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can be programmed to recognize frequencies and usually rotates through a number of programed frequencies at a certain rate. Any major field study should use receivers with search and track capabilities, especially if several transmitters are used. A search model is greatly desired for fish tracking; it-can prevent loss of a fish if temperature changes or other factors cause a transmitter to emit a slightly different frequency. The search receiver, however, cannot be tuned as precisely for individual transmitters as a pinpoint receiver and cannot have as great a range in terms of signal strength received or distance. Because radiotelemetry was developed principally for wildlife applications, many tracking receivers are not tunable except for 10 or more narrow frequency bands. Thesa should be avoided in fish tracking work in favor of "search" and tunable "tracking" receiver types. Programmable receivers require the exact frequency of the transmitter to be input as a known. If temperature changes .cause the frequency to change very much (as is often the case with small transmitters implanted in cold-blooded animals), the receiver may not be able to detect the signal. Also, if the unit requires much time to rotate through the frequencies one at a time, a fish location could be overflown in aerial tracking. CASE STUDY: RADIOTELEMETRY OF COLORADO SQUAWFISH In March 1980, the U.S. Fish and Wildlife Service initiated a radio- telemetry study of endangered fishes in the Green River as part of the Colorado River Fisheries Project (Tyus and McAda 1984). The study area included the Green River from Jensen, Utah, to its confluence with the Colorado River, about 500 km downstream. Within this area, the river flows through long stretches of flat water, enters whitewater in Desolation and Gray Canyons, and passes through another flat water reach on its way to join the Colorado River. The river has a relatively high conductivity (ranging from about 200 to 2,000 umho) and is full of underwater objects and obstructions. The river is shallow (<10 ft) through most of the study area. This case study evaluates radiotracking success in a large river, compares habitat data based on radiotagged fish with data collected by electrofishing, and discusses habitat data partitioning. The potential effects on the fish of surgically implanted radios is evaluated by a comparison of growth rates between implanted and nonimplanted fish. METHODS "Antenna-less" (transmitting antenna sealed within a coated capsule) radio modules (AVM 1979) were obtained from the AVM and Smith-Root companies. These radios consisted of a transmitting antenna, radiotransmitter, and battery, all sealed in a water-tight capsule. Each had a magnetic switch and was activated when implanted. Short-life and long-life radios were evaluated from each company: 143