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Movement and Miarations Historical accounts document spring spawning movements of razorback suckers in various locations in the basin (Jordan 1891, Hubbs and Miller 1953, Sigler and Miller 1963). Spawning migrations and other movements presumably evolved in the context of flow regimes, pluvial events, and the diversity of available habitats (Smith 1981; Tyus 1986, 1987; Tyus and Karp 1989, 1990). Similar spawning migrations have been studied for other riverine catostomid and appear to be a major part of their reproductive ecology (Dence 1948, Breder and Rosen 1966, Werner 1979). The factors controlling migratory behavior and homing in the razorback sucker have not been studied, but there is some evidence that learned behaviors (e.g., imprinting) may have an influence (Scholz et al. 1992, Modde et al. 1995). Razorback suckers may travel long distances in both lacustrine and riverine environments during the spawning season, and exhibit some fidelity to specific spawning areas. In Lake Mojave, razorback suckers move throughout the lake, which is about 100 km long (Marsh and Minckley 1989). Spawning migrations of 30 to 106 km (one way) have been recorded in the Green River near Jensen, Utah, and in the lower Yampa River in Dinosaur National Monument (Tyus 1987, Tyus and Karp 1990). Extensive movements also have been observed in juvenile razorback suckers stocked in Lakes Mojave and Powell, and rapid dispersal was observed in sonic-tagged fish (Mueller and Marsh1998). Fish stocked in the San Juan River arm of Lake Powell displayed a preference for the river inflow area, which they moved into and remained for an extended period (C. Karp, pers. comm., 1998). Razorback suckers travel mainly during the spring spawning season and are more sedentary during the remainder of the year. In summer, razorback suckers in the Green River were relatively sedentary, traveling only a few kilometers upstream or downstream (Tyus 1987, Tyus and Karp 1990). Little is known about movements of razorback suckers in winter, but Valdez and Masslich (1989) reported a net movement of less than 5 km (3 mi) between 1 December and 31 March. Valdez and Masslich (1989) also documented that changing flows stimulated fish movements in winter. Diet Razorback sucker diet varies depending on life stage, habitat, and food availability. When larvae hatch, the mouth is terminal, which appears to facilitate great diversity in feeding behavior. In the laboratory, larvae may feed at the surface, in the water column, and on the surface of the substrate (unpublished USFWS records, Vernal, Utah). In Lake Mojave, larvae begin exogenous feeding at about 9-10 mm total length, and feed mostly on phytoplankton and small zooplankton (Minckley and Gustafeson 1982, Marsh and Langhorst 1988, Papoulias and Minckley 1990). However, larvae stocked in a backwater in the Salt River, Arizona, consumed mainly chironomid larvae (J.E. Brooks, USFWS, pers. comm., 1994). No information is available regarding food habits of larval razorback suckers in riverine habitats. However, larvae of other 13