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RESOURCE PUBLICATION 177, 1990 19 CHAPTER 4 Toxic Substances Joseph B. Hunn and Rosalie A. Schnick Introduction Fish kills caused by toxic substances fall into several categories, each with its own set of accom- panying environmental evidence. Highly toxic sub- stances act quickly and cause abrupt, extensive mortalities. Some chemicals kill both plants and animals and thus severely and dramatically affect the ecosystem. Other compounds may affect only plants, only animals, or only certain species or sizes of fish. Kills associated with these substances may be abrupt, progressive, or lingering, and may trig- ger a chain of adverse environmental changes. If toxic substances enter the ecosystem at sublethal levels over an extended time, the environmental ef- fects are more subtle. Fish kills associated with such changes may appear at unexpected times of the year or long after the discharge has ended. Biological Responses to Toxic Substances Species of fish vary in their susceptibility to toxic substances. Unless the substance is so highly toxic or the concentration is so high that virtually all fish are killed shortly after contact, a progression of selectivity among fish species is usually evident. Because toxic substances may kill all of the biota, it is important to also check whether other organ- isms, such as algae, zooplankton, sandworms, snails, insects, crabs, crayfish, frogs, turtles, or snakes, are still alive. Often, some species are less sensitive than others to a toxicant, at least in the early stages of the kill. Unless the substance is herbicidal or algicidal, the dissolved oxygen, pH, and other water chemistry characteristics may appear normal. If the substance also kills plants, the picture becomes confused by misleading indicators, such as low oxygen, low pH, high C02, and dying algae. The observer must be A fish kill is sometimes the result of long-term, chronic introduction of toxic material. The rusting 55-gallon drums shown here contained hazardous materials that were re- leased over several years. alert and consider all of the evidence to determine the true cause of the fish kill. An array of information is needed before an in- vestigator can determine whether a toxic substance was responsible for a fish kill. Evidence used to make such a determination must come from on-site investigations and laboratory analyses of samples taken during the investigation. Information devel- oped from preliminary observations may include the following: I