Natural and Anthropogenic Influences on theDistribution of the Threatened Neosho Madtomin a Midwestern Warmwater Stream

-We attempted to discern the contributions of physical habitat, water chemistry, nutrients, and contaminants from historic lead-zinc mining activities on the riffle-dwelling benthic fish community of the Spring River, a midwestern warmwater stream that originates in Missouri and flows into Kansas and Oklahoma. The Spring River has a fish community that includes the Neosho madtom Noturus placidus, a species federally listed as threatened. Although anthropogenic factors such as contaminants limited populations and densities of fishes, an integrated assessment of natural and anthropogenic factors was necessary to effectively estimate the influence of the latter. Fish populations in the Spring River, especially Neosho madtoms, seem to be limited by the presence of cadmium, lead, and zinc in water and in benthic invertebrate food sources and by physical habitat. The population density and community structure of fish in the Spring River also seem to be related to water chemistry and nutrients. Concurrently, diminished food availability may be limiting fish populations at some sites where Neosho madtoms are not found. Many of the natural factors that may be limiting Neosho madtom and other riffle-dwelling fish popUlations in the Spring River probably are characteristic of the physiographic region drained by the upper reach and many of the tributaries of the Spring River. Our results indicate that competition between the Neosho mad tom and other species within the riffle-dwelling fish community is an unlikely cause of Neosho mad tom population limitation in the Spring River. Relationships between stream fish communities and their habitats have been well documented (Angermeier and Karr 1984; Matthews and Heins 1987; Kessler and Thorp 1993). Physical habitat complexity has been correlated with fish species diversity (Gorman and Karr 1978). Habitat factors such as water depth, velocity, and substrate composition are important to stream fishes (Aadland 1993). Moreover, habitat utilization by stream fishes varies with community composition (Fausch and White 1981; Finger 1982), and water chemistry and nutrients affect the distribution and abun* Corresponding author: mark..wildhaber@usgs.gov Received March 19, 1998; accepted May 14, 1999 dance of stream fishes (Layher and Maughan 1985; Layher et al. 1987; Maret et al. 1997). Habitat has been the primary focus of studies that target factors limiting the distribution and density of stream fishes, especially threatened and endangered fishes (Kessler and Thorp 1993; Freeman and Freeman