US Rivers Are Filling Up With Invasive Fish

The rivers of the U.S. and other countries around the world are becoming more diverse and filling up with a wide range of invasive fish, but that may pose a threat to the "delicate balance" of native species, a study has found.

New research from the University of Sheffield with Illinois State University, the University of Tennessee and University of Washington found that there are more diverse species of fish emerging in rivers. At first, scientists thought this was because of improved water quality and ecosystems.

However, the research, published in Nature Ecology Evolution, points to a different explanation—an increase in invasive species. It uncovered a 13 percent increase in fish communities per decade, and a 7 percent rise in species diversity, but a 30 percent decrease in "riverine fish community similarity."

The increase in river fish diversity may not be good news, the authors said. It could be threatening native fish populations and reflect that invasive species often thrive in ecosystems degraded as a result of human behavior.

"In the Western U.S., species such as largemouth bass, smallmouth bass, and walleye are considered invasive because they have been moved from their native habitats in the Eastern U.S. and are now having detrimental impacts on Western streams and rivers by feeding on native salmonids," study authors Alain Danet, of the University of Sheffield, and Xingli Giam of the University of Tennessee, told Newsweek in a statement.

"Some invasive species have come from outside the U.S. and their first introductions were intentional and authorized by governmental agencies," they continued. "Notable examples include brown trout and common carp, both of which were introduced to the U.S. in the latter 1800s from Europe and/or Asia. Another notable invasive species is the northern snakehead, a more recent introduction to the U.S. from East Asia. Unlike the brown trout and common carp, the northern snakehead was introduced likely via unauthorized releases."

The researchers also found that the areas where fish communities changed more quickly were the places that were suffering from habitat degradation due to human activities.

A rich fish community was more common in areas where there were significant human impacts. And here, crucially, there appeared to be fewer native species.

"We often rely on basic metrics to evaluate the health of an ecosystem, such as the number of species and the number of fish individuals, such increases in those numbers are generally interpreted as good news in terms of ecosystem recovery," Danet said. "This is particularly so in historically industrialised countries such as in North America and Western Europe, which have seen massive habitat degradation.

"Our study shows that increases in the number of fish species and individuals over the last decades in those historically degraded areas are accompanied by an upsurge of non-native species and rapid changes in the identity of the dominant species."

The introduction of invasive species, whether accidental or deliberate, has had a negative effect on ecosystems. Invasive species can be incredibly harmful to certain ecosystems. Sometimes they wipe out native species by preying on them, or destroying their food sources.

"This is concerning because it means that we are seeing important and rapid changes toward riverine fish communities more dominated by non-native species," Danet said. "It is well established that non-native species thrive in degraded areas, can trigger the extinction of native species and have significant effects on ecosystem functioning. So we are seeing important and rapid changes in fish communities, triggered by non-native species and linked to long-lasting and persistent human pressures on rivers."