Electroshocking‐Induced Injuries in Newly Transformed Juvenile Fish

Electroshocking‐induced injuries in fish have not been described histologically, and little is known about such injuries in fish that have recently completed the transformation from larva to juvenile. We electroshocked newly transformed juvenile channel catfish Ictalurus punctatus , rainbow trout Oncorhynchus mykiss , striped bass Morone saxatilis , largemouth bass Micropterus salmoides , bluegill Lepomis macrochirus , and Nile tilapia Tilapia nilotica (also known as Oreochromis niloticus ). Fish were exposed for 5 s (bluegills only) or 20 s to homogeneous electric fields of 30‐Hz to 120‐Hz pulsed DC. Only 2.8% of the survivors ( N = 800) and 0.7% of the fish that died immediately after electroshocking ( N = 600) had gross injuries. Hemorrhage in the posterior trunk or anterior tail was the most common gross lesion, occurring in 17 fish. Nine of these fish were examined histologically, and 8 had an injured vertebra, sometimes associated with a notochord hernia, at the location of hemorrhage. In largemouth bass, bluegills, and channel catfish that survived electroshocking without gross injuries, a vertebral lesion occurred in 17% ( N = 12) of the bluegills exposed for 5 s (8 V/cm) and in 40% ( N = 15) of the channel catfish (4 V/cm). No vertebral injuries occurred in 16 bluegills exposed for 20 s (4 V/cm) or in 22 largemouth bass (2 V/cm). Necrotic skeletal muscle was the most common histological lesion, occurring in 60% of the channel catfish, 64% of the bluegills, and 18% of the largemouth bass that survived electroshocking without gross injuries. Necrotic muscle was also found in one control largemouth bass (8%) and one control bluegill (20%). The types and frequencies of lesions were similar in fish that survived and in those that died after electroshocking.