A new study by researchers at Oregon State University and the U.S. Environmental Protection Agency found that more than half of the stream and river miles surveyed in 12 western states contained non-native fish and amphibians.
The proliferation of non-native species is even more prevalent in Arizona, Colorado and Montana, where the rate averages more than 80 percent of the stream miles surveyed, and in the northern Great Plains and western arid lands, where two-thirds of the stream miles contained non-natives.
Oregon and Washington had fewer non-native species, yet 20-21 percent of the stream miles in the Pacific Northwest still had fish or amphibians that were not present when Lewis and Clark first explored the region.
Results of the study have just been published in the North American Journal of Fisheries Management.
"As an ecologist, these numbers are disturbing – even when you take into account that some of these non-native species have value to anglers and others," said Bob Hughes, a senior research professor with OSU's Department of Fisheries and Wildlife, and a co-author of the study. "Things that evolve naturally tend to use resources more efficiently. When things are introduced, you run the risk of depleting native species.
"You never know which native species loss will trigger a domino effect on an ecosystem," he added. "How many rivets can you remove from the wing of an airplane and still have it fly?"
The Environmental Monitoring and Assessment Program study was designed as a survey of western streams to gauge the overall health of the region's waterways, the authors say. It was not intended to evaluate individual river systems. More than 1,300 sites were surveyed during a four-year period from 2000-04.
The most widespread non-native species were brook, rainbow and brown trout, which were found in 14-17 percent of the streams surveyed. While the rainbow trout is considered something of an iconic species, it is not native in many portions of the West. It is one of the most widely introduced fish and has taken a foothold in many streams and lakes because it adapts well, is aggressive, and hybridizes with others species.
"In the Pacific Northwest, the prevalence of rainbow trout may be having an impact on native trout," said Gregg Lomnicky, a project scientist with the Dynamac Corporation, which contracted with the EPA for portions of the research. "If we want to maintain populations of cutthroat trout and salmon, we may not be helping them with the introduction of these non-native species."
Some of the more common warm-water non-natives include the common carp, found in 10 percent of western stream miles; fathead minnows, 5 percent; green sunfish, 4 percent; and smallmouth bass, 4 percent.
Other non-native species were less common, but could be even more problematic the researchers say. The non-native bullfrog was found in only 1 percent of the stream miles, but this voracious predator has been blamed for depleting populations of native fish, frogs and pond turtles.
Weighing the value of one species versus another is a societal decision, the researchers say.
"The native Oregon chub is at risk from smallmouth and largemouth bass, neither of which are natives," said Thom Whittier, a faculty research assistant in the OSU Department of Fisheries and Wildlife. "Yet bass are a popular sport fish and the chub seems kind of pedestrian in comparison. Another example is the brook trout, which also is a popular fish, but creates real problems for native cutthroat and bull trout in Oregon.
"Attitudes change over time," Whittier added. "In the 1880s, they hauled carp out west by the trainload from back east to serve as a food and game fish. Now they're considered a pest."
Once introduced, it can be difficult to rid lakes and streams of non-native species. State officials had to use poison in southern Oregon's Diamond Lake to get rid of an exploding population of tui chubs, which marred water quality and temporarily devastated a well-known trout fishery. California has spent tens of millions of dollars over the last 13 years in an attempt to rid Lake Davis of northern pike; the results of this year's eradication project are not yet known.
In Arizona, eight of the top 12 most prevalent species in the rivers are now non-native.
Oregon's Willamette River has seen an increase in smallmouth bass, says Hughes, who has spent 20 years studying its fish as part of different research projects.
"In the years I've spent studying the Willamette, I've seen the number of bass increase and their range move gradually upriver," Hughes pointed out. "And the smallmouth may now be the dominant species in the Umpqua and John Day rivers."
Human activity has changed the look of many lower-elevation western river systems, which historically were braided and complex with multiple channels that created sanctuaries for migrating and resident fish. Dams, channelization, development and the transition from forest to pastures and housing tracts have eliminated that complexity in many rivers, which now look more like long ponds than multi-channeled streams.
"One question that comes to mind is whether everything should look the same," said Whittier. "Western rivers are slowly becoming homogenized and losing some of their individual identity as non-native species gain a foothold and threaten native fish. It is a serious challenge for people who do restoration work and manage the fisheries."
At least 50 sites were surveyed in each of the western states that included Washington, Oregon, California, Arizona, Nevada, Utah, Colorado, Wyoming, Idaho, Montana, North Dakota and South Dakota.