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1 76 canals makes young razorback suckers suscep- tible to collectors. Now that the species is known from such places, biologists are alert for their occurrence. Interest among biolo- gists along the lower river and extensive infor- mation exchange stimulated by active reintro- duction programs have contributed to and increased the probability that razorback suck- ers will be noted. Drains transport excess water used for leaching of salts from agricultural lands back to the river. They are fed by over-surface flow during irrigation cycles and subsurface perco- lation the rest of the time, which results in slow-moving, enriched aquatic habitats that are often densely vegetated by algae and macrophytes and may be characterized by chemical and thermal extremes (Minckley 1979). Drains are far more permanent than canals. Large drains rarely dry and are only sporadically disturbed by cleaning and maintenance operations in ways, such as dredging, that do not involve dewatering. There are, however, no known records for razorback suckers from drains. Origins of Recruits Larvae are most likely passively entrained into canals. Currents at intakes are substan- tial, and larval razorback suckers tend to be near shorelines, at least in reservoirs (Lang- horst and Marsh 1986) and also in the only historic collection of aggregated larvae and juveniles of the species recorded from the mainstream Colorado River in 1950 (R. R. Miller, in Sigler and Miller 1963). Drifting catostomid larvae (Catostomus insignis, Pan- tosteus clarki) in the Gila River, New Mexico, were concentrated by a factor of 6.5 near banks compared with samples in midstream (Bestgen et al. 1987). Reintroduced juvenile razorback suckers also show a marked procliv- ity to move downstream (Brooks 1985). Such behavior would obviously enhance the proba- bility of encountering a withdrawal point. The absence of razorback suckers in drains may result from a lack of sampling. As noted above, these habitats are far more permanent than canals. In addition, they are more com- plex, and thus exceedingly difficult to sample. Drains may also suffer seasonal chemical and physical extremes that are lethal to fishes. They nonetheless often support substantial populations of nonnative species (Minckley GREAT BASIN NATURALIST Vol. 49, No. 1 1979, Matter et al. 1986), including cen- trarchids and ictalurids that are demonstrated predators on young razorback suckers (Os- mundson 1987, Marsh and Brooks, in press). Drains furthermore flow into the river, and larval or juvenile razorback suckers may ei- ther not actively ascend against current or may be blocked from ascent by structures designed to prevent headward erosion. Young razorback suckers in the lower Colo- rado River system may have hatched in a number of places. Downstream, the Senator Wash Reservoir population occupies a small (190 ha) pump-storage impoundment, where they behave similarly to fish in Lake Mohave. A small number of adults (estimated popula- tion 54 ± 22 individuals [95% confidence limits] in 1980-81, which averaged -60 cm TL in 1973-74 [10 fish]) spawn and produce larvae each year, which then disappear before achieving juvenile size (Ulmer and Anderson 1985). Some of these could conceivably pass through penstocks of the reservoir and enter downstream intakes that lead to Imperial Val- ley canals. Fish appearing in the Parker, Ari- zona, and Blythe, California, areas could simi- larly originate from reproduction in Lake Havasu (Marsh and Papoulias, in press), pass through epilimnetic penstocks of Parker Dam, and then be diverted into canals at Headgate Rock Dam or Palo Verde Irrigation Diversion, respectively. Other spawning areas are un- known but certainly may exist (Loudermilk 1985). Occurrence in the Parker, Arizona, area in 1987 of 38 juveniles of a size attributable to the 1986 reintroductions, and three in 1988 that were wild fish, underlines a number of needs and factors to consider. First, it is imperative that reintroduced fish be marked, by fin re- moval or with oxytetracycline, for example, so they may be certainly and readily discrimi- nated from naturally produced individuals. Only in this way can the relative contributions of natural and reintroduction recruitment be evaluated. Second, assuming that some or all fish caught in 1987 were reintroduced, stocked and naturally produced larvae and juveniles must behave similarly, since they both appear to have passed from the river into canals. This provides information that razor- back sucker larvae and/or juveniles drift or move downstream after hatching (or introduc- tion), and likely did so in the natural state.