Effects of Electrofishing Fields on Captive Juveniles of Two Endangered Cyprinids

Abstract

We conducted laboratory experiments to address concerns about potential harmful effects of electrofishing on juvenile humpback chubs Gila cypha. Four types of square‐wave pulsed DC in homogeneous fields were tested: 30 Hz (12% duty cycle), 60 Hz (24% duty cycle), 80 Hz (40% duty cycle), and a complex pulse train of three 240‐Hz, 2.6‐ms pulses delivered at 15 Hz (12% duty cycle). We first determined peak‐voltage gradients for each current sufficient to induce the electroshock responses of taxis, narcosis, or tetany in captive‐reared early juvenile humpback chubs (49–96 mm total length) and bonytails G. elegans (46–79 mm). Bonytails were intended as surrogates for humpback chubs in most subsequent tests. However, mean voltage‐gradient response thresholds were 8–43% lower for humpback chubs than for bonytails. We then exposed 30 humpback chubs for 10 s to the complex pulse current at mean tetanizing field intensity and 60 bonytails for 10 s to one of the four currents at mean field intensities required for each of the three responses. All humpback chubs and half of the bonytails were euthanized, frozen, and later examined for internal injuries; remaining bonytails were reared for 98 d to assess effects on growth and survival. No mortalities, external injuries, or vertebral injuries were observed in either species. Moderately severe spinal hemorrhages were found in 20% of shocked humpback chubs and 13% of shocked bonytails. Shocked humpback chubs had a significantly higher (P ≤ 0.05) incidence of injuries than unshocked control fish. The number of injured bonytails was significantly higher (P ≤ 0.05) than controls in the 80‐Hz taxis treatment and the 30‐, 60‐, and 80‐Hz tetany treatments. Differences in the number of shocked bonytails with injuries among currents at each response threshold and among response thresholds for each current were not significant. No significant differences in injury rates were detected between humpback chubs and bonytails exposed to the complex pulse current at tetanizing field intensities. Growth of bonytails was not affected by shocking. Results suggest that electrofishing could cause spinal hemorrhages in some early juvenile humpback chubs but does not affect short‐term growth or survival. Studies are needed to evaluate the significance of electrofishing injuries in humpback chubs at the population level.

Electroshock‐Induced Injury and Mortality in the Spotfin Chub, a Threatened Minnow

“…Lack of vertebral damage in our study was consistent with results from other studies on small cyprinids (Cape Fear shiner: Holliman et al 2003; juvenile humpback chub Gila cypha and bonytail G. elegans: Ruppert and Muth 1997). Electroshock can induce hemorrhage associated with the vertebral column in small fish (Ruppert and Muth 1997;Cooke et al 1998).…”

Electroshocking-Induced Mortality of Four Fish Species during Posthatching Development

“…No delayed mortality occurred for channel catfish observed for 5 d after exposure, even though 68% had died during the first hour after electroshocking. Previous studies reported low rates of delayed mortality (Ͻ10%) for other species (Hudy 1985;Barrett and Grossman 1988;Schneider 1992;McMichael 1993;Habera et al 1996;Ruppert and Muth 1997;Cooke et al 1998), although previous studies did not consider the developmental stages we found to be the most sensitive to the lethal effects of electroshocking.…”

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

“…On the other hand, neither single nor multiple exposures to DC electrofishing had significant effects on survival of mottled sculpin Cottus bairdi over a 30-d period (Barrett and Grossman 1988). Survival of humpback chub Gila cypha and bonytails G. elegans, endangered cyprinids, was not affected by electroshock though there was evidence of broken blood vessels along the spinal column in some bonytails (Ruppert and Muth 1997). Cooke et al (1998) found that electrofishing induced little mortality (4-9%) in greenside darters E. blennoides and noted broken blood vessels and vertebral damage in some fish.…”

“…Based on prior studies on small fish, we identified electrical waveform, pulse frequency, voltage gradient, electroshock duration, and fish length as potential risk factors for mortality of electroshocked Cape Fear shiners. Whether these variables influence injury in small fish is not known, but injury to Cape Fear shiners was a concern because electroshock-induced injury has been reported in fish about 40-100 mm long (Ruppert and Muth 1997;Cooke et al 1998).…”