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.. 154 CooseMtion of Slream Fishes tive (Fig. 5) fish faunas are now connected by the Ten- nessee-Tombigbee Waterway (Boschung 1987). Although mainstem impoundments on the Tennessee and lentic environments associated with locks on the Tombigbee may limit dispersal of many stream fishes, the stage is set for invasions, hybridization, species re- placement, and ultimately a reduction in total species richness of the combined fauna (Preston 1962). Conclusions The common perception of all small non-game fishes as "minnows" and their existence in environments where few persons ever see them makes it difficult to muster public support for their conservation. In conflicts over development of increasingly valuable water resources, the fishes have few advocates. Beyond the serious issue of public awareness, however, lie other problems intrin- sic to river systems and their biota. In any species-rich fauna, most species are rare, and stream fishes are no exception (Sheldon 1987). In such cases reductions in the number of species by environ- mental stress or habitat fragmentation may be more pre- dictable than the identities of censored species, so fau- nal management, rather than a focus on single species, is required. Species-area relationships (Fig. 4) imply that extinc- tions will follow fragmentation of drainage networks. The impoundment of virtually the entire 1000 km length of the Tennessee River caused the disappearance of the minnow Hybognathus nucbalis from the entire system including tributaries not directly affected by res- ervoirs (Etnier et al. 1979). Other species may have been lost in some tributaries in the 50 years since con- struction began. Records and collections, as is inevitable when many rare species are involved, are probably in- adequate to fully document such extinctions if they oc- curred. Both theory and experience with other taxa and habitats suggest that extinctions are inevitable. The between-drainage component of fish diversity is large (Fig. 5), but introductions and extinctions will eventually homogenize the fish fauna to the point where much of the variety and evolutionary history has been lost. Prohibition of interbasin transfers of water and more effective controls on introductions could greatly slow this process. The geometry of rivers further complicates preserva- tion. Rivers are open, directional systems so protection of any segment requires control over the entire up- stream network and the surrounding landscape. There is little likelihood that such protection can be given to very many streams of order 4 or higher, yet it is these streams that support the greatest diversity of fishes. Op- timistically, some degree of riparian control and preven- tion of industrial and agricultural pollution, channeliza- Conservation Biology Volume 2, No. 2,June 1988 Sheldon tion, and impoundment may be sufficient to maintain diverse assemblages. Still larger streams, such as the Tennessee, have been irreparably modified. Conserva- tion efforts should be focused on the largest reasonably natural drainages in as many major river systems as pos- sible. McDowall (1984) and Maitland (1985) suggest useful criteria for site selection and protection of fresh- water fishes. However, the size of North American rivers and the diversity of their fishes make the problems far more difficult. (Buck Creek [Cicerello & Butler 1985] supports more native fishes than the British Isles.) A comprehensive plan for the conservation of North American fishes lies in the future. A biogeographic over- view, as opposed to piecemeal species management, is a necessary component of such a plan although single- species management is compatible with a more inclu- sive approach since streams containing rare species of- ten support diverse assemblages. The conservation of fishes is also compatible with the protection of other organisms, such as molluscs and crayfishes, and of ripar- ian vegetation, water quality, amenity, and the entire spectrum of values of running-water ecosystems. The evolutionary wealth of fishes that so impressed David Starr Jordan ( 1922) and earlier scientists is an important component of that value spectrum. Acknowledgments Peter Moyle and Jim Williams made helpful comments on the manuscript. DeWayne Williams prepared the fig- ures. Dave Etnier, Herb Boschung, and the Corps of En- gineers, US. Army, provided information and slides for the oral presentation of this material. I thank them all. literature Cited Balon, E. K., S. S. Crawford, and A. Lelek. 1986. Fish communi- ties of the upper Danube River (Germany, Austria) prior to the new Rhein-Main-Donau connection. Environmental Biology of Fishes 15:243-271. Bermingham, E., and). C. Avise. 1986. Molecular zoogeogra- phy of freshwater fishes in the southeastern United States. Ge- netics 113:939-965. Boschung, H. T. 1987. Physical factors and the distribution and abundance of fishes in the upper Tombigbee River system of Alabama and Mississippi, with emphasis on the Tennessee- Tombigbee Waterway. Pages 184-192 in W.). Matthews and D. C. Heins, editors. Community and Evolutionary Ecology of North American Stream Fishes. University of Oklahoma Press, Nonnan, Oklahoma. Brown, Co). D. 1971. Fishes of Montana. Big Sky Books, Bozeman, Montana. Cicerello, R R, and R S. Butler. 1985. Fishes of Buck Creek, Cumberland River drainage, Kentucky. Brimleyana 11:133- 159.