Fish, which share many physiological properties with mammals, are a highly visible part of most aquatic habitats with over 775 species in North America alone. Critical to many natural food chains, they are a source of food, recreation and economic growth and affect the plankton, macrophytes and other aquatic organisms with whom they coexist. Fish often are one of the first indicators of change in the environment and can highlight in dramatic fashion pollution in streams, rivers or lakes. Changes in the types and numbers of fish often indicate a change in pH, salinity, temperature, solutes, flow, clarity, dissolved oxygen, substrate composition or pollution level.
Tagging: Marking a fish and releasing it for recapture yields information on movement, rates of growth, genetic strains, and behavior of individual fish.