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Spawning occurs in the Lower Basin from January through April (Ulmer 1980; Langhorst <br />and Marsh 1986; Mueller 1989). Water temperatures during spawning in the Lower Basin <br />ranged from 53° to 64°F (Douglas 1952; Ulmer 1980; Langhorst and Marsh 1986). <br />Spawning has been extensively documented in Lower Basin reservoirs in flat or gently <br />sloping offshore areas over gravel, cobble, or mixed substrate types (Douglas 1952; Ulmer <br />1980; Bozek et al. 1984; Minckley et al. 1991). Ulmer (1980) observed spawning activity in <br />Senator Wash Reservoir in 10 to 18 feet of water. In Lake Mohave, razorback suckers were <br />observed during spawning in water depths of < 3 to 16 feet, with most fish in < 7 feet of <br />water (Bureau of Reclamation, unpublished data). <br />Bestgen (1990) concluded that razorback suckers may spawn in a variety of flow conditions, <br />from rivers to impoundments. He also concluded that their longevity allows them to persist <br />through several consecutive seasons of no or low reproduction; an adaptation to the pristine <br />conditions of the highly fluctuating and unpredictable Colorado River system. <br />Nursery Habitat <br />Habitats used by young razorback suckers have not been fully evaluated because of the low <br />number of young fish present in the river system. However, most studies agree that the <br />larvae prefer shallow, littoral zones for a few weeks after hatching, then they disperse to <br />deeper water areas (reviewed by Minckley et al. 1991). Laboratory studies indicated that, in <br />a riverine environment, the larvae enter stream drift and are transported downstream (Paulin <br />et al. 1990). <br />A number of investigators have collected viable fertilized eggs and larvae in the areas of <br />known spawning activity (Ulmer 1980; Bozek et al. 1984; Tyus 1987; Marsh and Langhorst <br />1988), but few have collected larvae larger than 0.6 inches in the wild. This indicates little <br />or no successful recruitment of wild razorback suckers. Several researchers have observed <br />predation on razorback sucker eggs and larvae by carp, channel catfish, smallmouth bass, <br />largemouth bass, bluegill, green sunfish, and redear sunfish (Jonez and Sumner 1954; Ulmer <br />1980; Marsh and Langhorst 1988; Langhorst 1989). Other researchers hypothesized that <br />predation is a major cause underlying the lack of recruitment to the adult razorback sucker <br />population throughout the basin (McAda and Wydoski 1980; Minckley 1983; Tyus 1987). <br />Loudermilk (1985) observed that young razorback sucker larvae inhabited the upper water <br />column for the first few days after swim-up and exhibited no defensive behavior from <br />potential predators. Karp and Tyus (1990) observed that several fish species readily attacked <br />razorback sucker larvae in lab experiments. Marsh and Langhorst (1988) found larval <br />razorback suckers in Lake Mohave survived longer and grew larger in the absence of <br />predators. <br />Juvenile and Adult Habitat <br />Habitat needs of young and juvenile razorback suckers in the wild are little known because <br />they have rarely been encountered by researchers, particularly in native riverine habitats <br />13