Electroshocking-induced mortality of four fish species during posthatching development

Immediate mortality of electroshocked fish was related to the stage of development after hatching. Larvae and juveniles of four species of fish were exposed to homogeneous voltage gradients (2-16 V/cm) of 60-Hz pulsed DC in water having an ambient conductivity of 100 muS/ cm. Recently hatched fish and fish older than 100 d did not die after electroshocking. The developmental period most susceptible to electroshocking-induced mortality was near the time of transformation from larvae to juveniles for all four species. The highest predicted mortality occurred 22 d after hatching or at 19-21 mm total length (TL) for bluegill Lepomis macrochirus, 36-41 d or 29-32 mm TL for largemouth bass Micropterus salmoides, 42-43 d or 27-30 mm TL for channel catfish Ictalurus punctatus, and 38 d or 21 mm TL for Nile tiapia Oreochromis niloticus. During a 5-d observation period, there was no delayed mortality of channel catfish. A field validation experiment with 25-d-old largemouth bass (14-22 min TL) indicated that mortality in the laboratory or around an electrofishing boat did not differ if voltage gradients were similar.