow trout, a level of 0.10 ppm nitrite is a concern. More resistant species such as the channel catfish may survive up to 30 ppm nitrite.
In marine systems, nitrite is less likely to be toxic. Studies suggest calcium in sea water protects against toxicity, while chloride prevents the uptake of nitrite, thus enabling fish in salt water environments to withstand levels of nitrite that would be toxic in freshwater.
Treatments In ponds, nitrite toxicity is routinely prevented by adding sodium or calcium chloride at five times the nitrite concentration. To increase the nitrite concentration by 1 ppm per acre-foot of water, use 4.3 pounds of calcium chloride or 4.5 pounds of sodium chloride.
Other methods are available to keep nitrite levels under control, for example, feed rates may be reduced to lower the ammonia available for conversion to nitrite. Increased aeration may accelerate nitrification rates and prevent nitrite from accumulating. In many recirculation systems or smaller pond systems, replacement of part or all of the water with nitrite-free water is a practical solution.
Treating a Pond for Nitrite: An Example Assume that a one-acre pond with an average depth of six feet contains 8 ppm nitrite. The chloride required for treatment is 5 (parts chloride) x 8 ppm nitrite, which equals 40 ppm chloride.
To determine how much salt to add to the pond, multiply 4.5 pounds sodium chloride by the acre feet of water; then multiply by the ppm of chloride required. In the case of the one-acre pond, 4.5 pounds x 6 acre feet x 40 ppm chloride equals 1,080 pounds sodium chloride.
Biological filtration units can be inoculated with commercially available nitrifying bacteria to improve or reestablish activity.
Testing:
Because nitrite is toxic to many fish species at relatively low concentrations, you should test frequently. In recirculation systems that support high densities of fish, you may need to test daily. In open pond or cage culture systems, test nitrite several times a week.
Nitrite tests available from test kit manufacturers generally involve a simple one or two-step procedure. Under acidic conditions, nitrite reacts with sulfanilamide to produce sulfanilic acid.
Reaction with N-(2 Naphthyl)-ethylene diamine dihydrochloride (NED dihydrochloride) produces red-purple azo dye that can be measured using visual colorimetric devices or electronic colorimeters. This procedure is very sensitive and permits visual detection of nitrite levels below 0.1 ppm in fresh and salt water.
Author:
Daniel E. Terlizzi
