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20 JOSEPH B. HUNN AND ROSALIE A. SCHNICK Fish • Rate of mortality was abrupt and most fish died within 24 hours • Small fish died first • Some species were affected more quickly than others, although all fish eventually died • Behavioral changes noted were indicative of tox- icant poisoning (Tables 4.1 and 4.2) Invertebrates • Zooplankters dead, dying, or absent (suspected in- secticide poisoning) • Benthos numbers greatly reduced with a marked change in species composition • Crabs, crayfish, and sandworms dead, dying, or absent Other animals • Signs of poisoning observed among other verte- brates (e.g., frogs, turtles, snakes) • Invertebrates (e.g., snails) show signs of poisoning Algae • Algae alive and normal • Algae absent or dead (suspected herbicide poisoning) Table 4.1. Some observed fish behaviors and water chemistry characteristics associated with fish mor- talities (modified from Davis 1986). Observations or water chemistry Possible cause Large fish coming to surface, gulping air; low dis- solved oxygen. Small fish alive and normal Large fish coming to surface and gulping air in the presence of adequate dissolved oxygen Fish swimming erratically and moving up tributary streams to avoid pollution Fish dying after a heavy rain Oil sheen on water Streambanks and bottom covered with orange- colored substance; high conductivity readings in water samples Low pH, orange discoloration of water but good water clarity Fish hyperexcitable, rapid movements followed by death; fish may attempt to swim onto shore High levels of chloride, high conductivity, high salinity, and high osmolality in nonmarine waters Low levels of chloride, low salinity, and low conduc- tivity in estuarine or marine waters Oxygen depletion caused by excessive organic mat- ter; look for a sewage treatment plant, livestock feedlot, irrigation runoff, decaying plant material, or dying algal bloom after several days of hot, calm, cloudy weather May be same as above but enough time has passed to allow for reoxygenation of water. Ammonia kills may also have these characteristics; look for possible drainage from livestock feedlot Usually a heavy metal or chemical wastes discharged from a chemical complex or through a sewage treatment plant May be a pesticide or herbicide that has washed off adjacent agricultural fields; a spill dumped from spraying equipment; or chemicals from an aerial spraying operation Drilling and refinery operations; ruptured pipeline in the area; wash water discharged from oil barges; or a leaking barge Drilling operations; look for discharge of brine water into the stream Acid water discharge from coal mining operation High levels of ammonia or low pH Possible return flow of irrigation waters that are hyperosmotic to fish Intrusion of fresh water that is hypoosmotic to fish L