Nitrite poisoning commonly called “brown blood” disease is an environmental condition that affects a number of fish species cultured for food today including channel catfish. Its cause is elevated levels of nitrite in the pond or culture system water.
The presence of nitrite at abnormally high levels, over the years, has resulted in loss of millions of pounds of catfish and a sub-lethal stress of millions more.
Nitrite, a product of the breakdown of ammonia by bacteria, is a compound that can enter the circulatory system of fish through the gills. Its presence at elevated levels can cause the fish blood to take on a dark brownish color, thus the name “brown blood.”
Chemically, nitrite can oxidize hemoglobin in the fish red blood cells and convert it into another compound called methemoglobin. Methemoglobin does not transport oxygen as is the function of normal hemoglobin and, as a result, affected fish show signs of low oxygen stress even in the presence of saturated levels of dissolved oxygen.
The fish are actually suffocating because their blood cannot take up oxygen as is the case normally.
Even if the fish do not die from the immediate stress, they are affected just like fish which are stressed from classical low dissolved oxygen levels. The intensity of the condition can be evaluated by the color of the fish blood.
Slightly affected fish have reddish-brown blood, whereas more acutely affected fish have chocolate brown colored blood. The percentage of hemoglobin in the fish blood that has been converted to methemoglobin combined with the dissolved oxygen concentration present in the water will decide the number of fish that will survive or suffocate and die.
For example, fish which moderately affected with nitrite poisoning in a pond with a dissolved oxygen level of 7 ppm should survive but, if the same fish is placed in a pond with a dissolved oxygen level of 2.0 ppm likely will not.
Sources of Nitrite in Ponds:
Nitrite is an intermediate fish waste compound that is formed when ammonia is broken down through bacterial activity. Particular groups of bacteria use ammonia as their food source. The ammonia decomposing bacteria then produce a waste product called nitrite.
Still other groups of bacteria use nitrite as a food resource and produce nitrate as waste. Nitrate is a compound that is not toxic to fish at concentrations typically found in ponds. Another alternate route of ammonia decomposition and uptake in commercial catfish ponds is through direct utilization by phytoplankton (microscopic plants).
While most trouble from elevated nitrite presence arises during the cooler months, it can occur any time of year. More commonly, when abnormally large amounts of ammonia build up during cooler water temperatures caused by reduced phytoplankton and bacterial activity, similar but rapid build up of nitrite will sometimes follow. To add to the dilemma, the bacteria that consume ammonia produce nitrite work faster than the bacteria that convert nitrite to non-toxic nitrate in cool water.
The outcome of this situation is that significantly elevated nitrite levels can build-up and possibly result nitrite poisoning. It is important to be aware that lethal concentrations of nitrite can develop in cool water that has elevated levels of ammonia within 24 hours.
Prevention and Treatment:
Nitrite toxicity is easily preventable does not have to be a significant problem in catfish pond production. The solution is using salt (NaCl) as an amendment to water.
More explicitly, the chloride (Cl), which is found in common salt, is the fraction of salt that is responsible for the prevention and can affect treatment of nitrite poisoning in fish. The chloride competes with nitrite at the fish gill surface for absorption.
If a concentration ratio of 9 to 1 of chloride to nitrite is maintained in water, many more chloride ions are absorbed than are the nitrite molecules therefore; nitrite poisoning or “brown blood” disease does not develop.
Farmers who routinely maintain a minimum of 60-150 ppm of chloride in water at all times seldom experience losses from nitrite build-up and effectively prevent nitrite poisoning.
Prevention is far more economically desirable than trying to treat fish already stressed and dying. Although salt and freight prices have risen significantly in recent months, it’s still the best management practice to use salt preventatively.
It is a prudent business practice to retain as much salt as possible by retaining as much water as practical. Setting standpipe elevation to capture rainfall and reduce dilution by excessive rainfall should be a conscious goal.
As a management task, pond water should be monitored for chloride levels several times each year. Often, ponds will receive more rain or well water during the course of the year than is realized and chloride levels will become reduced.
It is typically an effective strategy to be sure to enter the fall season with a minimum of 60-100 ppm chlorides. Most effective managers monitor chloride levels at least 2-3 times per month through the winter and spring to maintain a minimum of 60 ppm chlorides.
The economic consequences of not practicing a nitrite management strategy can be severe. Both the direct loss of fish from nitrite poisoning as well as indirect losses brought about by stress induced losses from bacterial and parasitic disease more than justify the cost of salt.
If salt costs $100 per ton, it takes only 133 pounds of fish sold at 75 cents per pound to cover its cost. Some areas of Alabama have the good fortune of having access to salty well water. Although this water doesn’t have the same chemical make-up as seawater, it is still of great value to most freshwater fish species.
Most traditional agricultural water users do not find the saline water valuable but it can be of great value to those growing fish or crustaceans. Farms having access to this type of saline water seldom see problems with nitrite induced mortality.
Author:
Jesse A. Chappell. Extension Fisheries Specialist, Assistant Professor
