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Cl c., ~ .......} 0"; i\) Platania. Dudley, and Maruca. 2000. Drift of Fishes in the San Juan River 199] -1997. FINAL INTRODUCTION Colorado pikeminnow (Prychocheilus 11Icius) is a federally endangered species (U.S, Department of the Interior, 1974) endemic to the Colorado River Basin where it was once abundant and widespread (Tyus, 1991), This species now occupies only about 20% of its historic range (Tyus, 1990). The Green River sub-basin apparently support the majoriry of remaining Upper Basin individuals (Holden and Wick, 1982; Bestgen, 1998). Conversely, no Colorado pikeminnow have been reported in the Lower Basin since Ihe 1960s (Minckley and Deacon, 1968; Minckley, 1973; Moyle, 1976). A small but self-susIaining populaIion of this species occurs in the lower-most 225 river Ian (between Cudei Diversion Dam and Ihe inlet of Lake Powe-ll Reservoir) of the San Juan River. The decline of this and other naIive fishes in the San Juan River has been attributed to flow modifications and Ihe resulIant changes to the Ihennal regime, instream barriers, and non-native predation- compeIiIion for habiIat and resources. Understanding the condiIions necessary for spawning in Colorado pikeminnow and other naIive fishes was deemed necessary 10 stabilize and increase the populaIion size of this species. Much has been reported regarding Ihe life-history and reproductive behavior of Colorado pikeminnow (Vanicek and Kramer, 1969). Studies in Ihe Upper Colorado River Basin (Yampa and Green rivers) have demonstrated that this species spawns as spring runoff is receding and at water temperatures between 18'C and 20'C (Haynes et aI., 1984; Nesler eI aI., 1988). Larval Colorado pikeminnow employ drifI as a dispersal mechanism and are presumed to begin this passive movement approximaIely five days after hatching. The five-day time-frame corresponded with Ihe swim-up period of Ihis tish as reported by Hamman (1981, 1986). This life-history phase (drifIing larvae), the focus of several investigations in the Upper Colorado River Basin, has been investigated to provide infonnation on spawning bar location, reproductive Sllccess, and the effects of various flow-regimes on reproduction. The collection of a juvenile (177 mm TL) Colorado pikeminnow in 1978 (Minckley and Carothers, 1979) and rediscovery of a reproducing population of Colorado pikeminnow in the San Juan River in 1987 (Meyer and Moretti, 1988; Platania and BesIgen, 1988; Platania et aI., 1991) demonstraIed a need for studies to ascertain infonnaIion such as that obtained for this species in the Upper Colorado River Basin. Such studies would also ptovide comparable information on other members of the ichthyofaunal communiry. In 1991, passive drifI-netting for larval and young-of-year (YOY) fish was initiated in the San Juan River. The primary nbjectives of the passive drift-netting study were to I) detennine the temporal distribution of San Juan River ichthyoplankton in relaIion to the hydrograph, 2) provide comparative analysis of the reproductive success of San Juan River fishes, 3) attempt 10 characIerize downstream movement ofichthyoplankIon, and 4) attempt 10 validate the presumed spawning period of Colorado pikeminnow. AcquisiIion of these data would be integrated with hydrologic and other biological studies to develop management options for the San Juan River. Biological data could Ihen be compared with San Juan River hydrology patterns to develop models that correlate Colorado pikeminnow reproductive success and annual discharge patterns, These objectives meI goals 5.3,2 (Detennine the sIatus and trends of resident fish species), 5.3.3 (Detennine Ihe life history of endangered and other native fish species and relaIionships to all oIher residenI fish species), and 5.3.5 (Characterize fish communiry response to different annual flow regimes) as defined in the San Juan River Recovery Implementation Program (SJRIP) document. 2