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Utah in October. This is the first record of this species in the San Juan River. Catch rates for all species were down riverwide from 1992 (Table 9 in Appendix I, Table 10 on page 34 in Ryden and Pfeifer: 1992 annual progress report). Catch rates were highest for flannelmouth sucker and bluehead sucker in Reach III. Channel catfish had their highest catch rates in Reach IV, while common carp were most abundant in Reach II. Catch rates for the six most abundant species showed a downward trend from Reach III to Reach VII. However, flannelmouth sucker and red shiner showed a large spike in catch rate in Reach VII (Figures 5 and 6). Catch per unit effort for all species was probably affected by very clear water conditions on all trips and in all reaches, except for Reach VI (Bluff to Mexican Hat) which was sampled one day after a major storm and was very turbid with no visibility below the water surface. Size Structure Collections made in OM's contained slightly more than 20% of all fish captured for the four most abundant species in the last three years: flannelmouth sucker = 21%; bluehead sucker = 21%; channel catfish = 29.2%; common carp = 24.8% (Figure 7). In terms of sheer numbers, flannelmouth sucker made up the bulk of these fish in all river reaches (Figures 7 and 8). In each of the last three years, more juvenile (JUV) flannelmouth sucker were collected than adults (1991 = 60.6% JUV, 1992 = 69.4% JUV, 1993 = 68.1% JUV). Bluehead sucker have also shown greater numbers of juvenile fish over the last three years (1991 = 58.2% JUV, 1992 = 53.0% JUV, 1993 = 59.0% JUV). The channel catfish population had fewer juvenile fish in 1993 than in previous years collections (1991 = 42.3% JUV, 1992 = 41.1% JUV, 1993 = 29.9% JUV). Considerably fewer juvenile than adult common carp were collected in 1991-1993 (1991 = 1.1% JUV, 1992 = 4.8% JUV, 1993 = 29.0% JUV), with the highest number being collected in 1993. Both flannelmouth sucker and channel catfish showed distinct shifts in population size-structure from larger to smaller fish in more downstream reaches. Flannelmouth sucker populations were dominated by juveniles in Reaches II through VII with this domination becoming most pronounced towards Lake Powell (Figure 9). This corresponds with a decline in the mean TL from Reaches I to VI in 1991 and 1992 (Figure 10). Juvenile channel catfish were almost non-existent in the upper two reaches of the river but had become the dominant size class by Reach IV in 1991 and 1992, and by Reach VI in 1993 (Figure 11). Channel catfish also showed a pronounced decline in mean TL from Reaches I to VII (Figure 12). This suggests that young channel catfish may be more prone to drift or displacement by high flows than young flannelmouth sucker, and are being transported to lower reaches of the river. Bluehead sucker have a much less distinguishable pattern for size class distribution riverwide. Like flannelmouth sucker the population is dominated by juvenile fish in Reaches II-VII, with the exception of Reach V in 1993 (Figure 9). However, the juvenile to adult ratio remains fairly constant in reaches II-VII within a given year. This is reflected in the fairly straight trend in mean TL for 1991-1993 (Figure 10). Of the four most abundant species in the San Juan River, bluehead sucker seem to have been least affected in 12 1 I ,I I ,I f i I I I ,I t I I 1 I I I I