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. . - 3 - The documented existence of a remnant humpback chub population in Yampa Canyon justifies further efforts to determine the reproductive success of this population. Examination of the existing preserved fish collection will permit the determination of the potential reproductive success of the adult population in the recent past without requiring further field collection of fish samples. The results of this work would also provide the basis for determining if and what type of future field study may be needed. If a substantial number of !UJA ~ are found, this may warrant a more comprehensive screening of this and the existing Fish and Wildlife Service fish collections at Grand Junction, Colorado and Yernal, Utah. Reproductive success in the established humpback chub population at Black Rocks has been assumed in the past due to the presence of smaller-size humpback chub in the samples. Due to the taxonomic difficulties and costs of determining the presence of larval of early juvenile humpback chub in seine collections at Black Rocks and elsewhere, indices of young-of-the-year abundance have not been estimated. An important aspect of monitoring the stability of the Black Rocks' humpback population and their continued welfare would be to establish relationships between environmental variables such as flow regime with indices of reproductive success by the adult population. This concern would also apply equally to the Yampa River population if sufficient numbers of young Gila cvoha are found. This work would be complementary to the major ~ taxonomy study that is ongoing, and would provide supplementary data to this high priority project. The approach will be to take a subsample of the Colorado Division of Wildlife collection of over 27,000 preserved Gila and screen those fish for possible Gila cvoha using fin ray and myomere counts. Unpublished Ph.D. data on the taxonomy and development of early-life stage Qil! sp. collected by Robert Muth, Larval Fish Lab, Colorado State University, Fort Collins, ColoradO, indicates that 90 percent of the young-of-the-year Gila examined that had fin ray counts of 10 or more were ultimately determined to be Gila cvoha after detailed morpho-meristic analyses. This sample index will be used to permit work study students to examine approximately 10,000 Gila (5,000 from each river) and screen for potential humpback chub. The smaller subsample of suspected g. cvoha would then be examined in detail using morpho-meristic and developmental criteria developed by R.T. Muth's doctoral study to verify taxonomic identification. The Gila collection used in this study will consist of seine-collected specimens from both rivers during 1981-1983. Only metalarval and juvenile Gila from seine samples collected in July and August of these three years will be used for screening. These years of collection represent the bulk of the existing CDOW samples and the sample months represent the best opportunity for finding the above life stages of Gila in the seine samples. This stratified subsample approach was selected as having the highest probability of determining if significant numbers of Gila ~ were preeent, short of examining the entire collection. Subtask A: compile samples containing seine-collected Gila sp. from CDOW collection at CSU LFL and determine a subsampling scheme. Subtask B: hire work study assistants, train in fin ray screening process, subsample collection, and screen for suspected Gila cvoha. Subtask C: examine suspected g. ~ sample using morpho-meristic and developmental criteria developed in Muth, R.T. 1990. Ontoaenv and taxonomy of humoback chub. bonvtail, and roundtail chub larvae and early iuveniles. Ph.D. dissertation, Colorado State University, Fort Collins. Subtask D: determine number and relative abundance of g. cvoha young-of-the year by river and year of collection, and relative to USGS gage records for flow and temperature regime. L/ll