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Life History General The razorback sucker and other endemic Colorado River fishes are adapted to the fluctuating hydrologic environment of the historic Colorado River (Minckley 1973, 1983; Carlson and Muth 1989), with its periodically extreme flow conditions and high turbidities. Historic riverine systems provided a wide variety of habitats including backwaters, sloughs, oxbow lakes, and seasonally inundated flood plains, which were used to satisfy various life history requirements (Holden and Stalnaker 1975; Minckley 1983; Lanigan and Tyus 1989). The fish also occurs in reservoirs, where it is capable of surviving for many years. Although the fish has been successfully propagated in hatcheries, some of the life history needs of the fish in nature, including certain habitat requirements of the various life stages, and other attributes of this fish remain unknown. Reproduction Razorback suckers have been captured in breeding condition in many different habitats and environmental conditions. Spawning has been documented in mainstream rivers, riverine-influenced areas of large impoundments, and wave-washed shorelines of reservoirs. The ability to use a variety of habitats and flow conditions may reflect adaptations to conditions in the historic Colorado River system, which may now be changed. Absence of any substantial level of recruitment makes it difficult to determine if the present habitat and flows are the most suitable for recovery efforts. It is not known whether all life history requirements can be met in either the mainstream river or an impoundment. For clarity, the following discussion of what is known about reproductive requirements and habitat use is separated into lacustrine and riverine habitats. Lacustrine habitats. The largest surviving stock of razorback suckers exists in Lake Mojave in the lower Colorado River basin. Successful spawning has been documented in Lake Mojave and numerous larvae have been collected (Bozek et al. 1990, Marsh and Langhorst 1988). However, juveniles have been extremely rare in collections (Minckley et al. 1991). Spawning in Lake Mojave occurs early in the year, from January through April/May (Langhorst and Marsh 1986, Mueller 1989). Water temperatures during spawning ranged from 11.5-18°C (52.7-64.4°F) (Douglas 1952, Langhorst and Marsh 1986). Spawning fish congregate and spawn in flat or gently sloping shoreline areas over gravel, cobble, or mixed substrate types (Douglas 1952, Bozek et al. 1990, Minckley et al. 1991). Medel-Ulmer (1983) observed similar spawning activity in Senator Wash Reservoir in water depths of 10 to 18 m. In Lake Mojave, razorback suckers were observed spawning in several locations water up to 5 m deep, with most fish in less than 2 m of water. Minckley et al. (1991, p. 320) summarized spawning in Lake Mojave as follows: 8