7170 - Laserfiche WebLink

Home Browse Search

12 FRED P. MEYER AND ROGER L. HERMAN See Chapter 7 for instructions on what additional data are needed and how the information should be documented. An analysis of this information often makes it possible to rule out several potential causes of a fish kill and may make it possible to distinguish between one or two likely or suspected causes. This reduces the number and types of sam- ples that are required and helps reduce the person- nel, equipment, and laboratory work needed when time is critical. In recording fish kills, it is important to establish the magnitude of the mortality. The significance of a fish kill is always directly related to economic, geographical, and political factors associated with the site, as well as to the ecological effects. The losses of 100 fish in a prime trout stream or any losses due to a possible toxic discharge are always important; in other situations, the loss of thousands of gizzard shad may be of little public concern. The American Public Health Association (APHA) et al. (1985) offers the following guide for reporting fish kills: Minor kill: less than 100 fish Moderate kill: 100 to 1,000 fish in 16 km of stream or equivalent lentic area Major kill: more than 1,000 fish in 1.6 km of a stream or equivalent lentic area The rate or pattern of loss is a helpful indicator (Fig. 3.1). If all fish died abruptly or within a short time (24 hours or less), it is likely that the kill was caused by a sudden, catastrophic event that made the environment fatally toxic to fish. If the mortal- ity began slowly and then rose sharply over the next 5 to 7 days, the most likely causes would be a slow- ly developing oxygen depletion or a highly virulent infectious agent. Mortality that continues at a low rate over an extended period may be due to a mar- ginal environment, a low-virulence infective agent, or chronic exposure to sublethal concentrations of a toxic substance. A second important piece of information is that of the sizes and species of fish affected (Table 3.1). In kills caused by toxic substances, small fish usual- ly die before larger ones of the same species; in oxygen depletion, the reverse is true. Establishing when a kill began and how long it continued is also often important. It is useful to know whether the kill began at night, how long it continued, and whether it was interrupted for a time and then began anew. 100 1 Oxygen depletion l th l i 1 or e a tox cant highly t 1 1 virulent bacteria O 1 or virus infection ? i O D 1 a? 1 1 C U external parasites, 1 low-virulence bacteria, I poor environmental conditions, or 1 chronic exposure to pollution 1 2 3 4 5 6 7 8 9 10 11 Time (days) Fig. 3.1. Curves (mortality versus time) associated with three major categor- ies of fish mortality (Wede- meyer et al. 1976).