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2 INTRODUCTION I NTRODUCTI ON Fish eggs and larvae are a principal focus of many ecological studies. Their sensitivity to environ- mental changes and susceptibility to entrainment are major concerns of siting, environmental impact, and monitoring surveys now required of many industries and utilities. Their distributions and densities are indicative of spawning and nursery areas, spawning seasons, larval behavior, and year-class strength. Even in baseline surveys designed to determine the presence and relative abundance of fishes, larval collections can often fill gaps in fish collection caused by gear selectivity, behavior, or habitats that make certain species difficult to collect or observe as adults. Studies of fish larvae can also provide information on morphological development, systematics, growth rates, survival rates, food habits, predation and various other ecological relationships (Snyder 1976a). One of the major obstacles to including fish larvae in studies of the Upper Colorado River System, as well as in most other waters of the country, is the lack of adequate descriptions, keys, or guides to facilitate identification. In the report of a work- shop group chaired by Snyder (1976a) it was estimated that of about 700 species of fish found in North America's freshwaters, the eggs or larvae of only about 15% had been described and illustrated in the published literature. The report also discussed the limited number of regional keys and guides that included freshwater species, and noted that most were far from complete in coverage. The illustrated guides cited were Fish (1929a, 1929b and 1932, Lake Erie), Winn and Miller (1954, "postlarval" cyprini- forms of the Lower Colorado River Basin), Mansueti and Hardy (1967, Chesapeake Bay Region), May and Gasaway (1967, Oklahoma), Taber (1969, Lake Texoma, Oklahoma) and Lippson and Moran (1974, Potomac River Estuary). Since that report, numerous descriptions have been added to the published literature as well as several major works including the following illustrated guides or keys: Lippson (1976, family characteristics, Great Lakes); Hogue, Wallus and Kay (1976, Tennessee River); Loos and Fuiman (1977, numerous species of the genus Notpopis); Fuiman (1978 and 1979, Northeastern catostomids); Conner (1979, centrarchids, Louisiana); Loos et al. (1979, cyprinids, Upper Potomac River); Perry and Menzel (1978, cyprinids, Iowa); Wang and Kernehan (1979, Delaware Estuaries); Drewry (1979, punch card key to families of larvae with yolk, Great Lakes); and three of the six volumes of the Mid-Atlantic Bight series, Jones, Martin, and Hardy (1978), Hardy (1978a) and Hardy (1978b). The latter series is the most compre- hensive synthesis of previously-published descriptive information and illustrations on the eggs, larvae, and juveniles of the species covered that is currently available; it also includes some ori~inal work. Unillustrated works such as Snyder (1971, preliminary family key, lower Susquehanna River), Nelson and Cole (1975, many species, western end of Lake Erie) and Dorr, Jude, Tesar and Thurber (1976, several species, southeastern Lake Michigan) are generally less useful than illustrated guides and keys. Many, perhaps the majority, of species described in pub- lished literature, theses, and reports are not covered in the keys and guides cited above. Bibliographies of early life history literature by Mansueti (1954), Werner (1976) and Kernehan (1976, up-date in progress) are most valuable in recognizing much, but by no means all, of this widely scattered and often obscure 1 i tera ture. Not all of the information in the publiShed literature is entirely reliable. Some publiShed descriptions are based on misidentified specimens. For example, the 7.0, 7.7, 8.4, and 9.7 mm TL specimens illustrated as MicPOpterus saZmoides in Taber (1969) and reprinted as such in Lippson and Moran (1974) and Wang and Kernehan (1979) are a Lepomis species, possibly L. megaZoti~. .Also, . some guides include errors in transcrlptlon of certaln information from the original descriptions (e.g. lengths of illustrated Mopone amep~cana in Lippson and Moran 1974 and Wang and Kernehan 1979), Some of these misidentifications and transcription errors have been reported in the published literature (Snyder and Douglas, 1978) while others await publi- cation or discovery. Of the 20 cypriniform fishes found in the Upper Colorado River System in Colorado, only Catostomus commepsoni, Cyprinus carpio, PimephaZes ppomeZas, NotPOpis Zutpensis and Riahapdsoni~ baZteatLfs, all non-natives, were adequately descrlbed and lllustrated as larvae for identification purposes when work on the planned guide was begun in 1977. The descriptions of the latter two species were only marginally adequate. None of the native species were adequately described throughout a major portion of their larval develop- ment. Until work on the guide is completed, these contributions are intended to provide additional descriptive information useful in identification of the basin's cypriniform fish larvae. THE UPPER COLORADO RIVER SYSTEM AND ITS FISHES All of Colorado west of the Continental Divide is drained by the Colorado River System. For water management purposes, the system has been politically divided at Lee Ferry, Arizona (just below the Glen Canyon Dam and Lake Powell) into upper a~d lower. systems or basins. The Upper Colorado Rlver Basln consists of three hydrologic subbasins, the Green River Basin, the Upper Main Stem Colorado River B~sin, and the San Juan-Colorado River Basin, each of WhlCh is Significantly represented in Colorado (Fig. 2). The nature of the Upper Colorado River System, its aquatic inhabitants and man's impact on it were summarized by Joseph et al. (1977). Historically, the river system has provided rigorous aquatic environments typified by great fluctuation in flow, velocity, turbidity and temper- ature. Joseph et al. (1977) described three major habitat zones within the river system: 1) an upper zone of cold, high mountain streams; 2) an intermediate zone of small and medium-sized streams or rivers; and 3) a lower zone of larger, more turbid rivers. The latter includes both steep- gradient canyon areas and meandering river stretches in flat terrain with low-gradient canyons. The numerous reservoirs constructed in relatively-recent years in all three zones should be considered a fourth habitat zone, All habitat zones are represented in Colorado. Man has dramatically changed and continues to alter the nature of the Colorado River System physically, chemically and biological~y. Due,t? his need to provide potable water to dlstant clt~es and to make small portions of the desert and seml- arid regions of the Southwest green with lawns, orchards, and other agricultural crops, the once