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e PREFACE The habitat use information and Habitat Suitability Index (HSI) models presented in this publication aid in impact assessment and habitat management activities. Literature concerning a species' habitat requirements and preferences is reviewed and then synthesized into subjective HSI models, which are scaled to produce an index between 0 (unsuitable habitat) and 1 (optimal habitat). Assumptions used to transform habitat use i nformat ion into these mathematical models are noted, and guidelines for model application are described. Any models found in the literature which may also be used to calculate an HSI are cited, and simplified HSI models, based on the most important habitat characteristics for the species, are presented. Also included is a brief discussion of Suitability Index (51) curves as used in the Instream Flow Incremental Methodology (IFIM), and a discussion of SI curves ava il ab 1 e for the I FIM ana lys is of white sucker habi tat. Use of habitat information presented in this publication for impact assessment requires the setting of clear study objectives and may require modification of the models to meet those objectives. Methods for modifying HSI models and recommended measurement techniques for model variables are presented in Terrell et al. (1982).1 A discussion of HSI model building techniques, including the component approach is presented in U.S. Fish and Wildlife Service (1981).2 e The HSI models presented herein are complex hypotheses of species-habitat relationships, not statements of proven cause and effect relationships. Results of mode lPerformance tests, when avail abl e, are referenced; however, models that have demonstrated reliability in specific situations may prove unreliable in others. For this reason, the FWS encourages model users to convey comments and suggestions that may help us increase the utility and effectiveness of this habitat-based approach to fish and wildlife planning. Please send comments to: Habitat Evaluation Procedures Group Western Energy and Land Use Team U.S. Fish and Wildlife Service 2627 Redwing Road Ft. Collins, CO 80526-2899 1Terrell, J. W., T. E. McMahon, P. D. Inskip, R. F. Raleigh, and K. L. Williamson. 1982. Habitat suitability index models: Appendix A. Guidelines for riverine and lacustrine applications of fish HSI models with the Habitat Evaluation Procedures. U.S. Fish Wildl. Servo FWS/OBS-82/10.A. 54 pp. 2U.S. Fish and Wildlife Service. habitat suitability index models. Ecol. Servo n.p. 1981. Standards for the deve 1 opment of 103 ESM. U.S. Fish Wildl. Serv., Div. e i i i