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.