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186 WESTERN NORTH AMERICAN NATURALIST [Volume 60 -5 -10 -15 -20 -25 30 25 20 15 moved to more productive habitats following spawning (Valdez and Wick 1981, Tyus and Karp 1990, Modde and Wick 1997). Increased activity (Table 2) in shallower depths (Fig. 3) may indicate planktonic feeding (Marsh 1987). Wydoski and Wick (1998) suggested floodplain habitats and associated feeding activity are critical for fish to regain body condition after spawning. Homing patterns of spawning and non- spawning populations were typical of those described by Gerking (1958): "Spawning migra- tions are directed toward a specific location ... [while] the fish are usually scattered over a wide area during the non-reproductive por- tion of life." While fish used common areas during spawning, they typically dispersed after spawning, with some fish returning to specific locations in which they had been found the previous summer. The distance between spawn- ing and summer use areas ranged from 20 km (4 fish) to 30 km (6 fish), with no correlations with sex or size. Fish exhibited a great deal of individuality, some being quite active while other were quite sedentary and remained in specific coves for weeks. -30 10 May June July Aug. Sept. Oct. 1995 Fig. 3. Monthly comparison of average depth (m) and body temperature (°C) of Xyrauchen texanus during ther- mal stratification (May-October) of Lake Mohave, Ari- zona-Nevada, in 1995. spawning groups that can number in the hun- dreds. Males typically dominate (2:1) spawn- ing sites (Minckley 1983). Females normally join males from adjacent deeper waters when ready to release their eggs (Minckley 1983, Mueller 1989), a behavior common with other catostomids (Moyle 1976). Prolonged spawning combined with excel- lent observation and field conditions allowed us to more closely examine spawning and post- spawning movements. All study fish returned to spawning sites used the previous year, although they were also tracked to other spawning groups. In some cases the distance between spawning sites exceeded 50 km; 9 of 10 fish were found crossing the reservoir, par- ticipating in spawning groups on both the Ari- zona and Nevada shorelines. Fish movements during peak spawning (1-28 February 1995, 1996) suggested males typically remain on specific spawning sites while females roam substantial distances between spawning sites. We were unable to determine whether females spawned at multiple sites, but reports of females spawning over extended periods combined with the species known fecundity (> 100,000 ova; Minckley 1983, Minckley et al. 1991) sup- port the contention that multiple-site spawn- ing occurs. Multiple-spawning-site behavior also helps explain the high genetic diversity of this population (Dowling et al. 1996). Travel distances and movement rates peaked post-spawning as the majority (70%) of study fish moved immediately up-reservoir to the warm/cold water mixing zone where nutrient- rich hypolimnetic releases from Lake Mead stimulate algal production (Baker and Paulson 1980). Post-spawning movement supports ear- lier speculations that X. texanus historically ACKNOWLEDGMENTS We thank numerous colleagues: Tom Burke (Bureau of Reclamation), Kent Turner (National Park Service), and George Devine (U.S. Fish and Wildlife Service) contributed initial review and administrative support, Tom Burke pro- vided larval fish data; numerous National Park Service staff assisted in field surveys; Mike Burrell (Nevada Department of Wildlife) col- lected study adults; Bob Waltermire helped analyze GIS and GPS data; and the Bureau of Reclamation supplied field equipment and logistical support. We thank Tim Modde and Dennis Shiozawa for reviewing the manu- script. Work was conducted under appropriate state and federal permits and was funded by U.S. Geological Survey. LITERATURE CITED BAKER, J.R., AND L.J. PAULSON. 1980. Evaluation of impacts associated with reregulation of water levels in Lake Mohave. Technical Report 4, Lake Mead Limnological Research Center, University of Nevada, Las Vegas. BozEK, M.A., L.J. PAULSON, AND G.R. WILDE. 1990. Effects of ambient Lake Mohave temperatures on develop- ment, oxygen consumption, and hatching success of