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Surgical implantation was completed with AVN1 fish modules (AVM 1979) dipped in melted beeswax. These transmitters (Sm-1) weighed approximately 11 g and are of "padlock" shape. The units were powered by a mercury (HG-630) battery and had a theoretical life of about 215 days. The units transmitted in the 40.660-40.700 MHz frequency range.' Radio receiver sets used were Smith-Root RF-40 and SR-40 models. 'The RF. type enabled the investigator to identify any particular frequency transmitted in the 40 MHz range. The SR receiver was a "search" type and received all emitted frequencies in the tuned range. Omni-directional and directional antennas were employed. A Larson- Kulrod whip antenna wag used to search for implanted fish. This antenna` was mounted on a 20 cm steel plate elevated about 4 m above the water when mounted on a boat, or mounted directly to an airplane wing strut. The loop antenna (directional) was used only to triangulate fish location by boat or from shore. It was necessary to use aircraft to keep contact with several fish at one time. The most successful aircraft used was a helicopter, since airplanes cannot fly in Yampa Canyon. The radiotelemetry program utilized two different methods for recording information about habitat preferences and movement. Long distance movement was determined by attempting to locate all radiotagged fish during a 1-2 day interval. The location of a fish was recorded on a one-time basis when encountered. In order to obtain more specific information on movement and habitat preferences, a diel sampling proce- dure was employed which consisted of observations made continuously over a 24 h period as follows: 1. The day was divided into three, eight-h periods with one selected at random as a start. A fish was then observed during this period, with depth, velocity and substrate determinations made every 15 min. Habitat measurements were not recorded unless fish were stationary for 30 min. 2. After a fish had been monitored for three, eight-h periods, the fish with the next tag number was then selected for study. Survey of the Little Snake River Surveys of the fishes of the Little Snake River were planned as special investigations, in order to relate the fish fauna of the Little Snake with that of the Yampa River. The lower 9 mi (14.4 km) of the Little Snake were sampled. It was possible to use electrofishing in the lower portion of the river at times, and seining of shore habitats was accomplished each time. No standardized sampling program was developed for these surveys and they were accomplished when manpower was available. 15