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<br />INTRODUCTION
<br />
<br />Importance of Early life History Investigations and Identification
<br />
<br />F or most fishes, larval and early (young-of-
<br />the-year) juvenile development includes a few to
<br />several life-history phases that are ecologically
<br />distinct from each other, as well as later juve-
<br />niles and adults (Snyder 1990; such phases do not
<br />necessarily correspond with the morphologically
<br />based developmental intervals defined below).
<br />Accordingly, knowledge of fish early life history
<br />is often essential for better understanding
<br />aquatic ecosystems and communities and more
<br />effectively monitoring, protecting, or managing
<br />fish populations and habitat. Such knowledge is
<br />particularly valuable in assessing environmental
<br />impacts and recovering endangered species.
<br />The collection and study of fish eggs,
<br />larvae, and early juveniles are or should be inte-
<br />gral parts of many fish and aquatic ecology
<br />investigations. Their spatial and temporal distri-
<br />bution and densities are indicative of spawning
<br />and nursery areas, spawning seasons, larval pro-
<br />duction, nursery habitat, behavior, and potential
<br />year-class strength. A single specimen is proof
<br />of at least some reproductive success. Even in
<br />baseline surveys to determine presence and
<br />relative abundance of fishes, larval-fish collec-
<br />tions can sometimes provide information on
<br />species that are difficult to collect or observe as
<br />adults because of gear selectivity, behavior, or
<br />habitat.
<br />Research or monitoring based on collec-
<br />tions of fish larvae usually requires accurate
<br />identification of collected specimens. Inland
<br />fishery managers and researchers often exclude
<br />
<br />potentially critical larval-fish investigations
<br />specifically because they haven't done it before
<br />or they don't have the taxonomic tools needed
<br />for the job. Unfortunately, adequate description
<br />of larvae, determination of taxonomic criteria,
<br />and development of keys for identification are
<br />time-consuming and expensive tasks. Although
<br />the inventory of such information is gradually
<br />increasing, much descriptive and taxonomic
<br />research is piecemeal, uncoordinated, and often
<br />"a labor of love."
<br />Of approximately 800 species of freshwater
<br />and anadromous fishes in the United States and
<br />Canada (Lee et al. 1980, Robins, et al. 1991)
<br />less than 25% have been adequately described as
<br />larvae for identification purposes (Snyder 1996,
<br />extrapolated from 15% reported by Snyder
<br />I 976a). In a relatively comprehensive listing of
<br />regional larval-fish guides, keys, and compar-
<br />ative descriptions by Simon (1986), only about
<br />80 of230 citations (35%) pertain to freshwater
<br />species. Kelso and Rutherford (1996) listed 18
<br />regionally oriented larval-fish identification
<br />manuals for or including North American fresh-
<br />water species (some for the same regions and all
<br />incomplete in coverage at the species level).
<br />Not included in the list were guides by Sturm
<br />(1988), Snyder and Muth (1988, 1990- probably
<br />treated as comparative descriptions rather than
<br />regional guides), and most recently, Simon and
<br />Wallus (2004). No guides to or including North
<br />American freshwater fish larvae were published
<br />between 1994 and 2004.
<br />
<br />This Guide and Prior Descriptions
<br />
<br />The purpose of this publication is to de-
<br />scribe and better facilitate identification of the
<br />larvae and early juveniles of Catostomidae
<br />(suckers) in the Upper Colorado River Basin
<br />(UCRB, Fig. I )-the native razorback, flannel-
<br />mouth, bluehead, and mountain suckers, and
<br />non-native white, longnose, and Utah suckers
<br />(Xyrauchen texanus, Catostomus latipinnis, C.
<br />disco bolus, C. platyrhynchus, C. commersoni,
<br />C. catostomus, and C. ardens respectively; com-
<br />mon and scientific names used herein follow
<br />
<br />Robins et al. 1991). All belong to subfamily
<br />Catostominae and tribe Catostomini. Xyrauchen
<br />is a monotypic genus. Among the Catostomus
<br />species, bluehead sucker and mountain sucker
<br />belong to subgenus Pantosteus, a distinctive
<br />group known as "mountain suckers" and treated
<br />as a separate genus prior to study by Smith
<br />(1966); the others belong to subgenus Catos-
<br />tomus, the "valley suckers" (Smith 1987).
<br />Winn and Miller (1954) published the ear-
<br />liest comparisons oflarvae for native cyprinid
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