H. Steve Killian
Extension Fisheries Specialist

Introduction

    "Brown blood" disease is a condition that affects channel catfish and some other species as a result of elevated levels of nitrite in the water. Hundreds of thousands of pounds of channel catfish have been lost as a result of this condition.
    Nitrite is a waste product that enters the blood stream of fish through the gills and turns the blood a dark brownish color, thus the name "brown blood." Nitrite can oxidize hemoglobin in red blood cells and convert it into a compound (methemoglobin) that can not carry oxygen. As a result, affected fish show signs of low oxygen stress such as gasping at the surface or hanging near aeration equipment. The affected fish exhibit these signs even when the dissolved oxygen levels in the water are normally adequate.
    The intensity of this condition can be judged by the color of the fish blood. Slightly affected fish have reddish-brown blood, and severely affected fish have chocolate brown blood. The percentage of the hemoglobin that has been converted to methemoglobin combined with the dissolved oxygen concentration in the water will determine the number of fish that will suffocate and die. For example, fish moderately affected with "brown blood" disease in a pond with a DO reading of 7 ppm should survive, however, the same fish in a pond with a DO reading of 2.0 ppm may not.

Sources of Nitrite in Ponds

    Nitrite is a intermediate waste compound that is formed when ammonia is broken down through bacterial activity. There are groups of bacteria that use ammonia for food. The ammonia eating bacteria produce a waste product called nitrite. There are other groups of bacteria that use nitrite for food and produce nitrate as waste. Nitrate is a compound that is not toxic to fish at concentrations typically found in ponds. An alternate fate of ammonia in commercial catfish ponds is through direct utilization by phytoplankton (microscopic plants).
    Problems arise during the cooler months when large amounts of ammonia build up as a result of reduced phytoplankton and bacterial activity. To compound the problem, the bacteria that convert ammonia into nitrite work faster than the bacteria that convert nitrite to non-toxic nitrate in cool water. As a result of this situation, a large amount of nitrite can build-up and possibly result in "brown blood" disease. It should be noted that high concentrations of nitrite can develop in cool water that has elevated levels of ammonia within 24 hours.

Prevention and Treatment

    Nitrite toxicity does not have to be a problem in catfish ponds because it is one of the easiest diseases to prevent. The answer is salt (NaCl), and more specifically, chloride (Cl), which is the portion of salt that is responsible for the prevention and treatment of "brown blood" disease in fish.
    Chloride competes with nitrite at the fishes’ gill surface for absorption. If a 9 to 1 ratio of chloride to nitrite is maintained in water, many more chloride ions are absorbed than are nitrite molecules and, therefore, "brown blood" disease does not develop. It is recommended to maintain at least 60 ppm of chloride in water at all times to prevent nitrite induced brown blood. In extreme cases where nitrite concentrations have risen to 10 ppm, 100 ppm chloride concentrations or more may be necessary for complete protection.
    Ponds should be checked for chloride levels several times each year. It is very important to enter the fall with at least 60 ppm chlorides. Monitor chloride levels monthly through the winter and spring to maintain 60 ppm. Make sure to check chloride levels after periods of heavy rain, as rain water will lower chloride levels.
    If "brown blood" disease develops, the producer pays for this condition in several ways. First, the producer could lose his entire investment directly through nitrite induced suffocation. Also, nitrite toxicity weakens and stresses the fish that do survive which makes them more susceptible to bacterial infection, parasitic infestations and anemia (white-lip).
    Some producers have the good fortune of having salty well water. It is common for producers in some locations in southeast and southwest Arkansas to have chloride levels greater than 300 ppm. In these fortunate situations, the producer should never experience nitrite induced "brown blood" disease.

Calculations

    Salt (NaCl) is the cheapest source of chloride for producers. Adding 4.5 lbs of salt to 1 acre-foot of water increases the chloride level 1 ppm. The following examples show how to calculate the amount of salt needed to prevent nitrite induced "brown blood" disease.
    The first step is to measure the chloride level in the existing pond or water source. This can be done by using an inexpensive simple water quality kit or test strip. The next step is to calculate how much salt is necessary to bring the chloride level to 60 ppm or higher. For example:

Problem 1

– The pond has 25 ppm chlorides
– Need 60 ppm chlorides
– Must increase 35 ppm chlorides
– Pond is 12 acres with 4 feet average depth

* 4.5 lbs. of salt X 35 ppm chlorides needed = 157.5 lbs. of salt/acre-foot
* 157.5 lbs. of salt/acre-foot X 48 acre feet = 7560 lbs.
* 7560 lbs. ÷ 2000 lbs. = 3.78 tons

OR

* Add about 4 tons of salt to the pond

Problem 2

– Nitrite 8 ppm
– Chloride 60 ppm
– Pond 15 acres with 4.0 foot average depth

* 8 ppm nitrite X 9 = 72 ppm chloride required
* 72 ppm chloride needed - 60 ppm chloride present = 12 ppm chloride to add
* 12 ppm chloride X 4.5 lbs. salt/acre-foot = 54 lbs. salt/acre-foot
* 54 lbs. salt/acre-foot X 60 acre-feet in pond = 3240 lbs.
* 3240 lbs ÷ 2000 lbs = 1.62 tons

OR

* Add about 2 tons of salt to the pond

Refer to Chloride/Nitrite table for additional information on salt requirements for "Brown Blood" protection.

    Custom delivery trucks are available for large quantities of salt in Arkansas. These trucks can haul and dispense up to 18 tons per load. Price will vary between $55 and $60 per ton delivered (1996 prices).

Other Species Affected

    Channel catfish are not the only fish sensitive to nitrite poisoning. Tilapia and trout are also quite sensitive to this waste product. Goldfish and fathead minnows are not as sensitive as catfish, and green sunfish and largemouth bass are very resistant to nitrite poisoning.

Remember

_ Prevention is the key.
_ Maintain at least 60 ppm chlorides at all times in commercial catfish ponds.
_ Always maintain a 9 to 1 chloride to nitrite ratio.

Salt Additon Table for Prevention of Nitrite Toxicity

* It is recommeded that in winter there be enough chloride to protect against 7 ppm nitrite.

Accredited By

North Central Association of Colleges and Schools
Commission on Institutions of Higher Education
30 N. LaSalle, Suite 2400
Chicago, Illinois 60602-2504
1-800-621-7440 FAX: 312-263-7462

H. STEVEN KILLIAN is Extension fisheries specialist for the Cooperative Extension Program, University of Arkansas at Pine Bluff. He is headquartered in Lake Village.

FSA9000-2M-5-97RV