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Surgical Procedure Each radio transmitter was dipped in melted beeswax to cover any abrasive surfaces which might irritate the fish. Frequency and pulse rate of the transmitter were checked after dipping and before insertion. The fish to be tagged was placed in 'a large tub containing 150-250 mg/L of tricain methane sulfonate (MS-222) and watched until opercle movement stopped. The fish was then placed on a measuring board that was washed with MS-222 solution. During surgery the fish's gills were also irri- gated with MS-222. Operating instruments were sterilized with isopropyl alcohol after boiling. A short 3-4 cm incision was made with a number 15 scalpel blade in the side of the fish just anterior to the pelvic fin and ventral to the rib cage. The radio tag was inserted into the body cavity with the antennae loop toward the head of the fish. The incision was then closed with a monofilament nylon 000 suture with a FS-1 cutting needle. Five individual sutures were tied with surgeon's knots and each knot was sealed with "super glue" and inspected for tightness. After the incision was closed the fish was held in the quiet water of Gypsum Bay until it recovered from the anesthetic. Recovery was rapid (3-4 minutes) and the fish was released immediately into the bay. RESULTS A single squawfish large enough for surgical implant (817 mm, 4,082 grams) was captured by gillnet on June 9 at 10:00 a.m. in a back eddy 3-4 meters deep near river mile 196. The fish was in the net for less than 30 minutes and was in excellent condition both before and after surgery. The fish was released after tag insertion and tracked in Gypsum Canyon where it moved around the bay during the daytime on June 9. On June 10 extensive tracking failed to locate the fish in Gypsum Canyon. Boats tracked upstream to the Cataract Canyon rapids and downstream approxi- mately 5 km without receiving a signal. This search procedure was repeated on the following day (June 11). On June 12 two boats tracked from Gypsum Canyon to Hite Marina (river mile 168). Four signals were received with the tracking receiver (RF-40) near Rock Canyon (river mile 173) and then the signal was lost. Repeated attempts to relocate the fish failed. Water was approximately 37 meters (120 ft) deep at Rock Canyon; if the fish sounded deep, range of the signal was probably too low to permit detection. Tracking was continued on alternate days through the month of June and into early July without locating the tagged fish. Several theories have been presented to explain the relatively high capture rate of this endangered species at the Colorado River/Lake Powell mixing zone during the spring of 1980. One theory suggests that the squawfish captured during 1980 were residents of the reservoir and moved upstream to the headwaters, perhaps on a spawning migration. The high condition factor (KTL of many fish captured in 1980 suggested they were 277