News Release

Release of captive-bred native fish negatively impacts ecosystems, study finds

Reports and Proceedings

University of North Carolina at Greensboro

Greensboro (March 7, 2023) — A new study in the Proceedings of the National Academy of Sciences finds that large-scale fish releases negatively impact ecosystems as a whole, while offering little benefit and some harm to the species they seek to support. 

For over a century, fisheries and natural resource managers have bred native fish in captivity and then released them, en masse, into the wild. It’s a popular method for supporting commercially important or threatened populations: more than 2 billion captive-bred Pacific salmon were released in the U.S. in 2016 alone. 

Unfortunately, the 150-year-old practice may be doing more harm than good, say researchers at UNC Greensboro, Hokkaido Research Organization, Hokkaido University, and the National Institute of Polar Research in Japan. 

UNCG freshwater ecologist Dr. Akira Terui, who led the study and whose research focuses on community ecology, was not surprised by his team’s results. "Many resource managers believe that releasing captive-bred native species into the wild is always a good thing,” he says. “However, ecosystems are delicately balanced with regards to resource availability, and releasing large numbers of new individuals can disrupt that. Imagine moving 100 people into a studio apartment — that's not a sustainable situation." 

The researchers used mathematical modeling to predict how massive releases influence surrounding species of fish in the wild. They then tested and confirmed their model predictions using 21 years of stream monitoring data from 97 rivers in Japan. 

“In an ecosystem, the balance that allows different species of fish with similar needs to co-exist is fragile,” Terui says. “When there is a massive release of members of one species in an ecosystem without the capacity to support them, then the other species populations decline due to greater competition for resources.” 

Moreover, the native species that the releases are designed to aid were also negatively impacted. Over the last two decades, Terui says, studies have already shown that a major issue with releasing captive-bred fish is the spread of genes reducing the target species’ survival in the wild. 

"We found that competition with a vast number of captive-bred members of a species leads to reduced numbers of naturally-bred members of the same species. Replacing naturally occurring members of a species with captive-bred individuals has the potential to reduce genetic diversity and reproductive fitness."

The researchers observed that fish communities exposed to hatchery salmon releases had more fluctuations in population density across time — an unstable dynamic that increases the risk of various populations dying out entirely. As expected, these communities contained fewer species overall. 

As evidence mounts that captive-bred releases negatively impact population health and ecosystem biodiversity, Terui says he hopes methods will change. “The U.S. Fish and Wildlife Service is currently spending hundreds of millions of dollars a year on fish hatcheries. Natural resource managers need to be considering alternate priorities like habitat conservation.”


About UNC Greensboro

Located in North Carolina’s third largest city, UNC Greensboro is among the most diverse, learner-centered public research universities in the state, with nearly 18,000 students in eight colleges and schools pursuing 175 areas of undergraduate and 250 areas of graduate study. UNCG continues to be recognized in national publications for academic excellence, access, and affordability. For the fourth consecutive year, U.S. News and World Report ranked UNCG No. 1 in North Carolina for social mobility — helping more first-generation and lower-income students find paths to prosperity than any other public university in the state. With a portfolio of more than $56M in research and creative activity, UNCG’s nearly 1,000 faculty and 1,700 staff help create an annual economic impact for the Piedmont Triad region in excess of $1B.


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