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72 GREAT BASIN NATURALIST provided by USFWS, USBR, and ASU. We thank the many biologists working on the lower Colorado River (acknowledged in text) for information leading to this publication. RAZORBACK SUCKERS IN THE LOWER COLORADO RIVER Natural Occurrences, 1962-88 Razorback suckers (>50 cm total length [TL]) are presently common in Lake Mohave, Arizona-Nevada, and larvae of the species (<15 mm TL) are seasonally abundant. Size- groups between the larval and adult life stages are, however, essentially absent from col- lections, despite intense sampling. Adults comprised an average of ^-25% of total fishes caught in annual trammel net samples be- tween 1975 and 1988 (Minckley 1983, Minck- ley and Marsh, unpublished data). Larvae occupy the littoral zone of Lake Mohave (Bozek et al. 1984, Marsh and Langhorst 1988), where 10-100 or more can be attracted to a strong light in a few minutes at night (Langhorst and Marsh 1986). They rarely occur in open water of the reservoir; Langhorst and Marsh (1986) captured only a single specimen in 22 tow-net hours in 1985, although larvae were at the same time com- mon in near-shore habitats. Four juvenile specimens (three preserved, 33 to 54 mm TL; ASU 11567 and 11568), collected by AZGFD personnel in July 1987 (T. Liles, AZGFD, personal communication), are the only natu- rally spawned juveniles recorded from Lake Mohave since the 1950s. In marked contrast, except for a small resi- dent population in Senator Wash Reservoir, California (Medel-Ulmer 1980, Ulmer 1987), there are confirmed records since 1962 for only 42 adult razorback suckers from the en- tire lower Colorado River mainstream and as- sociated habitats downstream from Davis Dam (Fig. 1), despite intensive fisheries sur- veys in that area. Sixteen adults were from Lake Havasu proper: five averaging 56.9 cm TL were caught in 1962; four > 50 cm TL were observed in 1975; three averaging 65.4 cm were electrofished in 1976; one (56.9 cm) was collected from the Bill Williams Arm of the reservoir in 1979; two (unmeasured) were caught by anglers in 1984 (Ulmer and Ander- son 1985); and a single fish 50.6 cm TL was Vol. 49, No. 1 gill-netted in 1986 (M. Giusti, CADFG, personal communication). Riverine reaches yielded 23 individuals: 12, all >50 cm TL, but unmeasured, were taken by various means from Blythe, California, downstream to Impe- rial Reservoir from 1969 to 1985 (Ulmer and Anderson 1985); nine others, mostly >60 cm TL, were angled, electrofished, trammel- netted, or observed in the Needles-Topock Gorge reach from 1972 through 1985 (Minck- ley 1983, Ulmer and Anderson 1985); and two, 57.2 and 61.0 cm TL, were trammel- netted from Laughlin Lagoon, Nevada, an ar- tificial backwater about 8 km below Davis Dam in 1986 (M. Burrell, NVDOW, personal communication). An additional three adults, -50 cm TL, were caught from the Central Arizona Project (CAP) Granite Reef Aqueduct in October 1986 (USBR 1986), which began withdrawing water in 1983 from the Bill Williams Arm of Lake Havasu. Two of the last were 25+ years of age, as determined by otolith analysis (original data; following methodology of McCarthy and Minckley 1987), and thus originated from Lake Havasu. There are no indications that adult razorback suckers in the lowermost Colorado River are occurring less frequently in the 1980s than in the 1960s, which is likely a reflection of low adult mortality and individual longevity (to at least 44 years; McCarthy and Minckley 1987). Larval razorback suckers are as rare as adults in the lower Colorado River down- stream from Lake Mohave. None was found in shoreline surveys with bright light at night in Lake Havasu in 1988. Razorback suckers ac- counted for only 0.56% of 6,617 larval speci- mens caught in tow-net samples in Lake Havasu and upstream riverine reaches in 1985 and 1986 (Marsh and Papoulias, in press). Eight individuals were taken in 1985 and 29 in 1986. Although catch per unit effort varied between years and among stations, similar abundances were indicated in riverine and reservoir habitats, and no areas of larval con- centration were evident (Marsh and Papou- lias, in press). Two larval razorback suckers, - 15 mm TL, were also identified among 5,036 larval specimens from the CAP canal in 1987 (G. Mueller, USBR, personal commu- nication). Twenty-four juvenile razorback suckers, - 15 to 37.1 cm TL, fish of sizes not otherwise known from Lake Mohave or elsewhere in the