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A bioassay involves the use of a biological organism to test for chemical toxicity. Probably the oldest and most familiar example of a bioassay is the caged canary that coal miners traditionally carried with them into a mine. Because canaries are more sensitive than humans to methane gas, they provided an advanced warning of dangerous methane levels in the mines. In a similar way, aquatic bioassays are used to provide information about the toxicity of contaminants present in effluents being released into natural streams, rivers and lakes. Suitable test organisms are held in a sample of the effluent, and their survival, or growth, or some other physiological parameter is monitored. Such aquatic bioassays are widely used today to measure the toxicity of industrial or municipal wastes. Many different test organisms can be used for aquatic bioassays. It is important that the species used be sensitive to changes in water chemistry and be representative of organisms in the natural habitat. Organisms commonly used include single-celled algae, cladocerans (water fleas), and fish such as the fathead minnow.
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