What indirect cause of fish kill is addressed by applying pesticides in smaller portions and starting at shallow edges?

• A. Oxygen consumption from decay of dead plants; treat only one-third to one-half of the pond at a time and start from shallow edges toward deeper water
• B. Direct chemical poisoning of fish by the pesticide
• C. Increased water temperature
• D. Complete absence of plant decay

Answer: (A)

The main idea is preventing fish kills by avoiding a sudden spike in oxygen demand from plant decay after treatment. When pesticides kill aquatic vegetation, the dead plant material begins to decompose, and bacteria use a lot of dissolved oxygen in the process. If the whole pond were treated at once, that oxygen demand can be large enough to drop dissolved oxygen levels quickly, especially at night or in warmer water, harming or killing fish. By applying the pesticide in smaller portions and starting at the shallow edges, the amount of vegetation dying at any one time is reduced and decomposition occurs more gradually, with better chances for the oxygen in the water to be replenished through mixing and photosynthesis. This approach targets the indirect cause of fish kill, rather than the pesticide’s direct effects on fish.