Fish, like macroinvertebrates, are susceptible to degradations in water quality and are sensitive to pollution and habitat loss. Unlike macroinvertebrates, fish are relatively mobile. If the stream reach does not provide adequate protection from predators, sufficient food supply or acceptable water quality, fish will search out more suitable conditions. The presence or absence of fish provides commentary on water quality as well as habitat stability and food supply.
Fish inhabit different living spaces within the stream ecosystem and have evolved to thrive in different flow regimes. A stream with a variety of flow conditions such as riffles, pools and undercut banks will host a more diverse population, which is indicative of a healthy stream.
Several additional factors make fish excellent monitors of local steam conditions. Because fish have adapted different eating habits, a diverse fish population may indicate a well balanced ecosystem which sustains aquatic vegetation, macroinvertebrates and smaller prey species. Additionally, fish live longer than macroinvertebrates and spending a longer life cycle exposed to variations in water quality can provide valuable information regarding the consistency of water quality over many years. This is particularly true of predator species higher on the food chain which may exhibit the effects of bioaccumulation.
The longevity of the fish life cycle, their dependence on a variety of stable habitat to support different species, and their reliance on nourishment from plants and animals make fish communities a very useful tool when assessing stream health or impacts to water quality.