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Colorado squawfish captured were measured for length and weight, and examined for identification tags. Scale samples were taken for age-growth analyses in another study (Hawkins 1991). All Colorado squawfish were then released in the river at the capture site. Nongame fish species collected were enumerated by species and sample location and released. Starting in 1990, all nongame fish collected were also measured for total length. In 1991, all Colorado squawfish and other nongame fishes were measured for body depth and width between the head and insertion of the dorsal fm as part of the assessment of vulnerability to gamefish predation using comparison of predator gape and prey cross-sectional body areas. Radio transmitter imnlantation and tracking From 1986 through 1989, select northern pike were surgically implanted with radio transmitters for movement and habitat use investigations. This effort started in October 1986. Candidate northern pike were determined as suitable for implantation by a minimum size of 600 mm total length. These fish were placed in a large oval tub and examined for injury or abnormal behavior following capture. Fish selected as suitable would be anesthetized with MS- 222, and processed for individual measurements prior to implantation surgery. The anesthetized fish would be placed belly-up in a V -shaped wooden trough to support the fish and provide stability during the operation. Surgical procedure consisted of removing scales from the ventral area just posterior to the pelvic girdle using forceps to permit a 30-35 mm length incision through the body wall using a curved blade scalpel. The curved blade was used to lift the body wall away from the internal organs while cutting to minimize risk of internal injuries from the blade. A capsule-shaped, 30 MHz radio transmitter, approximately 5 em long by 2 cm diameter and coated in beeswax, was placed through the incision and pushed forward in the body cavity over the pelvic girdle. The incision was then closed using 4-5 sutures of 3/0 or 4/0 dissolving catgut or permanent surgical thread and curved needles held by a hemostat clamp. All surgical instruments, the transmitter, and incision site on the fish were cleansed with disinfectant iodine solution and rinsed prior to surgery. During the operation, an assistant would repeatedly pour water over the fish's gills and upper body to maintain ventilation and prevent drying. Either water dosed with MS-222 from the holding tub or fresh river water was used for this process. The flSh's status during surgery was monitored using the fish's gilling activity. Reduced gilling activity signified increased influence of anesthetic or shock and therefore increased risk of death. Fresh river water was used primarily for ventilation during surgery unless the fish showed signs of recovery and increased movement before implantation was complete. Anesthetic-dosed water was used periodically throughout the procedure to keep the fish motionless. Upon completing suturing of the incision, the implanted fish was placed in an elongate mesh bag and placed in the river to recover. After 30-60 minutes the fish was removed, the surgical wound checked for adequate closure, and the fish allowed to swim away under its own power. The fish was released when it was capable of swimming strongly enough to elude recapture by hand. Searching for the presence of radio-tagged northern pike was included as a task during all subsequent river trips made with boats during the open-water season. Searches were also 9