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Introduction Electrofishing is a commonly used sampling method that can harm fish. Most studies on adverse effects of electrofishing have investigated short-term mortality or spinal and soft-tissue injuries in yearling or older fish (e.g. Spencer 1967; Sharber and Carothers 1988; Holmes et al. 1990; Sharber et al. 1994). Although spawning fish are sometimes targeted for electrofishing, relatively little is known about impacts of electric fields on reproduction, gametes, or subsequent offspring (Snyder 1992). Lamarque (1990) noted that information on effects of electrofishing currents on early life stages of fish is sparse and limited primarily to salmonid embryos, but he suggested that electric fields detrimental to adult fish would probably also harm young. Exposure to electrofishing currents caused significantly lower survival in embryos of salmonids (Godfrey 1957; Dwyer et a!. 1993; Dwyer and Erdahl 1995) and walleye Stizostedion vitreum (Newman and Stone 1992). The largest extant riverine population of federally endangered razorback sucker Xyrauchen texanus occurs in the middle Green River system of northeastern Utah and northwestern Colorado, but number of adults is low and recruitment is minimal (Lanigan and Tyus 1989; USFWS 1991). Successful reproduction by this population has only recently been confirmed through collection of larvae (R.T. Muth, unpublished data) and early juveniles (Guttermuth et a!. 1994). Sampling for adult razorback sucker is most successful in spring when fish aggregate to spawn, and electrofishing over known spawning areas in the Green and Yampa rivers is effective (Tyus 1987; Tyus and Karp 1990). However, there are concerns within the Recovery Implementation Program for Endangered Fish Species in the Upper Colorado River Basin that electrofishing of spawning aggregations might harm adults and reduce their reproductive success. Muth and Ruppert (in press) assessed effects of electrofishing currents on captive ripe razorback sucker and subsequent egg hatching success. Our objective in this study was to conduct laboratory tests to determine if electroshock by pulsed DC at selected pulse frequencies and peak-voltage gradients affected survival of razorback sucker embryos and survival and growth of early larvae. Methods Razorback sucker eggs at 28 h postfertilization were obtained from Dexter National Fish Hatchery and Technology Center, New Mexico, and placed in a flow-through incubator receiving well water at 17.60C. Samples of eggs were removed from the incubator at hourly intervals and examined under a stereo-zoom dissecting microscope to determine developmental state of embryos. Embryos were exposed to treatment conditions at early epiboly (33 h postfertilization), early tailbud (78 h), or finfold (122 h) developmental stages; stages were identified following descriptions by Minckley and Gustafson (1982). Eggs not used in the experiment on embryos were allowed to hatch, and larvae were treated at 36 h posthatching, about 96 h before swimup. Electric systems were powered by a Honda'" model EG5000X, 5000-W, 240-V generator. Alternating current from the generator was transformed to square-wave pulsed DC by a Coffelt" WP-15 or Coffelt Mark 20 Complex Pulse System (CPS™) electrofishing control unit. Treatment settings for the WP-15 were 30 Hz and 12% duty cycle (4-ms pulses), 60 Hz and 24% duty cycle (4-ms pulses), or 80 Hz and 40% duty cycle (5-ms pulses). Settings at 60 Hz and 24% duty cycle are commonly used with the WP-15 in boat electrofishing for adult razorback sucker in the middle Green River system (E. Wick, U.S. 6