Some fish species are more sensitive to environmental degradation than others. The collection of species living in an ecosystem is called an assemblage. Fish assemblages are used as indicators that are sensitive to habitat degradation, environmental contamination, and overall ecosystem productivity. Ecosystems with more fish species tend to be healthier, and the presence of certain, more sensitive species is a sign that the ecosystem is in good health.

Fish surveys conducted by the Ohio Sea Grant. Photo by Eugene Braig via Flickr CC BY-NC