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22 JOSEPH B. HUNN AND ROSALIE A. SCHNICK • Significant differences exist in the chemical com- position of water between the site of the kill and the reference (control) site • On-site toxicity tests indicate that water from the kill site is toxic, whereas that from the reference site is not Sediment • A suspect toxicant is present in sediments from the site of the kill • The suspect toxicant was not found in sediments from the reference site or is present in equal or lesser quantities at the reference site • Toxic chemical levels at the site of the kill are higher than those of background samples from the area (Kelly and Hite 1984) Tissues • Activity of enzymes (e.g., acetylcholinesterase in brain, ATPase in gills) is reduced in fish from the kill area Concentrations of toxic metals (e.g., Cd, Cu, Hg, Zn) in gill tissue are higher in fish from the kill area than in fish from the reference site (sus- pected metal poisoning) Concentrations of the suspect toxicant in tissues are greater in fish from the kill site than in those from the reference site Concentrations of the suspect toxicant in fish tissues are known to be toxic Investigations of kills suspected to have originated from a toxic substance must proceed as though the cause is unknown. All factors must be checked or eliminated unless there is firm evidence that certain causes are not involved. The investigation should proceed through a process of elimination. The use of autoanalyzers provides rapid and highly sensitive water chemistry deter- minations.