Barriers to Fish Migration

June 19, 2017

One reason fish populations struggle is that barriers prevent them from reaching the upstream habitat where they breed and grow. Learn why fish migration is important and what we’re doing to help.

Why is fish migration important?

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Every year, millions of fish— salmon, steelhead trout, shad, alewives, and sturgeon, among others—migrate to their native habitats to reproduce. Some fish need to swim thousands of miles through oceans and rivers to reach their destination. They are often blocked from completing their journey by engineered barriers, such as dams and culverts. When fish can’t reach their habitat, they can’t reproduce and build their populations.

What types of barriers affect fish migration?


Common types of barriers are dams and culverts. Although dams provide benefits, such as hydroelectric power and irrigation, many are now obsolete. Some of these dams have fallen into disrepair and are now considered hazards to humans.

Hydropower dams use the energy of river water spilling down the height of a dam to power turbines, which generate electricity. These dams can also prevent fish from swimming upstream.

Culverts (tunnels that carry a stream under a road) can also be barriers to fish migration when they are too small or steep or are perched too high to allow fish to pass.

How do barriers impact fish?

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.

What is NOAA Fisheries doing to improve fish migration?


NOAA Fisheries works to protect and restore river habitat through a variety of programs and partnerships. We work with conservation organizations, energy companies, states, tribes, and citizens to evaluate the best option for each barrier. We understand that most barriers have the same general impact on fish—blocking their migration—but each requires a specific set of conservation actions.

In cases where there is support for removal and where barriers are no longer serving their intended purpose, we open rivers and streams for fish by completely removing barriers. This can mean removing them with heavy equipment like bulldozers and excavators, or even blasting them away with dynamite. Sometimes—with hydroelectric or historic dams—we can’t remove the barriers so we help fish swim around them instead. We also enlarge culverts to allow a more natural flow of water for fish to swim through.

What is the role of NOAA Fisheries in hydropower facilities?

Because hydropower dams can affect habitat and marine resources, we work with partners including other federal agencies, industry, states, non-governmental organizations, and tribes to identify and implement solutions to reopen rivers to migratory fish while preserving hydropower generation. These facilities generally operate under a license from the Federal Energy Regulatory Commission. These licenses typically last decades—30 to 50 years. When licenses need to be renewed, we have an opportunity to review how the power plant operates.

We review information about the facility’s operations and evaluate project impacts on migratory fish and their habitat. We can require improved fish passage to ensure the safe, timely, and effective upstream and downstream passage of migrating fish, and recommend conditions to the license that will protect or improve habitat and fish populations.

After a license is issued, we work closely with the hydropower companies to monitor the progress of fish migration at these facilities.

How successful have we been at improving fish migration?

With the input, expertise, and support of our partners, we have conducted more than 700 fish migration projects, including dam removals and hydropower facility updates. Together, these projects have opened nearly 7,000 miles of rivers and streams for fish migration.

Case Study: Penobscot River Restoration in Maine


The Penobscot River is New England’s second largest river—home to 11 migratory fish species, including three listed under the Endangered Species Act. The river also hosts the largest run of Atlantic salmon left in the United States. But dams and other barriers and water pollution have severely reduced numbers of many migratory fish in the watershed.

Since 2003, we have supported the efforts of the Penobscot River Restoration Trust and other partners to restore the river. We have provided funding and technical assistance for a large-scale effort to improve fish passage in more than 1,000 miles of river and stream habitat. We helped remove the lowest two dams on the river and are working with partners to identify priority areas for fish passage projects throughout the watershed. These efforts will help Atlantic salmon and other fish reach their upstream habitat, while maintaining hydropower production and increasing opportunities for recreation.

In 2015, endangered shortnose sturgeon reached habitat in the Penobscot River that had been blocked by dams for more than a century. More than half a million river herring—45 times more than in 2013—were counted at a former dam site, and Atlantic salmon numbers are the highest since 2011.