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8 I I I I I I I I I I I I I I I I I I I boat electroftshing (Table 8) were for flannelmouth sucker (294.2), carp (155.5) and bluehead sucker (100.6). Differences in catch rates between the two types of electrofishing boats were related to two factors: 1) catch rates were reflective of actual differences in species composition between the upper Dolores River (above the confluence of the San Miguel River) where the canoe was primarily used and the lower Dolores River (below the confluence of the San Miguel) where the Jon boat was the primary electrofishing craft, and 2) higher catch rates of smaller species with the Jon boat may reflect differences in electrofishing efficiency between the Jon boat and canoe. Effectiveness of electrofishing from either boat was influenced by conductivity, flow, turbidity and channel morphology. High conductivities associated with particular areas probably had the greatest influence. Electrofishing the Paradox Valley reach (Reach IV) was less effective because of saline groundwater inflow. Conductivities in the other reaches were more suitable for electrofishing. High turbidity associated with tributary runoff from storm events also affected electrofishing efficiency, primarily during Trips 2, 4 and 5 when extremely high turbidities were encountered. This influenced electrofishing success by impairing the netter's ability to see fish, and by possibly reducing fish activity. Higher catch rates for most species were observed during the second and third trips in 1990 compared to corresponding trips in 1991 (Table 9). Differences in catch rates between similar trips on different years may be associated with differences in flows and timing of sampling. Lower flows during the second trip in 1990 compared to the second trip in 1991 may have concentrated fish and predisposed them to capture. Lower catch rates during the third trip in 1991 compared to the same trip in 1990 were probably associated with behavioral differences of fish between late summer and early fall. During the third trip of 1991 fish were probably in deeper habitats, were less active and therefore less susceptible to capture by electrofishing. 4.2.2 Gill aDd Trammel Netting Netting efforts were higher during the first year of the study (1990) because of poor sample conditions in 1991. Factors affecting efficiency of gill and trammel netting included river flow, channel morphology, floating debris and excessive turbidity. Netting was ineffective in shallow habitats and during periods of high debris flow. Catch rates for experimental gill nets (Table 10), trammel nets (Table 11) and floating trammel nets (Table 12) are presented separately as number of fish/1oo feet of net/1oo hours. The highest catch rates for experimental gill nets, which were used most frequently, were for flannelmouth sucker (6.4), roundtail chub (1.4) and bluehead sucker (0.8). Trammel nets (both sinking and floating) also produced relatively high catch rates for flannelmouth sucker (1.5 and 43.8, respectively). High catch rates for carp and channel catfish in trammel nets, were probably more indicative of gear effectiveness on spined fishes than actual differences in densities. Netting with gill and trammel nets was conducive to river reaches with greater flow and deeper channels. Low releases from McPhee Dam above the confluence of the San Miguel River made sampling with nets ineffective. This situation was particularly evident during Trips 1 and 6 when releases from McPhee Dam were 20 and 32 cfs, respectively. Under these conditions nets could be used in few locations where deep pools or runs were found In reaches where the channel was wide and shallow, netting was impractical and not attempted. Below the confluence of the San Miguel River gill and trammel nets were more effective because of higher water volume. Floating debris associated with tributary runoff from storm events affected netting during Trip 2, 4 and 5.