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More than 300 years later and not far from the original fish passage site there on the river, U.S. Geological Survey researchers have been working with dam operators and other cooperators to expand scientific understanding of the effects these barriers and dams can have on migratory fish, and what can be done to maximize their chance to travel freely up and down rivers, to and from oceans.

It’s a scene happening throughout the U.S., with the USGS providing science to help build and maintain healthy fish populations, and with them, healthier ecosystems. The fish passage research the USGS conducts provides critical information that helps build fish passages that enable key species to get past dams, navigate waterways and complete migration cycles. These specially designed structures enable fish to bypass blocked waterways, giving migratory fish access to key spawning areas, nurseries and other crucial habitats.

When migratory fish are able to use effectively designed fish passages, it strengthens inland and ocean ecosystems, fishing communities, and enhances conservation efforts of these important fish. This is significant because traveling fish are ecologically and economically important in the oceans where many species spend most of their adult lives, and in the freshwater habitats where a majority are born and reproduce.

The spawning seasons of migratory fish attract saltwater and freshwater anglers, supporting a sport fishing community that made nearly 63 million recreational fishing trips in 2016, adding tens of billions of dollars to local economies. Healthy populations of these fish in the ocean are also important for U.S. commercial fishing, which had a $51 billion sales impact on the U.S. economy in 2015 while also providing food to millions of people around the world.

Why Are Migratory Fish Important?

In addition to the value of migratory fish to sport and commercial fishing, and as a vital staple for human consumption, these fish are also ecologically important to ocean and stream environments.

“As a core component of healthy streams and oceans, migratory fish play crucial roles in stable ecosystems for a lot of different reasons,” said Ted Castro-Santos, a Research Ecologist at the USGS Leetown Science Center. “The fact they are a major food source for so many different species is probably one of their most important roles.”

Adult migratory fish bring nutrients with them into fresh water habitats where many of these fish become prey to other fish, birds and animals. The fish that are able to make it to upstream spawning grounds and successfully reproduce become part of a spawning cycle that can potentially create trillions of eggs each season.

These fish eggs can nourish almost every level of the food web and even with the eggs’ high attrition rate, millions of baby fish may hatch each season. The juvenile fish are also a major food source in fresh water habitats until they grow large enough to attempt their own journey to ocean waters, where they become part of ocean food chains and complete the migration cycle started by their parents.

But the delicate balance of these migration cycles has been disrupted as dams and barriers block waterways across the nation and cut off many migratory fish from their ancestral spawning areas.

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How Does USGS Fish Passage Research Help?

To help migratory fish travel up and down waterways, USGS scientists conduct research on fish behavior and use this knowledge to design and develop new and improved fish passages, as well as work with states and other partners to identify the best locations for fish passages to be installed. Ongoing fish passage research conducted by the USGS is important since man-made barriers, which can slow or prevent migration, have played a role in the shrinking fish populations.

“Over time, it has been observed that man-made barriers have been one of the major stressors leading to population declines for many migratory fish species,” said Tom O’Connell, USGS Leetown Science Center director. “Effective fish passages are a key component needed to restore migratory fish populations to healthy levels.”

The USGS research helps not only species recovery, but also state resource managers and dam operators since new and improved fish passage techniques can be more cost effective, allow more fish to complete migration, and require less time or water usage, which can save operators money.

Thinking Like a Fish

In order to design structures migratory fish can find and use, it is important to understand what drives them and how they interact with man-made barriers. USGS scientists study migratory fish to better understand fish behaviors so scientists and engineers can use this knowledge to create more effective fish passages.  

Since the size, usage and design of dams and barriers can change over time, creating effective fish passages is a constant challenge. USGS scientists must understand why some fish successfully use fish passages to bypass a barrier and others don’t.

“We have a tremendous diversity of fish species out there and current techniques are only allowing for a fraction of them to pass barriers,” Castro-Santos said. “Researching new ways to improve designs and get fish past barriers will help everything connected to these ecosystems.”

USGS experts evaluate dam designs, barrier uses, cooperator management efforts and fish behavior to create tailored fish passages that vary as needed across the nation. Studies of some older fish passage designs found a significant percentage of migratory fish failed to use them, often because they couldn’t find the entrances to the structures, Castro-Santos said. This knowledge led to the development of new techniques to draw fish into passage entrances, such as altering water-flow rates in the passage to match what a target species prefers, simulating certain noises that can entice fish to swim toward the passage, and using pheromones to attract fish.

While most fish passages are used to help fish get past manmade barriers to complete their migratory cycles, experts also believe one potential use for fish passage research is to limit the spread of invasive species up or downstream. As the USGS continues to research new and improved ways of getting native fish upstream, scientists will also be looking for ways that prevent invasive fish from utilizing those same fish passages. 

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One Size Doesn’t Fit All

Barriers come in all shapes and sizes, and there are dozens of different species of fish affected by them, so a single fish passage solution will not work for all fish in all locations.

“Scientists have learned a great deal over the past several decades when it comes to fish nature and fish passage design,” O’Connell said. “Most species need passages that are designed for their unique physical traits.”

For instance, most migratory fish face problems with barriers when they are heading downstream out to sea, such as getting caught in power generating turbines, which results in many fish kills each year. This is especially true for American eels because they spend most of their adult lives in fresh water and travel out to sea for spawning, which is the opposite of most migratory fishes’ movement pattern and could require solutions that might not work for other fish species.

“Because of the complexity and needs of so many different migratory fish there is a tremendous amount of work left to be done to improve passage for the whole range of species,” Castro-Santos said.

Why Not Just Remove the Dams?

In light of some of the problems caused by man-made barriers, there has been growing interest around the world in dam removal. However, most dams provide significant services for society like energy production, flood control and water retention for reservoirs, so removing every dam is not a viable option. Designing effective fish passages is one of the best ways to balance human needs while also supporting ecosystems and migratory fish.

“In the future, it is likely that more dams will be built around the world, not less,” said Mona Khalil, USGS Energy and Wildlife program specialist. “So it is more important than ever to continue studying migratory fish, so the best techniques and fish passage designs can be used to help them complete their migratory cycles.”

Casto-Santos believes fish passage research will have lasting benefits.

“Hopefully new science and fish passage designs will increase fish passage effectiveness,” Castro-Santos said. “Restoring access to upstream habitats is important -- and not just to the fish, but to the streams, the rivers and ocean ecosystems these fish live in, and the communities, fisherman and world populations that rely on migratory fish.”