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INTRODUCTION ACKNOWLEDGEMENTS Appreciation is given to Barbara Kermeen who provided a review of the theoretical section. Charles C. Coutant and Alexis E. Knight furnished guid- ance and expert review comments. Rudy R. Ringe gave insight into many poten- tial problems of field application. Cathy Short provided technical editing assistance. vii The U.S. Fish and Wildlife Service (FWS) is evaluating the status of four endemic fishes of the Colorado River system. Three of these fish are clas- sified as endangered under the Endangered Species Act of 1973 (P.L. 93-205): the Colorado squawfish (Pt chocheilus Lucius), the humpback chub (Gila cypha); and the bonytail chub (Gila ele ans The fourth fish, the razorback sucker (Xyrauchen texanus), is proposed as threatened. Because of the difficulty of working in the remote upper Colorado River system with its whitewater sections, the FWS became interested 1n the feasibility of using radiotelemetry to learn more about the life histories of these fishes. The Colorado squawfish was selected for initial work because of its large size and availability in flat water sections of the Green River. In 1978, the FWS employed a contractor to study the spawning requirements of Colorado squawfish in the Green River of Utah. Results of this preliminary study were reported by Holden and Selby (1978) and are discussed in this report. The FWS also .sponsored a study to investigate the feasibility of radio tracking Colorado squawfish in the Green River, Utah (Sinning 1979). Sinning's report included theoretical information on radiotelemetry, an evalua- tion of the previous radiotelemetric study in the Green River, and synthesized information from other studies. Sinning concluded that radiotelemetry results would-be marginal in the Green River because high conductivity impedes radio- signal transmissions in water. Further feasibility studies were recommended. The FWS began a radiotelemetry study on the Green River in March 1980. This study was initiated because of an urgent need for data on habitat pref- erences of Colorado squawfish. These data were needed for incorporation into recovery plans required by the FWS for all endangered species and the informa- tion base used for evaluating proposed upper Colorado River Basin development projects. Theoretical information assembled by Sinning (1979) and data obtained during the FWS study (Tyus et al. 1981) are included in this report. Material prepared by Sinning (1979) has been rewritten, simplified, and oriented toward the needs of other investigators. Basic radioreception information obtained by the FWS is presented in an effort to relate theoretical considerations to field applications. Practical information on the effectiveness of radio- telemetry has significance for other investigators working in rivers of high