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I II I rl~' t 11 'LI fI fI [I ~.I il rl' i ~. 'I II .1 'I II ~I I Both flannelmouth and razorback suckers were observed during visual surveys. Flannelmouth suckers were generally restricted to a 32-Ian reach downstream of Davis Dam while razorback suckers were distributed throughout the entire study area. Based on observed densities (fish/ha) and surface area of the Colorado River channel, large adult flannelmouth suoker numbers exceeded 4,000 fish while razorback sucker numbers in the main stem were estimated at <100 (Table 1.8). We suspect this is a fairly accurate depiction for channel densities, especially since razorbacks prefer backwater habitats that were not included in these calculations. (Initial sampling in 2003 indicated that juvenile flannelmouth suckers were fairly common in the Topock region. This would suggest the extent of the flannelmouth sucker population may be larger than that used in the estimate.) Razorback Suckers Far less was learned about razorback suckers due to the relatively low numbers captured. A multi-agency razorback sucker 'round-up' was held in 2001 and 2002 to mark and recapture razorback suckers found between Davis and Parker Dams. Approximately 30 razorback suckers were captured in the reservoir; the majority were in the upper portions of Lake Havasu and the river immediately upstream to Davis Dam. Several fish stocked at Winsor Beach (Lake Havasu) were recaptured more than 100 Ian upstream. The vast majority of the 238 fish captured were taken from Park Moabi Lagoon (Ill), while fewer numbers were taken at Pulpit Rock (19), Blankenship Bend (19), Laughlin Lagoon (71), below Davis Dam (6), and scattered locations. The majority (67) of razorback suckers taken from Laughlin Lagoon were fish recently stocked in 2000. There were also approximately 30 razorback suckers collected by AGFD, which were recent stocks. The recently stocked fish were excluded from Table 1.9. We encountered only three fish over 600 mm that could have been considered 'wild fish'. Spawning. Unlike the relic population in Lake Mohave or the flannelmouths below Davis Dam, we were unable to actually observe spawning aggregates. Attempts to locate spawning groups using telemetry equipment failed due to equipment failure or poor signal strength. Helicopter over-flights during 2002 and 2003 located what appeared to be a large school offish off the Blankenship Bend sand bar and other locations, but subsequent sampling only encountered common carp. We suspect fish are spawning in the more productive and turbid backwaters where observations may not be possible (Figun: 1.9.). For instance, in Park Moabi where we suspect spawning occurred, water visibility was <1 m. Trammel netting did produce relatively high numbers of ripe males in Park Moabi Lagoon and the Blankenship Bend area (Table 1.10). Net sets during the spawning season typically yielded one or two fish per set. One set along a rocky shoreline in Park Moabi yielded five ripe males. We captured razorback sucker larvae at this location in 2000, 2001, and 2002 and at Pulpit Rock and Blankenship Bend. We suspect razorback suckers were spawning at or near these locations. Table 1.8. Estimation of flannelmouth sucker and razorback sucker population size based on visual surveys conducted in the Colorado River channel between Davis Dam and Lake Havasu during August 2000. Species Density (ha) Area (ha) Population size Flannelmouth sucker Razorback sucker 480a 1.350b 4,128 81 8.6 0.06 aBased on the estimated surface area of the 32 Ian of the Colorado River channel (150 m width) reach downstream of Davis Dam. bBased on estimated surface area of the Colorado River channel between Davis Dam and Lake Havasu. 16