CANCELLED - Structural differences between fish assemblages in natural and altered major rivers

Background/Question/Methods

Anthropogenic disturbances such as damming, channelization, and water withdrawals have altered many large rivers worldwide.  Several competing theories exist, but many aquatic biologists contend that altered systems are more susceptible to invasion by nonnative species than unaltered systems.  However, recent terrestrial studies indicate that naturally functioning systems, which tend to maintain greater habitat diversity, support more species—both native and nonnative.  The Yellowstone and Missouri rivers in Montana provide a unique opportunity to evaluate how anthropogenic disturbances influence the establishment and proliferation of nonnative species, as well as the preservation of native fish assemblages.  They are adjacent watersheds with proximal headwaters, parallel river courses, and similar lengths and watershed areas.  However, the two rivers have experienced vastly different histories of anthropogenic alteration—the Yellowstone River remains the longest free-flowing river in the conterminous United States whereas multiple dams impound the mainstem Missouri River. We sampled the fish assemblages of both rivers to determine if the naturally-functioning Yellowstone River supported more native and nonnative species than the highly altered Missouri River.

Results/Conclusions

We collected 74,892 fish in the Yellowstone River (n = 49 sampling reaches) and only 13,786 in the Missouri River (n = 84 reaches) from 2008 to 2010.  Much of the difference was the result of large schools (i.e., > 500 fish in a single mini-fyke net) of native cyprinids in Yellowstone river samples.  Despite nearly half as much sampling effort as in the Missouri River, we collected more native and the same number of nonnative species in the Yellowstone River.  However, nonnative fish comprised only 1.1% of the total catch in the Yellowstone River as opposed to 7.5% in the Missouri River.  We also observed higher relative abundances of degradation-tolerant and coldwater species in the Missouri River.  Our results indicate that natural functioning rivers may in fact support more nonnative species than altered rivers while preserving the structure and function of the native fish assemblage.