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Discussion Single exposure to homogeneous electric fields significantly reduced survival of razorback sucker embryos in most treatments and growth of early larvae in all treatments. Razorback sucker embryos and pre-swimup larvae in the Green and Yampa rivers would be subjected to heterogenous electric fields possibly several times during their development with electrofishing operations proven effective for collecting adults. Tyus and Karp (1990) successfully captured adult razorback sucker by electrofishing over active spawning areas at 2-3-d intervals. Razorback sucker eggs are demersal, and larvae remain hidden in the substrate after hatching until they swimup and disperse from spawning areas (Snyder and Muth 1990). Mean water temperatures during razorback sucker spawning in the Green and Yampa rivers ranged 14-150C (Tyus and Karp 1989, 1990). At 150C, eggs hatch in 8.0-13.5 d postfertilization, and larvae swimup 13 d after hatching (Bozek et al. 1984; Marsh 1985). Early life stages of razorback sucker concealed in the spawning substrate (coarse sand to cobble) might be at least partially protected from electrofishing current. However, Dwyer et al. (1993) demonstrated that eggs of Yellowstone cutthroat trout Oncorhynchus clarki bouvieri placed in artificial redds and covered by about 15 cm of gravel can be killed by the same level of electric current having a negative effect on survival of eggs placed free in nylon-mesh baskets within a treatment chamber. Sensitivity of razorback sucker embryos to electric fields decreased with advancing developmental stage in all treatments, but mean survival at each developmental stage beyond early epiboly remained significantly lower than that for controls except in the 30-Hz and CPS treatments. Mean survival of embryos in the 60-Hz treatments was inversely related to peak-voltage gradient. These observations are consistent with results of similar studies on other fishes. Godfrey (1957) found that sensitivity of embryos of Atlantic salmon Sa/mo safar and brook trout Sa/velinus fontinalis to 550-V, 1.7-A DC was low during the first few hours of development (water hardening; pre-cleavage stages), high for stages from early cleavage to end of epiboly, and low again thereafter. He also reported that mortality increased with increasing exposure time (30-300 s) and field intensity (150 V, 0.6 A or 550 V, 1.7 A). Newman and Stone (1992) evaluated the viability of 24 and 48-h (postfertilization) walleye eggs exposed to quarter-sine wave, pulsed DC at 120 Hz, 400 V, and 3 A. Eggs were put in nylon-mesh bags that were placed on typical walleye spawning substrate in about 46-cm deep water and subjected to a single pass of an electrofishing boat (minimum exposure duration that electrofishing in the wild would produce). Mortality was 64% for the 24-h eggs and 56% for the 48-h eggs; mortality for shocked eggs at each stage of development was significantly higher than that for eggs in controls. Incubation temperatures and developmental state of eggs at treatment were not reported by Newman and Stone, but walleye eggs incubated at 150C reached mid-cleavage in about 24 h after fertilization and early epiboly in about 48 h (McElman and Balon 1979). Dwyer et al. (1993) exposed eggs of rainbow trout O. mykiss at 2-26 d postfertilization for 10 s to a homogenous, pulsed-DC (square-waveform) electric field of 240 Hz and 0.9-1.0 mean V/cm; eggs hatched on day 28. Mean percent mortality for eggs treated on days 4-10 was significantly greater than that in treatments on days 2 or 12-26 and controls; mortality on days 2 or 12-26 was not significantly different from corresponding controls. Among treatments, highest mortality occurred in eggs shocked on day 8, and differences were significant. In another experiment, Dwyer et al. (1993) determined that mortality of Yellowstone cutthroat trout eggs exposed 8 d after fertilization for 5, 10, or 20 s to CPS current in a homogenous field increased with increasing voltage intensity ranging 12