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INTERPRETING THE SCENE 13 Table 3.1. Physical signs associated with fish mortality problems caused by oxygen depletion, toxic algae blooms, and pesticide toxicity (modified from Wedemeyer et al. 1976). Physical signs associated with fish mortality Cause of mortality Oxygen depletion Toxic algal bloom Pesticide toxicity Fish behavior Species selectivity in fish kill Size of fish Time of fish kill Plankton abundance Dissolved oxygen Water pH Water color Algal bloom Gasping and swim- Convulsive, erratic ming at the surface swimming, lethargy None if depletion is total; common carp and bullheads may survive if depletion is partial Large fish killed first, eventually may kill all sizes and species Night and early morning hours Algae dying, little zooplankton present Less than 2 ppm, usually less than 1 ppm 6.0-7.5 Brown, gray, or black Many dead and dying algal cells None, all species affected Small fish killed first, eventually all sizes Only during hours of bright sunlight, about 9:00 a.m. to 5:00 p.m. Abundance of one algal species, little zooplankton present Very high, often saturated, or supersaturated near surface 9.5 and above Dark green, brown, or golden, some- times with musty odor Abundant algae, predominately of one species Kills caused by toxic substances are usually abrupt. The mortality may begin at any hour and continue until all fish have died or until the substance has been degraded, neutralized, or diluted. Small fish usually die first and affected fish often have convul- sions, lose equilibrium, or show other signs of tox- icosis (see Chapter 4). A quick check of limnological or water quality characteristics will yield highly useful information (Table 3.1). If algae are alive and thriving but zoo- Convulsive, erratic swimming, lethargy; if organophosphate pesticide, pectoral fins extended anteriorly Usually one species killed before others, depending on fish sensitivity and pesticide level encountered Small fish killed first, eventually may kill all sizes Any hour, day or night If insecticide, no zooplankton present, but algae normal. If herbicide, algae may be absent Normal range 7.5-9.0 Normal color and little or no unusual odor Normal bloom of mixed species unless herbicide involved; then algae absent or reduced plankton and insects are dead or absent, you should suspect an insecticide as a potential cause. On the other hand, the presence of dead or dying algae, but live zooplankton, would suggest that the substance was herbicidal. If both types of plankton are dead, dying, or absent, an acid, strong alkali, heavy metal, or other highly toxic substance should be suspected. A review of the previous information should en- able the investigator to reach a judgment as to the likely cause of a fish kill and guide decisions about I