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AVM 180 d and 1;-year modules with "Sm 1" fish transmitters and loop antenna, and powered by 3 types of mercury batteries (630, Hg 1, and 828). These were coated with acrylic resin. Smith-Root 150- and 300-day modules with "P 40" fish transmitters with a modified dipole antenna, powered by one type lithium battery in series (B body) and parallel (C body). These were encased in a polycarbamate body. All radios were :grouped by the general battery rating provided by the companies. Theoretical transmitting life within each group was calculated for radio type by dividing the average current drain of the transmitters in milli- amps by the average milliamp-day rated capacity of the battery used (Table 1). Table 1. Specifications of radiotransmitter modules implanted in Colorado squawfish. Type Rated Weight (g) Length Type Company battery life (d) In air In water (cm) A Smith-Root Lithium 150 15 4 B Smith-Root Lithium 300 22 5 C AVM ~'ercury 180 11 3.5 0 AVM Mercury 550 23 6 Colorado squawfish were captured and surgically implanted in April and early May (Bidgood 1980; Tyus and McAda 1984). Fish modules were tested for transmitter frequency and pulse rates and dipped twice in melted, purified beeswax before intraperitoneal implantation. All surgery was performed by the author or under his supervision so that the surgical technique did not vary between fish. Care was taken to insure that internal organs were not inadvertently cut during the surgery. In 1980, fish were held about 5 days before release, to test surgical procedure, fish recovery, and suture retention. From 1981 to 1985, all fish were released imiediately after implantation. Nine razorback suckers were implanted with three types of radios and released during this time period, but because of low numbers they were not used for comparisons of radio performance. Fish were tracked weekly with Smith-Root Model RF-40 and SR-40 receivers tu~;ed to the 40.600-40.700 MHz range. Fish were detected primarily with Larsen-Kulrod whip antennas, but Yagi and loop antennas were also tested. Receivers were tested each dsy. A difference in auditory quality was evident between the radio transmitters, but all were judged acceptable. Signal attenuation with increasing water depth and conductivity (Tyus 1982; Winters 1983) was noted. Tracking was usually done with two boats traveling slowly downstream on opposite sides of the river, although some tracking was done 144