Effects of Pulsed and Continuous DC Electrofishing on Juvenile Rainbow Trout

Three hundred fifty juvenile rainbow trout Oncorhynchus mykiss, reared under captive conditions, were exposed to 300-V continuous DC or low-frequency (30-Hz) pulsed DC (PDC) for one or three electroshocking passes to evaluate the effects of electroshock on mortality, injury, and growth over 147 d. Mortality was negligible (∼1%) in all treatments. Injury rates varied from 15% to 39%, with PDC causing a greater number (but typically less severe) injuries than analogous DC sampling. Multiple-pass sampling designs caused more spinal injuries than single-pass designs. Longer (heavier) fish sustained more spinal injuries. Electroshocking reduced mean growth rates, but there were no statistically significant differences in growth between treatment groups. Growth in length was significantly reduced with increasing severity of injuries. Thus, it appears that growth was not directly impaired by electroshocking but rather by the occurrence of spinal injury, the severity of which was directly proportional to the magnitude of the growth depression. Extrapolation of the experimental data to field studies in which 20% or less of the population is sampled suggested reductions of 3% or less in mean population growth with DC or low-frequency PDC electroshocking.