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I FRED P. MEYER were the most common and killed the most fish, mining, pest control, and reservoir operations also killed large numbers. Among natural causes, oxygen depletion during winter or summer was the most fre- quent problem. Unfortunately, discerning what may be relevant information is often difficult. The evidence may point to several possible causes because the observed signs are common to more than one cause. Searching out the cause of a fish kill can be similar to a Sherlock Holmes investigation. Only careful observation, ac- curate recording, and complete laboratory analyses will enable an investigator to piece together the critical bits of information that eventually pinpoint the exact cause. In the past, it has sometimes been difficult to prove the cause of a fish kill in court because of carelessness, lack of timely action, and a series of common failures: failures to record observations, conduct appropriate tests, collect the needed sam- ples, maintain chain-of-custody procedures, or docu- ment evidence properly. The vital evidence on which a definitive decision must be based may be short- lived, especially in rivers or areas of tidal influence. It is critical that the investigator know and under- stand the need for prompt, precise action to record or preserve the relevant evidence. Quality assurance to protect the validity of data, samples, and other evidence is a critical part of any fish kill investigation. Before beginning work at a fish kill site, every investigator should be thoroughly familiar with quality assurance requirements and rules of evidence (discussed in Chapter 7). Before attempting to explain the cause of a fish kill, an investigator should carefully study the en- tire environmental picture. Seemingly insignificant factors of weather, water flow, vegetation, algal blooms, pollution, water chemistry, and other activ- ities in the area may play important roles. The in- vestigator should try to determine what factors in the environment suddenly changed and why. No evidence should be overlooked. Frequently, the first indication that something is wrong is the presence of dead fish. Such evidence is after the fact and the investigator must mentally reconstruct the environmental situation that led to the kill. Unfortunately, dead fish often look alike, whether they were killed by a toxic substance or died of asphyxiation from an oxygen depletion. However, the site of the fish kill usually offers clues to the nature of the cause. It is the responsibility of the in- vestigator to watch for and to recognize these clues. This fish kill investigation manual is intended to serve as a guide to field fishery biologists to help them through the entire investigative process. It begins at the point of first notification of a fish kill, proceeds through the various stages, discusses the types of causes and the evidence associated with them, provides guidance at the various decision- making stages, and culminates in the preparation of a completion report.' Additional information on the physiological re- quirements of fish, how changes in the environment affect fish, and why fish kills occur was given by Wedemeyer et al. (1976) in their book Environmen- tal Stress and Fish Diseases. This useful reference discusses the causes and effects of most environmen- tal changes, provides the optimal and stressful limits of environmental variables for a number of fish species, and discusses the activity and effects of a number of toxic substances. Other useful references have been published by the Aquatic-Life Advisory Committee (1956); U.S. Environmental Protection Agency (EPA; 1971, 1972); Bouwkamp (1980); American Fisheries Society (1982); and Tracy and Kittle (1982). In the following chapters, each of the several types of fish kills is discussed in detail. Information is given about clues to watch for, data to collect, tests to run, equipment needed, the kinds of samples to collect, how to handle samples properly, where to have samples processed, and how to proceed when the results are received. All units of measure listed in this manual are presented in metric units; numerical equivalents for their conversion to the English sys- tem are shown in Appendix A. For more detailed information, see Moore and Mitchell (1987). IThe principal publications of interest in fish kill investigations that are referred to in this manual are listed in alphabetical order in the references that follow Chapter 13. In the text, the name of the author and year of publication (e.g., Hill 1983) identify the publication referred to.