Over the past hundred years, people have introduced dozens of non-native fish species into the Colorado River and its tributaries. During that time, populations of native fish species have dropped, in some cases dramatically. It’s easy to guess at the causes of native species decline, like predation and competition for food. But it’s far more difficult to prove.
Researchers from Northern Arizona University and Arizona State University recently studied two native fish species and their presumed non-native competitors. Two of the fish are bottom-dwellers: native Sonora sucker and non-native common carp.
The other two live in the pools, runs, and rapids of the Colorado River drainage: native roundtail chub and non-native smallmouth bass. Populations of both Sonora sucker and roundtail chub have plummeted in recent decades. Both species now occupy only a fraction of their historic range.
Researchers tested the feeding behavior of all four fish in over 600 trials. They found that the native fish actually ate a broader range of foods than non-natives—a trait that would seem to give them a competitive edge. However, the non-natives had larger mouths and were more aggressive feeders. They also rely more on vision to locate prey than natives, an advantage in rivers where dams have increased water clarity.
Painstaking studies like this one help clarify the complex interactions between native and non-native fish and wildlife. That’s an essential part of developing management strategies to protect native species and maintain the region’s biodiversity.