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24 JOSEPH B. HUNN AND ROSALIE A. SCHNICK Results of the analyses of samples taken for Prior- ity I testing can be used to determine whether the water chemistry is within the normal range for substances that are involved in most fish kills-for example, low dissolved oxygen and high ammonia. If all characteristics are within the normal range, it may be necessary to seek further analyses such as those listed as Priority II. If the values resulting from analyses of samples from Priority I and II testing are within the normal range for the area sampled, it is a strong indication that the kill was caused by a toxic substance not usually found in the waters concerned. Fish kills sometimes occur in situations where all environmental factors seem to be normal. Favorable water chemistry characteristics and high dissolved oxygen concentrations indicate good water condi- tions; the fish are normal in color and physical con- dition and have no lesions. The mortality rate may be slow, but continuous. Generally, predatory or om- nivorous species older than 2 years are the only fish affected, and small fish and forage species may be alive and well. Such mysterious kills are most com- monly seen in late fall or early winter, depending on the latitude. These seasonal fish kills often occur in waters ad- jacent to areas where chemicals are used, stored, or applied. Spills, accidental spraying, or runoff can in- troduce sublethal pesticide levels to the environment that then become involved in the food chain by bio- magnification. In kills of this type, the key indicator is that only large predatory fish are affected, where- as young-of-the-year and forage fishes seem to be thriving. Water conditions will appear to be good to excellent. The most common cause of these unexplained fish kills is chronic exposure to sublethal levels of a pesti- cide. Although the daily exposure may be low, fish L The loss of large predators may indicate a fish kill caused by biomagnification of contaminants through the food chain. In such kills, young-of-the-year fish of all species may survive. (Photo courtesy of the Missouri Department of Conservation.)