Some US streams and rivers facing severe declines in fish populations, new study suggests
The composition of fish populations in streams and rivers across the U.S. has been severely altered as a result of changing water temperatures and human-driven introductions of fish, according to new research.
Cold-water streams have experienced disproportionate impacts, experiencing more than a 50% decrease in fish abundance over the last three decades, according to a study published Wednesday in Nature.
Some of the largest threats include climate change and fish introductions -- both by invasive species or game fish stocking, scientists say.
Researchers from the U.S. Environmental Protection Agency tracked trends in freshwater fish biodiversity in streams and rivers all over the U.S. from 1993 to 2019. During that time, they monitored 389 species across 2,992 sites.
They found that species abundance, or the number of individual fish, and species richness, the number of unique species, declined by 53.4% and 32%, respectively, in cold streams -- where temperatures are typically below 60 degrees Fahrenheit.
Communities within these ecosystems became more distinct, the researchers said. The proportion of larger, late-maturing fish increased, but the abundance and richness of smaller fish decreased, likely due to the introduction of large game fish, according to the paper.
However, the trends reversed for warm streams, with average temperatures above 75 degrees Fahrenheit. The abundance and richness of fish in warm streams increased by 70.5% and 15.6%, respectively, according to the paper.
Communities became more similar in warm streams as well, with smaller fish replacing larger fish.
Impacts from temperature were likely to be greater in cold streams because those ecosystems are more sensitive to increasing temperatures, the researchers said. When both were present, it resulted in increased rates of degradation to the local fish biodiversity.
In addition, changes in temperature and abundance of introduced game fish magnified the impacts of each other, according to the paper.
Fish biodiversity is under threat globally as a result of warming temperatures and other environmental factors.
More than 18,000 fish species live in the freshwater systems around the world -- accounting for nearly 25% of all vertebrates on the planet, according to the paper. Freshwater fish are "vital" sources of food and income for humans, the researchers said.
A Nature study published in January found that a quarter of the world's freshwater species are at risk of extinction.
Environmental agencies will need to develop strategies to mitigate warming and control fish introductions in order to conserve the natural biodiversity, the authors said. The need is "urgent" given the size of the changes over less than three decades, the researchers said.
The EPA did not immediately respond to ABC News' request for comment.
