A Predictive Risk Model for Electroshock‐Induced Mortality of the Endangered Cape Fear Shiner

Abstract

We evaluated the effects of a single electroshock on injury and mortality of hatcheryreared Cape Fear shiners Notropis mekistocholas (N ϭ 517), an endangered cyprinid. Groups of 18-22 Cape Fear shiners were exposed to DC, 120-Hz pulsed DC (PDC), or 60-Hz PDC at voltage gradients of 1.1, 1.9, or 2.7 V/cm for 3 s. Mortality occurred only among fish exposed to 120-Hz PDC (25%) and DC (38%) applied at 2.7 V/cm. Because no mortality occurred in Cape Fear shiners exposed to 60-Hz PDC, this waveform was selected for further study of electroshock duration (3, 6, 12, 24, or 48 s) and voltage gradient (0.9, 1.6, or 2.3 V/cm). Most fish electroshocked in the experiments were immobilized (ceased swimming motion). No physical injury was detected by necropsy or radiography in any fish. Electroshock-induced mortality of Cape Fear shiners showed a strong multivariable relationship to voltage gradient, electroshock duration, and fish length. Fish subjected to 60-Hz PDC at 0.9 or 1.6 V/cm for 6 s experienced low mortality (Ͻ10%). Our results demonstrate that Cape Fear shiners can be immobilized by 60-Hz PDC electroshock without injury or significant risk of mortality. We propose that electrofishing may be safely used to sample similar small cyprinids, imperiled or otherwise, when electrofishers select an appropriate waveform (DC pulsed at 60-Hz or less) and use it judiciously (minimal exposure at, or below, the immobilization threshold).

Unintended Effects of Electrofishing on Nongame Fishes

“…An understanding of the harmful effects is important because electrofishing is possibly the most widespread method for sampling game fish populations in the United States (Bonar et al 2009). Moreover, such surveys frequently encounter populations of rare and occasionally imperiled fish species (Nielsen 1998;Holliman et al 2003;Poos et al 2007). For some of the study species, our findings show no obvious patterns.…”

“…Depending on the power of the generator and the size of the electrified field required to collect the target species, electrofishing with continuous DC is feasible in water with intermediate water conductivities but is usually unfeasible in locations with low or high conductivities (Reynolds 1996). Considering that mortality for bystander fish was highest at low frequencies while injuries were highest at high frequencies, intermediate frequency levels may be most appropriate when pulsed DC is applied, a fact that has been recognized by other authors (e.g., Holliman et al 2003;Dolan and Miranda 2004). However, to substantially reduce harmful effects, electrofishing must use continuous DC.…”

Susceptibility of Ten Fish Species to Electroshock‐Induced Mortality

“…In electrofishing situations, voltage gradients are heterogeneous, and the position of the fish changes relative to the electrodes. While the effect of exposure duration on fish mortality has not been evaluated in heterogeneous electric fields, prior research indicates that in homogeneous fields, longer exposure durations lead to higher fish mortality for all species tested (Collins et al 1954;Whaley et al 1978;Holliman et al 2003). The ability to quantify the electroshock exposure is an advantage of studies conducted in homogeneous electric fields.…”

“…Homogeneous electric fields with defined electrical characteristics have been used to evaluate electroshock effects on fish (Collins et al 1954;Dolan and Miranda 2003;Holliman et al 2003). However, except for a comparison of two species by Whaley et al (1978), multiple-species comparisons of mortality after exposure to homogeneous electric fields have not been conducted.…”

“…In a study of four fish species, newly metamorphosed juveniles had higher mortality following electroshock than younger or older fish (Henry et al 2003a). Studies of electroshock-induced mortality of older fish offer conflicting results, including higher mortality for smaller fish (Habera et al 1996), higher mortality for larger fish (Holliman et al 2003), or no difference in mortality with respect to fish size (Collins et al 1954;Pratt 1955;Whaley et al 1978). The frequency of spinal injury can increase with fish size (Hollender and Carline 1994;Thompson et al 1997); however, there is no established relationship between spinal injury and mortality of electroshocked fish (Spencer 1967;Habera et al 1996;Henry and Grizzle 2003).…”