Early detection of disease is necessary for the effective treatment and control of fish health problems. Intensive production of fish increases the likelihood of parasite and disease outbreaks. With careful daily observation of fish activity and pond conditions, a producer has a better chance of recognizing and treating problems before major losses occur.

The ability to recognize early stages of disease is a key management technique that will ensure production of healthy fish and the economic success of the producer. Early detection will allow the producer to provide a diagnostic laboratory with sufficient information to accurately evaluate and treat fish health problems before sizeable fish losses are incurred.

To provide such information to a diagnostic laboratory the signs of disease and the associated mortality rate must be recognized. Some disease have similar symptoms and must be diagnosed via a rate of mortality.

Mortality patterns can be grouped into three general categories. (Figure 1). The first category involves the rapid mortality of fish in the pond. This indicates that some catastrophic event made the environment noxious to fish.  The second category involves a slow build up to a sharp rise in mortality. This is an indicator of a substance that must be developed throughout the pond to cause high mortality. Mortality that continues at a low rate indicates exposure of a sublethal level of substances that causes stress. A chronic exposure to a number of these substances elevates the mortality rate.

Fish size and species also affect the mortality rate. When high mortality is caused by a toxic substance, small fish die before larger ones of the same species. However, the opposite is true when a total dissolved oxygen (D.O.) depletion of a pond occurs. In terms of species, trout are stressed at 5 ppm while catfish are not severely stressed until oxygen levels drop below 2 ppm. Tilapia, on the other hand, may be sustained overnight at a D.O. measurement of only 1 ppm.

Abnormal behavior and physical appearance are the first recognizable signs of fish disease. Although recognition of these signs is essential for early detection, some are associated with many diseases which in turn may lead to a false diagnosis (Table 1). The ability to associate signs of disease and the correct pattern of mortality is an important key in diagnosing fish health problems. For accurate diagnosis, information on mortality rates should be submitted with fish and water samples to a qualified diagnostician for diagnosis and treatment recommendations.

The Cooperative Extension Service maintains three diagnostic laboratories to assist with fish health and water quality problems: Lonoke Agricultural Research and Extension Center, Hwy 70 East, Lonoke, Arkansas; Chicot County Extension Office, 434 South Cokely, Lake Village, Arkansas; and S. J. Parker Agricultural Research Complex, University of Arkansas at Pine Bluff, Pine Bluff, Arkansas.

MELISSA S. HOBBS is fisheries assistant specialist, University of Arkansas Cooperative Extension Service, located at Lonoke.

FSA9064-11-3M-12-91-S263