By John A. Plumb.
Since the beginning of commercial catfish farming in the United States in the late 1950's, the industry has grown to where it was worth about $200 million in 1983. Projections place the potential value of the catfish industry at $1.5 billion by the end of the century.
In 1984 about 85,000 acres of water are in catfish production in the United States, with channel catfish the principal culture species.
The catfish industry is becoming a significant agricultural enterprise in certain parts of the United States, especially the South where water temperatures are warm enough for a 6- to 8-month growing season each year.
In a rapidly expanding agricultural industry such as catfish farming there are many problems. Any time a fish is taken from an extensive environment (such as a farm pond) and placed in an intensive environment (culture pond), health problems occur.
Normal catfish-carrying capacity of a fertilized farm pond is no more than a few hundred pounds per acre.
However, the carrying capacity of an efficient catfish culture pond may be 5,000 pounds or more. These high densities result in increased susceptibility to infectious disease. Although there are many potential infectious diseases of cultured channel catfish, only the most serious and frequent will be discussed here.
Fish diseases have some basic characteristics that separate mortalities caused by infections from those caused by low oxygen and other water quality problems or pesticides. Diseased fish usually have external sores, bloody areas in the skin, swim slowly at the surface, stop feeding before death occurs, and have a gradually increasing mortality pattern over a period of days or weeks. Pesticide- or oxygen-caused fish kills usually occur overnight or in a very short period during which a high number of fish die.
CCV Has High Kill
1) Virus Diseases. Channel catfish virus disease (CCV) is the most serious virus disease of these fish. It occurs in fingerling production facilities and may result in 50 to 90 percent mortality in fry and fingerling populations. CCV causes disease during the first summer of the fish's life, while water temperature is above 80 F (27 C). The disease is more severe while fish are less than 6 weeks old. As the fish get older and grow to 4 inches and larger they become more resistant and the mortality is less severe.
Channel catfish infected with CCV have bloody areas at the base of fins and in the skin, swollen bellies, and are popeyed. A clear straw-colored fluid is present in the body cavity. Gills and internal organs are generally pale. Just before death, infected fish swim slowly at the surface with occasional erratic movements.
CCV is easily transmitted from infected fish to noninfected fish by contact through the water. But it is not known how the virus is transmitted from one generation to another. It is presumed, and there is strong circumstantial evidence, that CCV is passed from the carrier broodfish to their offspring via reproduction.
Impact of CCV on the catfish industry as a whole is not as severe as that of some other diseases. However, due to the often high rate of mortality, the effect on fish farms where it occurs can be devastating.
2) Bacterial Diseases. There are four major bacterial diseases of farm-raised channel catfish. The most common is "Motile Aeromonas Septicemia" (MAS), usually caused by Aeromonas hydrophila. This widespread water-borne bacterium is present in nearly all fresh water.
Stress Is Factor
The disease most often occurs after fish have been stressed due to some environmental problem such as low oxygen, excessively low or high temperatures, improper handling, or some other debilitating infectious or noninfectious disease.
Symptoms of MAS vary. Infected catfish may have frayed fins and/or deeply eroded open sores in the skin and muscle. The body cavity often contains a bloody fluid, and internal organs are mottled in color and very soft. Fish of all ages are susceptible to MAS. It may occur at any time of the year, but most frequently in spring and fall. Mortalities are low to moderate (5 to 30 percent). The disease usually is chronic, with a few fish continuously dying over a long period of time, but mortality may be acute.
columnaris is a bacterial disease that most frequently causes problems in young fish, but occasionally infects production-size fish. It may occur at any time of the year.