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however, almost all fish that moved, oriented to the chemical to which they had been exposed, and presumably imprinted, as larvae. ~ Razorback suckers in the UCR basin spawn in spring, when flows are increasing and water temperatures in the main channel are about 14-15°C (Tyus 1987, Tyus and Karp 1990), but there appears to be some plasticity in both timing and temperature (Hinckley et al. 1991). Early in the spawning season, razorbacks congregate in flooded shorelines, flooded bottomlands and gravel pits, and in the mouths of tributary streams. ~ These areas, which are probably used for staging, resting, or feeding, have higher temperatures (mean: 19.6, range: 17.5-21 C; Tyus and Karp 1990) than the main channel, and the warmer conditions may enhance maturation of gametes or othervvise prepare the fish for reproductive activities. There has been some controversy in the literature about the habitat that razorback ~ suckers select for spawning (see Hinckley et al. 1991 for review)., The issue is sign cant because it provides part of the justification for major investments by the RIP for acquisition of bottomlands. The typical spawning substrate for catostomids consists of gravel or cobble (Breder and Rosen 1966). This is true of suckers in the western US (Moyle 1976), including obligate lacustrine suckers that ascend streams to spawn on ~ gravels and cobbles (Scoppertone and Vinyard 1991). The razorback sucker conforms to the general catostomid pattern: it deposits eggs in flowing water over substrate that is predominantly gravel or cobble (reviewed by Hinckley et al. 1991). Even in Lake Mohave, spawning adults aggregate over areas of coarse substrate (cobble mixed w~h gravel), and the spawning fish sweep away fine materials before creating the ~ depressions in which the eggs are deposited. There is no indication that razorback suckers deposit eggs over flooded vegetation in Lake Mohave, nor is it likely that they will do so in flooded bottomland where the substrate may be vegetated or composed of fine material. Except for induced spawning of captive fish in hatchery ponds (e.g., Dexter NFH, Dexter, NM), catostomids do not deposit eggs over vegetation or fine ~ substrate. Confusion about the sites chosen by razorback suckers for egg deposition may be due in part to the extended period of time during which the females remain ripe. It has been speculated that razorback suckers spawn in silty backwaters, flooded pasture, river oxbows, and flooded bottomlands (Valdez et al. 1982, Osmundson and Kaeding 1991). ~ To be sure, razorback suckers in an advanced reproductive state (tuberculate with expressible sex products) have been found in habitats of that kind, leading to the logical conclusion that egg deposition also occurs there. However, egg deposition has not been observed in those habitats, and it would not be expected based on the reproductive biology of that or related species. Amore likely explanation is that the fish ~ used those habitats for staging, and that the advanced reproductive state was a misleading clue. Because razorback suckers remain ripe for a long time, ripe fish captured in the staging area would still have time to move into the main channel and spawn over coarse substrate; this behavior has been documented previously (McAda 20 •