In the United States, more than 2 million dams and other barriers block fish from migrating upstream. As a result, many fish populations have declined.

For example, Atlantic salmon used to be found in every river north of the Hudson River. Due to dams and other threats, less than half of 1 percent of the historic population remains. The last remnant populations of Atlantic salmon in U.S. waters exist in just a few rivers and streams in central and eastern Maine. They are an endangered species.

Reduced fish populations affect the entire ecosystem, since they are often important prey for other animals. They are often crucial to commercial and recreational fisheries, so reduced numbers can impact local economies as well.