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256 locations (Minckley 19S3), and rare in the upper Colorado River (Holden 1978, Holden & Stalnaker 1975, Tyus 1987). Spawning by razorback suckers is widespread in Lake Mohave, and while successful reproduction and emergence of larvae have been repeatedly documented (Paulson et al. 19&Jb, Bo- zek 19M. Marsh & Langhorst 19X8), ther,e has been no evidence of recruitment (McCarthy & Minckley 1987). Inappropriate temperature regimes have been suggested as one reason for the poor reproductive success of the razorback suckers (Johnson & Rinne 1992. Marsh 19H5). In Lake Mohave, spawning occurs from lo-noC (Bozek J984), a temperature regime which appears largely incompadble with the optimal incubation temperature of WOC ob- tained by Marsh (1985). This suggests th~lt success- ful embryonic development and emerg1ence may not occur or may be reduced during II large part of the spawning season, and contribute to re,cruitment failure. The purpose of this study was to determine temperature ranges under which razorback sucker larvae emerge in Lake Mohave and Jescriibehatch- ing success, development rates, and oxygen con- sumption in the laboratory under corresponding temperatures. I;..~. . . '" -! j f ;~ Methods ,t~ ::d L.u1Y'.ae were collected from Luke Mohav,c at night in known spawning areas using hand-held dive lights during J982 and J983. The lights were held stationary, 0.25 m above the water surlface, and larvae entering the illuminated area durilllg 15 min sample periods were netted by hand. Q)lIections were usually made in water approximately 1 meter deep. but three water depths were sampled in the Six Mile Coves area in February 1983. Species iden- tification and interval of development was deter- mined for each larvae following Snyder (11981) and FUlman (1979), and by using laboratory-cultured razorback sucker larva as reference material. Early and late protolarvae correspond to free embryos while early mesolarvae correspond to larvae (Ba- lon 1984). When collections at a site exceeded 300 t I 't 01 .~ 'iJ. ~ ~, ~:', t~ j ~.r, ., ., :>~ ~ ,"';" -:~l. )} ~ i individuals. only 50'Yo were processed '0 dctcrmine stages of development. To as~'SS development under controlled thermal regimes, razorback sucker eggs were inseminated in the field with the sperm fn1l11 ripc ..dulls nelted from Arilona Bay and the Six Mile Coves afl~a. Gametl.'S were collected from 6 adult p..irs during 1982 and 8 adult pairs in 1983. Sperm was collected and mixed prior to stripping females to insure max- imum genetic variability. All fish used were nat- ur.ally ripe individuals. No hormonal injections were used to induce egg maturation. FertjJjl~..d eggs were tr.ansported at ambient lake temperature immediately following insemination. In the laboratory, fertilized eggs were nmdomly placed in experimental chambers at lake temper- ature. Total time from insemination to placement of eggs into chambers was approximately 4 h. Eggs were then acclimated at the rate of JO C per hour to experimental temperatures (8, 10. J2. 15 and 20" C depending upon experiment) in 18.9 liter aquari.l. Temperatures were held within IS'C of reported temperature during the dumtion of thc experi- ment. All acclimation was completed prior to the morula stages of development. Temperature was regulated using a Masterline Model 2tJ95 c1oscd- system. watcr recirculating and cooling unit. with individual offsetting thermostatic heating units. Water supply was dechlorinated municipal w.lter from Lake Mead. In 19M2. eggs were dispersed owr cobble sub- strates in individual aquaria. During these experi- ments, fungus developed on some eggs in intersti- tial pockets that were adjacent to non-developing eggs. To alleviate this, in 1983, eggs were placed in individual chambers on 8O-p.m-mesh Nitex netting and suspended from a rocker-arm assembly having a rocking amplitude of 5 em and periodicity of 7 cycles per minute. Rocking action provided water movement through the neuing and furnished oxy- genated water to the eggs. This design prevented fungus from developing on the eggs and precluded treatment of the eggs with fungicide. Experiments between years were not comhined bcCClUse wc did not know how the increased watcr now associated with thc rocker arm assembly would ..ffcct oxygcn I. "'." . . " ';':' ':':" :. .,:'.,.. ..' . . . . . . .. ..... . .). . ,. . . ~.c . . ,,;1