When a number of dead fish are found in one place, the incident is referred to as a fish kill, and there is significant reason to suspect pollution. The three main causes of fish kills are poisoning, disease, and suffocation.

Sanitation worker using a rake to remove dead fish from the Rodrigo de Freitas lake in Rio de Janeiro, Brazil. (©Reuters New Media Inc./Corbis. Reproduced by permission.)

Poisoning

Fish may be poisoned by a wide range of polluting substances, including pesticides, acids, ammonia, phenols, cresols, compounds of metals, detergents, or cyanides. Many of these substances are used in industrial processes or in agriculture and are released through drains or are accidentally spilled into waterways. Acid rain, derived from industrial pollutants in the atmosphere, causes rivers to become toxic for various kinds of fish. Some types of toxic algal blooms kill fish. During the 1990s the...