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Temperature Main channel temperatures were monitored at six sites, one within each of study strata 1, 6, 7, 8, 10, and 11 (see Figure 1); two additional sites, one upstream and one downstream of Stratum 12 in the Gunnison River were also monitored. At five sites, thermographs (Ryan Instruments, Redmond, Washington) were deployed and the data downloaded twice yearly. Data from strata 7 and 10, and from the site downstream of Stratum 12, were collected by the U. S. Geological Survey (USGS) at their gaging stations (Colorado River near Colorado-Utah State Line and near Cameo, and Gunnison River near Grand Junction, respectively). Mean daily values were calculated from readings taken every two hours (to the nearest 0.1 ° Q. We compared temperature indices for Colorado pikeminnow growth among thermograph sites for the 1992-1997 period to measure spatial variation in thermal regime suitability along the river continuum. We derived these indices by using an approach developed specifically for Colorado pikeminnow by Kaeding and Osmundson (1988) in which mean daily temperatures are converted to values relative to the maximum potential (1.0) for growth at the optimum temperature (25 ° C); these thermal units are then summed to provide an annual value. RESULTS Fish Abundance Fish catch rates varied both spatially and temporally. To examine spatial differences (among strata), an overall catch rate was calculated for each stratum using data from all sampling periods. To do this, data (catch and effort) from the various sampling periods were first pooled for each shoreline sample. The catch rate for each grand shoreline sample was then calculated and the mean of these was then taken. Data were pooled only from the three periods during which sampling was done in all strata, (i.e., spring 1994 data were excluded). Catch rates within strata varied with period, and inclusion of data from the four strata sampled in spring 1994 would bias results of those strata relative to the other strata for which there were no spring 1994 data. To examine changes in distribution through time (temporal variation), catch rates are also presented by individual sampling period. Spatial variation All fish.--Mean total catch rates (species combined) in strata upstream of Westwater Canyon (`upstream strata') were considerably higher (-2-5 fish per electrofishing minute) than those in strata downstream (< 1 fish/min) of Westwater Canyon (`downstream strata'). The lowest catch rate (0.1 fish/min) was in Stratum 1 (Figure 2). Log-transformed catch rates in strata 7, 8, 9, 11 and 12 were significantly higher than in downstream strata 1-6 (Tukey-Kramer [T-K] multiple comparison test, P < 0.05). Strata 8, 9, 11 and 12 catch rates were not statistically different from one another. The Stratum 10 catch rate was significantly higher than in downstream strata 1-5, but significantly lower than catch rates in upstream strata 8, 9, 11 and 12 (Appendix Table 3). 8