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Habitat Suitability Curves Twedt and Holden (1980) developed habitat suitability curves for Colorado squawfish in the San Juan. The curves indicate that adult fishes habitat use varies. Adults used depths of 2-4 feet during all seasons. Velocities used by adults varied by time of year. In early spring, prior to runoff, fish were found in velocities of 1-1.5 feet per second (ft/s). During runoff squawfish used velocities from 0 - 2 ft/s. Adult substrate use also varied between seasonal periods. Colorado squawfish predominantly used silt and sand substrates found in backwaters during spring runoff. A variety of substrates (silt, sand, gravel with cover) were used in pre and post runoff periods. Age-0 and juvenile curves showed a much narrower habitat use than adults. Young squawfish use backwaters 1-2 feet deep with velocities less than 0.2 ft/s and silt substrate. Because adult curves were developed with a limited data base (n = 156), Twedt and Holden (1980) expressed concern over the validity of such curves. This data, like the habitat availability data, was compiled from daytime observations. METHODS The project consisted of tracking fish implanted with transmitters to determine their location, habitat use, and movement within a reach of river. Ten adult Colorado squawfish, five adult channel catfish, and five adult northern pike were opportunistically implanted with radio transmitters in a cooperative effort by personnel from Miller Ecological Consultants, Colorado Division of Wildlife, and U.S. Fish and Wildlife Service (USFWS) in 1996 and 1997 (Table 1). Four Colorado squawfish and three northern pike were collected by angling, all other fish were collected by electrofishing. Radio transmitters were implanted using procedures developed by the USFWS. Fish were anesthetized with MS-222 and transmitters were surgically implanted into the gut cavity. Fish were observed in a recovery tank for a minimum of ten minutes. When fully recovered, they were released back into the habitat from which they were captured. Radio tracking was conducted on a biweekly basis from July 22, 1996 through October 26, 1996 and July 20, 1997 through September 26, 1997. Observations were made on five consecutive days each week that tracking was conducted. An attempt was made to locate all fish at least once each observation week. After a fish was located it was monitored for a minimum of 30 minutes (Colorado squawfish were monitored for a minimum of one hour). During this monitoring period, the date, time of day, weather conditions, water and ambient air temperature, time monitored, sketch of the surrounding habitat including fish locations, and any observations of local movement were recorded. Other physical habitat data that were collected whenever possible included habitat type, total water depth, water velocity (mean column and bottom), substrate type, proximity to cover, description of cover at the location, general description of the site, and measurements of the habitat including length, width, bank features, shoreline vegetation, dominant substrate and cover. Discharge was obtained from the USGS gaging station near Maybell, Colorado. The 24-hour observations were conducted in both 1996 and 1997 on Colorado squawfish. One Colorado squawfish was monitored for a continuous 24-hour period during each observation week. This fish was located and observed for movement approximately every hour except for some short periods when weather interfered. All observed activity and movement by fish during this time was recorded. Final Report - Colorado squawfish habitat Page 6 Miller Ecological Consultants, Inc. December 17, 1997