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Winter Kill in Channel Catfish

Disease Symptoms

Winter Kill Syndrome is a serious disease affecting farm-raised channel catfish during colder months. Clinical signs of the disease are easily recognized and are exhibited as dry skin, sunken eyes, and skin lesions that appear as white to yellow cottony patches. The cottony patches result from the growth of a fungus (genus, Saprolegnia). Internally, the intestinal tract is empty, and a clear, watery fluid is present in the gut. Mortality associated with Winter Kill is variable and usually results in chronic low-grade losses; however, mass mortalities and significant financial losses have been observed. Although all sizes of fish can be affected, the disease occurs most frequently among fish of harvestable size.

Clinical Signs

  • Dry skin lacking a slime coat
  • Sunken eyes (endopthalmia)
  • Empty stomach and intestine with watery fluid
  • Diffuse skin hemorrhage
  • White to yellow cottony patches on skin

Causes of Disease

Previous exposure to significant stress during the summer or fall, physical injury, preexisting illness, and inadequate nutrition appear to predispose fish to disease in the winter. Although bacteria (Aeromonas hydrophila and Cytophaga columnaris) have been isolated in approximately 25 percent of Winter Kill cases, infection with fungus appears to be the most consistent feature of the disease. Fungi are everywhere in natural waters and commonly grow on dead organic material. However, several species can act as primary disease producers for catfish.

Research at the University of Mississippi Medical Center indicates that certain species of fungi (Saprolegnia) found in catfish ponds can penetrate and damage the skin and muscle of relatively healthy fish. Evidence from diagnostic records and from laboratory and field studies involving commercial catfish ponds indicates that the occurrence of Winter Kill appears to depend on two major factors. These factors are (1) a rapid drop in water temperature, which reduces immunity in the fish and (2) the production of high number of motile (moving) fungus spores in the water.

Seasonal Influence on Pond Temperature

Because the high water temperature observed during the summer appears to optimize catfish defenses, development of the disease rarely occurs in summer.

Winter cold weather fronts cause tremendous flunctuations in pond temperatures, preventing catfish from adjusting to pond environmental conditions and resulting in a continual state of immunosuppression (suppression of the immunse system). Generally, mortality remains low until pond temperatures begin to increase and stabilize in late winter and early spring. A stable pond temperature favors the growth of the fungus (Saprolegnia) and increases the likelihood of exposure and disease. The mortality observed during the winter is the result of environmental and biological interactions that took place several months earlier.

Disease Prevention

Although temperature appears to be the primary predisposing factor resulting in the development of this disease, other forms of stress (poor environmental quality, seining stress, preexisting illness, and inadequate nutrition) are likely to compromise the health of the fish and increase the severity of the disease. Low alkalinity and pH also appear to influence the frequency and severity of the disease.

Optimizing water quality and reducing stress have been recommended for lessening the severity of Winter Kill in pond-raised catfish. Unfortunately, under conditions of heavy stocking and feeding rates, optimal water quality is difficult to maintain. It is also impossible to control rapid temperature fluctuations in the pond water because of the large surface area and shallow depths characteristic of most commercial catfish ponds.

Several ongoing research projects are developing methods to prevent this disease. Laboratory experiments indicate that various chemicals may be effective in controlling the development of fungal infections. Studies conducted at Delta Western Research Center, Indianola, MS, in cooperation with the Mississippi Agricultural and Forestry Experiment Station, Mississippi State University, indicate that copper sulfate and formalin may be effective in preventing mortalities associated with the fungus (Saprolegnia), whereas potassium permangante and moderate concentrations of salt have little effect. Formalin was also effective as a postinfective treatment in reducing mortalities. Copper sulfate appears to be toxic to catfish held at low temperatures under laboratory conditions. Studies are underway to evaluate the strategic use of chemicals to control this disease in commercial catfish ponds. Although copper sulfate and formalin appear to have some potential for controlling Winter Kill, their routine use cannot be recommended until pond studies are completed. These studies will evaluate their effectiveness under typical culture conditions and will study the effect of long-term use of chemicals on the fish and the environment. Although approved for use against parasites, these chemicals need further FDA approval in treating fungus (Saprolegnia) in commercial catfish ponds.

Prevention

  • Minimize stress and handling.
  • Clear up ongoing disease problems before winter.
  • Maintain adequate water quality (alkalinity greater than 30 ppm).
  • Maintain adequate witner feeding schedules.

References

Bly, J.E., L.A. Lawson, D.J. Dale, A.J. Szalai, R.M. Durborow, and L.W. Clem. 1992. Winter saprolegniosis in channel catfish. Diseases of Aquatic Organisms. 13:155-164.

Bly, J.E., S.M.A Quiniou, L.A. Lawson, and L.W. Clem. 1995. Winter saprolegniosis in channel catfish: Prophylaxis and treatment. Research and review: A compilation of abstracts of research on channel catfish. Catfish Farmers of America Research Symposium, Memphis, TN.

Bly, J.E., L.A. lawson, A.J. Szalai, and L.W. Clem. 1993. Environmental factors affecting outbreaks of winter saprolegniosi channel catfish Ictalatus punctatus (Rafinesque). Journal of Fish Diseases. 16:541-549.

Durborow, R.M. and D. Crosby. 1988. Monitory Winter Kill conditions can cut losses. The Catfish Journal: 3:9.

Li, M.H., D.J. Wise, and E.H. Robinson (In press). Chemical prevention and treatment of winter saprolegniosis (Winter Kill) in channel catfish Ictalurus punctatus. Journal of the World Aquaculture Society.

Marking, L.L., J. Rach, and T.M. Scheier. 1994. Evaluation of antifungal agents for fish culture. Progressive Fish-Culturist 56:225-231.

By Dr. David Wise, Area Extension Aquaculture Specialist, Delta Research and Extension Center; Dr. P.R. Waterstrat, College of Veterinary Medicine; and Dr. Richard Hopper, Extension Veterinary Medicine

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Information Sheet 1392
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director

Copyright by Mississippi State University. All rights reserved.

This document may be copied and distributed for nonprofit educational purposes provided that credit is given to the Mississippi State University Extension Service.

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