Evaluation of the Incremental Methodology for Recommending Instream Flows for Fishes

Application of the \u27incremental methodology\u27 is limited in many situations because quantitative information on microhabitat preferences of fishes is scarce, and because few field tests of the validity of the assumptions have been conducted. In this study, two critical assumptions were tested and the incremental methodology was applied in a warmwater stream in southeast Oklahoma. Habitat-suitability curves were developed for the freckled madtom Noturus nocturnus, the central stoneroller Campostoma anomalum, and the orangebelly darter Etheostoma radiosum. The assumption that depth, velocity, and substrate are perceived independently in the selection of microhabitats by these fishes was tested. The relation between standing stock and usable habitat (weighted usable area) was then investigated for these species as well as for adult and juvenile smallmouth bass Micropterus dolomieui. Fish population estimates were made quarterly over 2 years at four study sites concurrent with estimates of weighted usable area for each species. Instream flow recommendations then were developed on the basis of the incremental methodology, the wetted-perimeter method, and the Montana (Tennant) method. For the freckled madtom, the central stoneroller, and the orangebelly darter, the assumption of independence of habitat-suitability functions was violated for at least one of the three possible two-way combinations. Violation of this assumption was greatest for the interaction of depth and velocity. There were no significant correlations between weighted usable area and standing stock for adult and juvenile smallmouth bass in any season. For the freckled madtom, the central stoneroller, and the orangebelly darter, correlations between weighted usable area and standing stock were consistently significant during the summer. Monthly instream flows recommended for Glover Creek equaled or exceeded the historical flows in typical water years. Flow recommendations based on the Montana and wetted-perimeter methods were similar to those based on the incremental methodology for the low-flow season (July-December) only