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FOREWORD One of the mysteries that cloud the full understanding of the life history and ecology of many fish species is found in that segment of the life cycle from hatching as larvae to the stage at which juveniles can be readily caught and identified. Understanding that ephemeral piece oflife history for a fish species has been the primary goal and career pursuit for the Larval Fish Laboratory and Darrel Snyder at Colorado State University. While perhaps not the most exciting field of fishery science to young, new professionals, understanding larval and early juvenile fish population ecology is recognized by conservation and sport fishery managers alike as fundamental. Most limiting factors determining recruitment success and year class strength exert themselves at this life stage. Comparison of the June 1990 publication of the identification key for the early life stages of sucker species in the Colorado River Basin with this new edition demonstrates the characteristic persistence of scientific endeavor and the skillful adaptation of computer technology. Instead of a book that sits on a shelf until needed, opened, read, and oft-times interpreted, this revised edition carries a compact disk that provides a new identification tool to update and replace the former printed key and transform one's involvement into a flexible interactive experience. The user can define the set of candidate species and selects characters to be evaluated from a continually updated list of best available characters. The book is still there for comparison of specimens with detailed descriptive information and illustrations, but new generations of field biologists consider their computer as integral to field work as nets, seines and electro-fishing boats. Also new is the addition of another sucker species not covered in the 1990 publication, the longnose sucker, Catostomus catostomus. Passed over as the lowest information priority in 1990, this introduced species in the Colorado River Basin has shown up more frequently in the past decade in rivers targeted for reintroduction of the federally-endangered razorback sucker. Our low priority became a "need-to-know" information gap. What has not changed is quality of the product. The definitive information is still there and has been improved with new information from a decade of continuing research by the CSU Larval Fish Lab and others. The 1990 publication was predicted to be invaluable to the community of researchers and biologists working in the Upper Colorado River Basin for native fish conservation and endangered fish recovery. Indeed, that publication was soon sold out and copies required replacement binding from continual use. A small supply of nearly 100 copies was recently discovered in boxes in the Division of Wildlife warehouse. They were made available upon request, and were gone within a week. Good prediction. What distinguishes this and the previous publication are the extraordinary drawings and pictures that accompany and clarify the extensive technical jargon required to navigate your way through identification of organisms that can be wholly draped over your thumbnail. The drawings of the fish and pictures of the skeletal features are what one actually sees of these semi-transparent fish under the light of a dissection microscope. A key to successful conservation of native and endangered fishes starts with the survival of the larval fish as they emerge from the gravels of Colorado River Basin Rivers, are swept downstream to nursery habitats, and face high mortality from a myriad of sources. Are the fish you collected the endangered razorback sucker or the abundant flannelmouth sucker? This identification tool, this software program, this publication gets you there with clear and credible support and documentation. ~ Pf)uiw Thomas P. Nesler Native Fishes Conservation Program Manager Colorado Division of Wildlife III