7874 - Laserfiche WebLink

Home Browse Search

250-700 V (2.4-6.7 peak V/cm). They also demonstrated a significant positive relationship between mortality and exposure duration at 400 V but not at 250, 550, or 700 V. Dwyer and Erdahl (1995) concluded that voltage level is more critical to mortality of cutthroat trout eggs than either waveform or pulse rate. We found inverse relationships between mean percent survival of razorback sucker embryos and both voltage gradient and pulse rate. Survival of embryos shocked at early tailbud or finfold was significantly greater in the 30-Hz (1.2-V/cm) treatment compared to the 80-Hz (1.2-V/cm) treatment, but effects of pulse duration (4-ms pulses at 30 Hz and 5-ms pulses at 80 Hz) could not be evaluated with our study design. Our results and those of other studies suggest that fish embryos are most susceptible to electro-shock mortality during the period from early cleavage through end of epiboly (Le., when the blastoderm has overgrown the yolk, near beginning of the eyed stage). This is a critical period of cell division and morphogenesis (Long 1984). Mechanisms (physiological reactions or physical damage) responsible for increased mortality in early embryos exposed to electric currents are unknown. However, Dwyer et al. (1993) compared effects of electrical and mechanical shock on developing rainbow trout embryos and found that ontogenetic patterns in sensitivity to either type of shock were similar. Hayes (1949) and Post et al. (1974) reported that salmonid embryos are most sensitive to mechanical shock during the early to mid-epiboly stage. Hayes (1949) suggested that mortality induced by mechanical shock was due to rupturing of the vitelline membrane. Reduced growth of razorback sucker larvae exposed to electrofishing fields in the wild could lower their chances of survival. Miller et al. (1988) and Rice et al. (1993) suggested that lower mean growth rates of fish early in life can cause reduced survival by prolonging the period of vulnerability to starvation and predation. Starvation and predation by nonnative fishes have been hypothesized as factors limiting survival of razorback sucker larvae, and existing evidence suggests that predation is a significant cause of larval mortality (Minckley 1983; Marsh and Langhorst 1988; Minckley et al. 1991). Survival of fish in early life stages often determines year-class strength of populations. Houde (1987) reported that relatively minor variations in survival, growth, or developmental rate of fish early life stages can cause substantial fluctuations in recruitment success. Muth and Ruppert (in press) found that exposure of ripe razorback sucker to homogeneous, square-wave 60-Hz (24% duty cycle) or CPS currents at 1.0 peak V/cm for 10 s injured fish, caused premature expulsion of gametes, and significantly reduced egg hatching success. Results of this study suggest that electrofishing of razorback sucker spawning aggregations could significantly harm embryos and early larvae, and in doing so possibly further limit recruitment. Results support the recommendation by Muth and Ruppert (in press) that the need for electrofishing over active razorback sucker spawning areas should be carefully evaluated. References Bozek, M. A., L. J. Paulson, and J. E. Deacon. 1984. Factors affecting reproductive success of bonytail chubs and razorback suckers in Lake Mohave. Final report. U.S. Bureau of Reclamation, 14-16-0002-81-251, Boulder City, Nevada. 13