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Final Report 4-1 September 2000 4 FISHES OF THE GREEN RIVER The major physical disruption to native fishes and their habitats in the main-stem Green River was caused by the construction and operation of Flaming Gorge Dam (Section 1.2.1). The fish community downstream of the dam, which consists of trout and native and nonnative cool-water and warm-water species, is strongly affected by water releases from the dam and the inflow of the Yampa Ri ver at Echo Park. With the exception of usually minor flow contributions from tributaries, the flow and temperature of the Green River upstream of the Yampa River are completely regulated by the dam. Releases of cold, clear water allowed for the establishment of a tailwater trout fishery and eliminated most, if not all, successful reproduction by most native fishes. Downstream of Echo Park, the Green River is more similar in flow, sediment loads, and temperature to pre-dam conditions because of the inflowing Yampa River. Trout become incidental to rare, and native and nonnative warm-water species become the major components of the fish community. This chapter describes the fish fauna of the Green River from Flaming Gorge Dam downstream to its confluence with the Colorado River, with an emphasis on the endangered humpback chub, Colorado pikeminnow, and razorback sucker. The chapter is divided into four main sections. The first (Section 4.1), is an overview of the distribution, qualitative relative abundance, and habitats of the native and nonnative fishes that provides a general characterization of the overall fish community. The last three sections present detailed accounts of the ecology and habitat requirements of Colorado pikeminnow (Section 4.2), razorback sucker (4.3), and humpback chub (4.4). Information presented in these three sections was used to develop the integrated flow and temperature recommendations (Chapter 5) to ensure that the habitat needs of the endangered fishes are met. Although the species accounts (Sections 4.2-4.4) are organized similarly, their content and format do vary because different levels of ecological information were available. Brief overviews of rangewide historic and present distributions, reasons for decline, and life-history attributes are followed by a summary of research on the species in the Green River system, including studies conducted in support of the 1992 Biological Opinion (USFWS 1992) and this synthesis report. A description of the species' ecology in the Green River system is the centerpiece of each account. Each account begins with a discussion of the changes in distributi on and abundance patterns associated with the construction and operation of Flaming Gorge Dam. Unique to the Colorado pikeminnow account (Section 4.2) is a conceptual model describing a range of factors that may currently affect the fish's growth, survival, and recruitment. Information on the life history of each species is presented by season - spring (21 March to 20 June), summer/autumn (21 June to 20 December), and winter (21 December to 20 March) - because distinct biological processes occur in each seasonal period. The Colorado pikeminnow account includes discussions of recruitment dynamics (long-term monitoring data are used to describe abundance trends for several life stages) and enhancement of thermal regimes to benefit the species. ~