The fauna of the Neosho and Marais des Cygnes rivers is capable of a wide range of adjustment in response to marked environmental changes. As these rivers become low and clear they take on many of the faunal characteristics of smaller tributaries and ponds. Species such as black bullhead, spotted bass, largemouth, white crappie, red shiner, rosy-faced shiner, blunt-faced minnow, mimic shiner, and slender madtom assume a more prominent position in the total population. Other species such as channel catfish, flathead, freshwater drum, blue sucker, and such riffle-dwelling species as the gravel chub, Neosho madtom, and slender-headed darter hold a less prominent position in the total population.

When permanent flow is re-established the more mobile and the more generalized species (with respect to habitat) are able to utilize the available space immediately. As a result, these species increase rapidly in numbers. This increase occurs both by movement from more permanent waters and by reproduction. Channel catfish, flathead, freshwater drum, and river carpsucker are mobile species (Funk, 1957; Trautman, 1957) and long-nosed gar probably are mobile. Individuals that move supplement those that survive in residual pools, and provide brood stock adequate to produce a large year-class in the first year of permanent flow.

The five species last mentioned are found in diverse kinds of streams, indicating that they are adaptable to varying habitats. A sixth species, the red shiner, although probably less mobile, is able to utilize opportunistically nearly any kind of habitat in Plains streams. Although this species seldom is abundant in riffles, it was, in 1957, abundant in both pool and riffle situations at all my stations. These riffles were almost unoccupied by other species in 1957 until mid-summer, when hatches of channel catfish and flatheads occurred. Although adult channel catfish and flatheads live well in pools, the young occupy mainly riffles. This age- and size-segregation, in different habitats, was an advantage to the rapid re-establishment of these species in the Neosho and Marais des Cygnes rivers in 1957.

Species that occupy restricted habitats, especially riffle-dwellers such as the Neosho madtom, gravel chub, and slender-headed darter, were slowest to increase following drought. These species seem less capable of adapting to the variable conditions prevalent in the Neosho and Marais des Cygnes rivers than species that have more generalized habitat-requirements.

In the Neosho and Marais des Cygnes rivers nearly all species that were found in years just prior to the drought of 1952-1956 were again found in the last year of my survey; however, some species that live in a restricted habitat may eventually be extirpated in these two rivers. The high-finned carpsucker Carpiodes velifer, common shiner Notropis cornutus, horny-headed chub Hybopsis biguttata, and johnny darter Etheostoma nigrum all have specific habitat requirements and have disappeared or become restricted to one tributary in the Wakarusa River System (Deacon and Metcalf, 1961). The disappearance or reduction of these species implies long-term changes in the environment.

Suckers, minnows and catfishes constitute the main fauna of the Neosho and Marais des Cygnes rivers, because these families contain many species that have generalized habitat-requirements. Many of these fish are able to live successfully in either ponds or flowing waters and others are capable of long migrations. Because these fish predominate in the streams of Kansas, attempts should be made to utilize them more effectively.

In years such as 1957, large numbers of young channel catfish could be collected and used to stock new ponds and lakes. So doing would not affect the numbers of adults produced in the stream, and, if enough young could be removed, those remaining in the streams might grow faster.

Suckers and carp are abundant in the two rivers and mostly are unused at present, because current regulations preclude the use of methods effective for the capture of these species.