Electrofishing-induced injury to fish: Effect of species, developmental stage, and electric-field characteristics

The electric fields generated by electrofishing equipment can injure fish; however, the electrical and biological variables important in determining injury are not well understood. Objectives of this study were to define electric-field characteristics of electrofishing boats and to evaluate the effects of similar electric fields on fish in relation to species and stage of development. The overall objective was to provide recommendations for reducing electroshocking-induced injury of fish.; For seven electrofishing boats, voltage gradients were heterogeneous around the boat and ranged from 2.1–3.4 V/cm within 1 m of the bow of the boat to 16–20 V/cm within 5 cm of the anode droppers. Three boats had malfunctioning equipment indicating the need to test electroshocking equipment routinely.; In laboratory experiments with homogeneous electric fields, survival of fish was related to species, stage of development, and electric-field characteristics. Embryos of largemouth bass Micropterus salmoides and bluegill Lepomis macrochirus had low survival rates after exposure to DC, but PDC had little or no effect on survival. Channel catfish Ictalurus punctatus embryo survival was reduced by both DC and PDC and were more susceptible than either largemouth bass or bluegill embryos. Newly transformed juvenile fish was the developmental stage with the highest immediate (1 h) mortality after electroshock, and no delayed (5 d) mortality occurred. Mortality increased with voltage gradient, pulse frequency, pulse width, and duration of exposure. At constant electrical power, mortality of newly transformed juvenile fish was highest when electroshocked in water of 72–230 μS/cm ambient conductivity. Largemouth bass mortality in the laboratory did not differ from mortality induced around an electrofishing boat in a pond when voltage gradients were similar. Blackbanded darters Percina nigrofasciata were more susceptible to electroshocking-induced mortality than 9 other species, while newly transformed juvenile paddlefish Polyodon spathula and adult western mosquitofish Gambusia affinis were most resistant. Grossly visible injuries were observed in fewer than 3% of fish that were electroshocked, but 27–7% (depending on species) had injuries detected during histological examination. Lesions described by histopathology included: necrosis of skeletal muscle, vertebral injury, notochord hernia, and hemorrhage. Results indicate that fish early life stages can be injured during electroshocking; however, electrofishing procedures can be modified and electric fields can be selected to reduce electroshocking-induced injury.