Abstract

In the last 15 years the channel catfish industry has experienced phenomenal growth as an aquaculture commodity. Annual production has increased from less than 2,750,000 kg in 1970 to over 90,080,000 kg in 1985 with 80% of the industry located in Mississippi. Commercial fish losses due to various diseases exceeded $10,000,000 in 1985, emphasizing the importance of fish health management programs. The fish disease diagnostic laboratory at the Delta Branch Experiment Station was presented with 1672 cases in 1985. Current methods of catfish disease treatment and management are discussed.

The Industry

Farm-raised channel catfish (Ictalurus punctatus) is the fastest growing industry in Mississippi, employing over 4,000 people with a payroll in excess of $40,000,000. Mississippi currently has 30,350 ha committed to channel catfish production accounting for eighty percent of the acreage and 907 of the U.S. production. Land and water is available to support a total of 200,000 ha in this state alone. The processing plants in Mississippi dominate the industry by processing over 95% of the catfish sold in the U.S. In 1985, this amounted to 90,080,000 kg of farm-raised fish. The two major catfish feed mills will produce approximately 250,000 metric tons of catfish feed in 1986. With feed conversion 1.7:1 to 2.0:1, over 125,000 metric tons of fish will be produced.

Most commercial catfish production in the U.S. is accomplished in 8 ha rectangular earthen ponds 1.2 m deep. The majority of farms range in size from 30 to 260 ha with some over 1600 ha. Initial investment costs for land, wells and pond construction are approximately $6,000/ha. The ponds are filled with agricultural well water and the only addition of water is to replace that lost to evaporation.

Fingerling production is accomplished in hatcheries or by "wild spawning" brood fish in open ponds. Brood females reach maturity at 3 years of age and produce 1300-1800 eggs/kg body weight. Eggs batch in 8 days and the fry begin eating 3 days later. Seines are used to harvest large fish and the ponds are not drained. As large fish are removed, smaller stocker fish are added as replacements. Harvest is year-round although most of the fish growth occurs when water temperatures are near 30°C. Annual production of 4000 kg/ha is common although some ponds exceed 7000 kg/ha.

At the high stocking densities found in commercial catfish production, the fish are dependent on the feed provided by the farmer. The feed consists of a high protein (32%) formulated ration of corn, soybeans, wheat, millett, fish meal and various vitamin and mineral packs. It is produced by an extrusion process which gives it the appearance of dry dog food and makes it float.

The current trend toward health, diet and exercise is unmistakable in our society today and corresponds to the increase in consumption of fish in the U.S. The per capita consumption of farm-raised catfish is currently 0.23 kg/year, or one-tenth of the total per capita consumption of all fish portions, steaks and fillets. Market projections show these figures possibly doubling in the next three years, further indicating a strong growth potential for the channel catfish industry.

The Problems

Production of channel catfish is similar TO intensive production schemes used for other food animal industries. Successful management requires a constant effort and is labor intensive, not unlike commercial dairy production. Crowding (heavy stocking rates) and high-feeding rates result in stress to the animal and to the production system. Each channel catfish pond represents a unique ecosystem which must be treated as an individual unit. Struggles to balance maximum production with minimum pollution and stress become the basis of day-to-day management.

Large amounts of feed and high stocking rates result in the release of large amounts of nutrients into the water producing a eutrophic system, dense algal blooms can produce rapid fluctuations in dissolved oxygen, carbon dioxide and pH. Various environmental changes may result in rapid changes in concentrations of potentially toxic nitrogen compounds. A manager must avoid accumulations of toxic compounds as his ability to flush toxins from the pond is negligible because of the large pond volume.

In addition to water quality a manager must monitor the fish closely to prevent uncontrollable disease outbreaks. Individual fish are not available for observation or handling. The process of "topping", which is the addition of young fish (fingerlings) to replace older fish which are harvested, results in a continuous mixing of fish populations and age classes. Fish stocked into a pond may introduce a disease agent or may be susceptible to an organism endemic to the environment or resident fish population. Either situation can result in serious losses should a disease outbreak occur.

Specific problems encountered by catfish producers become of greater or lesser importance at different times of the year. During a twelve-month period the environment changes dramatically as numerous factors interact. The fish themselves, being ectothermic animals, experience marked changes in metabolic rate and immunocompetence at different times of the year. Not surprisingly, many disease entities are seasonal with spring and fall being of particular concern as the environment and the fish are in a state of flux at these times. A brief overview of production problems and economically important diseases are presented for each of the four seasons.

Spring (March through June)

Water temperatures are changing with a consistent warming trend predominating. Warmer water results in increased metabolic rates and consequent increased activity by fish. Feeding activity increases resulting in an increase in nitrogenous wastes entering the system. Bacterial flora must adapt to these changes to prevent accumulations of toxic ammonia and/or nitrites. By mid-April it may be warm enough for producers to begin sporadic night-oxygen checks of their ponds.

Farm activity increases as personnel must be available for regular feeding of each pond and night work to monitor dissolved oxygen levels. Spawning season involves increased attention to broodstock husbandry and careful monitoring of spawning containers so that egg masses can be collected and brought into the hatchery. Hatchery operation is complex and personnel must be on hand 24 hours. This is a labor intensive process which requires constant shifting of personnel from brood ponds to hatchery to nursery ponds. Successful fry production is essential for a large producer to have sufficient numbers of fish to stock his ponds for the coming year.

Economically significant disease problems which occur in the spring include: methemoglobinemia, Enteric Septicemia of Catfish (ESC), proliferative gill disease, anemia ("white lip disease"), Icthyopthirius multifiliis (Ich), and miscellaneous parasitic problems.

Methemoglobinemia ("brown blood disease") is caused by a population imbalance of the nitrogenous bacteria Nitrosomas sp. and Nitrobacter sp resulting in accumulation of nitrite. Nitrite crosses the gill epithelium, enters the red blood cell and complexes with hemoglobin to form methemoglobin. The net result is a decreased oxygen carrying capacity of the blood. The presence or absence of fish mortality is dependent upon the percent methemoglobin present and the concentration of dissolved oxygen in the pond.

Enteric Septicemia of Catfish (ECS) is a bacterial disease caused by Edwardsiella ictaluri. Disease occurs as water temperatures move through the 22° to 28°C range. A controversy exists as to whether or not the bacterium is an obligatory pathogen but there is no doubt as to its economic impact. In 1985, fish mortality attributed to ESC alone was estimated at $2,000,000, and the disease represented more than 50% of the 1672 cases seen at the diagnostic lab at the Delta Branch Experiment Station in Stoneville, MS. Early diagnosis is essential as affected fish rapidly develop anorexia.

Proliferative gill disease ("hamburger gill disease") and catfish anemia ("white lip disease") are two disorders of undetermined etiology which frustrate catfish producers and fish health specialists alike. An unidentified organism has been consistently associated with gill pathology seen in proliferative gill disease. Mortality due to the disorder can approach 100% within an affected pond. Flypertrophy and necrosis of gill tissue result in respiratory distress and death due to anoxia. Channel catfish affected with "white lip disease" develop profound anemia and picked cell volumes as low as 1% are not uncommon. Mortality within a pond is variable. A multiplicity of factors have been implicated in development of the disease but most are currently considered speculative. Ichthyopthirius multifiliis ("Ich") is a large ciliated protozoan with a complex life cycle. It can infest skin and gills; spreads rapidly and mortality within a pond can be very high. "Ich" can be easily transferred from pond to pond by fomites (i.e., nets, treatment boats, boots) and by predators (i.e., birds, raccoons, dogs). An outbreak can quickly affect an entire farm. Treatment is expensive, labor intensive and not always effective. A diagnosis of "Ich" on a catfish farm is truly an emergency situation.

Summer (July through September)

Hot summer weather contributes to two environmental problems which, although not specific disease entities, may represent two of the most important problems which affect catfish production. Warm water temperatures are optimal for catfish growth and feeding rates of >3% body weight per day are common. Fish in a 8 ha pond may be fed 500 kg of feed daily. These high feeding rates result in rapid eutrophication and dense algal growth. The warm water is unable to hold sufficient dissolved oxygen in solution to support fish and algal respiration when photosynthesis is not taking place. Consequently at night and during cloudy weather, emergency aeration becomes mandatory to prevent fish kills due to anoxia. Dissolved oxygen must be monitored at least every 2 hours in each pond every night and this becomes an expensive, labor intensive process which begins in April and continues through October.

Algal blooms, particularly blue-green algae, flourish in nutrient rich catfish ponds and are implicated in the occurrence of "off-flavor". Up to 70% of all catfish ponds may be "off-flavor" in August and September. A pond which is "off-flavor" is a trite liability in that fish cannot be sold, but must be fed. The longer a pond is "off-flavor" the greater the risk as fish growth compromises the carrying capacity of a pond. Fish are crowded, environmental conditions may deteriorate and stress increases. Such a situation is very conducive to disease outbreaks. Additionally, as fish grow the feed conversion rate declines, and the return on a producer's investment decreases accordingly.

Immunocompetence of channel catfish is optimal during warm summer weather. Occurrence of infectious diseases declines unless fish are stressed by low oxygen, handling or other factors. However, channel catfish virus disease (CCV), caused by a herpes virus, is a disease of major concern during the summer months. Subclinically infected fingerlings may "break" with this disease when stressed in hot weather, and mortality rates can be high.

Fall (October through December)

Decreasing temperatures favorably affect the oxygen carrying capacity of large bodies of water and the need for emergency aeration subsides. Ponds which have been "off-flavor" may come back "on" as the algae population changes. However, fluctuating water temperatures adversely affect the fish's immune system (1) and the balance among nitrifying bacteria within a pond.

A cold front can cause water temperature to drop sharply. Channel catfish have been shown to lose T-cell function completely following a sudden drop in water temperature (2). Concurrently the kinetics of the humoral DO levels associated with phytoplankton abundance and increased standing crop due to fish growth (3). Since the nutrients being utilized by the phytoplankton are ultimately derived from the feed, limiting feed reduces the algae blooms and subsequent die off.

Channel catfish virus occurs primarily in fingerlings when water temperatures are highest. Therefore, preventing the handling stress of moving fingerlings during the middle of the summer is an effective control measure for the disease. The management practice of setting minimum DO levels before operating emergency aeration equipment, especially during Edwardsiella ictaluri season is an example of using stress reduction as a preventive measure. The stress of nutritional inadequacy is avoided by providing the highest quality feed available. Consequently, "broken back" disease, or vitamin C deficiency is rare.

The management practice of reducing standing crops during the fall months not only reduces crowding stress for the winter months but also lowers the financial risk of losing fish to untreatable winter diseases. The practice of not mixing populations of fish that have had previous Edwardsiella outbreaks with populations with no prior exposure makes sense.

Therapy

Drugs cannot take the place of good management. However, the intensive production conditions to which catfish are subjected are conducive to disease outbreak and when it occurs, prompt therapeutic action can save the producer time and money. Mixed infections are common, therefore, accurate diagnosis becomes essential for development of a logical treatment protocol. Few drugs are legal or effective and improper use of an agent can further jeopardize fish health.

For commercially produced channel catfish the only two routes of practical drug administration are in the feed or in the water. Antibiotics are administered per os, via the feed. Terramycin® (Pfizer), an oxytetracycline compound, has been the only legal antibiotic available for use in channel catfish. A potentiated sulfonamide, Romet-30® (Hoffmann-LaRoche) received full FDA clearance for use in channel catfish on May 23, 1986. Terramycin® must be incorporated into sinking feed and a fish must eat 1.5% body weight (BW) daily to achieve therapeutic drug levels. Romet-30® retains its activity when subjected to high temperatures and, therefore, is available as a floating feed. This is advantageous as feeding activity can be easily observed. In addition, therapeutic levels of Romet-30® can be achieved by a fish eating only 0.5% BW per day. This is an important consideration as fish affected by bacterial disease, particularly ESC, rapidly become anorexic.

Ectoparasites are generally treated by using chemicals intended for algae control. Copper sulfate (CuSO 4 ) can be applied as a prolonged bath and is effective against ciliated protozoans like "Ich" and triophyra. However, the copper ion is toxic to fish and water quality must be tested prior to use of the compound. In addition, the algicidal properties will cause a drop in DO and emergency aeration is frequently necessary following its use. Potassium permanganate (KMnO 4 ) can also be used as an ectoparasiticide and an external fungicide. It has less deleterious effects on water quality but excessive treatment can be lethal. Formalin is EPA approved for use in channel catfish ponds but is of little practical value because of its expense and because of its property of chemically removing oxygen from the water.

Salt (NaC1) is used to increase the chloride ion concentration in catfish ponds; a means of preventing methemoglobinemia as the chloride and nitrite ions compete for receptor sites on the epithelial cell membranes which line the gills. Three to five parts chloride to one part nitrite is usually sufficient for prevention of clinical brown blood disease.

Training and Education

Education of producers allows for implementation of management practices consistent with successful preventive medicine programs. Mississippi State University provides an Extension Service which functions as a liaison between university scientists and the producer-user group. Needs and concerns of producers are made known to researchers, and scientific progress is presented to producers in a less-technical format. This exchange is accomplished through publications, workshops, short-courses, and formal education. The most recent innovation has been implementation of a post-DVM residency program in fish health management by the College of Veterinary Medicine. It is hoped that the 12-month program will equip veterinarians to provide full animal health care services to aquaculturists in the state. The availability of veterinary specialists should help create a veterinary-client relationship which will benefit individual farmers and the industry as a whole.

References

1.  Collins, M. T., Dawe, D. L. and Gratzek, J. B. Immune response of channel catfish under different environmental conditions. J. Am Vet Med Assn 169(9):991-994 (1976).

2.  Clem, L. W., Faulmann, E., Miller, N. W., Ellasaesser, C., Lobb, C. J. and Cucheus, M. A. Temperature-mediated processes in teleost immunity: differential effects of in vitro and in vivo temperature on mitogenic responses of channel catfish lymphocytes. Dev and Comp Immun 8:313-322 (1984).

3.  Tucker, C. S. and Boyd, C. E. Water quality. In: Channel Catfish Culture, C. S. Tucker (ed.), Elsevier, Amsterdam. 1985, pp 161-163.