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946 OSMUNDSON ET AL. 200 mm TL and longer. Because condition changes monthly (Hawkins 1992), month-specific length- weight relationships were developed by using the 1990-1994 data. Only fishes weighed with an elec- tronic balance were included. Excluded from the June calculations were 1994 data because we as- sumed gonads matured early that year and would have biased fish weights. Relative condition of each fish was calculated using the month-specific constants from the length-weight relationships (fish not weighed with a balance and those cap- tured in June 1994 were again excluded). We com- pared mean condition factors between upper and lower reaches within 100-mm length-classes and among length-classes within reaches. Relative abundance of potential forage spe- cies.-Estimates of relative abundance of small prey fish less than 100 mm TL were from unpub- lished fall seine survey data collected for an in- teragency annual monitoring program (see McAda et al. 1994 for methods). We used electrofishing and trammel netting to develop catch-rate indices of larger prey fish (>100 mm TL) likely to serve as forage for large Colorado squawfish. Electro- fishing surveys were conducted from 20 April through 7 July, 1993, in two or more 0.8-km sub- reaches within each stratum (Table 1). The starting points (rkm) for electrofishing subreaches were se- lected by using a random numbers table. Within each sample subreach, both shorelines were elec- trofished in a downstream direction. After identi- fication and length measurement, fish were re- leased at the lower end of the subreach. Catch per unit effort was expressed as the number caught divided by the time electric current was applied, as metered by the VVP-15. Numbers of each spe- cies captured in trammel nets during Colorado squawfish sampling were recorded during 1992- 1994, and CPUE was calculated as the mean num- ber of fish caught per net set. We developed an index of relative abundance for soft-rayed fusiform fish by pooling CPUE of the common species within each stratum. Spitted species were excluded, as were rare species. Though prey preference of Colorado squawfish is unknown, soft-rayed fish were assumed to be pre- ferred over spined or spiny-rayed fish, as is the case with northern pike Esox lucius and muskel- lunge E. masquinongy (Beyerle and Williams 1968; Mauck and Coble 1971; Wahl and Stein 1988). These two large, cool-to-warmwater, north temperate piscivores have morphologies and eco- logical roles similar to Colorado squawfish. Spined and spiny-rayed fish are more costly, in terms of both energy required for ingestion and risk of mor- tality from throat or stomach puncture (Gillen et al. 1981; McAda 1983). We conducted an addi- tional relative abundance analysis by first parti- tioning potential prey so that only soft-rayed fish 100-300 mm long were considered, based on the assumption that Colorado squawfish can only con- sume fish up to about half their own length (e.g., Juanes 1994). Diet.-Stomach contents of Colorado squawfish captured during 1994 were examined by using a Seaburg stomach sampler (Seaburg 1957) as mod- ified by Gengerke et al. (1973). Back-flush tubes of various diameters were used based on the size of the fish. Empty stomachs were recorded in the field. Stomach contents from those fish containing food were preserved in 10% formalin and sent to the Larval Fish Laboratory (LFL) at Colorado State University for analysis. Food items were identified to lowest practical taxon and measured in standard length (SL), when possible; visual es- timates were made of the percentage of total vol- ume of stomach contents contributed by each tax- on. The `aggregate percent method' (Swanson et al. 1974) was used to calculate an overall percent volume for each food item for those fish containing food. Temperature.-Main channel temperatures (nearest 0. VC) were monitored at sites within stra- ta 1, 3, 5, 6, and 7 and at two sites upstream of stratum 7 (Cameo and Rulison, Colorado) in a reach historically used by Colorado squawfish but presently unoccupied due to the long-term effects of three instream barriers that block migrations (Figure 1). At five sites, thermographs (Ryan In- struments, Redmond, Washington) were deployed and downloaded twice yearly. Data from stratum 5 and Cameo were collected by the U.S. Geolog- ical Survey at the Colorado-Utah state line and Cameo gauging stations, respectively. Mean daily values were calculated from readings taken every 2 h. We compared temperature indices for growth among thermograph sites for the 1992-1996 pe- riod to measure spatial variation in thermal regime suitability along the river continuum. We derived these indices by using an approach developed spe- cifically for Colorado squawfish by Kaeding and Osmundson (1988) in which mean daily temper- atures are converted to values relative to the max- imum potential (1.0) for growth at the optimum temperature (25°C); these thermal units are then summed to provide an annual value.